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

I've been reading about amps and the seemingly endless choices that designers make, and found myself wondering this, but haven't been able to find the answer. It would seem, if I'm correctly understanding the definition of differential, also called push-pull, that bridging the two sides of a stereo amplifier would, by necessity, be creating exactly this topology. Unless I'm missing something, of course, which may well be the case.

Thanks to those who understand such things much better than I for any clarification.

Also, those who'd rush to weigh in about the likely sonic benefits -- or detriments -- of such arrangements needn't bother, as that's not what I'm wondering about.


-- Howard

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.
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.

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

Here's why I posed this question in the first place. I was reading (in 6moons) a review of the McCormack DNA-500 and read the following from designer Steve McCormack:

"In the context of its power rating, how I can achieve this size and weight factor while still delivering this type of power is because unlike previous DNA designs, the DNA-500 is a fully differential/balanced amplifier from input to output," McCormack explains. "Speaker drive is thus differential or push-pull as well. We combine two complete amplifier sections per channel, each of which is driven from an input phase-splitter. (The incoming signal is split into a pair of opposite-polarity waveforms, each of which drives one section of the amplifier pair.) In most conventional amplifiers, the speaker is connected to a single positive output and ground. In the DNA-500, the speaker is connected across a pair of positive outputs of opposite polarity, thus driving the speaker in push-pull mode. This arrangement delivers a degree of speaker control that is not possible otherwise.

"Having experimented extensively with this design approach over the years, it has come to be my favorite method for producing high-performance amplifiers. It creates what is often referred to as a bridged amp. There are four independent amplifier channels, two per side, each channel pair in bridged/balanced configuration to double the voltage over a conventional amplifier. An ordinary 100-watt stereo amplifier in bridged mode will net you a 400-watt amplifier if the power supply can process the current. That's the principle of how the DNA-500 works.

"Many audiophiles feel that such a bridged-style amplifier design is a bad approach. Their prejudice is based on past experiences with stereo amplifiers that were switchable into bridged mono. These amps often had poorly designed input phase splitters and were not optimized to handle the reduced load impedance in bridged mode. The result was often compromised audible performance, instability, overheating, blown fuses and more than a few damaged amplifiers. It's no wonder that bridged amps have acquired such a bad reputation. However, the DNA-500 demonstrates that this need not be the case."

This all got me to thinking, and wondering. As I'm nobody's electrical engineer (or any other type of engineer, for that matter), I may well have been confused by some of the terminology, perhaps conflating this with that and the other thing. 

Thanks to all for your thoughtful and considered responses.

-- Howard
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

If you are talking about one of Steve McCormick's designs in the context of this question, you have no worries. He knows what he is doing and his inputs work correctly.
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.

-- 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!