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

@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

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
 

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

Hi Howard,

Engineers as well as non-engineers can easily conflate some of these terms incorrectly, at least in their wording if not in their understanding. In particular, the terms "differential" and "push-pull" are often used in an excessively loose manner IMO.

As I see it "push-pull" is best used to distinguish an amplifier from one having a "single-ended" output stage, such as a SET (single-ended triode) tube amplifier, or the single-ended pentode tube amplifier Whart referred to.

In the first paragraph of Steve M.’s writeup that you quoted he says:

Speaker drive is thus differential or push-pull as well.

Yes, the + and - outputs can indeed be thought of as pushing and pulling current, each of them doing so alternately (with one pushing while the other is pulling) on the positive and negative halves of each cycle of the waveform. And yes, the **outputs** (i.e., the "speaker drive") operate in a differential manner. But his wording misleadingly conflates the two terms, IMO. And I’m not sure if his adjacent reference to the DNA-500 being a "fully differential/balanced amplifier from input to output" is worded as precisely as it should be. That wording could be construed to mean that the internal signal paths of the amp are comprised of differential stages, which does not appear to be the case.

The bottom line to my interpretation of the description you quoted is that I would characterize the amp as being equivalent to two robustly designed bridged mono amplifiers on a single chassis.

Also, the first full paragraph of the post by GS5556 is worded a bit more ambiguously than it should be, IMO:

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.

First, a push-pull amp can be class A or class AB, but a class A amp is not necessarily push-pull. A single-ended audio amplifier of necessity operates its output stage with class A bias. (If it didn’t there would be little or no output for part of each cycle of the waveform). A push-pull amplifier may bias its output stage in either class A or class AB, or it may bias the output stage in class A up to a certain power level, and then switch to AB. (Or it may operate in class D or other classes, but those are different animals altogether). The distinction between these bias classes is explained in this Wikipedia writeup.

Also, the quoted statement could be interpreted to mean that a push-pull design cuts off the current that is conducted by a given output device during part or all of either the positive or negative half of the output waveform. However such cutoff may or may not occur in a push-pull design, depending on the bias class.

Not sure if all of that adds to or lessens the confusion, but FWIW. :-)

Best regards,
-- Al

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