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Old 7th August 2009, 08:17 PM   #1
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Default Bridged vs Conventional Amps

There have been some interesting discussions about the benefits of bridged amps in other threads lately, such as the Bridged Capacitor PSU Thread. Various forms of bridging seem increasingly popular in high-end designs.

Historically I think bridging has mostly been used as a way to either save money in high power amps or to increase the flexibility of multi-channel amplifiers (i.e. allow a 2 channel amp to be used as a higher power monoblock). But the trend in many flagship high-end power amps is towards more "balanced"--i.e.bridged--output stages. Lots of companies are doing it now including Bryston, Outlaw, Emotiva and some, like Crown, have long used it for their flagship products.

If this has already been well discussed in another thread, let me know. But many of the bridged amps on the market are relatively new designs. And I haven't seen bridging presented as a cost-no-object ultimate amplifier topology by anyone. I also searched AES and the last paper on the topic was from 1984 by Sansui and it was largely in reference to using bridging instead of a transformer.

The marketing hype implies bridging is done to reduce distortion. Bryston, for example, says of their 7b and 14b: "employs a balanced-output design that reduces THD and IMD to unprecedented low values." But, from what I know, a bridged amp can only cancel even order distortion products--and that's not the dominant source of distortion. Crossover distortion typically dominates and it's mainly odd order.

A bridged amp could also, in theory at least, have a better slew rate. But slew rate, in practice, rarely causes real world distortion in modern amps. I suppose slew induced distortion *might* show up with full power sine wave testing at 20 Khz of an extremely powerful amp but even that doesn't seem very likely until you get up to obscene power levels.

In general, I would expect a bridged amp to have *more* distortion, not less. Generally distortion rises into lower impedance loads due to things like beta droop. And I would expect some of the non-linearities to be additive between the two halves of a bridged amp--i.e. perhaps thermal related distortion, phase distortions and/or mismatches between the sides.

So am I missing something as to why more and more flagship amps are bridged besides they're cheaper to build that way? Some of them, like the $8000 Bryston 14b and flagship Crowns, are supposed to be "no compromise" cost-of-little-object designs.

Perhaps once you get past a certain power level, the other benefits of a bridged design (like the broader choice of output transistors operating at half the voltage) allow it to genuinely outperform a non-bridged design? Or are the manufactures still trying to save money or just want to be different for marketing reasons?

Does anyone have any conclusive data on a bridged amp offering overall better performance than a similar conventional single-ended design?
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Old 7th August 2009, 09:03 PM   #2
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In addition, I should add that most of these high-end amps, like the Brystons, likely end up connected to similarly high-end speakers that often present relatively difficult low impedance loads from lots of drivers and/or complex crossover networks, etc. To me, this makes a bridged design seem even less ideal. So why would a well respected amp manufacture like Bryston choose a bridged design for an $8000 amp?
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Old 7th August 2009, 09:28 PM   #3
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There have been hi end bridged amps around since at least as far back as the mid 70s. Properly done, they are NOT cheaper. In fact, they can cost almost twice as much to build.

Some of us believe the extra expense is worth the performance gain achievable. Others do not.
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Old 7th August 2009, 09:45 PM   #4
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Quote:
Originally posted by Steve Dunlap
There have been hi end bridged amps around since at least as far back as the mid 70s. Properly done, they are NOT cheaper. In fact, they can cost almost twice as much to build.

Some of us believe the extra expense is worth the performance gain achievable. Others do not.
Yeah, the bridged Crown's for example go way back. But still, for the reasons I listed above, any sort of bridged amp would seem to be at a disadvantage. So what I'm after is how can a bridged amp outperform a single ended design? Where does the "performance gain" come from?
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Old 7th August 2009, 09:50 PM   #5
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one could build an amp with more dynamic headroom within a given supply voltage.
regards
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Old 7th August 2009, 10:14 PM   #6
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Quote:
Originally posted by Juergen Knoop
headroom?
Hmmm... That's an interesting point. In a real world design, because a bridged amp works the power supply twice as hard, there would likely be more DC voltage "droop" with sustained output. That would likely give a bridged amp more measurable headroom compared to its sustained output. But power supply droop generally isn't considered a good thing in a high-end amplifier.

