Amplifier Anatomy - Part 1

By: Patrick Quilter,
Chief Technical Officer QSC Audio Products, Inc.

What do power amplifiers do? Power amplifiers drive loudspeakers. After an audio signal has been mixed, equalized and otherwise processed at a standardized line level, it is sent to the power amplifier. Its job is to increase the power of the signal until we get the desired sound level from the loudspeakers, without otherwise altering the waveform of the signal.

We need to talk about loudspeakers for a moment. A loudspeaker is an electromagnetic device that converts electric current into motion at audio frequencies. Because of the weight of the cone and the unavoidable resistive losses in the voice coil, it takes a lot of power to produce a high sound level. We know (especially if we read last month’s article, “Mr. Ohm and His Talking Electrons”) that electric power is a product of voltage and current.

Loudspeaker cone movement is proportional to the current in the voice coil. The amount of heat in the amplifier components is also proportional to current. However, it takes voltage to make the current flow, so a power amplifier must deliver high voltage and current simultaneously. Loudspeakers are generally made with a voice coil resistance of about 8 ohms, so the amplifier must produce 8V across the loudspeaker terminals to cause a 1A current to flow. This means that ideally the amplifier works into an impedance of 8 ohms.

In the real world, the impedance of loudspeakers is more complex. As the cone moves, it generates electrical back pressure, which can increase or decrease the current flow from the amplifier.

Because of interactions with air pressure in the loudspeaker cabinet, the cone motion varies greatly, especially in the bass region. Therefore, the loudspeaker’s impedance varies at different frequencies, and may range from 4 ohms to 20 ohms, averaging about 8 ohms. In addition, more than one 8 ohm loudspeaker may be connected to the amplifier. For these reasons, most professional amplifiers are built to work into impedances as low as 2 ohms, which will draw up to four times the normal current. The amplifier must also withstand very high impedances in case the load is disconnected. This requirement is normally not a problem because the current flow with no load is zero.

AMPLIFIER POWER We all know that the amplifier’s power rating tells us how loud the amplifier will get. A “200W at 8 ohm” amplifier is designed to deliver 40V to an 8 ohm loudspeaker, resulting in 5A of current (40V divided by 8 ohms). Of course, 40V times 5A yields the amplifier rating of 200W.

If we want to double the cone motion, we have to double the current, from 5A to 10A. Because the loudspeaker impedance is still 8 ohms, it will take 80V to get 10A. Therefore, the power rating must increase from 200W the 800W (80V multiplied by 10A). You can see why the power ratings escalate pretty quickly in high-output systems.

HOW DO POWER AMPLIFIERS WORK? An amplifier is basically an ac-to-dc power converter. It takes ac power from the wall outlet (at fixed frequency and voltage) and converts it to audio power at the loudspeaker terminals (with variable frequency and voltage). The audio output is supposed to be a faithful replica of the line-level audio input, only larger.

Let’s look a little further into the block diagram, at some of the major sub systems inside a power amplifier. We will explain more about each sub-system later in the article.

First we need a power supply. This subsystem accepts the ac power from the wall, isolates the audio circuitry from shock hazard, raises or lowers the ac voltage to suit the needs of the amplifier power rating, converts the ac power to dc, and stores it in an energy reservoir.

The other major subsystem is the output section. This is the electronic circuitry that accepts the linelevel audio input and uses this information to control high-power transistors. These convert the energy contained in the dc reservoir to a high-power audio waveform that is a magnified replica of the input signal AMPLIFIER PERFORMANCE LIMITATIONS All amplifiers have a maximum power limit. The voltage at the amplifier output can only go as high as the voltage in the dc power supply. If the signal tries to exceed this limit, it ”hits the ceiling,” and the waveform becomes flattened. This problem, called clipping because it looks like the top of the waveform has been clipped off, results in the familiar ”blatting” sound of an overdriven amplifier.

Increasing the supply voltage adds cost and weight to the amplifier, so amplifier power has a big effect on price.

Amplifiers have a minimum rated output impedance, which should be equal or less than the impedance of the loudspeaker load. As the impedance of the loudspeaker gets lower, more current will be drawn from the amplifier. This is why, up to a point, the amplifier power rating increases into lower impedances. However, the increased current puts a greater strain on the amplifier components and the power supply. At some minimum impedance, the strain will get so high that the power-supply voltage sags or the transistors overheat. Any further decrease in impedance will cause the amplifier circuitry to collapse, resulting in less power, or it could even cause amplifier failure.

