Quote:
Originally Posted by Matt Syson  Hi
When fed with the centre tap floating yes indeed there would be no signal COMPARED TO GROUND but the transformer only cares about the potential from the end of the primary winding to the other end, which is in this case is the algebraic sum of the 2 amplifier outputs. The transformer does not care if it is a balanced signal or unbalanced, only the Voltage difference presented to the ends of the windings.
Since these transformers have half primaries (1+1:2) ratio then indeed the voltage compared to ground at the centre tap will be theoretically zero with only manufacturing tolerances and imbalance allowing some signal to appear.
Common mode 'rejection' works because the transformer only 'transmits' a potential (current) DIFFERENCE as presented to the ends of the primary. If both ends have the SAME signal (voltage and phase) then there is no DIFFERENCE so it will not transmit any signal to the secondary.
This situation is only spoiled by imbalance of the windings, usually capacitive.
Matt S |
Yes, this is my understanding as well, so for this reason, the 100uF are not in series as such, but they do see 1/2 of the output load each.
And, omitting the transformer from the summing circuit may cause problems if an unbalanced input is used. The transformer ensures that all inputs are "re-balanced" despite the input format.
Also, the network on the secondary of the transformer should tame any high frequency rise due to the transformer being loaded higher than 600 ohms.