Balanced vs Unbalanced?

Herman, look at it this way: a differential amplifier does not care if it is getting a single-ended or balanced input signal. It will act exactly the same in either case. So if the input signal has two opposing inputs that have slightly different amplitudes, the differential amplifier will still not care.

IOW, 1 volt at one input while the other is at ground is the same as two 0.5V at both inputs. Or 0.25V and one input and 0.75 at the other. The differential amp does not care- it just amplifies what is different between its two inputs, regardless of differing amplitudes.

In the meantime, the CMRR is not dependent on the signal, its dependent on the differential amplifier, and it gets an awful lot of that from how effective its Constant Current Source (CCS) is.

The bottom line is you can have an imbalance and it will work out fine, so you don't need loop feedback in the circuit to insure perfect balance.

BTW in general this issue is really poorly understood, so that was an excellent question!

Ralph, I agree with all of that. I wasn't very clear when I said

for everything to work as designed you have to have a circuit capable of producing 2 signals that are exactly the same except for their polarity.

I never meant to make any correlation between signal and CMRR. I should have said

to maximize CMRR the circuit must amplify any common mode signal to produce 2 noise signals that are exactly the same except for their polarity.

If one side amplifies the common mode signal i.e. noise more than the other then they don't completely cancel. That can't be corrected in the next stage. Sorry for the confusion.

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Herman, Yes, that's true. I've found in practice though that it does not seem to come up. Also, again the CCS circuit plays a huge role in this- even if there is a mismatch in the tubes, you don't seem to get any extra noise.

BTW, any tube amp with a single-ended input can be modified to have a true balanced input, often without any change to the way it operates with a single-ended input, and without using an input transformer. IOW its possible to have a balanced front end and an SET as well, with all the benefits.