Unbalancing Acts

From Balanced to SE
    We are all familiar with phase splitters circuits that convert an unbalanced signal into a balanced pair of signals. But where are the circuits that convert balanced  signals into a single unbalanced one?

IC balanced to unbalanced conveter

   First we might ask ourselves, Why would we need a circuit to make the conversion from unbalanced to balanced? The answer is simple enough: complex audio systems are seldom unalloyed SE or balanced, but often a mix of both. Even if you are running all tube gear, it is likely that your setup is mixed as the output stage of most tube amplifiers are push-pull although the output signal to the speakers is un-balanced.
  The output transformer of a tube amplifier serves this function, but seldom is a transformer used for line level signals. Cost and size are the transformer disadvantages. Still the transformer can perform a wonderful audio magic trick: it rejects common mode signals wonderfully. If a signal is shared (in both phase and amplitude) on the primary winding leads, very little of it shows up on the secondary winding. And as power supply noise is usually equally present in both phase legs of a balanced pair of signals, this noise drops out of the equation. Even harmonic distortions also cancel in the primary and drop out of the output signal.

Differential input signals are amplified

Common mode signals are rejected

   The Op-Amp output works to eliminate any voltage differences between its inputs. When the input signals are in anti-phase as they are in the middle figure above, output must sum the absolute difference between the input signal in order to keep the input pins of the Op-Amp matched.  Conversely, when the input signals match in voltage as they do in the last figure above, the only way the pins of the Op-Amp could match up is if the output where to mimic ground; any voltage at the output would unbalance the input pins. This circuit works well at mimicking a transformer. In fact, it works so well that it needs to be translated in tubes.

Tube Op-Amps
   Although the first Op-Amps were made up of tubes,  tubes are rarely used in an operational configuration today. (This is probably a mistake on the part of many tube circuit

Balanced to unbalanced with a transformer

   The question now is, How do we mimic the transformer's magic trick in a tube circuit? With ICs the circuit is quite straightforward: 4 resistors and an Op-Amp-Amp is all that is needed.

pg. 10


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