rogs
Well-known member
It will depend on the configuration........ The paper you've linked to essentially describes the details for a true differential amplifier.
The Alice OPA circuit is designed to take the output from the primary op-amp, and then uses a second op-amp to invert that signal, to create a differential audio output.
This will add some extra noise..... The signal from the first op-amp will be fed to the input of the second. That signal will include the noise from the first amp. The resistors fitted into the signal path of this second inverting amplifier will also add noise. (The higher the value, the more noise they will add).
Although some of this noise will be uncorrelated, and cancel out, much will not.
(There is an excellent explanation of the maths involved in this post on the Micbuilders forum from Goran Finnberg: Pin 2 and 3 SIgnal vs Impedance Balanced )
The output signal level will be increased by 6dB of course, so less preamp gain will be required, but adding extra gain on most modern mic preamps should add very little noise to the signal. Certainly less than than that created by the inverting audio output.
And if the extra gain is added digitally in a DAW then there should be even less (or even no?) extra noise added!
This audio sample shows the difference in noise level between one of my single sided OPIC circuits and a fully differential ('Alice OPA') version of the same circuit: opa42.noise.single.then.differential.wav
In both cases the noise level is 'way down in the weeds' (a description used by Alice OPA designer Jules Ryckebusch!
) but with the single sided version the noise is around 3dB lower.Single sided audio ouputs are often seen as being unbalanced. This is not necessarily true.
If the output impedance is balanced - even passively, as with the OPIC circuits - then an acceptable CMRR can still be achieved.
Single sided balanced audio outputs are used quite extensively on professional audio equlpment ... the names Neumann, dbx and Soundcraft spring to mind for example. For them it's a 'win win' -- lower noise and less cost!
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