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

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.

Howard (Hodu), when I wrote my previous posts I had overlooked the reference in your original post to push-pull. As I indicated, my responses pertained to the relations or lack thereof between bridged, differential, and balanced amps. Thanks to Cleeds and GS5556 for addressing push-pull.

Also, Ralph’s (Atmasphere’s) post reminds me that the statement in my previous post referring to "two differential receiver stages" would also be applicable to a design in which the balanced input is routed to a single differential receiver stage which provides both inverted and non-inverted outputs.

Bill (Whart), I took a quick look at reviews of the Audiopax 88 in Stereophile and Soundstage, which I see is a single-ended pentode design. The Stereophile review states that its two amplifier circuits are utilized in series, and I certainly don’t know what to make of that. But the controls you refer to, called "Timbre Lock," apparently simply vary the bias of each of the pentodes, thereby changing the amp’s distortion characteristics. The stated intent being to allow those distortion characteristics to be adjusted by the user so as to be as complementary as possible to the distortion characteristics of the speakers that are being used. Although at the same time John Atkinson’s measurements make clear that for several reasons one’s choice of speakers that would be suitable matches for the amp is particularly limited.

Best regards,
-- Al

Thanks for explaining, Al. They were interesting to use.