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!
As an engineer I like to use "rules of thumb" to make quick estimates that help to explain the physical world around me.
These rules of thumb are easy-to-remember approximations that eliminate the need for complicated and needlessly precise calculations.
If you feel discombobulated by the complexities of high school physics, there is hope! I encourage you to step back and take a fresh approach.
If you learn a few simple rules of thumb, you can unravel mysteries of the physical world, amaze your friends, and yourself.
In this paper I will present 15 simple rules that I find useful when working with music and audio.
- John Siau
The Benchmark AHB2 power amplifier and HPA4 headphone amplifier both feature feed-forward error correction. This correction system is an important subset of the patented THX-AAA™ (Achromatic Audio Amplifier) technology. It is one of the systems that keeps these Benchmark amplifiers virtually distortion free when driving heavy loads. It is also the reason that these amplifiers can support 500 kHz bandwidths without risk of instability when driving reactive loads.
This paper explains the differences between feedback and feed-forward systems. As you read this paper, you will discover that you already understand the benefits of feed-forward correction because you use it instinctively to improve a feedback system commonly found in your automobile. If feed-forward correction can improve your driving experience, it may also improve your listening experience!
If you look at the back of any Benchmark product, you will find balanced XLR analog-audio connectors. As a convenience, we also provide unbalanced RCA connectors on many of our products. In all cases, the balanced interfaces will provide better performance.
We build our unbalanced interfaces to the same high standards as our balanced interfaces, but the laws of physics dictate that the balanced interfaces will provide better noise performance.
This application note explains the advantages of balanced interfaces.