It's always been amusing to watch the band set up. The guitarist brings his amp, a few pedals, and maybe a couple of guitars. The bass player brings his instrument, and often his own amp. The drummer uses the church's drum kit, but he brings his own sticks and takes the time to tune and position the drums to his liking.
But the vocalist just uses whatever mic is handed to them.
My experience has been that the choice of microphone for the vocalists, especially the lead vocalist, has a substantial effect on her sound in the house, her intelligibility, and even her confidence in front of a crowd. Using "whatever they give me" would be like the the guitarist playing "whatever guitar they hand me," whether it's a Fender Squire or a Paul Reed Smith Custom 24 guitar, or the sound guy saying, "Yeah, whatever. Behringer, Midas, Yamaha, Digico: they're all the same."
The point: if you're a vocalist, find a mic that really lets your voice give its best in your facility. If you're the sound guy, then give real thought to what mics sound best on which vocalist, particular the main vocalists. Try out some new ones if you need to, and teach your team that "This is John's mic!" Or encourage John to buy his own vocal mic.
And of course, audio engineers love working with untrained vocalists, who sing away from the mic, lean into the mic for their loud notes, and cup the grille. The reality is that a good sound system will clearly amplify whatever sound (good or bad) that the vocal mic picks up. It is not to the vocalist's advantage to send a poor signal to the sound system.
Audix created this video, and they make some excellent vocal microphones (and some amazing instrument mics), including some at modest prices. Of course, they use Audix mics in these brief clips. But the techniques are appropriate for any handheld vocal microphone.
Note: this post contains a video clip. If you're having a hard time seeing it, click on the title ("Vocal Microphone Technique") to watch the video on the post's home page. And if you want to share the video with your vocalists, use this link: http://j.mp/VocalTechnique.
Two Days Discovering Nexo
I’ve been hearing a lot of talk about Nexo speakers over the
past few years. I’ve listened to various of their models, and I’ve been impressed
enough to recommend them for a couple of rooms, particularly given the
outstanding support I’ve been getting from them.
But I haven’t really known the Nexo lineup; until I hadn’t had
opportunity to listen critically and extensively to their whole selection of
speakers.
I have now.
I spent Monday of this week designing speaker systems for rooms using various software, and I was liking the way the Nexo speakers worked in the planning: the plans looked good, but what did they sound like?
The Cerritos Center in the greater Los Angeles
I had a .dwg model of the Cerritos Center
The NX1 software was easy to work with, so I imported some
smaller real-world rooms that I’m working on in the real world. Yep. Looked easy. Looked like the PS series might sound good.
I have to admit, there’s a fair bit of skeptic in me. Any box that promises me
a rectangular coverage pattern (in this case, Nexo’s PS series) had better do more than just advertise well! It needs
to offer actual rectangular coverage pattern. And more importantly it needs to sound good! But
they sure looked good on the computer screen: a couple of boxes and a sub in a room that seats 400.
 |
| Nexo's PS series (Apologies for the cruddy cell-phone-camera photos.) |
Finished the designs, closed the computer for the night, and headed out for dinner with 40 engineers and another dozen or so from Yamaha/Nexo (Yamaha distributes Nexo in the
Next morning, we head over to the
I fell in love.
Let me just cut to the chase: I never knew “Sound on a Stick” could sound that good. I measured 112 dB at the back of the listening room, and I have to say, it sure didn’t feel like 112 dB. We were playing vocal tracks (I got so tired of “Bird on a Wire” this weekend! I still don’t know the artist.), and it didn’t sound like a PA playing. It sounded like a woman there singing to me. Singing well.
Randy Weitzel had put up a very nice drum kit behind the speakers, and brought in a fine drummer to show them off. We listened to the same drum kit: voiced the same as the original kit, with zero EQ, zero compression: same drum kit, but more of it. We listened with speakers, without speakers. Even the little 8” 2-way sounded way bigger than it was.
(Note: I’m not big on stage monitors, but the Nexo wedges [45N?] were clear, loud, and were so tight in their pattern that even the drum overheads were in the drum monitor!)
The boxes' horns reportedly put out square pattern: I didn’t measure the exact edges of the pattern, but it sure seemed square to me. The coverage was clearly narrower on one end than the other; I could hear that. The previous days' computer exercises seemed to match real world applications.
Then we listened to Nexo’s Baby Line Array: the Geo-S8. An 8” 2-way box, in a couple of 12-box arrays. Pretty good! Clear, articulate, musical, at 90 feet. Fifteen hundred seats of modest folk music would be a fine fit for the baby line arrays, or a few hundred seats of music with teeth! I had done a project with another small line array recently; I'll bet these could have served that room at least as well.Then we needed to pull these down so we could put up another array to test. We took down two dozen S8 boxes and hung two dozen S12 boxes in less than an hour. OK. I’m impressed. That was easy. Let's go to lunch.
 |
| Line Arrays: Geo-D, Geo-T and Geo-12 (outside to inside) |
After lunch, we came back and listened to the S12 boxes, the very boxes I had been using in the design software to fill the computer model of the
They fired them up, and behold: they sounded as the modeling showed: clear, articulate, in the entire room. Well, most of the room; the third balcony was a little weak, but the software predicted that. When we added the subwoofers, it was clearly nothing to complain about. Remarkably smooth. Remarkably even, throughout the room. I like those boxes. And yes, the room might have been a little toward the big side for these speakers. I will have no problem recommending these speakers for a medium size church, and they'll make it sound good!
We listened to the Geo-D boxes (it’s kind of weird, in that it’s pronounced: “G O D”). These are the main boxes for this room, and I can see why: effortless excellence. I walked the entire floor, and three balconies, and maybe a dozen of the loge boxes: the entire room sounded the same! It was a little (!) bit louder in some seats than it was behind them, but the voicing was clear everywhere. I’ve heard it said before, but it was true: there wasn’t a bad seat in the house! These boxes are amazing!
I had taken time to talk with Jack Hayback (away from the Yamaha/Nexo boys) about his experience with the Nexo Geo-D speakers, and how they compared to the two other brands before them. His eyes lit up! He had lots of good things to say, a number of stories, and he compared them to the two (other brands) that he had had before he got the Nexos. This was a sincerely happy audio guy!
More significant, the Lighting Director, John Palmer, told me how clear the audio was when he first heard them. (In my experience, it takes a lot for quality sound to impress the LD!)
Lastly, we listened to the Geo-T: the big dogs. These are the famous boomerang shape that you’ve probably seen on major arena tours around the world, and I can believe it. They shook me to my core at more than 100’, both in clarity and in the solidity of their sound. I can see why the Big Names tour with this gear.
 |
| Subs, in Cardioid (!!) configuration: RS 18 & RS 15 |
We actually spent the next two hours listening to this track
and that (and “Bird on a Wire” on every speaker in the house!), listening loud,
listening quiet, to all these speakers, not because we needed to, but because
we wanted to. Big line arrays,
sounding as good as I’ve ever heard, and more. I have never heard so much detail from James Taylor, Pink Floyd and even
Rammstein, and several I don't know. I’m not used to describing Rammstein as “beautiful,” but I
did this weekend!
