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

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.

Regards,
-- Al