By Allen H. Burdick
Let's face it, an Audio Distribution Amplifier (DA) is not absolutely necessary for the distribution of audio! You can daisy chain your audio from input to input these days, generally with minimal loading on the source. However, what happens if a piece of equipment on the chain fails, or someone inadvertently cuts the audio pair, or you wish to remove a piece of equipment while on the air? Well, of course, that's why we install DAs in the first place. The DA is an insurance policy. But like any insurance policy, you'd better be sure of your coverage before you need to make a claim. To examine our insurance coverage, let's review the basic criteria for good audio transmission.
In a historic paper to the Audio Engineering Society, presented in 1980, Richard Hess of National Tele-Consultants, then with ABC-TV, outlined the need to move from the 600 ohm power matched interconnect system that we inherited from the Bell Labs, to a 60 ohm voltage source interconnect for runs up to 3000' in length. This provides an increase in interconnect bandwidth of 5 times what the 600 ohm system would have under the same circumstances. It also provides a much lower noise pickup and reduces the quiescent power drain and heat generated in the equipment. Most equipment manufacturers have now subscribed to that understanding with the resultant improvement in interconnect bandwidth. See "A Clean Audio Installation Guide™" tech note for more information.
The next issue is the type of amplifier output needed. Most networks will not use audio distribution amplifiers that have multiple output drivers. Rather, they require DAs with a single output amplifier stage. The reason for not using multiple output drivers is the possibility of having the "On Air" output fail while monitoring a different output, and thus not being aware of the loss.
Mr. Hess also noted, in his paper, the requirement for a DA to be able to operate with up to 1/3 of its outputs in a shorted condition. This is a very important concept. In facilities, change is constant. New equipment is added, old equipment and cable runs are removed. The insurance policy must be robust, and be able to cover the unexpected.
These three requirements define the output stage design of an audio distribution amplifier. The output stage must be a single amplifier with "build-out" resistors that create the desired drive impedance. In this case, the use of 30 ohm resistors from two amplifiers is necessary to create the 60 ohm balanced output. If we have, say, ten balanced outputs on our distribution amplifier, then with three of those outputs shorted the amplifiers must be able to drive two 10 ohm loads, and still deliver audio to the other destinations. Remember, this is our insurance policy and it can't let us down during an emergency. The implication is obvious. To deliver full output into a 10 ohm load we need a small power amplifier, i.e. 10 watts per channel, relative to ground, 40 watts balanced.
Unfortunately, many of the devices being passed off as an insurance policy cannot survive this condition. Often, they are light weight designs that were created in the days of the 600 ohm power matched thinking, and simply had their output resistors changed from 300 ohms to 30 ohms. In other cases, while the amplifier itself might be able to drive a 10 ohm load, the power supply will not provide enough current to the amplifier under short circuit conditions.
Most unfortunate of all is the facility where the staff thinks they have an insurance policy, only to find in an emergency that they did not read the fine print and the coverage wasn't really available. Caveat emptor!
At Benchmark, listening is the final exam that determines if a design passes from engineering to production. When all of the measurements show that a product is working flawlessly, we spend time listening for issues that may not have shown up on the test station. If we hear something, we go back and figure out how to measure what we heard. We then add this test to our arsenal of measurements.
Benchmark's listening room is equipped with a variety of signal sources, amplifiers and loudspeakers, including the selection of nearfield monitors shown in the photo. It is also equipped with ABX switch boxes that can be used to switch sources while the music is playing.
Benchmark's lab is equipped with Audio Precision test stations that include the top-of-the-line APx555 and the older AP2722 and AP2522. We don't just use these test stations for R&D - every product must pass a full set of tests on one of our Audio Precision test stations before it ships from our factory in Syracuse, NY.
Paul Seydor of The Absolute Sound interviews John Siau, VP and chief designer at Benchmark Media Systems. The interview accompanies Paul's review of the LA4 in the December, 2020 issue of TAS.
"At Benchmark, listening is the final exam that determines if a design passes from engineering to production. But since listening tests are never perfect, it’s essential we develop measurements for each artifact we identify in a listening test. An APx555 test set has far more resolution than human hearing, but it has no intelligence. We have to tell it exactly what to measure and how to measure it. When we hear something we cannot measure, we are not doing the right measurements. If we just listen, redesign, then repeat, we may arrive at a solution that just masks the artifact with another less-objectionable artifact. But if we focus on eliminating every artifact that we can measure, we can quickly converge on a solution that approaches sonic transparency. If we can measure an artifact, we don't try to determine if it’s low enough to be inaudible, we simply try to eliminate it."
- John Siau