It’s not hard for a big line array to play loud music well.
But when they can play the same music softly and gently at 60 dB, with the same
clarity as at 120 dB, then I’m impressed. It still had solid, believable 60 Hz bass guitar at 60 dB! I'm not used to that.
Summary: These are very good speakers. The entire lineup is worth paying attention to, and in my opinion, probably worth their not insubstantial cost.
They are not the
right speakers for everybody; they’re not inexpensive, for one thing.
Anything beyond the PS series probably need professional
installation, and certainly need professional design. And while I haven’t seen
it, I imagine there may be a room that they just don’t fit well in. The PS series can handle aggressive worship music comfortably in a mid-size church, as can the Geo-8. The Geo 12 will do wonders in a large church. And if your room is big enough, the Geo-D will make it sing! The Geo-T is not, in my opinion, needed for anything much smaller than a stadium concert.
But if you have some room in your budget for quality
speakers, do NOT overlook Nexo.
(Obligatory plug: if you would like a speaker system
designed for your room – and you have a budget for it, drop me a line, and we
can talk about Nexo, or a dozen other lines. But we will talk about Nexo. At least for a little bit.)
Getting Started with Stage Design!
Veteran Technical Director and CCI Solutions Church Relations Director, Duke DeJong helps you get started creating modern worship stage designs.
The key is to find out what your resources are. I'm a huge fan of LED and intelligent lights but most churches aren't able to start with those. Many churches have a few spare dimmers and par cans lying around and that can be a great way to get you started. Pick some colors you like, put those gels in your lights and aim them at something reflective. You've now added color to your stage.
Next, find out what type of materials you have to light. I love a stage with darker colored walls so I can put something that lights well in front of it, helping any over flow light to disappear. Some of my favorite things to light include various fabrics like Poly Muslin, Poly Sheen and Spandex (must be fire retardant), Coroplast and even Bubblewrap.
Really anything that reflects light has potential as a design element. You can make great structures out of metal, wood or even PVC and then cover them with something light friendly. The opportunities are endless and to get started you should see what materials people in your church have access to or the ability to work with. Whatever you have access to should dictate what materials you start with.
If you can't come up with an original idea, find some designs from other people that look like something you can do a variation of and simply try it out. If it works first try, awesome! For most of the designs I've done I will see elements I like somewhere and then I do a little tweaking and adjusting to make it something that works right for my space. If something doesn't work just right one week, try tweaking it for next week. The key is to use whatever resources you have access to, find a concept you like and try it out. If you ever want to bounce ideas off of someone or need some ideas to get you started, let me know.
Churches across America both big and small are looking for ways to enhance their worship spaces with color and texture. I love this trend because it brings more visual artistry to the church and it helps our relevance to the younger generations who continue to become more and more visual. For churches looking to enter into the world of stage design, here are a few thoughts to get you started.
Finding Resources!
The key is to find out what your resources are. I'm a huge fan of LED and intelligent lights but most churches aren't able to start with those. Many churches have a few spare dimmers and par cans lying around and that can be a great way to get you started. Pick some colors you like, put those gels in your lights and aim them at something reflective. You've now added color to your stage.
Let there Be Light!
Next, find out what type of materials you have to light. I love a stage with darker colored walls so I can put something that lights well in front of it, helping any over flow light to disappear. Some of my favorite things to light include various fabrics like Poly Muslin, Poly Sheen and Spandex (must be fire retardant), Coroplast and even Bubblewrap.
Reflective Properties!
Really anything that reflects light has potential as a design element. You can make great structures out of metal, wood or even PVC and then cover them with something light friendly. The opportunities are endless and to get started you should see what materials people in your church have access to or the ability to work with. Whatever you have access to should dictate what materials you start with.
Wrapping it Up!
If you can't come up with an original idea, find some designs from other people that look like something you can do a variation of and simply try it out. If it works first try, awesome! For most of the designs I've done I will see elements I like somewhere and then I do a little tweaking and adjusting to make it something that works right for my space. If something doesn't work just right one week, try tweaking it for next week. The key is to use whatever resources you have access to, find a concept you like and try it out. If you ever want to bounce ideas off of someone or need some ideas to get you started, let me know.
Duke DeJong
Church Relations Director
CCI Solutions
Duke has more than 12 years of experience as a technical artist, trainer and collaborator for ministries. Duke travels around the country for CCI Solutions and is available to help your ministry. Join Duke on Facebook at www.facebook.com/ccisolutions.
The Art (and Necessity) of Compression
By Duke DeJong
Church Relations Director, CCI Solutions
A while back, I got the rare opportunity to work with the youth band at our church. These guys have an incredible heart and passion to worship and have loads of raw talent which translates into a powerful time of worship. When they lead, as a worshipper I feel free and emboldened to praise God the way He created me too. When they lead, as a sound man I have to work as hard and quick as ever to create a decent mix to help facilitate that worship.
As is the case with most youth bands and even many churches, they are not using state of the art or high dollar gear for their services. Now don't get me wrong, they are not operating with the bare minimum. The system includes an Allen and Heath console, JBL speakers and subs, and solid system and signal processing. What I longed for that night was individual compressors.
Maybe this never happens to you, but in a mix including 3 vocals, an un-caged drum set, two electrics, acoustic, bass, and keyboard, I had a hard time keeping the vocals out on top to lead the group in worship while keeping the music strong. The vocalists on the team are gifted in leading worship, but for a variety of reasons (key of the song, dynamic range, mic etiquette, etc) their volumes were all over the place that night and the second I took my finger off their faders I would either lose them or have way too much of them. With 7 stage monitors, acoustic drums, 3 amps and a very small stage, I was dreaming for a few compressors to help me layer the mix the way I wanted.
The compressor is one of the sound man's most useful tools - yet I am always surprised how few people seem to understand and know how to effectively use this critical piece of gear. I would like to help a few more of you get comfortable using compressors.
The clearest definition of compression that I've ever seen is this: "Compression is the art of making louder parts of a composition appear softer, and conversely, the softer parts appear louder." That night, if I would have left the lead singer's fader in one spot for the entire night his volume alone would have ranged anywhere from 85 dB to 120 dB. Alright that might be an exaggeration but he got loud. When he was closer to the 85-95 dB volume he could barely be heard over the drums and guitars. Neither end of the spectrum is really acceptable in a good mix, so compression comes along and makes it possible to narrow down that volume range to make things more mixable.
