Millivolt Meter Recommendation for Audio

[MakeMineVinyl]

You are 100% correct. Input levels, playback levels and final stage levels are all adjusted before messing with any VU trims.

I think we're getting to the same end. Admittedly I'm not familiar with this particular model, but I've calibrated zillions of Studers. The VU meters need to read zero for a particular output voltage, one way or another. How you get there is up to the user.

 

[March Audio]

So I'm not sure what to make of these results.

First, a DC battery test with a D cell battery. The packaging says 1.5V; I have no idea how loose D cell battery tolerances are.

View attachment 83831

6.7% off spec. Is it the meter or the battery? No idea...

Moving on the to the calibration tape.



View attachment 83832


The default reference fluxivity for the PR99 MK II is 250 nWB/m = 0 VU

So it's expected that a 355 nWb/m tape gives this result for a 1 kHz tone.

View attachment 83833

Looks like the left channel is about 0.5 dB too low.

When measuring the 1 kHz test tone at 355 nWb/m = +3 VU, we get this output at the XLR line out jacks, in CAL (calibrated / fixed) mode:

View attachment 83834


So the next part is where my math is making me less than confident.

At 0 VU on the meter, the output should be +4 dBU at the XLR jacks, which = 1.23 VAC, I think.

So +3 dB on the VU meter should be.....+7 dBU, which = 2.24 VAC??

Is that math correct?

If so, either the output is a little hot, the meter is off by .01V, or my measuring is doing something wrong.

Or my math is wrong.

Use this

http://www.sengpielaudio.com/calculator-db-volt.htm

 

[watchnerd]

Use this

http://www.sengpielaudio.com/calculator-db-volt.htm

2.25 V RMS = 9.3 dBU

I'm not sure what that tells me relative to operating levels, if 0 dB on the VU meter is supposed to correspond to +4 dBU / 1.23 V RMS at the line out.

 

[watchnerd]

You are 100% correct. Input levels, playback levels and final stage levels are all adjusted before messing with any VU trims.

I think we're getting to the same end. Admittedly I'm not familiar with this particular model, but I've calibrated zillions of Studers. The VU meters need to read zero for a particular output voltage, one way or another. How you get there is up to the user.

Isn't the manual implying you can't accurately calibrate the output to a reference standard without first calibrating the input circuit?

Maybe if you're only doing playback, calibrating output alone would be okay, but if we're trying to calibrate the record and monitor circuits, too, don't we need both input and output adjusted?

 

[MakeMineVinyl]

2.25 V RMS = 9.3 dBU

I'm not sure what that tells me relative to operating levels, if 0 dB on the VU meter is supposed to correspond to +4 dBU / 1.23 V RMS at the line out.

1.23V is +4dB above .775V which is equal to 1milliwatt into 600 Ohms. There is a rather complicated history why the +4 came about, but that's probably too far down in the weeds at this point.

 

[March Audio]

2.25 V RMS = 9.3 dBU

I'm not sure what that tells me relative to operating levels, if 0 dB on the VU meter is supposed to correspond to +4 dBU / 1.23 V RMS at the line out.

I'm not clear on what you need to know.

 

[MakeMineVinyl]

Isn't the manual implying you can't accurately calibrate the output to a reference standard without first calibrating the input circuit?

Maybe if you're only doing playback, calibrating output alone would be okay, but if we're trying to calibrate the record and monitor circuits, too, don't we need both input and output adjusted?

At this point, I'd just follow the manual step by step if you're not already familiar with calibrating pro machines. They're basically all the same in the end. You just want the VU meters to equal a known output voltage from the line out jacks. Then they are basically your reference.

 

[restorer-john]

Isn't the manual implying you can't accurately calibrate the output to a reference standard without first calibrating the input circuit?

Maybe if you're only doing playback, calibrating output alone would be okay, but if we're trying to calibrate the record and monitor circuits, too, don't we need both input and output adjusted?

Absolutely. Otherwise it's a circle of confusion.

Follow the manual. The steps are the same for all tape decks, be they open reel or cassette, two head, three head or six head. It gets really fun when you have bidirectional autoreverse three head decks or switchable 2/4 track decks, all with their own rec and playback amplifiers.

