The Audio MicroScope™
Introduction
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.
Devices Used
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.
Construction
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.
System Testing
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.
Conclusion
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.
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