Let's say I have a 20 dB range between a vocalist's quiet singing versus their loudest singing. With a compressor I can take that 20 dB range and make it as small as a 1 dB range, but since I don't want to eliminate the artistic dynamic range that the singer is using to create the mood or feel of what they are singing, I can get that range down to a very manageable 5-8 dB that will make mixing significantly less complicated but still leave some of that dynamic in place. So how do we get our compressor to do that? With some understanding of the compressor's settings you can be on your way to a smoother sound and a less stressful time behind the board.
In simple terms the threshold is the point where the compressor starts to do its thing. Since there is a wide range of compressor and mixer brands I'm going to talk about these settings more generically as opposed to using the numbers on the knob. If the input meter on your console (let's say negative infinity to +15 dB) matches that of your compressor, things will be a little clearer as the numbers will match. You must first set the gain (or trim) of your channel on your mixer (on my regular console that is around +3, or typically where the green lights first turn to yellow or maybe the yellow light just starts to glow on the meter). Now if your numbers match, and your vocal meter is showing signal between -5 and +10 dB, I'd start with my threshold set close to 0 dB. If your numbers don't match, once your gain is set turn your threshold knob and find the area where the gain reduction knobs just come on. Begin with your threshold there and if you find it's not compressing frequently or soon enough you can lower the threshold from there to make it kick in sooner.
This one is a relatively simple concept. The ratio simply says for every x dB the source goes up in volume, the compressor will only let the output go up y dB. For example, if you set a vocal mic with a 3:1 ratio, for every 3 dB the vocal increases coming into the board, the output will only increase 1 dB. You can think of the ratio as setting the size potential of the source. If you want it to be able to go bigger, you can leave your ratio smaller. If you want it to stay a little smaller, or perhaps be more under control, you can set your ratio higher. I tend to start with a ratio of 3:1 for most vocals and guitars, and often times I will go 4:1 or even 5:1 on drums or very dynamic guitars. My preference is to start low and if you need more compression (less range) you can always increase the ratio. The reason it is my preference is simply this, I don't want to take away any more control from the musicians than is absolutely necessary to make the mix work well. If I start it at 5:1 when 3:1 will do and don't adjust it down, I may be holding that source back. If I start low and it's still too big, I can easily adjust my ratio up.
The attack is how quickly the compressor responds to the volume change. A slower attack will sound a little smoother, rounding out the sound of your source a little bit and in essence making it sound a little "fatter". A slower attack will generally be less noticeable which can be good for vocals and some thin or scratchy guitars. Setting your attack to a faster setting can be great for instruments such as drums or any other very aggressive instruments. A faster attack will give an instrument more of an aggressive, pumping feel, and potentially bring out more of the high end edginess. The ultimate decider on where to set this is by listening. I tend to set vocals a little slower, guitars in the middle, and drums faster to start. From there, if you need a little more aggressiveness or snap you can speed up the attack, and if it needs to be a little smoother or fuller you can slow it down. As in all things with sound, let what you hear guide your settings and adjust until you are happy.
The release is the back side of the attack, and sets how quickly you want to release the compression once that loud burst is over. As with the attack, a slower release will sound smoother and less noticeable but could end up taking some of the aggressiveness out of aggressive instruments by compressing them when they don't need to be. I again will tend to start a little slower for vocals, middle of the road for guitars, and faster for drums. You'll want to again experiment with where to set this by listening to the sound. If the source sounds like it is pumping a little bit, slow the release down to help even it out a bit. If it feels like you might be losing something on the next note/beat, you likely need to speed the release up a bit. Again, let the sound of the source guide you to where it should be set. Listen and adjust until it sounds right to you.
Most compressors have an output to help boost the volume of the end result, and here's where I tend to see a lot of mistakes made. Now that you've taken that 85 to 105 dB vocal and compressed it down to a manageable 85 to 93 dB, you may need to increase the output a little to get it over those guitars and drums. Instead of reaching for the gain or trim knobs (which would then bring more signal into the compressor and would change how you've set your compressor), if you add 5 dB of gain to your output you just took that 85-93 dB and made it 90-98 dB.
Compressors are a huge help to the sound man and used right they will help you get great sound out of your sources and give you the ability to get the mix where you want it. Compressors are especially useful in the worship environment where the voice of those leading the worship must always be present but not piercing, where more and more guitars are being used to lead the music but can't overtake the vocals, and where many churches use acoustic drums.
I truly believe that no one setting is right for any vocal or instrument. If you start with a lower basic setting and then adjust based off of what you are hearing, your compressors can give you a great edge to get your mix balanced and layered according to plan. Just remember, you don't want to compress something more than you need to. If you're having trouble keeping a source in it's place in the mix the compressor is the tool to help you make that happen.
Church Relations Director, CCI Solutions
A while back, I got the rare opportunity to work with the youth band at our church. These guys have an incredible heart and passion to worship and have loads of raw talent which translates into a powerful time of worship. When they lead, as a worshipper I feel free and emboldened to praise God the way He created me too. When they lead, as a sound man I have to work as hard and quick as ever to create a decent mix to help facilitate that worship.More Compressors Please!
As is the case with most youth bands and even many churches, they are not using state of the art or high dollar gear for their services. Now don't get me wrong, they are not operating with the bare minimum. The system includes an Allen and Heath console, JBL speakers and subs, and solid system and signal processing. What I longed for that night was individual compressors.
Keeping the Vocals on Top!
Maybe this never happens to you, but in a mix including 3 vocals, an un-caged drum set, two electrics, acoustic, bass, and keyboard, I had a hard time keeping the vocals out on top to lead the group in worship while keeping the music strong. The vocalists on the team are gifted in leading worship, but for a variety of reasons (key of the song, dynamic range, mic etiquette, etc) their volumes were all over the place that night and the second I took my finger off their faders I would either lose them or have way too much of them. With 7 stage monitors, acoustic drums, 3 amps and a very small stage, I was dreaming for a few compressors to help me layer the mix the way I wanted.
Sound Man's Most Useful Tool!
The compressor is one of the sound man's most useful tools - yet I am always surprised how few people seem to understand and know how to effectively use this critical piece of gear. I would like to help a few more of you get comfortable using compressors.
So What is Compression Really?
The clearest definition of compression that I've ever seen is this: "Compression is the art of making louder parts of a composition appear softer, and conversely, the softer parts appear louder." That night, if I would have left the lead singer's fader in one spot for the entire night his volume alone would have ranged anywhere from 85 dB to 120 dB. Alright that might be an exaggeration but he got loud. When he was closer to the 85-95 dB volume he could barely be heard over the drums and guitars. Neither end of the spectrum is really acceptable in a good mix, so compression comes along and makes it possible to narrow down that volume range to make things more mixable.
