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 the 2023 AXPONA show in Chicago, I had the opportunity to see and hear the Hill Plasmatronics tweeter. I also had the great pleasure of meeting Dr. Alan Hill, the physicist who invented this unique device.
The plasma driver has no moving parts and no diaphragm. Sound is emitted directly from the thermal expansion and contraction of an electrically sustained plasma. The plasma is generated within a stream of helium gas. In the demonstration, there was a large helium tank on the floor with a sufficient supply for several hours of listening.
While a tank of helium, tubing, high voltage power supplies, and the smell of smoke may not be appropriate for every living room, this was absolutely the best thing I experienced at the show!
What happens when you ask ChatGPT to write a review?
I asked ChatGPT to review the Benchmark AHB2, and I was shocked at the results!
It turns out that ChatGPT wrote a review by gleaning information from the many publicly available reviews of the AHB2. I recognized phases and sequences of words that had appeared in prior reviews, and yet the text was new, original, and well written. The ChatGPT review seemed to summarize the collective impressions of the reviewers without directly quoting any particular reviewer.
It should be possible to ask ChatGPT to write a review on any well-known product and the result will be an AI-generated review that reflects the collective opinions of many human reviewers.
These AI-generated reviews may be more useful than traditional search results!
If I am not mistaken, ChatGPT seems to have golden ears!
Here is my dialog with ChatGPT:
Write a review of the Benchmark AHB2 complete with listening impressions.
Title: Unleashing Sonic Purity: A Review of the Benchmark AHB2 Amplifier
Introduction: The Benchmark AHB2 amplifier has gained a reputation for its exceptional audio performance and unwavering dedication to sonic purity. In this review, we delve into the heart of this powerhouse amplifier, exploring its features, design, and most importantly, the listening impressions it evokes...
Today, Benchmark's John Siau and OpenAI's ChatGPT had a conversation about star-quad cables. It turns out that ChatGPT got quite a few things wrong, but learned quickly when these errors were pointed out by John Siau.
ChatGPT is quick to recognize mistakes, and quick to provide detailed corrections. In each of the first few corrections, additional errors were made. As each of these were pointed out, ChatGPT began to provide accurate information.
Here is my conversation with ChatGPT:
How do star-quad cables reject interference?
Star-quad cables are designed to provide improved rejection of ...