Recording - May 2018 - 62

Trust,But
verify
BY MIKE RIVERS

I introduced this series as a how-to guide to checking out
your studio gear without the use of expensive laboratory test
equipment. The first installment (November 2017) described
how to measure frequency response and distortion using the
program Room EQ Wizard. Goal met, but now I'm going to
have to backpedal a bit. I really wanted to start this series with
voltage measurements, since there's nothing more fundamental,
however this requires a voltmeter, which isn't normally a part of
today's recording studio. So in this chapter, I'll be talking more
about tools than test procedures, but stick with me.
Frequency response and distortion are usually expressed
as relative measurements rather than in actual voltage. You're
interested in knowing that the output of a 12 dB per octave
low cut filter is 12 dB lower at 80 Hz than it is at 160 Hz,
not that its output at 80 Hz is 2 volts. On the other hand, the
maximum level of your interface's monitor output before clipping
is an absolute voltage (though usually expressed as dBu), so
we need to make a real voltage measurement.
In the old days, every studio had a maintenance shop, and
test gear was common. Contemporary studio hardware, however, isn't designed for repairability, so you might not have a
multimeter handy. A DMM (Digital Multi Meter) is very versatile-
besides proofing out your studio gear, it'll help you find and
repair a bad cable, test a loudspeaker or a fuse, troubleshoot
an amplifier (particularly one with tubes), or diagnose AC
power problems. Some DMMs include a temperature probe
so you can look for hot spots in your rack or check on what's
cooking on your barbecue grill. Some measure capacitance,
some even measure transistor characteristics.
A decent DMM can be had for as little as $10 or as much
as about $1,000, so it took some research to find a couple of
models I could recommend for what we'll be doing here, at
a price that won't discourage you from gearing up. See Fig.
1. There are a few things to know about a DMM that aren't
obvious, so before you jump on to the Web to purchase one,
let's detour a bit into specification-land.

DMM CHARACTERISTICS AND SPECS
To handle technical details that might be overwhelming if
presented here in this article, I've created an in-depth article
on my blog (mikeriversaudio.wordpress.com) in which I discuss
the dirty details of how DMM specs relate to real-world use.
I also review several meters kindly sent to me by Circuit
Specialists-a distributor of lots of good electronics shop
62

RECORDING May 2018

PART 2: MEASURING VOLTAGE (AND
THINGS YOU CAN SEE FROM THERE)
goodies that I've dealt with in the past-and make a couple of
recommendations at the roughly $30 and $60 level. (Figure
1) Here and now, let's cover the basics.
The "multi" in multimeter refers to its ability to measure, at a
minimum, DC and AC voltage, current, and resistance. DMMs
have page-long spec sheets, but the ones most important to us are:
Measurement tolerance-This is expressed as plus or minus
a percentage of the reading.
Digital ambiguity or uncertainty-This is expressed as ± a
couple of numbers in the lowest order digit.
Range-This is the largest number a DMM can display
before running out of digits. It's determined by a scaling factor
and relates to the most confusing characteristic of all...
Counts-This is expressed as the highest number that can
be displayed in the most significant digit. Sometimes it's
expressed as fractional digits, for example, 3½ digits.
Tolerance is straightforward. It's the "wiggle room" in the meter's
calibration and stability. Ambiguity is a function of the resolution
of the meter's analog-to-digital conversion. Better meters use
more bits and better clocking, but an inexpensive meter typically
has an ambiguity of ± 2 or 3 digits; that means 2.05 volts could
be displayed as anywhere between 2.02 and 2.08 volts.
Range relates to the largest and smallest values a DMM can
measure. By using a meter that actually measures 0 to 100
millivolts and attenuating (or scaling) higher voltages so that
full scale can be set to 1, 10, 100, or 1000 volts, we can read
small or large voltages with the same precision. Analog multimeters have a manual range switch, as do many DMMs, but
with the magic of digital logic, some DMMs are auto-ranging.
Counts describes how the display shows voltages within the
scale range; it's a holdover from the old days of very expensive
meters where each digit required its own subassembly or Nixie
tube. With the demand for less expensive bench-grade meters,
designers came up with a sneaky way to use only four digits in
the display-three real digits (0-9) and a "half" high-order digit,
which usually is blank or shows only a 1. This lets a display

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Table of Contents for the Digital Edition of Recording - May 2018