Back in “the good old days,” choosing and installing a power amplifier for church sound was a simple proposition. All you needed to do was select an amplifier (or several) with a total output power that came close to the combined power handling of the loudspeakers. Assuming everything was wired up to present a workable load impedance to the amplifier, there really wasn’t much else to do. First, you connected the input signal, either directly from the mixing console or from something like a graphic equalizer. Then, you wired the loudspeakers to the amplifier output terminals. That’s it. Done deal.
Oh, as for those two knobs on the front, there were basically two schools of thought: set them at two o’clock, or turn them all the way up.
In other words, the amplifiers of that era were essentially “dumb boxes” that were designed to work tolerably well when inserted between a variety of input signals and any connected loads. That approach worked reasonably well with “mid-fi” analog components at the front end, and with loudspeakers that were either columns of 6-inch cones or a fifteen-and-a-horn with a passive crossover.
However, that was way back when. To paraphrase a spiritually elusive songwriter, the times they have a’changed. Today’s new worship sound systems place more complex demands on the power amplifier, and require greater adaptability to provide peak performance and long-term reliability. A few premium power amplifiers now offer this adaptability through flexible configuration: adjustable input gain, and variable peak limiting in the output stage. These features allow the amplifier to be “custom tailored” to the input signal characteristics, and to the loudspeakers connected to each channel output.
UP FRONT: DIGITAL DYNAMICS
More often than not, the component directly in front of the power amplifier in a modern worship sound system will be a digital processor. It’s also increasingly likely that, in front of that processor, you’ll find a digital mixing console.
Compared to analog components once typical of church installations, the difference is notable. Noise levels are lower, and the dynamic range is wider. Also, with a flood of new processors coming into the market, the differences in nominal output voltages (input to the amplifier) between different units can be substantial.
Also, we must add to this mix of variables the wide dynamic range of sounds heard in contemporary worship. Input can vary from a solo flute or a pastor’s dramatic whispering all the way up to a flat-out rock sound. That’s much more demanding than a single voice preaching boldly into a fixed dynamic microphone.
The bottom line is that it’s vital for the input gain of the power amplifier be precisely matched to the output of the device in front of it, taking into consideration the dynamics of the program signal. This can only be accomplished with adjustable input gain.
But, wait a minute, what about those knobs on the front of the amp? Don’t those adjust the input gain? In a word, no. These are level potentiometers, and they serve a different purpose. They attenuate the signal between the input stage and the following power stage. They do not alter the input gain, and therefore cannot prevent the input stage from distorting or adding noise due to a mismatch with the connected processor.
A USEFUL ANALOGY: THE MIXER
You are probably familiar already with the concept of input gain. You use it to set up your mixing console for various input signals.
Every console has an input gain adjustment at the front of the signal chain. This is where you select either mic or line input, and for mic input you then adjust the sensitivity knob to match gain to particular microphones. In fact, you may be familiar with the result of plugging in a microphone with your channel input set to “line.” You might get the mic loud enough if you push the channel and master faders all the way up, but you will also hear a lot of noise.
The same principle applies with the power amplifier. In this case, the level potentiometers function like the channel faders: they merely attenuate the signal that has already passed through the input stage. (See Illustration 1.) Any signal degradation already introduced at the input cannot be remedied at this point.
Granted, the range of input signals presented by various signal processors is not as wide as that represented by a low sensitivity microphone versus a “hot” line input. (Console gain is normally about 50 dB.) But the difference is significant nonetheless, and if the amplifier input gain is not matched to the signal, the resulting performance will be less than optimal.
With power amplifier inputs, a gain adjustment range of 20 dB is more than adequate. In Lab.gruppen models, this adjustment is made using rear-panel DIP-switches. Since the same processor will feed a particular amp in two- and four-channel applications, one gain adjustment applies to all channels. In our eight-channel models, however, separate gain adjustments are provided for adjacent groups of four channels as it is more likely that different processors, or even different program material, might be applied.
ALL LOADS ARE NOT CREATED EQUAL
The need for “custom tailoring” of the amplifier is even more critical at the output end. Here, anything less than an optimum match between the amplifier output and the connected loudspeaker load will inevitably compromise both performance and long-term reliability.
