By Allen H. Burdick
Two inexpensive jack-mounted products from Benchmark Media Systems can be assembled into a very powerful test set we call The Audio MicroScope™. The Audio MicroScope™ is a tool to be used in setting up analog audio systems. Because of its extremely high gain capability, over 85 dB, it is particularly useful in hunting down signal interface problems, such as hum and buzz from RF interference, magnetic induction, and ground loops.
The first device in the test set is the MP-3, a single channel microphone preamplifier. The MP-3 is mounted on an XLR type jack and has a balanced input, variable gain from 26 to 65 dB, and a balanced output. It also has the ability to feed phantom power (with an external source of +48 V required) to a microphone. This can be useful even in this test set, particularly when testing microphone input cables. The performance of the MP-3 is excellent, with a 1 dB noise figure, a 2 kHz THD at A=40 dB of 0.005% and a bandwidth of greater than 200 kHz.
The second device used is the HPA-2. The HPA-2 is a high quality stereo headphone amplifier that has approximately 300 mA of very clean output capability per channel and is mounted on a 1/4" TRS jack. The HPA-2 is a improved headphone amplifier over the original HPA-1). Although the HPA-2 is a stereo device, in this application we will be using it as a dual channel mono amp. That is, both inputs to the HPA-2 will be connected to one half of the balanced output of the second product, the MP-3 microphone preamplifier.
Figure 1. MP-3 to HPA-2 Interconnect.
Mount an MP-3, an HPA-2, in a project box along with a DPDT switch for power, and four 9 V batteries or a PS-1. Be sure to include a switch for the +48 V if it is used. The Audio Microscope™ will allow you to "look" at your system and find minute problems.
To avoid damaging your hearing, since there is over 85 dB of gain available, you must be careful to have all normal audio fully removed from the path under test. Start with low gain and move cautiously upward as you begin the cleanup of the system.
Start at the beginning of the audio chain and listen to the microphone cable that feeds your console! Back terminate the wire, that is, replace the microphone with a 150 ohm carbon film or preferably a metal film resistor. Turn on phantom power, if available. Then turn up the gain and listen while you or someone else flexes the cable. If the cable is high quality, you will hear relatively little noise, if not, be prepared for a surprise. Next listen to the output of the first piece of equipment. It should be clean and free from hum. Move to the output of the next piece of equipment. Is the interconnect between the first two pieces of equipment free from mains-related hum and buzz? If not, work with the wiring and ground system, until they are. This, of course, assumes that the equipment by itself is clean, not always a safe assumption. Continue through the chain until the output of the final stage is just what you wanted.
You will find that the "Audio Microscope™" is one of the most valuable tools in your kit. In addition to using it with clip leads for direct connections, when used with a telephone pickup coil, Radio Shack Part # 44-533 it is most useful for doing a Magnetic Field survey of your equipment space. This piece of test equipment should be used hand in hand with the "A Clean Audio Installation Guide™" by Allen Burdick, a Benchmark Media Systems application note.
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