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!
If an audio system is composed of multiple components, we may have detailed specifications for each component, but we will not know the performance of the combined system without doing some calculations. You may have questions such as these:
Use Benchmark's online audio calculators to find answers!
For example, if we know the output power of an amplifier, as well as the sensitivity and impedance of our loudspeakers, we can calculate the maximum sound pressure level that our system can produce.
This application note provides interactive examples that help to answer the questions listed above.
We have added an "Audio Calculators" section to our webpage. Click "Calculators" on the top menu to see more like these:
Secrets contributor Sumit Chawla recently caught up with Benchmark’s VP and Chief Designer, John Siau to get a little more in-depth on several subjects.
Q: "Benchmark is one of the few companies that publishes an extensive set of measurements, but you also balance that with subjective testing. Can you talk about the equipment, the listening room, and the process for subjective testing?"
Q: "Was there ever a time where you learned something from a subjective test that was not captured by measurements?"
Q: "You conducted some listening tests to determine whether distortion in the “First Watt” was audible. What test material did you use for this, and what did you find?"
Q: "The AHB2 amplifier incorporates THX Audio Achromatic Amplifier technology. When and how did the partnership with THX come about?"
Q: "Linear power supplies have been and remain quite popular in high-end devices. You favor switch-mode power supplies. When and why did you make this switch?"
... and more!