Amplifiers also must reproduce all audio frequencies, from the highest to the lowest, at equal volume.

This ability is called flat frequency response because the graph of amplifier gain vs.

frequency is a flat line.

If the gain at low frequencies falls off, the sound will be thin or lacking in impact. If the high frequency gain rolls off, the sound will be dull or muffled. Most modern direct-coupled amplifiers are capable of very flat response, but sometimes the frequency response is intentionally limited to protect the loudspeakers from excessive power at frequencies we can’t hear.

MORE ABOUT THE POWER SUPPLY Why do we have to convert the ac power from the wall into dc power, and then back to ac? The ac power from the wall is at a fixed voltage and frequency, which are completely different from the audio voltages and frequencies. If we tried to use the ac voltage “as is” for a power supply, we would only be able to reproduce small parts of the audio waveform.

We have to convert the ac power into a fixed dc source, and provide enough energy storage to carry us through the periods where the ac voltage is passing through zero. This way, the audio output section has the power available to respond at anytime as required by the input signal.

So, let’s take a more detailed look at the amplifier’s power supply.

The ac power comes into the amplifier through the ac cord, is controlled by the on/off switch, and usually goes through a fuse or circuit breaker, which cuts off ac power in case of massive overload.

It then reaches the power transformer, which is in the heart of the power supply. A transformer consists of two coils of wire around a common magnetic core.

The ac power is connected to the first or primary winding, which converts electric energy into magnetic energy. This magnetism flows through the iron core to the secondary coil, which converts it back to electricity.

Why do we do all of this? The two coils are insulated from each other, so that the secondary coil is isolated from any shock hazard in the primary coil. The ac voltage and current can also be scaled up or down by changing the number of turns in the secondary coil. Transformers are son useful that they are the major reason we use ac power distribution instead of dc (transformers only work on ac).

The simplest and least expensive transformer is the E-I type, which is generally cubic-shaped (roughly equal height, length, and width). This type is widely used, but it has a tendency to give off hum, which might be picked up by nearby circuitry.

The U-I type is more expensive, but it is easier to make in a flatter shape that can fit into low-profile amplifiers. It also reduces the hum emissions. The toroidal type is built on a donut-shaped core, which has the best magnetic properties. It can be made quite flat, it weighs somewhat less and is has low hum emissions, but it is the most expensive. At QSC, we use the U-I transformer for most of our low-profile amplifiers because it offers most of the advantages of the toroid at a lower cost.

Once we have scaled and isolated the ac power through a transformer, we need to convert it to dc.

This is the job of the rectifier and dc filter capacitors.

The rectifier is a one-way ratchet that takes the back-and-forth flow of ac current and redirects it so that is always flows in the same direction. The rectifier uses diodes, which permit current flow in one direction and block flow in the reverse direction.

A full-wave bridge rectifier circuit uses four diodes. Each diode passes current only in the direction of the arrow. If you follow the current flow around the circuit, you will see that, no matter which way the ac current is flowing into the rectifier, it always emerges in the same direction. Now we have the dc instead of ac (the current only flows in one direction), but it still has big valleys in it. We need to smooth out this ripple voltage. This is the job of the filter capacitors.

Capacitors are like tanks that hold electricity. Once you fill them, it takes a while to drain them out.

Therefore, we connect a large capacitor to the output of the rectifier. The capacitor fills, or charges up, to the peak voltage of the rectified wave-form. If the capacitor is large enough, it stays pretty full between the peaks, and we get an almost perfectly smooth dc voltage. The electronic value of the filter capacitors determines how well the ripple voltage is removed.

In the last 20 years, high-value capacitors have been considerably reduced in size. The high-density filter capacitors are easier to mount on the circuit board right next to the power transistors, which helps improve the high-frequency performance of the amplifier. The length of wiring between the old-style capacitors and the transistors can introduce a slight inductance or electronic lag, which prevents the transistors from instantly drawing power from the supply.

POWER-SUPPLY REGULATION Another concern with power supplies is regulation: the ability to hold the dc supply voltage constant, despite changes in amplifier loading or ac voltage.

The first characteristic is called load regulation, or sometimes power-supply stiffness, and basically depends on the resistance in the transformer. An ideal transformer would have zero resistance and would be able to maintain a constant voltage (perfect regulation) no matter how little or how much current the amplifier needs. Real-world transformers have resistance in the wire coils, which causes the supply voltage to drop when current flow increases. To minimize this voltage drop, thicker wire must be used, which increases the size and weight of the transformer.

