Adjusting Min Output (Poor Man's CMRR) of Rec'vr DOA in 2-Stage Line Amp

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Active member
Oct 4, 2017
Statesboro, Georgia
Adjusting Min Output (Poor Man's CMRR) of Rec'vr DOA in 2-Stage Line Amp

I am assuming you have done this before as I am hitting on on the main points only. The detailed instructions can be found thanks to Jeff at the CAPI Online store.

While using the 'CMRR' special cable (similar to the one used in some CAPI modules, ie., VC528-ML2, VP28, a few others), I became curious about what is actually going on. I understand the concept behind the two closely matched R1 and R2 (~430<R<=1000) placed in series with XLR-M P2 and P3 in an attempt to assure the test signal voltage level will be the same at the + and - input terminals of the 1st-Stage DOA (A1). This is the matched R's primary purpose.

When using the special CMRR cable, the builder is expecting a very small signal level at A1 output, say down around -60dBu to -90dBu. RV1 is adjusted to obtain a minimum level or null. But when I started thinking about injecting ~+10dBu into R1 and R2, I now assume the very low voltage measured at A1 output was due to the voltage drop across R1 and R2.

I think this is a key point: If a large test signal was seen at A1 output, would it not obscure the ability to measure a low level voltage CHANGE when RV1 is being adjusted?

Keep in mind that R1 and R2 are removed (different cable or SW1 is NOT pushed in on CAPI's Extender Test Card) when RV2 is adjusted for unity gain. A1 output measurements are taken relative to GND, as at that point it is a single-ended signal path. The signal becomes balanced again at A2/T2 output, ie., module output.

This is not very news worthy, but I would like some feedback on whether I am correct or not.

Everyone have great New Year!



Well-known member
Oct 10, 2009
North New Zealand
Presumably the test signal is injected between ground and both the precision resistors connected together, thus putting in only common mode signal. This should have a very low value. You could put in -10 as long as the first stage does not distort. Better to put in -40 or -50 an then do your null. You dont really need two resistors, and unless really well matched, they will degrade your common mode ratio.

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