Example of How an Audio Compressor Works
Let's say I have a 20 dB range between a vocalist's quiet singing versus their loudest singing. With a compressor I can take that 20 dB range and make it as small as a 1 dB range, but since I don't want to eliminate the artistic dynamic range that the singer is using to create the mood or feel of what they are singing, I can get that range down to a very manageable 5-8 dB that will make mixing significantly less complicated but still leave some of that dynamic in place. So how do we get our compressor to do that? With some understanding of the compressor's settings you can be on your way to a smoother sound and a less stressful time behind the board.
The Control Elements Found on an Audio Compressor!
Threshold
In simple terms the threshold is the point where the compressor starts to do its thing. Since there is a wide range of compressor and mixer brands I'm going to talk about these settings more generically as opposed to using the numbers on the knob. If the input meter on your console (let's say negative infinity to +15 dB) matches that of your compressor, things will be a little clearer as the numbers will match. You must first set the gain (or trim) of your channel on your mixer (on my regular console that is around +3, or typically where the green lights first turn to yellow or maybe the yellow light just starts to glow on the meter). Now if your numbers match, and your vocal meter is showing signal between -5 and +10 dB, I'd start with my threshold set close to 0 dB. If your numbers don't match, once your gain is set turn your threshold knob and find the area where the gain reduction knobs just come on. Begin with your threshold there and if you find it's not compressing frequently or soon enough you can lower the threshold from there to make it kick in sooner.
Ratio
This one is a relatively simple concept. The ratio simply says for every x dB the source goes up in volume, the compressor will only let the output go up y dB. For example, if you set a vocal mic with a 3:1 ratio, for every 3 dB the vocal increases coming into the board, the output will only increase 1 dB. You can think of the ratio as setting the size potential of the source. If you want it to be able to go bigger, you can leave your ratio smaller. If you want it to stay a little smaller, or perhaps be more under control, you can set your ratio higher. I tend to start with a ratio of 3:1 for most vocals and guitars, and often times I will go 4:1 or even 5:1 on drums or very dynamic guitars. My preference is to start low and if you need more compression (less range) you can always increase the ratio. The reason it is my preference is simply this, I don't want to take away any more control from the musicians than is absolutely necessary to make the mix work well. If I start it at 5:1 when 3:1 will do and don't adjust it down, I may be holding that source back. If I start low and it's still too big, I can easily adjust my ratio up.Attack
The attack is how quickly the compressor responds to the volume change. A slower attack will sound a little smoother, rounding out the sound of your source a little bit and in essence making it sound a little "fatter". A slower attack will generally be less noticeable which can be good for vocals and some thin or scratchy guitars. Setting your attack to a faster setting can be great for instruments such as drums or any other very aggressive instruments. A faster attack will give an instrument more of an aggressive, pumping feel, and potentially bring out more of the high end edginess. The ultimate decider on where to set this is by listening. I tend to set vocals a little slower, guitars in the middle, and drums faster to start. From there, if you need a little more aggressiveness or snap you can speed up the attack, and if it needs to be a little smoother or fuller you can slow it down. As in all things with sound, let what you hear guide your settings and adjust until you are happy.
Release
The release is the back side of the attack, and sets how quickly you want to release the compression once that loud burst is over. As with the attack, a slower release will sound smoother and less noticeable but could end up taking some of the aggressiveness out of aggressive instruments by compressing them when they don't need to be. I again will tend to start a little slower for vocals, middle of the road for guitars, and faster for drums. You'll want to again experiment with where to set this by listening to the sound. If the source sounds like it is pumping a little bit, slow the release down to help even it out a bit. If it feels like you might be losing something on the next note/beat, you likely need to speed the release up a bit. Again, let the sound of the source guide you to where it should be set. Listen and adjust until it sounds right to you.
Output
Most compressors have an output to help boost the volume of the end result, and here's where I tend to see a lot of mistakes made. Now that you've taken that 85 to 105 dB vocal and compressed it down to a manageable 85 to 93 dB, you may need to increase the output a little to get it over those guitars and drums. Instead of reaching for the gain or trim knobs (which would then bring more signal into the compressor and would change how you've set your compressor), if you add 5 dB of gain to your output you just took that 85-93 dB and made it 90-98 dB.
Especially Useful in Worship Environments!
Compressors are a huge help to the sound man and used right they will help you get great sound out of your sources and give you the ability to get the mix where you want it. Compressors are especially useful in the worship environment where the voice of those leading the worship must always be present but not piercing, where more and more guitars are being used to lead the music but can't overtake the vocals, and where many churches use acoustic drums.
Wrap Up!
I truly believe that no one setting is right for any vocal or instrument. If you start with a lower basic setting and then adjust based off of what you are hearing, your compressors can give you a great edge to get your mix balanced and layered according to plan. Just remember, you don't want to compress something more than you need to. If you're having trouble keeping a source in it's place in the mix the compressor is the tool to help you make that happen.
Duke DeJong
Church Relations Director
CCI Solutions
Church Relations Director
CCI Solutions
A Detailed Guide To Constant-Voltage ("70v") Audio Systems
Clarifying and defining key power aspects with constant-voltage (or high-impedance) systems
Electric power companies have a good idea that has been applied to audio engineering.
When they run power through miles of cable, they minimize resistive power loss by running the power as high voltage and low current.
To do this, they use a step-up transformer at the power station and a step-down transformer at each customer’s location. This reduces power loss due to the I2R heating of the power cables.
The same solution can be applied to audio communications in the form of a constant-voltage system (typically 70 volts in the U.S. and 100V overseas).
Such a system is often used when a single power amplifier drives many loudspeakers through long cable runs (over 50 feet). Some examples of this condition are distributed speaker systems for PA, paging, or low-SPL background music.
BACKGROUND
The label “constant voltage” has been confusing because the voltage is really not constant in an audio program. A better term might be “high impedance.”
A typical high-impedance system is shown in Figure 1. A transformer at the power amplifier output steps up the voltage to approximately 70 volts at full power.
Each loudspeaker has a step-down transformer that matches the 70-volt line to each loudspeaker’s impedance.
The primaries of all the loudspeaker transformers are paralleled across the transformer secondary on the power amplifier.
Figure 1. A typical high-impedance system using a step-up transformer on the amplifier output.
There are three options at the power-amp end for 70-volt operation:
• an external step-up transformer
• a built-in step-up transformer
• a high-voltage, transformerless output
These options are covered in detail later in this article.
The signal line to the loudspeakers is high voltage, low current, and usually high impedance. Typical line values for a 100-watt amplifier are 70 volt, 1.41 amperes, and 50 ohms.
How did the 70-volt line get its name? The intention was to have 100-volt peak on the line, which is 70.7 volts rms.
The technically correct value is 70.7 volt rms, but “70-volt (or “70V") is the common term. There are 70 volts on the line as maximum amplifier output with a sine wave signal. The actual voltage depends on the power amplifier wattage rating and the step-up ratio of the transformer. The audio program voltage in a 70V system might not even reach 70V. Conversely, peaks in the audio program might exceed 70V.
