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

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.

Howard, 

Incase you're shopping, ModWright KWA150SE amp should be on your top 5 list. The amp is transformer coupled at the balanced inputs then direct coupled throughout, fully dual mono, fully-differential and uses no global feedback. It is a powerhouse in bridge mode capable of whopping 450W in 8ohms. 

The short answer is no.

Push-pull means the output stage is has complementary devices that alternately control about half the signal by sourcing and sinking current -- not voltage -- between the positive and negative voltage rails. How much of the signal each control depends on the biasing so both Class A and Class A/B amps are push pull. 

The term differential is usually applied to the input stage, where two transistors share a common emitter load or current. Each transistor is fed a signal (source and feedback, e.g.) and the difference is amplified and then converted to current and sent to the voltage amplification stage. The VA stage then sends the voltage gain to the output stage where the push pull takes place (the OS is a low impedance load that has unity voltage gain and current amplification). The current is then sent from the positive (red) terminal to the speaker with the voltage referenced to ground. The current is in phase with the voltage.

When the two channels of the output stages are bridged, one channel receives the input signal in the normal fashion and the other channel receives the signal inverted. Both channels still operate in their differential input and push-pull output stages as they would, however the output voltage is mirrored (i.e., fully balanced). But the difference is that the inverted channel's red positive output is connected to the negative terminal of the speaker.

The speaker is no longer referenced to ground. It is referenced to the negative voltage of the inverted signal channel. Since each channel is out of phase, the peak to peak voltage is doubled. Therefore the power is quadrupled (power supply willing and able, of course).

The reason it is not push pull is because the current is not sourced between the two channels through the speaker load. The current is simply doubled through the speaker and in phase with the positive (non-inverted) channel.

So, let me throw in a question about that Audiopax amp- the 88, if memory serves, which consisted of two mirror image amp circuits that were combined in a single channel with controls that allowed you to vary their performance in relation to each other. I had these amps for a year or so, and it was fascinating to essentially 'dial in' their performance.