Earlier solid-state amplifiers used rather large transformers to keep the no-load voltage (at rated power). The designers needed to minimize no-load voltage because high-voltage transistors were expensive, if not impossible to get. In recent years, the cost of high-voltage transistors has come down, so the trend has been toward somewhat smaller transformers to reduce the weight of amplifiers, even though the no-load voltage rebounds to a higher level.

A side effect of the voltage drop is that, because the filter capacitors charge up to higher voltage during periods of low demand, the amplifiers can deliver a momentary burst of power above its normal rating. This feature, called dynamic headroom, can add 2dB or 3dB of peak undistorted power, which is equivalent to having up to 100% more wattage.

The ability to hold a constant voltage despite ac voltage fluctuation is called line regulation. Ordinary passive power supplies, such as the transfomer-rectifier-capacitor system discussed earlier, do not offer line regulation. The dc supply voltage changes along with any change in the ac voltage. The power company usually tries to maintain a constant voltage, but heavy loads or long ac cables can cause voltage drops, which result in loss of amplifier power.

Companies that use a great deal of amplification, such as touring companies, must invest a lot of money in heavy-gauge ac cabling (distro systems) to minimize this effect. Until switching power supplies become more practical, the correction to this problem would unfortunately add cost and weight to the amplifier, so the tendency has been to spend the same money making bigger amplifiers.

You get the desired minimum power under worst-case conditions, and you come out ahead when ac service is normal.

SWITCHING POWER SUPPLIES The size and weight of power-supply components has been somewhat reduced over the last 20 years, but progress has been slow because we are only refining the same basic technology. Meanwhile, other industries, such as the computer industry, have been perfecting light-weight switching supplies reduce the size and weight of the power transformer by operating it at a much higher frequency. For reasons beyond the scope of this article, high-frequency transformers are much smaller that low-frequency transformers. However, we are stuck with the 50Hz or 60Hz ac power supplied by the power company, so if we want to use high-frequency transformers, we must generate our own high-frequency power, which results in a fairly complicated block diagram.

First, we rectify the incoming AC and smooth it with capacitors, just as we did with the passive supply described above, but without an ac transformer. Then we use a high-speed switching transistors to convert the dc power to a high-frequency ac waveform, usually 50kHz to 100kHz (about 1,000 times higher than normal ac power). This high-frequency ac is fed to a small high-frequency transformer, which isolates the secondary from ac shock hazard and scales the voltages, just as the large ac transformer did in the passive supply. This high-frequency ac voltage is the rectifier and filtered again, resulting in the final dc supply for the amplifier. The active supply is much more complicated than the passive supply, but the weight of the components is much less.

Although active supplies are more expensive, costs are slowly coming down, and there are important advantages. In addition to the primary benefit of greatly reduced weight, we can control the operation of the high-frequency transistors to compensate for variations in ac voltage and load currents, thus improving both kinds of power-supply regulation. The ultimate result will be more consistent amplifier performance, but the audio industry must solve problems of cost, reliability and radio/TV interference caused by the high-frequency switching. This will undoubtedly be an active area of progress in the decade of the 1990s.

MORE ABOUT THE AUDIO OUTPUT CIRCUIT The story of the actual power amplifier circuit begins with the input connectors. These humble components are crucial to getting a high-quality signal into the amplifier because garbage in is garbage out. In addition to corrosion-proof plating and strong mounting, it’s a big help to have balanced inputs, which are now fairly standard. Balanced inputs permit the amplifier to ignore most forms of interference that occur in the cabling between electronic units.

Most amplifiers also have a gain control. It is usually operated full up, but it is handy to be able to reduce gain for testing or to lower the noise floor when you know you have input volume to spare.

After the balanced-input and gain-control circuitry, we enter the actual power amplifier circuit.

The main function of this circuit section is to increase the input signal from about 1V to about 100V, and to increase the current from about 0.1mA to about 30A. This is a power gain of about 30 million! To understand how this occurs, we need to discuss how transistors work.

Transistors (and tubes in earlier years) are variable-resistance elements that are connected between the dc supply and the load (the loud-speaker). The transistor acts as a valve. A small input signal causes a much larger amount of current to flow from the dc supply to the load. The device controls a load current about 50 to 100 times greater that the input current, so the device has a gain of 50 to 100.

To increase the gain, we can cascade devices by driving a second transistor with the output of the first transistor, and so on. This way, we can build up the tremendous gains we need. The exact method of cascading is on of the major differences among amplifier designs, and it would double the length of this article to fully review these methods, However, we can give some basic terminology.