Other high-voltage systems might run at other voltages. Although rare, the 200V system has been used for cable length exceeding one mile.
ADVANTAGES OF 70V OPERATION
As stated before, a 70V line reduces power loss due to cable heating.
That’s because the loudspeaker cable carries the audio signal as a low current.
Consequently you can use smaller-gauge loudspeaker cable, or very long cable runs, without losing excessive power.
Another advantage of 70V operation is that you can more easily provide the amplifier with a matching load. Suppose you’re connecting hundred of loudspeakers to a single 8-ohm amplifier output. It can be difficult to wire the loudspeakers in a series-parallel combination having a total impedance of 8 ohms.
Also it’s bad practice to run loudspeakers in series because if one loudspeaker fails, all of the loudspeakers in series are lost. This changes the load impedance seen by the power amplifier.
With a 70V system you can hang hundreds of loudspeakers in parallel on a single amplifier output if you provide a matching load. Details of impedance matching are covered later. In addition, a 70V distributed system is relatively easy to design, and allows flexibility in power settings.
Let’s compare a standard low-impedance system to a constant-voltage system. Imagine that you want to provide PA for a runway at an airshow. A low-impedance system might employ 30 speaker clusters spaced 100 feet apart, each cluster powered by a 1000W amplifier for extra headroom. A high-impedance version of that system might use only one amplifier providing 140V. The cost savings is obvious.
DISADVANTAGES OF 70V OPERATION
One disadvantage of a 70V system is that the transformers add expense. Particularly if you use large transformers for extended low-frequency response, the cost per transformer may run $70 to $200. Low-power paging systems, or those with limited low-frequency response, can use small transformers costing around $4.95 each. Many loudspeakers are sold with 70V transformers included.
Another disadvantage is that transformers can degrade the frequency response and add distortion. In addition, a 70V line may require conduit to meet local building code.
TRANSFORMERS
The main component of a 70V system is the loudspeaker transformer.
Its secondary winding has taps at various impedances. You choose the tap that matches the loudspeaker impedance.
For example, if you’re using a 4-ohm loudspeaker, connect it between the 4-ohm tap and common.
The primary winding has taps at several power levels. These power taps indicate how much maximum power the loudspeaker receives. For example, suppose you have a 70V transformer with the primary tapped at 10W and the secondary tapped at 8 ohms. Then a loudspeaker rated at 8 ohms should receive 10W at its voice coil when the primary is connected to a 70V line.
Transformers have insertion loss mainly due to resistance. Precise system calculations should take insertion loss into account. These calculations are covered in the Appendix later in this article.
INSTALLATION
With this background in mind, let’s proceed to installation practices. Here’s a basic procedure that neglects transformer insertion loss:
1. Do NOT connect the 70V loudspeaker line to the power amplifier yet.
2. Install a transformer at each loudspeaker location, or use loudspeakers with built-in transformers.
3. Connect each loudspeaker to its transformer secondary tap. The tap impedance should equal the loudspeaker impedance.
4. Connect each transformer primary to the 70V line from the power amplifier. Choose the tap that will deliver the desired wattage to that loudspeaker.
5. Add the wattage ratings of all the primary taps. This sum must not exceed the amplifier’s wattage rating. If it does, change to a lower-wattage primary tap of one or more transformers, or use a higher power amplifier.
6. Connect the 70V loudspeaker line to the 70V output of the amplifier.
As an example, suppose you are setting up a 70V system with 8-ohm loudspeakers and a 60W power amp. Connect the 8-ohm secondary taps to each speaker. Suppose the total loudspeaker wattage is 55 watts. This is acceptable because it does not exceed the amplifier power rating of 60 watts.
Here’s a more detailed procedure that emphasizes impedance matching:
1. Compute the minimum safe load.
The minimum safe load impedance that can be connected to the amplifier is given by:
where
Z = minimum safe load impedance, in ohms.
E = loudspeaker line voltage (25V, 70.7V, 100V, etc.)
P = maximum continuous average power rating of power amplifier, in watts.
An example: For an amplifier rated at 100 watts continuous average power, the minimum load impedance that may be connected safely to the 70.7V output is:
2. Choose transformer taps.
Tap the primary at the desired power level for the loudspeaker, and tap the secondary at the impedance of the loudspeaker. The sum of all the power taps for all the loudspeakers should not exceed the power output of the amplifier.
Note: Changing the power tap also changes the load impedance seen by the amplifier. Raising the power tap lowers the load impedance, and vice versa.
Also, changing the power tap changes the SPL of the loudspeaker. Reducing the power tap by half reduces the SPL by 3 dB, which is a just-noticeable difference in speech sound level.
If a particular loudspeaker is too loud or too quiet, you can change its power tap. Just be careful that the total power drain does not exceed the power output of the amplifier.
3. Connect the loudspeakers together.
Connect all the loudspeaker-transformer primaries in parallel. Run a single cable, or redundant cables, back to the power-amplifier transformer secondary. But DON’T CONNECT IT YET.
4. Measure the load impedance.
Before connecting the load, first measure its impedance with an impedance bridge (a simple low-cost unit is adequate). Here’s why you must do this: If the load impedance is too low, the power amplifier will be loaded down and may overheat or distort. It’s a myth that your can connect an unlimited number of loudspeakers to a 70V line.
If the load impedance measures too low, re-tap all of the loudspeakers at the next-lower power tap. This raises the load impedance. Measure again.
Usually, it’s no problem if the load impedance measures higher than the matching value (the calculated minimum safe load impedance). The system will work, but at reduced efficiency. Typically there is more than enough power available, so efficiency is not a problem.
If for some reason power the power is limited, then the system should be wired for maximum power transfer. This occurs when the measured load impedance matches the calculated minimum safe load impedance. If the load impedance measures above this value, you can re-tap all the loudspeakers at the next-higher power tap and measure again. This tap change lowers the load impedance.
Many people don’t realize that a transformer labeled for use with a specific voltage will work just as well at other voltages. See the constant voltage calculator here. It determines the power delivered from a transformer tap when driven with other than the rated voltage.
PRECAUTIONS
Since a 70V line is relatively high-impedance, it is more sensitive to partial shorts than a low-impedance line. Consequently, you may want to avoid running 70V lines in underground conduit which may leak water.
Use high-quality transformers with low insertion loss. Otherwise, the power loss in the transformer itself may negate the value of the 70V system.
Avoid driving small transformers past their nominal input voltage rating. Otherwise, they will saturate, draw more than the indicated power (possibly overload the amplifier) and will distort the signal.
You may want to insert a high-pass filter ahead of the power amplifier to prevent strong low-frequency transients which can cause core saturation.
The CTs amplifiers include a high-pass filter that can be selected at 70 Hz, 35 Hz, or bypass. The CH amplifiers insert a 70 Hz high-pass filter when placed in high-impedance mode.
