Is a stereo amp, when bridged to mono, by definition differential?

Hi Howard,

That’s a very thoughtful question. And I guess the answer can be summed up as "sort of." :-)

First, as you realize balanced amps, bridged amps, and differential amps all have differential outputs. But the term "differential amp" is perhaps best used to refer to a particular form of "balanced amp," in which each stage of the amp’s signal path operates in a differential manner. There are other forms of balanced amps, in which an entirely separate signal path exists for each of the two signal polarities. Those designs would perhaps best be referred to as being balanced but not differential, at least internally.

So I would re-phrase your question as asking if a stereo amp, when bridged, constitutes a balanced amp. And again, the answer is "sort of."

If the amp provides only unbalanced inputs, when operated in bridged mono mode a phase inversion would obviously have to be introduced somehow in one of the channels. So in those cases there would be some difference in the circuitry of the two otherwise nominally balanced and identical signal paths.

If the amp provides balanced inputs, and if in bridged mode one of the two signals in the balanced pair of input signals that is being used is routed to the circuitry for one channel, and the other of the two signals in the balanced pair of input signals that is being used is routed to the circuitry for the other channel, then yes, I would consider that to be equivalent to one form of a balanced amplifier. At least in terms of topology, although as you realize not necessarily in terms of performance.

Best regards,
-- Al

Al's answer is a great answer to a tricky question. One clarification I would make to the OP is that differential amplifier and push-pull amplifier are not interchangeable terms. Push-pull amps can be balanced or unbalanced amplifiers.

Thank you both for your responses -- they are much appreciated.

-- Howard

A further elaboration to the last paragraph of my previous post. When I said:

If the amp provides balanced inputs, and if in bridged mode one of the two signals in the balanced pair of input signals that is being used is routed to the circuitry for one channel, and the other of the two signals in the balanced pair of input signals that is being used is routed to the circuitry for the other channel, then yes, I would consider that to be equivalent to one form of a balanced amplifier.

The same conclusion would apply to an alternative approach in which the single pair of balanced input signals that is being used in bridged mode is simultaneously routed into two differential receiver stages, that are identical except that they would be wired so as to result in the outputs of the two stages having opposite polarities. With the output of one stage going into the rest of the circuitry for one channel, and the output of the other stage going into the rest of the circuitry for the other channel.

Best regards,
-- Al
 

If the balanced input only operates when the amp is bridged, then it is considered balanced but not differential (even if the individual channels are differential). set up this way the Common Mode Rejection Ratio (CMRR) will likely not be very good. Some mbl amplifiers are set up this way.

If the balanced input works in both bridged and stereo mode, then its likely that it is a differential input and performance will be better.