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Measuring voltages in a balanced audio circuit using an oscilloscope
Measuring voltages in a balanced audio circuit using an oscilloscope
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Old 22nd May 2019, 02:42 AM   #1
mrmatt is offline mrmatt
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Default Measuring voltages in a balanced audio circuit using an oscilloscope

Hello All,

So, I designed a simple balanced line level audio attenuator circuit. I was hoping to use my oscilloscope to test voltages throughout the circuit and see how each stage affects the signal.

I generated a 1kHZ -18dBFS sine wave using Logic Pro. I sent it out of my DAW using an audio interface (Sound Devices USBPRE 2). My understanding is that -18dBFS = +4dBu = 0dBVU. Likewise if I measure that signal I should read ~1.23V RMS or ~3.47Vpp.

I set up my oscilloscope (Rigol DS1054) so that probes 1 and 2 are added using the MATH function, and probe 2 is inverted. I measured the voltage across pins 2 and 3 of the output XLR of the audio interface and ended up reading about ~2.2Vpp (~.777V RMS). I then tried putting a 10K resistor across pins 2 and 3 to simulate a load, measured again, and had the same result. I then tried measuring between pins 1 and 3 and then 1 and 2 and then adding the individual readings, and still had a similar result.

I connected the output of the audio interface directly to a VU meter just to make sure, and measured 0dBVU.

I'm somewhat new at taking measurements like this. I'm clearly missing or misunderstanding something here, and am very interested in hearing your thoughts about where I could be going wrong and how I could improve my testing methodology. Thanks in advance!
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Old 22nd May 2019, 04:31 AM   #2
thaumaturge is offline thaumaturge  United States
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I'd try measuring something else using differential mode on the scope. Something easy to check, such as output from a filament transformer. Try reading direct, using probe ground as reference, then try differential, using just probe tips. Might help you pin down problem..
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Old 22nd May 2019, 08:05 PM   #3
mrmatt is offline mrmatt
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Thanks for the suggestion! That actually helped me confirm I was measuring correctly.

Just a quick update as I think I solved the problem.

The specs for the USBPRE2 audio interface state that output clipping is at 18dBU. I'm assuming that this means that 0dBFS on this device is 18dBU, which puts 0dBVU (+4dBU) at approximately -14dBFS.


If I understand correctly, the headroom of the device determines which dBVU (and dBu) level that corresponds to 0dBFS which is the loudest level the device can reach. This means that 0dBVU (+4dBu) can be different dBFS levels on different devices, depending on the headroom.


Sure enough, I plugged this into my NTI XL2 meter and -14dBFS gets me to where I expect to be.
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