POWER-AMPLIFIER OPTIONS
As stated earlier, there are three power-amplifier options for 70V operation: The amplifier might have:
• an external step-up transformer
• a built-in step-up transformer
• a high-voltage, transformerless output
Let’s consider each option.
Amplifier with external transformer
This system is shown in Figure 1 (on page 1). If you use an external transformer, select one recommended or supplied by the amplifier manufacturer.
If you have a conventional amplifier with low-impedance outputs only, and you want 70V or 100V operation, Crown has the needed accessories. The TP-170V is a panel with four built-in autoformers that convert four low-impedance outputs to high impedance. The T-170V is a single autoformer for the same purpose.
Choose a transformer with a power rating equal to or exceeding the wattage of the power amplifier. The turns ratio should be adequate to provide 70.7V at the secondary when full sine-wave power is applied to the primary. Use the following formula for a 70.7V line:
where
T = turns ratio
70.7 = voltage of constant-voltage line
P = amplifier power output in watts
Z = amplifier rated impedance
SQR means square root
Better yet, measure the amplifier’s output voltage at full power into its rated load impedance, and use the formula:
where
T = turns ratio
70.7 = voltage of constant-voltage line
E = measured output voltage at full power into the rated impedance.
Amplifier with built-in transformer
If the transformer is already built into the power amplifier, simply look for the output terminal labeled “70V,” “25V,” “100V,” or “high impedance.”
Amplifier with transformerless, high-voltage output
Figure 2 shows how a power amplifier with a high output voltage can power a distributed system without a step-up transformer.
Figure 2. A constant-voltage system using a high-voltage power amplifier.
Many high-power amplifiers can drive 70V lines directly without an output transformer. For example, Crown CH amplifiers have an auto transformer (except CH 4). CTs amplifiers can provide direct constant-voltage (70V/100V/140V/200V) or low-impedance (2/4/8 ohm) operation.
In Dual Mode, the CTs 600/1200 can power 25/50/70V lines; the CTs 2000/3000 can power 25/50/70/100V lines. In Bridge-Mono mode, the CTs 600/1200 can power 140V lines; the CTs 2000/3000 can power 140V and 200V lines.
With CTs Series amps, one channel can drive low-impedance loudspeakers, while another channel drives loudspeakers with 70V transformers. This makes it easy to set up a system with large, low-Z loudspeakers for local coverage and distributed 70V loudspeakers for distant rooms—all with a single amplifier.
The Crown CTs 2000 adept at providing constant power levels into various loads. In dual mode, it delivers 1000 watts into 2/4/8 ohms and into a 70V line. In bridge-mono mono, it delivers 2000 watts into 4, 8, or 16 ohms, 2000 watts into a 140V line, and 2000 watts into a 200V line.
Crown Commercial Audio series of amplifiers and mixer-amps provide both low-Z and constant-voltage operation. For example, the 180MA and 280MA mixer-amps offer 4-ohm, 70V and 100V outputs.
Pros and cons of transformerless systems
The high-voltage, transformerless approach eliminates the drawbacks of amplifier transformers:
• cost
• weight
• limited bandwidth
• distortion
• core saturation at low frequencies.
On the other hand, transformers are useful to prevent ground loops, ultrasonic oscillations and RFI. Some local ordinances require transformer-isolated systems.
Let’s look at the core-saturation problem in more detail. Sound systems can generate unwanted low frequencies, due to, say, a dropped microphone or a phantom-powered mic pulled out of its connector.
Low frequencies at high power tend to saturate the core of a transformer. The less the amount of iron in the transformer, the more likely it is to saturate.
Saturation reduces the impedance of the transformer, which in turn may cause the amplifier to go into current limiting. When this occurs, negative voltage spikes are generated in the transformer that travel back to the amplifier—a phenomenon called flyback. The spikes cause a raspy, distorted sound. In addition, the extreme low-impedance load might cause the power amplifier to fail.
Some Crown amplifiers are designed with high-current capability to tolerate these low-frequency stresses.
Production amplifiers are given a “torture test.” Each amplifier must deliver a 15-Hz signal at full power into a saturated power transformer for 1 second without developing a hernia!
Many transformers are reactive, so their impedance varies with frequency. Some 8-ohm transformers measure as low as 1 ohm at low frequencies. That’s another reason for specifying an amplifier with high current capability.
CONCLUSION
Using a high-voltage system greatly simplifies the installation of multiple-loudspeaker PA systems. It also minimizes power loss in the loudspeaker cables. If you take care that your load does not exceed the power and impedance limits of your power amplifier, you’ll be rewarded with a safe, efficient system.
APPENDIX: HISTORY OF CONSTANT-VOLTAGE SYSTEMS
In early industrial sound systems, multiple loudspeakers were carefully configured to provide a matching impedance load to the amplifier. But as these systems grew in size, several problems arose: how to connect multiple loudspeakers to the same amplifier without loading it down, how to individually control the sound power level fed to those loudspeakers, and how to overcome the power loss associated with the typically long lines that ran between the power amp and loudspeakers.
By the late 1920s and early 1930s the “step-up, step-down” idea has been applied to loudspeaker lines in what has become known as “constant voltage” distributed systems. (Radio Physics Course 2nd Ed., Radio Technical Publishing co., N.Y., 1931).
Various voltages have been tried such as 25, 35, 50, 70, 100, 140, and 200 volts, but the 70V system has become the most widespread.
After World War II, we find constant-voltage systems depicted in such reference works as Radio Engineering 3rd Ed. (McGraw-Hill, N.Y., 1947). By the end of that decade, several standards had evolved to regulate 70V specifications for amplifiers and transformers. (Radio Manufacturer’s Association, SE-101-A And SE-106, both from July 1949). In the 1950’s we find the use of 70V systems very well established as evidenced by Radiotron Designer’s Handbook 4th Ed. (RCA, N.J., 1953 and Radio Engineering Handbook 5th Ed. (McGraw-Hill, N.Y., 1959).
As component design improved, 70V systems began to achieve high-fidelity status, but there were two weak links in the chain: the step-up and step-down transformers. Good broadband transformers that could resist core saturation and distortion were expensive.
Half of this problem was solved in 1967 When Crown International introduced the DC-300. It was most likely the first high-powered low-distortion solid-state power amplifier capable of directly driving a 70V line without a step-up transformer. And in June 1987, the Macro- Tech 2400 was introduced with the capability of directly driving a 100V line. Thus, today only loudspeaker needs a transformer to step down the voltage.
APPENDIX: TRANSFORMER INSERTION LOSS
Transformers have insertion loss (power loss due mainly to resistance). This loss should be included in system calculations for precision.
Converted to a power ratio, insertion loss can be expressed as
PR = 10 (L/10)
where
PR = power ratio
L = insertion loss in dB (always a positive number).
