DMM / digital multimeter audio bandwidth capability

[cashhewn]

Hi.

I had a Klein MM700 digital multimeter go bad after only about 3-4 years. It powers on the LCD screen but doesn’t register continuity or voltage etc. I verified that the leads are good. It’s out of two year warranty. I need a replacement and probably want a different brand out of principle.

Someone mentioned that the MM700 is only rated for true RMS from 50-400Hz, something that had never occurred to me to matter before when measuring AC voltage.

I know that for measurement of generated audio sine waves I need a true RMS DMM, but how important is bandwidth capability actually? The price jumps to many hundreds of money units if full audio bandwidth capability is required.

Can I just test with low frequency tones, like 250 Hz or lower? It would be easier on my ears anyway, the mellow tone.

What are the implications?

I had thought that I seemed to be having success with the MM700: everything was measuring out roughly enough and making sense (with my converters which are +19dBu capable, -13.8 dBfs was measuring about 0VU/+4dBu, which technically should be -15dBfs, so maybe that’s explains the small discrepancy of 1.2dB if I was actually sending maybe a 1kHz tone and it was out of the spec range of the MM700?).

Thanks for any explanations and recommendations of a DMM for recording studio purposes.

[Wonks]

I think you are better off moving to an oscilloscope. Measuring over a wide range of frequencies is what they are designed to do. Older analogue ones can be picked up pretty cheaply, and the range of digital units at different price points is very extensive.

There are also plug-in adapters which can use an audio interface along with software, for a low cost solution. I know Hugh uses something like this.

What sort of budget have you got?

[ajay_m]

At around £150, the Brymen 867 (which I own) is a superb and highly regarded DVM and has a bandwidth for true RMS out to 100KHz. The manual with specs is online. I can recommend Telonic as a supplier and they also sell a wide range of other test equipment.

[resistorman]

You did check the fuse, right?

[Wonks]

If you are simply measuring sine waves, then measuring the peak value will tell you what to know. The RMS value is 0.707 (or more precisely 1 over root 2) of the peak value.

[ef37a]

Holy friggin test leads Batman! I just checked on their site for the successor to my venerable Fluke 83. It is the Fluke 87v and is the thick end of 900 quid!

As Wonks says, true rms only matters if you are measuring other than sine waveforms, distortion products say and only then if you need to present the figures to a learned journal.
I don't know about "digitals" but old moving coil meters responded to the average value of a sine wave but were calibrated to show the rms value. Clearly a distorted waveform will show an error. "The Trade" got over that problem in service manuals by giving certain voltages, e.g. a half wave rectified filament supply, as found on a specific meter, almost always an Avo 7 or 8.

But you do need the meter to be accurate to 20kHz and possibly beyond so, unless you have the odd 900 sovs handy, a careful check of specifications is indicated.

Dave.

[ajay_m]

Well, the Flukes are nice instruments but they have a lot of competition and not cheap Chinese junk but decent CAT IV certified DVMs. That said I do concur that a decent digital scope is a more versatile option and the entry level scopes from Rigol, Micsig and Siglent are good devices, with the edge probably going to Siglent. Certainly the SDS1202X-E Siglent scope I own is very capable and has functionality that would have cost the price of a family car when I was young.
The most recent models now have 12 bit resolution as well but be aware that, at least in the case of the Siglent models, you lose the external trigger input which for decades has been a pretty standard feature on scopes. Hence for those models I would go to a four channel scope for decent flexibility though you will be parting with a fair few hundred pictures of royalty for one of these. But. Buy once, cry once, as they say.....

[Hugh Robjohns]

I know that for measurement of generated audio sine waves I need a true RMS DMM...

Or... here's s thought... a proper audio meter of some kind.

...but how important is bandwidth capability actually?

Depends what you're planning on measuring... and why.

Can I just test with low frequency tones, like 250 Hz or lower?

Yes... if you only want to know about frequencies below 250Hz.... which I'd say was a bit restrictive in the world of audio testing.

Thanks for any explanations and recommendations of a DMM for recording studio purposes.

I'd use audio measurement software in your computer (like Room EQ Wizard)... or even just your DAW and its meters if you're trying to keep costs down. All you need to know is the maximum input/output analogue levels for your interface which will be in the published specs.

Alternatively, a calibrated professional VU or PPM analogue meter unit is very easy to use, very accurate, and generally very cheap in the secondhand market places these days.

Or for a decent long term investment look at the handheld test sets from the likes of NTi. The original ML1 is beginning to pop up on the marketplaces now at attractive prices, for example.

An oscilloscope is a very powerful tool, but with lots of scope to confuse and misinterpret (see what I did there? ) and you'll forever be converting voltages into decibels and back again. So it wouldn't be my first choice for general audio system alignment purposes.

[cashhewn]

Sorry for the delay in saying “thanks” to all who responded, I sincerely appreciate it.

Right, a scope or another meter (I need something external to the DAW)…

Lastly,

You did check the fuse, right?

Well… *grimace* I thought that since the LCD screen lit up, and that continuity wasn’t registering, it couldn’t be either of the fuses…but I didn’t check.

Lemme check the fuses…

[ef37a]

". So it wouldn't be my first choice for general audio system alignment purposes."

Quite honestly Hugh a digital meter is a pain as well. I haven't looked for some time but if OP can find a Levell AC mV meter on The Bay he will find it much faster for alignment purposes (and almost mandatory for analogue work such as tape machines). A basic scope is though very useful to ensure you are not clipping anything or getting spurious signals.

Or you can use a decent moving coil meter, Simpson, Avo* and drive it with a simple op amp/rectifier circuit. For "line" level signals you don't need a complex, highly accurate front end attenuator.

*Or indeed a nice big VU meter movement.

Dave.