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

I know this post is a bit old but I would like to ask a related question.

Looking at the Clayton M300 for example, promoted to be a pure class A balanced fixed bridged design, only with XLR inputs.  If each channel is composed of two 75 watt pure class A amplifiers in balanced design, is the output also class A as stated by the manufacturer? (rated at 300/600 wpc pure class A). Just curious, as 600 Watts class A into 4 ohms seems to be a lot. They are indeed great amps!  Thanks to any responders in advance.

@audiobrian Bridging two channels makes it possible for twice as much voltage to be applied to the speaker as could be provided by one of those channels, **IF** the amp can supply the correspondingly greater amount of current, and **IF** it can do so without overheating. Since for a given load impedance twice the voltage corresponds to four times as much power (P = E squared/R), two channels capable of supplying 75 watts into 8 ohms and presumably 150 watts into 4 ohms can potentially supply 300 and 600 watts into those impedances when bridged.

The majority of bridged amplifiers fall short of realizing that 4x factor, of course, due to current and/or thermal limitations.

In this case we are dealing with a very well regarded and robustly designed monoblock. However both its manual and a review I found in Positive Feedback state that in its high bias mode its power consumption is 800 watts. When providing 600 watts into 4 ohms that would mean an efficiency of 75%, and I’m not sure how an amp operating in class A can provide efficiency that is anywhere close to being that high.

Regards,
-- Al

Thank you Al,

I’ve always wondered how Clayton Audio gets 300 wpc class A into 8 ohms from a relatively small chassis/amplifier, albeit robustly built. I guess part of the answer is a fixed bridged design with 2 x 75 watt class A amps per channel. If power draw in high bias is indeed 800W, perhaps it truly delivers class A performance into 8 ohms but partly class A/AB into 4 ohms?  If one can build a class A amplifier of considerable power using bridged design (with the needed robust power supply and build parts), allowing for a much smaller amplifier, is this design any less desirable sonically than a non bridged class A amplifier such as Pass, Gryphon etc with considerably larger and heavier amplifiers? Manufacturers that have gone down the fixed bridged monoblocks design, not all in class A like Clayton, include Steve McCormack’s DNA 500, all Constellation amplifiers and Naim Reference Series, amongst others.
Thanks so much!

Some of the best amplifiers I have seen were unbalanced input but fully differential and complementary and direct coupled once past the input stage. One of the best I ever heard was a 'build it yourself' kit that was available many years ago thru "popular electronics" magazine. I had a couple of those amps for a while until a loose wire shorted the output and caused a chain reaction that took out every transistor in the chain. I fixed it at an expense greater than was reasonable but moved on pretty quickly to something less inflexible with regard to load impedance. :) Still, that was a fine sounding amp in its day. Today I don't know how well it would hold up against modern designs.