The last, or highest-power, set of transistors are called output transistors. These are high-power devices mounted on large heat sinks. The outputs are driven at a much lower power by driver transistors. Sometimes there are pre-drivers before the drivers, though in some cases it is possible to go to small-signal devices. QSC amplifiers use a very-high-gain integrated circuit (called an opamp), followed by a relatively simple 2-stage set of driver and output transistors.

The large load current is ideally a magnified replica of the small input current. However, for a number of reasons, the load current might no be an exact replica; it might be distorted. The most obvious kind of distortion is clipping, which occurs when the voltage across the load comes so close to the dc voltage that the transistor saturates, or bottoms out, and can’t go any further. A lesser form of distortion occurs because the transistor’s gain is not uniform: It varies because of temperature and current differences.

All of these effects are called non-linearities because the transistor deviates from the ideal of uniform magnification, much like wavy glass makes straight lines look crooked. We will explain how distortion is minimized later in the article.

Another major problem with transistors is that they are one-way devices: They only handle positive or negative currents. Therefore, we need a way to connect positive and negative devices together to deliver a complete audio waveform. This method is called push-pull operation and has been the key to high power performance since early tube amplifiers. There are a number of ways to combine push-pull currents.

Yet another problem is that of heat loss. Let’s say we connect a transistor to a 100V supply, but for the moment we only ask it to deliver 40V to an 8 ohm load. We know (from the example used earlier) that 5A of current will flow in the load, resulting in 200W of output power. That same 5A must flow through the transistor to get to the load. At the same time, the “unused” 60V appears across the transistor. We have a combination of 5A and 60V in the transistor, which results in 300W (5A x 60V) of power in the transistor.

A basic low of physics states that energy cannot be destroyed, only changed. Because we aren’t letting this unused power go to the load, it has to go somewhere, and the result is waste heat. This example show that it is very easy for the power wasted in transistors to exceed the power delivered to the load. This waste heat is the reason powerful amplifiers need large heat sinks, which dissipate the unwanted heat. Otherwise, the transistors would get so hot that they would fail.

Although I won’t burden the reader with excess detail about different ways to cascade transistors in amplifiers, the final push-pull output transistors can be combined in general ways that affect distortion and heat loss. These categories, called classes of operation, were defined many years ago to allow discussion of these trade-offs. You have probably heard of class A, class B, class AB and other lettered designations for amplifiers. Here’s a brief explanation of their meaning (and it has nothing to do with USDA meat grades).

CLASS A This is the easiest class to understand, so it leads the list. The positive and negative output transistors each handle 100% of the audio signal- they are biased so their zero-signal output current idles halfway between zero and maximum. When the audio current in one transistor increases, the current in one transistor increases, the current in the other decreases; as a result, their voltage move together. Each transistor can, therefore, deliver a faithful replica of the signal all by itself, except for the large idle current.

If we were to connect just one transistor to the loudspeaker, we would hear fair-quality sound, but that would force the cone way off center and would probably overheat the voice coil. When we connect both transistors to the load, the idle current from one transistor is absorbed by the other (rather than going through the load), but the audio currents reinforce each other and appear nicely in the load.

The primary advantage of class-A operation is inherent lack of distortion. The full waveform is preserved in the positive and negative transistors, so there is no trick to combining their currents.

However, a serious flaw is the extreme heat loss at idle. The transistors actually run hottest while “standing still”- sort of like controlling a car’s speed with the brakes while keeping the throttle mashed down. Naturally, amplifier designers have looked for other ways to ensure low distortion without such wasteful operation.

CLASS B If we are careful, we can let each transistor control only its half of the waveform. When the waveforms are combined properly, we still get the complete output waveform, but we have eliminated the large idle current. The amplifier runs much cooler because no power is used until it’s needed.

The trick, of course, is to get seamless combining. If the waveforms don’t joined together perfectly, we get zero-crossing distortion (frequently called crossover distortion). This kind of distortion is quite objectionable because it results in a slight gargling or rattling sound during quiet parts of the program, where the signal is near zero.

Fortunately, there are a number of ways to eliminate this problem. One popular method is to compromise between class A and B and operate the amplifier in class AB. Bu permitting a small idle current to flow, we get a small amount of idle heat, but we eliminate any chance of “dead space” between the positive and the negative waveforms.

CLASS C When each transistor controls less than 50% of the waveform, we call this mode class C. This mode is not usable for audio because of the large gap between the waveforms, which causes severe zerocrossing distortion. Class C is used where such distortion is unimportant or can be tuned out by other circuitry. In some amplifiers, the output transistors are run in class C for less idle heat, with the driver transistors filling in the gap. This method is called class ABC.