 

[MakeMineVinyl]

Absolutely. Otherwise it's a circle of confusion.

Follow the manual. The steps are the same for all tape decks, be they open reel or cassette, two head, three head or six head. It gets really fun when you have bidirectional autoreverse three head decks or switchable 2/4 track decks, all with their own rec and playback amplifiers.

Well, two head decks that have a combination record/play head are a particular pain in the ass!

 

[watchnerd]

I'm not clear on what you need to know.

Just general education on trying to figure out how VU readings map to output dBU / volts.

Simple curiousity.

 

[MakeMineVinyl]

Just general education on trying to figure out how VU readings map to output dBU / volts.

Zero on the VU meters = 1.23 volts which is 4dB above .775V which is 1 milliwatt into 600 ohms. Simple isn't it.

https://en.wikipedia.org/wiki/VU_meter

 

[March Audio]

Also this.

http://www.sengpielaudio.com/calculator-gainloss.htm

I will be first to admit I haven't calibrated one of these decks and I don't know the circuit implementation and where the adjustments are, playback gain, vu level adjustment.

My take on it would be that if 250nW is 0dB for that deck then 355nW played back from your reference cal tape is going to be +2.74dB on the VU meters.

This is 6.74dBu and 1.68 V RMS.

So set replay gain to give output of 1.68 v rms and meters to read +2.74dB

 

[March Audio]

Just general education on trying to figure out how VU readings map to output dBU / volts.

Simple curiousity.

Remember dB is just a ratio, using the second calculator I linked helps.

 

[watchnerd]

Zero on the VU meters = 1.23 volts which is 4dB above .775V which is 1 milliwatt into 600 ohms. Simple isn't it.

https://en.wikipedia.org/wiki/VU_meter

And then +3 on the meters =......?

 

[watchnerd]

Also this.

http://www.sengpielaudio.com/calculator-gainloss.htm

I will be first to admit I haven't calibrated one of these decks and I don't know the circuit implementation and where the adjustments are, playback gain, vu level adjustment.

My take on it would be that if 250nW is 0dB for that deck then 355nW played back from your reference cal tape is going to be +2.74dB.

This is 6.74dBu and 1.68 V RMS.

So set replay gain to give output of 1.68 v rms and meters to read +2.74dB

The VU meters seem to match the expected reference fluxivity, but based on what you wrote, the output voltage seems too high in comparison at 2.25V.

Then again, it's entirely possible I was measuring it wrong.

I was using alligator clips directly on the XLR pins at the line out.

 

[MakeMineVinyl]

And then +3 on the meters =......?

That would be +7dB above 0.775V (after you've calibrated them so they mean something).

20 log Vout/Vin.

 

[watchnerd]

That would be +7dB above .775V.

20 log Vout/Vin.

Thanks..... it's the log scale to voltage I always forget.

I'm too used to thinking in bits and digital reference levels.

 

[March Audio]

The VU meters seem to match the expected reference fluxivity, but based on what you wrote, the output voltage seems too high in comparison at 2.25V.

Then again, it's entirely possible I was measuring it wrong.

I was using alligator clips directly on the XLR pins at the line out.

Indeed, I may be missing something here. It's probably very obvious

The VU meters are behaving exactly as expected, minor trim issue.

Well we know that 0dB on the VU meters is 1.23 volts (+4dBu)
1.23 volts + 2.74dB is 1.68 volts. So where is the issue ?

Can you measure again to be sure?

 

[MakeMineVinyl]

Thanks..... it's the log scale to voltage I always forget.

I'm too used to thinking in bits and digital reference levels.

SEE that's why digital stinks and analog rules.

 

[watchnerd]

Indeed, I may be missing something here. It's probably very obvious

The VU meters are behaving exactly as expected, minor trim issue.

Well we know that 0dB on the VU meters is 1.23 volts (+4dBu)
1.23 volts + 2.74dB is 1.68 volts. So where is the issue ?

Can you measure again to be sure?

Is there a better way to measure than attaching alligator clipped probes to the #2 and #3 pins of the male XLR line out jack?

But for now, I need to cook a pork chop for dinner.