Some transformer manufacturers compensate for insertion loss by adding extra windings. In that case, the power delivered to the loudspeaker is the rated value of the tap. The primary draws the rated power times the power ratio of the insertion loss.
In this case, you can calculate the primary impedance as follows:
Pt = Ps + L
where
Pt = total power in dBm
Ps = power to the loudspeaker in dBm
L = insertion loss in dB
or
Pt = Ps * L
where
Pt = total power in watts
Ps = power to loudspeaker in watts
L = insertion loss (as a ratio).
Then the primary impedance is calculated as follows:
Z = (70.7)2/Pt = 5000/(Ps * 10(L/10))
where
Z = primary impedance in ohms
Pt = total power in watts
Ps = power to loudspeaker in watts
L = insertion loss in dB.
Other transformer manufacturers do not compensate for insertion loss. In this case, the primary impedance matches its rating. However, the power delivered to the loudspeaker is less than the power applied, due to the insertion loss.
Ps = Ptr/L
where
Ps = power to loudspeaker in watts
Ptr = power drawn by transformer in watts
L = insertion loss (as a ratio)
To determine whether a transformer is compensated, measure the power (E2/Z) delivered to the loudspeaker when connected to 70.7 volts. If it is less than the rated power, the transformer is not compensated for insertion loss.
When making loudspeaker SPL calculations based on sensitivity ratings, subtract the insertion loss in dB from the loudspeaker sensitivity rating (if the transformer is not compensated for insertion loss). In transformers that compensate for insertion loss, the speaker receives the power indicated. Consequently, each transformer draws a little more power from the line than is indicated. The final impedance will be too low if you add power equal to the amplifier power.
With non-compensated transformers, the labeled power is not the power received, so the loudspeaker SPL will be lower than calculated. The impedance will read correctly, but the acoustic output will be lower than expected.
APPENDIX: LINE LOSS
See the line loss calculator here.
REFERENCES
Daniels, Drew. Notes on 70-Volt and Distributed System Presentation, db, March/April 1988.
Davis, Don. Sound System Engineering, 2nd Ed., Indianapolis, Howard W. Sams Co., 1987, pp. 85-87, 402- 405. 138905-1 10-05
This article provided by Crown Audio, via ProSoundWeb.
Understanding Constant-Voltage ("70v") Audio Distribution Systems
25, 70.7 & 100 Volts; U.S. Standards; Just What is "Constant" Anyway?; Voltage Variations -- Make Up Your Mind; Calculating Losses -- Chasing Your Tail
by Dennis A. Bohn
Constant-voltage is the common name given to a general practice begun in the late 1920s and early 1930s (becoming a U.S. standard in 1949) governing the interface between power amplifiers and loudspeakers used in distributed sound systems.
Installations employing ceiling-mounted loudspeakers, such as offices, restaurants and schools are examples of distributed sound systems.
Other examples include installations requiring long cable runs, such as stadiums, factories and convention centers.
The need to do it differently than you would in your living room arose the first time someone needed to route audio to several places over long distances. It became an economic and physical necessity. Copper was too expensive and large cable too cumbersome to do things the home hi-fi way.
Stemming from this need to minimize cost, maximize efficiency, and simplify the design of complex audio systems, thus was born constant-voltage. The key to the solution came from understanding the electric company cross-country power distribution practices. They elegantly solved the same distribution problems by understanding that what they were distributing was power, not voltage.
Further they knew that power was voltage times current, and that power was conserved. This meant that you could change the mix of voltage and current so long as you maintained the same ratio: 100 watts was 100 watts—whether you received it by having 10 volts and 10 amps, or 100 volts and 1 amp. The idea bulb was lit. By stepping-up the voltage, you stepped-down the current, and vice-versa.
Therefore to distribute 1 megawatt of power from the generator to the user, the power company steps the voltage up to 200,000 volts, runs just 5 amps through relatively small wire, and then steps it back down again at, say, 1000 different customer sites, giving each 1 kilowatt. In this manner large gauge cable is only necessary for the short direct run to each house. Very clever.
Applied to audio, this means using a transformer to step-up the power amplifier’s output voltage (gaining the corresponding decrease in output current), use this higher voltage to drive the (now smaller gauge wire due to smaller current) long lines to the loudspeakers, and then using another transformer to step-down the voltage at each loudspeaker. Nothing to it.
U.S. Standards—Who Says?
This scheme became known as the constant-voltage distribution method. Early mention is found in Radio Engineering, 3rd Ed. (McGraw-Hill, 1947), and it was standardized by the American Radio Manufacturer’s Association as SE-101-A & SE-106, issued in July 1949 [1]. Later it was adopted as a standard by the EIA (Electronic Industries Association), and today is covered also by the National Electric Code (NEC) [2].
Basics—Just What is “Constant” Anyway?
The term “constant-voltage” is quite misleading and causes much confusion until understood. In electronics, two terms exist to describe two very different power sources: “constant-current” and “constant-voltage.”
Constant-current is a power source that supplies a fixed amount of current regardless of the load; so the output voltage varies, but the current remains constant.
Constant-voltage is just the opposite: the voltage stays constant regardless of the load; so the output current varies but not the voltage. Applied to distributed sound systems, the term is used to describe the action of the system at full power only. This is the key point in understanding. At full power the voltage on the system is constant and does not vary as a function of the number of loudspeakers driven, that is, you may add or remove (subject to the maximum power limits) any number of loudspeakers and the voltage will remain the same, i.e., constant.
The other thing that is “constant” is the amplifier’s output voltage at rated power—and it is the same voltage for all power ratings. Several voltages are used, but the most common in the U.S. is 70.7 volts rms. The standard specifies that all power amplifiers put out 70.7 volts at their rated power. So, whether it is a 100 watt, or 500 watt or 10 watt power amplifier, the maximum output voltage of each must be the same (constant) value of 70.7 volts.
Figure 1 diagrams the alternative series-parallel method, where, for example, nine loudspeakers are wired such that the net impedance seen by the amplifier is 8 ohms. The wiring must be selected sufficiently large to drive this low-impedance value.
Applying constant-voltage principles results in Figure 2. Here is seen an output transformer connected to the power amplifier which steps-up the full-power output voltage to a value of 70.7 volts (or 100 volts for Europe), then each loudspeaker has integrally mounted step-down transformers, converting the 70.7 volts to the correct low-voltage (high current) level required by the actual 8 ohm speaker coil.
It is common, although not universal, to find power (think loudness) taps at each speaker driver. These are used to allow different loudness levels in different coverage zones. With this scheme, the wire size is reduced considerably from that required in Figure 1 for the 70.7 volt connections.
Becoming more popular are various direct-drive 70.7 volt options as depicted in Figure 3. The output transformer shown in Figure 2 is either mounted directly onto (or inside of) the power amplifier, or it is mounted externally.