CLASSES D, E, & F These classes apply to switching amplifiers, which will be explained in the second half of this article.

CLASS G This mode uses two or more sets of output transistors connected to different supply voltages. The goal is to reduce the heat loss in class A or B amplifiers. Remember the example in which we had a 100V power supply but we only needed 40V into the load? We had a lot of waste heat because there was 60 “unused” volts wasted in the output transistors.

In a class G amplifier, we have on set of transistors connected to a lower voltage supply, say 60V, which supplies all output voltages up to this value. We then transfer to a second set of transistors connected to the 100V supply. With this method, the ”unused” voltage for a 40V output is cut from 60V to 20V, dramatically reducing the waste heat. Because an amplifier spends most of its time supplying only a fraction of its power, the average losses can be cut by 50% or more.

The main problem is to ensure seamless transfer from the low-voltage to the high-voltage transistors to avoid any small glitches similar to zero-crossing distortion. QSC Series Three and the original MX series amplifiers used this technique quite successfully.

CLASS H This class uses a single bank or output transistors connected to a low-voltage supply, along with some means of switching them to a higher-voltage supply when required. This method has the same thermal benefits as class G, but it avoids the second bank of output transistors, thus reducing the size and cost of the amplifier.

The QSC EX series uses this technique to pack more power in the same chassis. (The EX4000 has twice the power of the old MX2000.) The new MXa series uses the same technique to simplify the construction and to improve reliability.

Most of these methods require some alteration of the audio signal as it is broken apart and reassembled.

Not surprisingly, these alterations result in errors in the reassembled waveform. In Part II, we’ll discuss how error-correction circuitry, protection circuitry and other design features allow the amplifier to perform its function without altering the waveform.

Courtesy QSC Audio. Used by permission.
QSC manufactures high quality amplifiers and amplified loudspeakers.
Article in Sound & Video Contractor Feb. 20, 1993
David McLain | Loudspeaker Guy! | CCI SOLUTIONS
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PO Box 481 / 1247 85th Ave SE
Olympia, WA 98507-0481
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Recorderman Overhead Drum Mic Technique

Here is a simple way to capture an entire drum kit with just two microphones as pioneered by the infamous "Recorderman." By using a string to align both the bass drum and snare in the very center of the stereo image, you will get a very solid center channel and reasonably balanced sound from the kit. You can also compliment this setup with close mikes for a more traditional sound.

This has a whole lot of sound for a two mic system. Normally, I would probably add a snare mic and a kick drum mic for a bit more depth.

If the video's not working right, click on the title ("Recorderman Overhead Drum Mic Technique") or just visit

David McLain | Loudspeaker Guy! | CCI SOLUTIONS
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PO Box 481 / 1247 85th Ave SE
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The Ultimate Mic Shootout: a 12Gauge Shotgun!

You've probably figured out that "Shure Mike" is not entirely sane in his passion to prove that Shure mics - and especially the SM58 - are durable. Here his hunting buddies set up for "The Ultimate Mic Shootout!" Enjoy the mayhem.

As usual, if the video's not working right, click on the title ("The Ultimate Mic Shootout: a 12Gauge Shotgun!").

David McLain | Loudspeaker Guy! | CCI SOLUTIONS
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PO Box 481 / 1247 85th Ave SE
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Signal Processing Fundamentals: Dynamics

Dynamic Controllers

Dynamic controllers or processors represent a class of signal processing devices used to alter an audio signal based solely upon its frequency content and amplitude level, thus the term "dynamic" since the processing is completely program dependent. The two most common dynamic effects are compressors and expanders, with limiters and noise gates (or just "gates") being special cases of these.

The dynamic range of an audio passage is the ratio of the loudest (undistorted) signal to the quietest (just audible) signal, expressed in dB. Usually the maximum output signal is restricted by the size of the power supplies (you cannot swing more voltage than is available), while the minimum output signal is fixed by the noise floor (you cannot put out an audible signal less than the noise). Professional-grade analog signal processing equipment can output maximum levels of +26 dBu, with the best noise floors being down around -94 dBu. This gives a maximum dynamic range of 120 dB (equivalent to 20-bit digital audio) -- pretty impressive number -- but very difficult to work with. Thus were born dynamic processors.