The reason for this is the wide variation in demands placed on the amplifier by different types of connected loudspeakers. A newer church sound system might have a main PA with a dozen or more two- or three-way line array cabinets, several large subwoofers, and a number of smaller delay and fill speakers. Also, the system could incorporate distributed loudspeakers (narthex, cry room, fellowship hall) using 70 V, transformer-tapped ceiling speakers. All of these present very different load characteristics to the amplifier outputs.
Most amplifiers do not specifically accommodate these differences, except by adding a separate transformer – internally or externally— to accommodate the 70 V systems. Otherwise, it’s a matter of “one size fits all” at the amplifier output stages.
The exceptions are a few amplifiers that now offer a highly useful technique for matching the load to the amplifier. At Lab.gruppen we call it Voltage Peak Limiting (VPL), where another company turns the words around and calls it Peak Voltage Limiting. Either way, the effect is the same: peak output voltage is limited to maximize performance with a particular load.
OBSERVE THE LAW
To understand why this is important, let’s quickly review the basic rule that applies here: Ohm’s Law. Succinctly stated, it says that P = E x I, where P is power in watts, E is potential in volts, and I is current in amperes.
Now, at a slight risk of oversimplification, we can say that some loads prefer to see higher voltage and less current, while others prefer to see more current and less voltage. Most amplifier outputs have to be designed at a point of compromise in the middle, with an output characteristic that will work reasonably well with most connected loads.
The problem is that load characteristics for different loudspeaker drivers can be strikingly different. An example is the typical system with multi-way line array cabinets and subwoofers. The high-frequency compression drivers would all be connected to one channel (or channel group), the mid-frequencies to another, and the subwoofers to yet another. These driver types all work at their best when they are “looking at” different amplifier output characteristics.
With the Voltage Peak Limiter (VPL), each channel can be set to offer the optimum ratio of available current to available voltage. This has a number of benefits. At lower impedances, overall efficiency is markedly improved. The amplifier runs cooler and is less likely to run into protection modes. The VPL also facilitates output power management, preventing smaller loudspeakers or high-frequency drivers from being overpowered and damaged if the system in inadvertently pushed too hard. (This can be of critical importance in worship applications where the system may be operated by relatively inexperienced volunteers.) Perhaps most importantly, because the amplifier is a better match to the load, the loudspeaker’s sonic behavior is notably improved.
As an added bonus, VPL makes configuration for 70 V distributed systems a snap. Simply select the 100 Vpeak setting (70 Vrms) on that channel and it’s ready to go.
SET AND FORGET…FOR NOW
So, is the adaptable amplifier yet another piece of gear that you will have to fuss over on a weekly basis? Fortunately, the answer is no.
As long as the same drive processor is in front of the amplifiers, and the same loudspeakers connected to the outputs, there is no need to make any further changes
But, as we all know, worship sound systems do change. Loudspeaker systems are reconfigured, augmented, and replaced. Amplifiers once assigned to the main house system are reassigned to the stage monitors, or a distributed system in the fellowship hall. No matter what the new assignment, adaptable amplifiers can be re-set for the best possible performance.
A PREMIUM PACKAGE DEAL
As of this writing, “adaptable” power amplifiers – those with both adjustable input gain and configurable output characteristics – are offered by relatively few makers. This isn’t because trade secrets are involved. Rather, the reason most amplifier manufacturers have not implemented these features is a matter of cost. More circuitry and more switches do raise the price tag, and lamentably much of today’s power amplifier market has become a commodity business. “Total kilowatts for the dollar” is the name of the game, and too often performance is compromised to meet a price point.
Therefore, you won’t find this adaptability in bargain-basement amplifiers. However, by the same token, the same premium amplifiers that offer “custom-tailoring” also come standard with a host of other features appropriate for high-end worship sound. Along with the optimized performance, you can expect built-in networking for monitoring and control via Ethernet; an exceptionally high power density (power output for size and weight); a complete suite of protection features; and extensive front panel displays of amplifier status and fault conditions. In addition, the PC software program for the control system includes a calculator for determining optimal values for both the input gain and the output VPL settings.
The era of the “dumb box” amplifier is coming to a close. Your power amplifiers and loudspeakers are final components in delivering your worship sound. It’s worth a few extra dollars to ensure that the two relate to each other in the best way possible.