24.18.227.0
In Reply to: RE: Ping PJ: questions from last week posted by artsybrute on May 05, 2009 at 09:33:39
First, be aware of the difference between "balanced" and "push-pull".
Balanced usually refers to a balanced line, where the signal is the voltage difference between two lines and each line has the same impedance to ground. It's that balanced impedance which allows common-mode noise to be introduced from outside without affecting the (differential mode) signal. The voltages do not have to be equal and opposite relative to ground or any other reference point for this to work. Balanced lines are especially valuable in noisy environments, for tiny signals, and when long lines are used. They are pretty much universal in sound studios.
(Parenthetically there are two major refinements usually employed. The two lines are usually twisted together so that any noise induced by a magnetic field is canceled. And the two lines are usually surrounded by an electrostatic shield, used to connect the shields of the source and load devices so that the actual signal circuitry remains within a single shield enclosure.)
Push-pull is a circuit topology which has its own advantages and disadvantages, completely independent of whether or not such a circuit is connected to other devices by balanced or single-ended interconnects.
Now to the question. The only push-pull problem that I can think of that is unique to small-signal front ends is that the noise resistance is doubled since the inputs are effectively in series. Since the noise is uncorrelated, that adds 3dB to the noise figure. This is easily accounted for by proper choice of the SUT if you are using one. Other than that, the usual PP vs SE advantages and disadvantages apply - more tubes, even-order distortion cancellation, power supply rejection, phase splitting and combining, etc etc.
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