Compressors are signal processing units used to reduce (compress) the dynamic range of the signal passing through them. The modern use for compressors is to turn down just the loudest signals dynamically. For instance, an input dynamic range of 110 dB might pass through a compressor and exit with a new dynamic range of 70 dB. This clever bit of processing is normally done using a VCA (voltage controlled amplifier) whose gain is determined by a control voltage derived from the input signal. Therefore, whenever the input signal exceeds the threshold point, the control voltage becomes proportional to the signal's dynamic content. This lets the music peaks turn down the gain. Before compressors, a human did this at the mixing board and we called it gain-riding. This person literally turned down the gain anytime it got too loud for the system to handle.

You need to reduce the dynamic range because extreme ranges of dynamic material are very difficult for sound systems to handle. If you turn it up as loud as you want for the average signals, then along comes these huge musical peaks, which are vital to the punch and drama of the music, yet are way too large for the power amps and loudspeakers to handle. Either the power amps clip, or the loudspeakers bottom out (reach their travel limits), or both -- and the system sounds terrible. Or going the other way, if you set the system gain to prevent these overload occurrences, then when things get nice and quiet, and the vocals drop real low, nobody can hear a thing. It's always something. So you buy a compressor.

Using it is quite simple: Set a threshold point, above which everything will be turned down a certain amount, and then select a ratio defining just how much a "certain amount" is. All audio below the threshold point is unaffected and all audio above this point is compressed by the ratio amount. The earlier example of reducing 110 dB to 70 dB requires a ratio setting of 1.6:1 (110/70 = 1.6). The key to understanding compressors is to always think in terms of increasing level changes in dB above the threshold point. A compressor makes these increases smaller.From our example, for every 1.6 dB increase above the threshold point the output onlyincreases 1 dB. In this regard compressors make loud sounds quieter. If the sound gets louder by 1.6 dB and the output only increases by 1 dB, then the loud sound has been made quieter.

Some compressors include attack and release controls. The attack time is the amount of time that passes between the moment the input signal exceeds the threshold and the moment that the gain is actually reduced. The release time is just the opposite -- the amount of time that passes between the moment the input signal drops below the threshold and the moment that the gain is restored. These controls are very difficult to set, and yet once set, rarely need changing. Because of this difficulty, and the terrible sounding consequences of wrong settings, Rane correctly presets these controls to cover a wide variety of music and speech -- one less thing for you to worry about.

System overload is not the only place we find compressors. Another popular use is in the makingof sound. For example when used in conjunction with microphones and musical instrument pick-ups, compressors help determine the final timbre (tone) by selectively compressing specific frequencies and waveforms. Common examples are "fattening" drum sounds, increasing guitar sustain, vocal "smoothing," and "bringing up" specific sounds out of the mix, etc. It is quite amazing what a little compression can do. Check your owner's manual for more tips.

Figure 8. Gate/Expander/Compressor/Limiter Action


Expanders are signal processing units used to increase (expand) the dynamic range of the signal passing through it. However, modern expanders operate only below the set threshold point, that is, they operate only on low-level audio. Operating in this manner they make the quiet parts quieter. The term downward expander or downward expansion evolved to describe this type of application. The most common use is noise reduction. For example, say, an expander's threshold level is set to be just below the quietest vocal level being recorded, and the ratio control is set for 2:1. What happens is this: when the vocals stop, the signal level drops below the set point down to the noise floor. There has been a step decrease from the smallest signal level down to the noise floor. If that step change is, say, -10 dB, then the expander's output attenuates 20 dB (i.e., due to the 2:1 ratio, a 10 dB decrease becomes a 20 dB decrease), thus resulting in a noise reduction improvement of 10 dB. It's now 10 dB quieter than it would have been without the expander.


Limiters are compressors with fixed ratios of 10:1 or greater. Here, the dynamic action prevents the audio signal from becoming any bigger than the threshold setting. For example, say the threshold is set for +16 dBu and a musical peak suddenly comes along and causes the input to jump by 10 dB to +26 dB, the output will only increase by 1 dB to +17 dBu -- basically remaining level. Limiters find use in preventing equipment and recording media overloads. A limiter is the extreme case of compression.

You will hear the term pumping used in conjunction with poorly designed or improperly set limiters. Pumping describes an audible problem caused by actually hearing the gain change -- it makes a kind of "pumping" sound. This is particularly a problem with limiters that operate too abruptly. Rest assured that Rane limiters are designed not to have any audible side-effects.

