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May 2017

DMM vs. AVOM

I do mainly audio work and have been told by a few old timers that analog VOMs are the only way to go for testing and tuning gear. Problem is, I can get a new DMM for $20 on Amazon vs. an ancient Triplet or Simpson analog VOM on eBay for up to ten times that. My question is: Is it worth it? Would I be better off with a modern Fluke meter with true RMS reading?

#5173
DMM vs. AVOM
Providence, RI



Answers

Audio work can require the measurement of voltages down to the millivolt range. It can also require measurement of varying signals such as voice and music. To best measure these use an “AC VTVM.”

This is an electronic device with a large analog meter driven by an electron tube. They usually have a high input impedance and wide frequency range — often into the 100 kilohertz range. Their input voltage range is usually from 1 millivolt full scale to 300 volts full scale. The meter has scales in both volts and dB — with dB making it easy to measure amplifier gain.

Want to know what the output voltage of a microphone will be for a particular sound? Put the microphone at the desired location in relation to the sound and hook the microphone directly to an AC VTVM. Then just watch the meter scale. (A digital meter will just wildly blink random numbers.)

Mark Peterson
Plymouth, MN

As an audio engineer, I find that an AC millivolt meter, calibrated in decibels, is essential for audio engineering and repair. For example, to validate the dB attenuation of a Linkwitz-Riley crossover network, or to gauge flatness of a preamp. My 80’s vintage Leader LMV-187 is what I use for this work.

That said, it is easy to build much cheaper and available alternatives. Dual channel VU meter driver boards, and decent VU meters calibrated in dB, are available on eBay. Frontend one of these boards with the ‘Low cost PC Two Channel Oscilloscope’ circuit of the August 2016 N&V, and one has a decent AC metering system for audio engineering. Be sure to AC couple the input to the circuit, for example with 2.2uF non-polarized electrolytic capacitors.

I use my DMM’s for checking power supply voltages, etc. But for audio signal level work, an analog metering system, calibrated in dB, is indispensable.

Jim Lacenski
Bellevue, WA

New analog VOMs are certainly available, but quality is likely nowhere near what used to be standard. What the cheap DMMs won’t show you are fast transient signals; they will either average out the spurious signal (you won’t see it), or they will just completely miss it, due to a slower “sampling” rate.

A high-quality Fluke DMM can be a good choice, but there isn’t any reason not to have an analog meter on your bench, if you can find a good one (tested, calibrated, and known to work well), along with a Fluke.

Sometimes, the slight tremble (noise or hum over the signal) of an analog movement can be seen better by a human than a DMM can update. But for most things, you could use the Fluke (heck, for most things, you could just use an el-cheapo free-with-coupon Harbor Freight throw-away).

Another option to consider, depending on the signals being looked at of course, would be a digital o-scope; being able to sample a signal over a period of time, then going back and reviewing it can be very helpful in tracking down certain problems that can’t be seen otherwise, using other tools.

Andrew Ayers
Glendale, AZ

The use of analog versus digital to me is a personal choice. I work in the 2-way radio field and personally prefer analog meters for several reasons; the first is that it is easier to detect a peak or a null with a needle on a meter than it is trying to read changing numbers on a digital display. In fact, I have never seen a grid dip oscillator with a digital display.

The second reason is that while working on a circuit board and looking for voltages, I can glance up at an analog meter and watch the needle move, and with the proper range set, I have a good idea what the voltage is with just a glance. With some analog meters such as the B&K 290 electronic VTVM that I use on the bench, there is a scale on the bottom with zero centered mid-span which allows me to see if a voltage/current varies from what I have set as the standard I am using, which allows me to set voltage/current to that standard.

While I am an ‘analog guy’ there are times when I use a digital meter. When setting VCO voltages you need a digital meter to accurately set the voltages. Another use where digital is better (in my opinion) is reading the values of resistors and capacitors. A DMM has the advantage of reading capacitors, which analog can’t do. Although, it can be a pain to read resistors on an analog meter due to the logarithmic scale and crowding at the high end of the scale, it does excel at testing pots to see if the operation is smooth by watching the needle.

On my bench I have the B&K 290 analog electronic VTVM, a NLS TT-21 digital meter and for field work a Triplett model 60 VOM. What works for me or someone else may not necessarily work for you. As I said, it’s based on what you prefer and what your requirements are. I would suggest seeing if you can borrow a digital and analog meter, use them and see what works best for you.

I will offer this suggestion if you do go the analog route, go for a meter that has 20K ohms/volt or better, the higher the ohms/volt the better as it will not load down a circuit under test as a lower range would.

Craig Kielhofer
Plainfield, IL

There are many differences between DMMs and VOMs. The two which might justify the old timers advice are that VOMs have lower input impedance than the typical DMM and the dial display of a VOM display is easier to interpret for noisy or changing amplitude signals than a digital display. The higher input impedance of a DMM may make readings from old documentation based on using a VOM slightly inaccurate.

Neither of these characteristics really justifies spending lots more money. Many DMMs include a bar graph display which emulates a dial, mooting this difference.

Finally, analog meters from lesser makers are still available in economy markets and auction sites. If you do find those two differences compelling you can satisfy that need at costs comparable to most DMMs.

Warren Wilderson
Eagle Point, OR

The question is not Average vs RMS. When dealing with tuning; the question is measuring of Peaks and Deeps. You can’t measure Peaks and Deeps with digital Voltmeter, to do it you need an analog device.

About 50 years ago, Fluke came out with a nice digital multi-meter that couldn’t be used for tuning. After some time Fluke added an Analog Bar-graph that helps a lot for tuning purposes.

Try to look for Fluke Series 170 and you will find what you need. Buy the way, tuning is mainly done in RF equipment and not in audio.

Avnerdee
Tel-Aviv, Israel

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