Multimeter advice? Is true RMS feature important in dealing with audio signals?

[iandy4]

Hi Everyone,

I'm looking to upgrade from my shit Radioshack MM to a proper one.  I can't really afford or find a reason that justifies getting a fluke but I was doing a lot of research on multimeters and after watching this youtube video I think I'll go for the extech ex330:

The only thing is that I noticed that multimeters about 30 dollars more than this extech have the true RMS feature.  I looked up what that is and it seems like it could be very important for measuring small AC signals like audio.  So my questions are, 1) is RMS worth the extra money for someone who is just getting started but plans to get into designing audio circuits and working with microcontrollers?  2) What meter would you suggest that's under 150 bucks? Thanks guys!

[Hides-His-Eyes]

You could make a decent "RMS" box with a THAT2252 in a little box that you could connect to your cheap multimeter.

[slacker]

I don't think it's something you need for stompbox building, it's certainly not worth shelling out an extra $30 just for that. I don't know anything about the brand but that extech one looks like it does everything you're likely to need.

[iandy4]

Thanks for the advice guys! Leaning towards the extech.

[Gurner]

For me true RMS is important....as I'm always messing about with odd shaped waveforms (& amplifiers).

Having watching that (excellent) shootout, I shelled out for the Uni-T (which performed well in the true RMS measurement).....but I actually went for the later model the  UT-61E, I'm very happy with it.

[cpm]

i hardly use my multimeter since i got an oscilloscope, only for continuity and current
with the scope i look visually all the voltages and waveforms, and is a great improvement. i got mine a cheap used one for 100 euro

[R.G.]

I have never bought myself a true RMS meter, in over four decades. There are certain specific tasks it can do best, but they're isolated cases.

[Gurner]

I guess it's horses for courses, budget & what your intentions are....but I looked at it like this - I don't buy a multimeter that often (every 8-10 years!) ....the UT61e true RMS meter I bought only cost £40 (about $60)...granted, you can buy a very good multimeter for just £20-£25 ($40-$50), but for a mere £15-£20 more...I figured even if the true RMS aspect doesn't get brought into play that often (as it goes it has been handy for my situation!) , the fact that true RMS is typically only on the maker's flagship models, means you're getting all the other extra bundled 'refinement'  for just $20-$30 more  ....now if it had been $100 more, then I would have lived with normal RMS!

[R.G.]

I guess it's horses for courses, budget & what your intentions are....but I looked at it like this - I don't buy a multimeter that often (every 8-10 years!) ....the UT61e true RMS meter I bought only cost £40 (about $60)...granted, you can buy a very good multimeter for just £20-£25 ($40-$50), but for a mere £15-£20 more...I figured even if the true RMS aspect doesn't get brought into play that often (as it goes it has been handy for my situation!) , the fact that true RMS is typically only on the maker's flagship models, means you're getting all the other extra bundled 'refinement'  for just $20-$30 more  ....now if it had been $100 more, then I would have lived with normal RMS!

That's true. I never bought one because I still have the meter I bought over 20 years ago, and the need for true RMS was never urgent enough to buy a new meter.

It is indeed horses for courses.

[iandy4]

Well I'm a poor student and I just managed to get my hands on a free analog oscilloscope so I think I'll go with the extech ex330. Apparently this oscilloscope is enormous so the person giving it to me is, I assume, happy to help someone out and clear up some desk space at the same time.  Can't wait to play with my new toys when I move back to the US in a few weeks. Never had a meter that could even show amps let alone an oscilloscope.  Thanks for the help guys!

[harmonic]

....the UT61e true RMS meter I bought only cost £40 (about $60)...

Had my eye on this particular DMM. Do you have a link for it at that price? Ta!

[Gurner]

....the UT61e true RMS meter I bought only cost £40 (about $60)...

Had my eye on this particular DMM. Do you have a link for it at that price? Ta!

I saw it here...

http://cgi.ebay.co.uk/UNI-T-TRUE-RMS-Modern-Digital-Multimeter-UT61E-meter-/170646649780?pt=UK_Measuring_Tools_Levels&hash=item27bb553bb4

then noticed he that he only lived two miles from me here in London, asked him for a direct cash on collection price  .....£40 on the nose. (which is actually $65 not $60 ...of course this won't help if you're stateside!)

It's a nice meter....and lovely that it reads down to 1uA  (which is particularly handy when checking how low you can get the minimum draw down to in a PIC 'sleep' circuit, which fwiw in the latest circuit I've done is only about 10uA  ...or about 5-6 years 'standby' using an alkaline 9V battery!)

[cpm]

WTF

http://www.youtube.com/watch?v=7fFjiLU5bF0

[PRR]

> multimeters about 30 dollars more ...have the true RMS feature.

Because there is a $5 chip to compute RMS. $25 pure profit.

I'm going to say nobody here needs "true RMS".

RMS is mostly about heat. We never need the true heating value of oddball waveforms.

The ONE procedure I saw which NEEDED "true RMS" was calibrating remote control stage lamp dimmers. SCR dimmers make ugly waves, lamps respond to heat: RMS voltage.

In audio measurements, we mostly work with Sine waves. Waveform always the same.

If waveform is fairly consistent, then you want the most trouble-free and low-price rectifier possible, to leave time and money for other toys. Average is generally best.

> for measuring small AC signals

RMS converters have real trouble with small signals. Worse, they will be OK below 100Hz and falling at higher frequencies, frequency depending on level. I would have to have a lengthy check-session with an RMS meter before trusting it for audio.

Some of the older averaging DMMs had similar issues. Some deliberately roll-off above 400Hz. Some are OK at higher readings and read low for low readings. Any meter not extensively specified for audio, you need to check.

> I'm always messing about with odd shaped waveforms

OK, but if waveforms are different in odd ways, what do you really want to know? Heat? Surely not. Peak value for clipping? Average for loudness? But loudness has a lot to do with frequency distribution and number of partials. Nobody has ever perfected a loudness meter.

Some authorities state that "loudness" is related to RMS. But for usual speech/music signals, the difference between Average and RMS is nearly nothing. Knew a guy spent a lot of time developing an RMS meter, couldn't see a difference against plain average.

What _I_ would not be without, for audio, is Peak-to-Peak. In audio, the main limit of cleanliness is clipping. In many systems, clipping is plain P-to-P voltage excess. Whatever the waveform, if the P-to-P fits inside the limit, it will be clean. (And conversely: if you want distortion, your P-to-P usually must exceed some limit to get any dirt on it.)

Some older VTVMs measured Peak-to-Peak (but were marked 1/2.828 RMS). Most HeathKits do.

Problem with general purpose VTVMs (aside from being old and generally in need of fixing) is that the lowest scale is 1.5V. Somehow that was rarely a problem back in the day. That modern DMMs give readings down to 1mV makes 1.5V seem huge. When we needed small AC Volts we used the Boonton: an averaging meter with four stages of tube amplifier in front. One Boonton will read line noise in most older systems. For front-end noise work you could stack two for microVolt resolution. FWIW, correction factors for common non-Sine waveforms were in the manual.

> my sh*t Radioshack MM

What is wrong with the Radio Shed meter?

I own two. The DMM is a fine machine. The passive VOM has 20 years of garage crud in it and needs a cleaning.

Passive VOMs are awkward for small voltage in high impedances. This includes some parts of some pedals. I _would_ encourage getting a VTVM or DMM for such work.