Noise Gates

Noise gates (or gates) are expanders with fixed "infinite" downward expansion ratios. They are used extensively for controlling unwanted noise, such as preventing "open" microphones and "hot" instrument pick-ups from introducing extraneous sounds into your system. When the incoming audio signal drops below the threshold point, the gate prevents further output by reducing the gain to "zero." Typically, this means attenuating all signals by about 80 dB. Therefore once audio drops below the threshold, the output level basically becomes the residual noise of the gate. Common terminology refers to the gate "opening" and "closing." A gate is the extreme case of downward expansion.

Just as poorly designed limiters can cause pumping, poorly designed gates can cause breathing. The term breathing is used to describe an audible problem caused by being able to hear the noise floor of a product rise and lower, sounding a lot like the unit was "breathing." It takes careful design to get all the dynamic timing exactly right so breathing does not occur. Rane works very hard to make sure all of its dynamic processors have no audible funny business.

Another popular application for noise gates is to enhance musical instrument sounds, especially percussion instruments. Correctly setting a noise gate's attack (turn-on) and release (turn-off) adds "punch," or "tightens" the percussive sound, making it more pronounced -- this is how Phil Collins gets his cool snare sound, for instance.

PDF "Signal Processing Fundamentals" This note in PDF.

Courtesy Rane . Used by permission.

David McLain | The Processor Guy! | CCI SOLUTIONS
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PO Box 481 / 1247 85th Ave SE
Olympia, WA 98507-0481
Voice: 800/426-8664 x255 / Fax: 800/399-8273

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Cell Towers and Your Church

Navigating the challenges of a potential lease agreement
by Steve Kazella

“Catch for us the foxes, the little foxes that ruin the vineyards, our vineyards that are in bloom.” Song of Solomon 2:15

Besides the obvious financial benefits of having a cellular-phone tower built on your church property, there are many hidden pitfalls that churches entering into a cell tower lease with a wireless carrier need to be aware of.

My friend and church minister, Larry, got a new cell phone from his wife recently for his birthday. It was a model that didn’t have a camera, the ability to surf the Internet, send text messages, play games or do any high-tech applications. It was just a cool cell phone with a keypad, LCD screen and voice mail. He loved the gift – until he saw a commercial for the exact same phone being marketed to baby boomers as a cell phone with big buttons that was easy to dial.

The high-tech cell phones of today are mini wireless computers that eat up a lot more bandwidth than the devices of just a few years ago, fueling the increased demand for as many as 100,000 more new cell towers across the United States that will need to be built in the next five to 10 years. Churches that are prepared to lease space for cell tower development can avoid many of the problems experienced by other congregations and get rid of those annoying little foxes that want to ruin their vineyards. The first step is familiarizing yourself with some cell tower leasing and development tips.

1. Watch Your Lease and Tax Language Closely

Fortunately, most congregations have a good CPA on their financial committee who can provide localized tax advice on a state-by-state basis. Make sure that you are protected by having specific tax language included in your cellular carrier lease, which protects you from being hit with tax assessments because of an improvement on church premises. Most carriers want the lessor to pay the taxes upfront, and they then reimburse them within 60 to 90 days. Many churches don’t have the ability to do that, so you want to make sure the language in your lease protects the church from paying these fees upfront – and places the responsibility on the carrier, in the rare event your municipality decides to collect from you.

2. Count the Costs of Leasing before You Build

One of the biggest problems you’ll face with your initial cell tower lease and future co-location (bringing additional wireless carriers to your site) is having the budget to hire proper representation for dealing with wireless carriers. Carrier site acquisition contractors know that a church might not be able to afford a top-notch cell tower attorney to negotiate on its behalf, and the carriers take advantage of that. Many cell tower leasing consultants are actually paid big bonuses for bringing in a cheaper cell tower lease with better terms for the carrier. Churches often end up retaining good real-estate attorney generalists, or they’ll have an attorney who is a member review and negotiate the lease pro bono. However, they usually have no or very limited cellular site leasing experience and end up agreeing to unfavorable terms or negotiating their clients entirely out of the deal. Because cell tower leases are complicated and highly specialized, the added expense of retaining an experienced wireless attorney is recommended.

3. Select a Location Suitable for Tower Placement

Proper tower placement is essential. You want to make sure that a cellular site is not built anywhere that could impact future growth and expansion of your church.

