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Dual channel multimeter or another option?
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Swake:
I understand from OP that to goal is to align/match both channels output level one against the other. There is no statement about having to match an exactly measured level. We can assume classic analog line audio levels and frequencies.

Couldn't a simple differential opamp with a single mV sensitive (multi)meter on the output be a solution? Too much output on one side is a positive reading, too much on the other side is a negative reading. The reading is proportional to the difference indicating how much the mentioned pots have to be trimmed. Cheap, simple, small, portable. Do you need more?

Going the why-keep-it-simple-if-complicated-is-so-much-more-difficult-route: Genuine question: Can't a modern scope with math function (A minus B) also calculate and show the difference between the signals on 2 channels? The difference would be the representation of the deviation between the said channels. Any Siglent or Rigol does have this on board I believe. Obviously as 2 different channels on the scope are used these have to be matched first -> feed the same signal on both in parallel and any deviation between the 2 is to be taken into account when measuring the outputs of the cassette tape.
pope:
I'm having hard time to understand why a single good true RMS (although not necessary) meter won't work here.

All you need to do is to get a reading of 1.228Vrms (i.e +4dBu) at 1kHz sine wave on both channels. Most likely, this will be the reference level on most pro gear (there are some exceptions depending on the country).

You just have to repeat the process for each channel.
nctnico:

--- Quote from: pope on December 07, 2022, 10:32:11 pm ---I'm having hard time to understand why a single good true RMS (although not necessary) meter won't work here.

All you need to do is to get a reading of 1.228Vrms (i.e +4dBu) at 1kHz sine wave on both channels. Most likely, this will be the reference level on most pro gear (there are some exceptions depending on the country).

You just have to repeat the process for each channel.

--- End quote ---
It is quite possible that some adjustments affect both channels (think about left/right balance) so you'll want to see two readings at the same time. IMHO the OP's question is valid and he/she really needs to see two readings in order to do the job efficiently. Otherwise the question would not have been asked. I used to do some audio / telecom work every now and then and the dual channel AC voltmeter I have did come in handy several times.
bdunham7:

--- Quote from: nctnico on December 07, 2022, 08:01:27 pm ---Well, a true-RMS DMM rated for the frequency range to be measured, doesn't need pre-calibration of anything. It works out of the box to give you repeatable AND accurate results. For high frequencies you use a power meter.

@Tautech: no, an oscilloscope is the wrong tool for the job. For starters: what is the long term drift for the calibrator output? It's not specified in the datasheet of the SDS2000X+. Also the amplitude flatness at low frequencies is +/-1 dB which is over +/-10% error (which is 10 times worse than what the OP requires). Need I continue to educate you further or do you get the point?

--- End quote ---

I'm not sure how much this argument helps the OP, but my thoughts were that DSO is good enough for the job and since he already has multimeters, expands his capabilities.  A pair of reasonably matched inexpensive multimeters might do as well, although the ability of the scope to find issues other than signal level might be an eye opener.

As for your thoughts generally on scopes and signal levels, my statements are based on practical experience not spec sheets.  My SDS2354X+ is a year old and the calibrator output measures exactly 1.500V within my ability to measure.  It's probably good enough to sub for a DMM-Check.  As for meters being accurate out of the box, it isn't relevant to the OP's situation, but when you get into high crest factors your 'out-of-the-box' DMM will fail (to be accurate) quite quickly and worse, you won't realize it.  On the scope you'll be both be able to tell and to measure signals with crest factors of 10, 20 or even 30.  There aren't many DMMs that will do that.
nctnico:
The problem with measuring accurately is that your measurement system has to be designed to reach certain accuracy levels. Anything else is just anecdotal evidence based on luck. How can you be sure if the OP buys an oscilloscope that it has the same 'accuracy' as yours? You simply can't promise that or even pretend it will be. The specs say that loud & clear. It is similar to buying 1% resistors. A whole bunch of them will be better than 1% and some will be right on the edge. But you don't know what you get upfront and for as long as the resistors are within 1%, you can't claim anything.

For sure an oscilloscope can be used to look at the waveform of a signal and see whether it is very distorted but small distortions are better measured using a distortion analyser.
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