ron_swanson

Audio device calibration
« on: March 17, 2020, 08:47:29 PM »
Hello,

I've just recently finished  a stereo compressor build that I'm now in the middle of calibrating.  I did a very rough, ball-park calibration run through as a start to see if all seems to be working as expected  - switch actions, meter activity, ball-park measurements, buzzes, noise, etc. following a provided calibration document.  All looks / sounds good so far...

Now that I'm trying zero in the calibration more precisely, I'm finding an issue that maybe I've always had when calibrating previous builds, but never appeared obvious or I mis-understood. 

My electronics bench is not close by my studio DAW, so I've been using a stand alone bench signal generator that only offers a 20V max range that I now see may be a problem in how I've been applying.  Hoping someone can clarify in case I'm completely messing up my calibration steps in general.

Usually when calibrating, I'll be asked to set an input and / or measure an output signal of 0.775V (0dBu) which has seemed to do the job.  When using the bench signal generator, 0.775V (0dBu) seems to require something around a 2.1V setting on the generator in order to read 0.775V AC on my DMM.  Seems to have been adequate so far on previous builds.

On this build though, one of the calibration steps asks that I send a 1kHz 7.745V (+20dBu) from my signal generator.   My bench generator will only provide me with ~7VAC measured on my DMM at it's 20V max setting.   Close, but not 7.745V as desired.

So, I moved over to my DAW (ProTools HD + I/O16) only to find its max setting only offers 0.775V (0dBu) using the built in signal generator.

With this in mind, do I have my dB, dBu, dBFS, dBV, to V mixed up in my head and / or on my equipment?   If not, is my bench signal generator not up to the task requiring something more robust or maybe I just need to search for some alternate signal output setting on my generator that I've overlooked previously? 

Thanks in advance for any help here!

Cheers,
Greg


squarewave

Re: Audio device calibration
« Reply #1 on: March 17, 2020, 10:10:16 PM »
All of your observations sound correct to me. 7.75VAC RMS is 21.9 Vpp so your signal generator is just falling a little short. I think my $15 protek is about the same IIRC. If you have a largish 1:2 transformer you could step that up. Output Z of generators is usually 50 ohms so stepped up would be 200 ohms but your inputs are probably 10K so it should work fine.

Here's a good dBu, dBV, Vpp, etc online calculator:

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

ron_swanson

Re: Audio device calibration
« Reply #2 on: March 18, 2020, 12:06:50 AM »
All of your observations sound correct to me. 7.75VAC RMS is 21.9 Vpp so your signal generator is just falling a little short. I think my $15 protek is about the same IIRC. If you have a largish 1:2 transformer you could step that up. Output Z of generators is usually 50 ohms so stepped up would be 200 ohms but your inputs are probably 10K so it should work fine.

Here's a good dBu, dBV, Vpp, etc online calculator:

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

Hi squarewave,

Thanks for the reply and the link. 

I don't immediately have any transformers on hand that would fit the bill unfortunately, but seeing as the 'falling short' margin my generator is missing the mark by, I'm guessing if I just calculated the discrepancy and applied that proportion across the required tests I should be good within an acceptable margin.  Anything wrong with this thinking?

Seems there are very few signal generators that go beyond 20Vp2p without getting into very expensive units. 

Cheers,
Greg


Re: Audio device calibration
« Reply #3 on: March 20, 2020, 09:08:28 AM »
The newer models of Feeltech arb gens will put out 24v p-p .( FY6900- FY8300s)
I bought the original FY6600 which I modified and then got sent an FY6800 by the company for test and review purposes .
They are cheap and cheerfull but most who got one feel its good value for money  ,  I can get a sine with 0.00x% distortion out of mine although its only at sweet spot frequencies this is possible . The 15mhz model will do you fine for audio work and has plenty of other usefull functions too including modulation and counter inputs

Reviews of the newer 3ch model here ,

https://www.eevblog.com/forum/testgear/feeltech-fy8300s-quick-tear-down/

rackmonkey

Re: Audio device calibration
« Reply #4 on: March 25, 2020, 01:02:07 PM »
The situation you're describing is one reason it's good to have a high quality signal amplifier around. The vast majority of function generators, like you said,  aren't going to put out more than 20V pp. I did a post a while back on some low cost options for excellent performing signal amps. You also need them when testing/characterizing unknown output transformers, which is why I posted this in the Magnetics category. Maybe it should be in the Chamber. You run your function generator into one of these to get the output levels you need for a particular test:

https://groupdiy.com/index.php?topic=73237.0
« Last Edit: March 25, 2020, 01:16:30 PM by rackmonkey »
Whether you think you can or you think you can't, you're probably right.


 

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