4. Have a Professional Review Your Cell Site Design

At the end of the day, the congregation is not going to select the type of cell tower that gets built. Your municipality will ultimately decide on what design (steeple, bell tower, flagpole, fake tree, monopole, lattice tower) is acceptable, and within those parameters, you’ll have the choice to approve or reject what the carrier proposes. In many cases, a lattice tower or monopole looks better than a “stealth tree.” Decide whether you want a one- or two-carrier wireless steeple or steeple replacement, an 80-foot, three-carrier flagpole or a 110-foot to 150-foot, four- to five-carrier tower. As a rule of thumb, the higher the tower, the more potential you have for expansion and maximizing the revenue the site can generate. Have you ever heard the joke about the guy who hadn’t been to church in 20 years; he walked in one Sunday, and the roof crashed in on him? Make sure it doesn’t happen at your church. Spend $500 to have a local licensed structural engineer review all proposed wireless construction plans the carriers are proposing, especially on your roof.

5. Understand the Health Concerns

Your neighbors who you are reaching out to may be concerned about the rash of baseless negative articles that the wireless industry as a whole has not done a very good job of rebutting. For most, the biggest fear is the fear of the unknown, and the overwhelming majority of people don’t know anything about cell tower health issues except the negative articles that the cell-phone-owning media seem to publish regularly. Make sure your group is unified on the cell tower issue before you move forward.

6. Be Shrewd When Dealing with the Carriers

All wireless leases are heavily slanted in favor of the cellular service providers – and you don’t actually deal with the carriers directly, rather, with their subcontracted site leasing consultants and attorneys. Unfortunately, on the leasing end of the industry, the majority of vendors have limited wireless leasing experience, and have never worked both sides (working internally at a carrier or owning a cell tower property). Additionally, there are a number of charlatans negotiating cell tower leases. Carriers and their vendors are not going to tell you what kind of subletting language to include to maximize revenue for your church. Nor will they offer advice on how to protect your congregation’s ground space or rooftop space rights, proper right of first refusal, indemnification, tax language or proper co-location terms.

7. Avoid Death by Committee

Many cell tower leasing consultants can learn to recite the Book of Leviticus faster than it takes many churches to decide to move forward on a cell tower lease. If a carrier contacts your church about a cell tower but you only check your answering machine on Sundays, chances are you’ll be looking at the Elk’s Club’s cell tower for the next 25 years instead of it benefiting your church. Although the Bible teaches us to be quick to listen and slow to speak, it also tell us to be prepared to answer. Make sure that phone numbers of your pastor, deacon of facilities, administrator or church Web site are posted or listed, and that the carriers can reach you within 24 hours of initial site identification, or they will find another location.

How Can You Get a Cell Tower on Your Church Property?

Make sure that you are at least a mile from the nearest tower. Your church should not be in a flood plain or listed on a historical registry, have any environmental issues or have any endangered species nesting in your steeple.

Churches have several options for getting the cellular carriers to consider their sites for wireless development. They can: ...

•Wait for a wireless carrier or one of their real-estate site acquisition consultants to approach them. It’s essential to respond within 24 hours or the site acquisition consultant will go to the next location.

Market their sites on their own by submitting information online, directly to the carriers. Unfortunately, this is probably the least-effective method for getting your site selected, but as long as you have a good location and a mustard seed of faith....

Contact a tower construction company who then builds a tower at their expense and markets the site to the carriers. They keep 50 percent to 70 percent of all rental income if you are lucky enough to get a site built.

Use a wireless management company to tap into their industry contacts and attempt to bring a carrier to your site at the carrier’s expense.

Regardless of how you promote your church site, you should assign one point of contact (who is easy to reach) to deal with the carriers. Cell towers will need 600 to 2,000 square feet of ground space, and church steeple cell sites will require up to 1,200 square feet of ground space to locate equipment cabinets. Your facilities deacon or church office secretary should have copies on hand of documents such as site plans, easements, deeds and surveys. I suggest preparing a one-page flier containing a site photograph, church address, property information (block, lot, what zone it’s in), attached copy of a tax map with your church property highlighted, property elevation, steeple or bell tower height, and latitude/longitude.

As much as possible, avoid contacting your municipality to get their opinion about a proposed cellular site at your church. Let the cellular carriers approach the town on all matters pertaining to cell tower development.

These tips are just a starting point in helping churches make sure they protect themselves as they consider entering into a legally binding agreement that will easily fall to the next generation. Remember, if it were easy to get a cell phone tower built at your church, every congregation would have one.

Steven Kazella is the president of New Jersey-based AirWave Telecom Property Management (, a telecommunications consulting firm which partners with property owners and building owners to develop cell towers and rooftop antenna sites with wireless carriers. Kazella’s partners have nearly 50 years’ combined wireless lease negotiation and cell site development experience.

Courtesy Church Solutions magazine. Used by permission.

See also Spires for Hire, about building cell phone towers inside a church steeple.

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