In either case, its necessity adds cost, weight and bulk to the installation. An alternative is the direct-drive approach, where the power amplifier is designed from the get-go (I always wanted to use that phrase, and I sincerely apologize to all non-American readers from having done so) to put out 70.7 volts at full power. An amplifier designed in this manner does not have the current capacity to drive 8 ohm low-impedance loads; instead it has the high voltage output necessary for constant-voltage use—same power; different priorities.
Quite often direct-drive designs use bridge techniques which is why two amplifier sections are shown, although single-ended designs exist. The obvious advantage of direct-drive is that the cost, weight and bulk of the output transformer are gone. The one disadvantage is that also gone is the isolation offered by a real transformer. Some installations require this isolation.
Voltage Variations—Make Up Your Mind
The particular number of 70.7 volts originally came about from the second way that constant-voltage distribution reduced costs:
Back in the late ‘40s, UL safety code specified that all voltages above 100 volts peak ("max open-circuit value") created a “shock hazard,” and subsequently must be placed in conduit—expensive—bad.
Therefore working backward from a maximum of 100 volts peak (conduit not required), you get a maximum rms value of 70.7 volts (Vrms = 0.707 Vpeak). [It is common to see/hear/read “70.7 volts” shortened to just “70 volts”—it’s sloppy; it’s wrong; but it’s common—accept it.]
In Europe, and now in the U.S., 100 volts rms is popular. This allows use of even smaller wire. Some large U.S. installations have used as high as 210 volts rms, with wire runs of over one mile.
Remember: the higher the voltage, the lower the current, the smaller the cable, the longer the line. [For the very astute reader: The wire-gauge benefits of a reduction in current exceeds the power loss increases due to the higher impedance caused by the smaller wire, due to the current-squared nature of power.]
In some parts of the U.S. safety regulations regarding conduit use became stricter, forcing distributed systems to adopt a 25 volt rms standard. This saves conduit, but adds considerable copper cost (lower voltage = higher current = bigger wire), so its use is restricted to small installations.
Calculating Losses—Chasing Your Tail
As previously stated, modern constant-voltage amplifiers either integrate the step-up transformer into the same chassis, or employ a high voltage design to direct-drive the line. Similarly, constant-voltage loudspeakers have the step-down transformers built-in as diagrammed in Figures 2 and 3.
The constant-voltage concept specifies that amplifiers and loudspeakers need only be rated in watts. For example, an amplifier is rated for so many watts output at 70.7 volts, and a loudspeaker is rated for so many watts input (producing a certain SPL). Designing a system becomes a relatively simple matter of selecting speakers that will achieve the target SPL (quieter zones use lower wattage speakers, or ones with taps, etc.), and then adding up the total to obtain the required amplifier power.
For example, say you need (10) 25 watt, (5) 50 watt and (15) 10 watt loudspeakers to create the coverage and loudness required. Adding this up says you need 650 watts of amplifier power—simple enough—but alas, life in audioland is never easy. Because of real-world losses, you will need about 1000 watts.
Figure 4 shows the losses associated with each transformer in the system (another vote for direct-drive), plus the very real problem of line-losses. Insertion loss is the term used to describe the power dissipated or lost due to heat and voltage-drops across the internal transformer wiring. This lost power often is referred to as I2R losses, since power (in watts) is current-squared (abbreviated I2) times the wire resistance, R.
This same mechanism describes line-losses, since long lines add substantial total resistance and can be a significant source of power loss due to I2R effects. These losses occur physically as heat along the length of the wire.
You can go to a lot of trouble to calculate and/or measure each of these losses to determine exactly how much power is required [3], however there is a Catch-22 involved: Direct calculation turns out to be extremely difficult and unreliable due to the lack of published insertion loss information, thus measurement is the only truly reliable source of data.
The Catch-22 is that in order to measure it, you must wait until you have built it, but in order to build it, you must have your amplifiers, which you cannot order until you measure it, after you have built it!
The alternative is to apply a very seasoned rule of thumb: Use 1.5 times the value found by summing all of the loudspeaker powers. Thus for our example, 1.5 times 650 watts tells us we need around 975 watts.
Wire Size—How Big Is Big Enough?
Since the whole point of using constant-voltage distribution techniques is to optimize installation costs, proper wire sizing becomes a major factor. Due to wire resistance (usually expressed as ohms per foot, or meter) there can be a great deal of engineering involved to calculate the correct wire size.
The major factors considered are the maximum current flowing through the wire, the distance covered by the wire, and the resistance of the wire. The type of wire also must be selected. Generally, constant-voltage wiring consists of a twisted pair of solid or stranded conductors with or without a jacket.
For those who like to keep it simple, the job is relatively easy. For example, say the installation requires delivering 1000 watts to 100 loudspeakers. Calculating that 1000 watts at 70.7 volts is 14.14 amps, you then select a wire gauge that will carry 14.14 amps (plus some headroom for I2R wire losses) and wire up all 100 loudspeakers. This works, but it may be unnecessarily expensive and wasteful.
Really meticulous calculators make the job of selecting wire size a lot more interesting. For the above example, looked at another way, the task is not to deliver 1000 watts to 100 loudspeakers, but rather to distribute 10 watts each to 100 loudspeakers. These are different things. Wire size now becomes a function of the geometry involved.
For example, if all 100 loudspeakers are connected up daisy-chain fashion in a continuous line, then 14.14 amps flows to the first speaker where only 0.1414 amps are used to create the necessary 10 watts; from here 14.00 amps flows on to the next speaker where another 0.1414 amps are used; then 13.86 amps continues on to the next loudspeaker, and so on, until the final 0.1414 amps is delivered to the last speaker.
Well, obviously the wire size necessary to connect the last speaker doesn’t need to be rated for 14.14 amps. For this example, the fanatical installer would use a different wire size for each speaker, narrowing the gauge as he went. And the problem gets ever more complicated if the speakers are arranged in an array of, say, 10 x 10, for instance.
Luckily tables exist to make our lives easier. Some of the most useful appear in Giddings [3] as Tables 14-1 and Table 14-2 on pp. 332-333. These provide cable lengths and gauges for 0.5 dB and 1.5 dB power loss, along with power, ohms, and current info. Great book. Table 1 below reproduces much of Gidding’s Table 14-2 [4].
References
1. Langford-Smith, F., Ed. Radiotron Designer’s Handbook, 4th Ed. (RCA, 1953), p. 21.2.
2. Earley, Sheehan & Caloggero, Eds. National Electrical Code Handbook, 5th Ed. (NFPA, 1999).
3. See: Giddings, Phillip Audio System Design and Installation (Sams, 1990) for an excellent treatment of constant-voltage system designs criteria; also Davis, D. & C. Sound System Engineering, 2nd Ed. (Sams, 1987) provides a through treatment of the potential interface problems.
4. Reproduced by permission of the author and Howard W. Sams & Co.
Supplied by Rane via ProSoundWeb.
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