And, in some designs, a bridged amp is more likely to trigger its current limiting and/or suffer other losses into low impedance loads because of the effective load impedance being half of whatever is connected. So, as the load impedance drops, I would expect the single ended design to eventually have the headroom advantage all other things being equal.
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Old 7th August 2009, 10:23 PM   #7
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I did say properly done. That is why it can cost twice as much.
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Old 7th August 2009, 10:30 PM   #8
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Quote:
sustained output
this is probably an overrated feature. I would prefer more appreciation of dynamic headroom/music power.
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Old 7th August 2009, 10:30 PM   #9
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Quote:
Originally posted by Steve Dunlap
I did say properly done. That is why it can cost twice as much.
But how? No matter how high quality the halves of the bridge are, it seems to me the halves (or in the case of a grounded bridge design the "high side") will individually outperform the bridged version using the same design.

So even with an unlimited budget, if you set out to design the best amplifier in the world to drive a pair of real world speakers, why would you choose a bridged output over a single ended one?
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Old 7th August 2009, 11:15 PM   #10
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2nd breakdown limited SOA in older bipolar output Q really limits I as Vsupply goes up - more than linearly so bridged @ 1/2 the supply V the safe Ipeak could be much larger and use fewer devices in total

MOSFET or thermal limited Bipolar SOA devices come out about the same in total number of output Q for given output power ( bridged can cost a little more in fixed Vdrop for the 2X bias components)

the extra small signal and driver parts usually are a minor production cost compared to mounting/heatsinking the output Qs
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Old 7th August 2009, 11:30 PM   #11
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You can get reduced power supply noise, better common-mode noise
rejection, twice the slew rate, cancellation of 2nd harmonic and
lower (safer) voltages.

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Old 8th August 2009, 12:00 AM   #12
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Quote:
Originally posted by Nelson Pass
You can get reduced power supply noise, better common-mode noise
rejection, twice the slew rate, cancellation of 2nd harmonic and
lower (safer) voltages.
Ok, I agree with all of those except possibly common mode noise (it depends on where such noise enters into the signal path).

But, none of them AFAIK, will reduce real world distortion unless you're measuring *only* the 2nd harmonic. Given that 2nd harmonic distortion is generally much less of a problem (both subjectively and objectively) than the 3rd and higher orders, improvements there don't help the overall distortion or sound quality situation much. And I would expect the other factors I mentioned would more than offset any 2nd harmonic gains (i.e. the bridged amp having more 3rd and higher order distortion).

Quote:
Originally posted by jcx
MOSFET or thermal limited Bipolar SOA devices come out about the same in total number of output Q for given output power
OK, are you saying for a given power output you need roughly the same amount of output devices in a bridged vs single ended design? I realize the transistors are in a much better part of the SOA curve in a bridged amp compared to a conventional amp of the same power. But that's offset by the peak current being twice as high, and also of course, by the need for two mirrored output stages instead of one.

I haven't done the full analysis, but I would think it comes to how much output voltage and peak current you want. For example, I'm fairly sure a 200 watt/8 ohm bridged amp would need more output devices total if you want the same 2 ohm load performance as a conventional 200 watt/8 ohm amp. For a 1000 watt amp, however, it might be the other way around.

It would seem, at least based on commercial designs, there's a crossover point at some power level where the advantages of a bridged design overcome their disadvantages. But I've not seen any hard data to back that up or identify at what power level that might be true.
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Old 8th August 2009, 02:17 AM   #13
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I realized another potential advantage to the "grounded bridge" and similar topologies where the output stage power supply is not ground referenced: Such amps can have quieter grounds for the input stages of the amplifier. If the output stage power supply is floating, it avoids all the high ground currents in a conventional amp. As with SOA, this benefit is proportional to the output power of the amp.

Some also claim because the current drawn from the power supply in a bridged amp is full wave, vs half wave in a conventional amp, the bridged amp produces less induced (i.e. EMI coupled) noise in the rest of the amplifier. I'm less convinced this really helps. I'm not sure what the spectral energy differences are between the two waveforms, but I would suspect the twice higher higher currents in the bridged amp might more than offset the full wave advantage?
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Old 8th August 2009, 04:20 AM   #14
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Quote:
Originally posted by Nelson Pass
You can get reduced power supply noise, better common-mode noise
rejection, twice the slew rate, cancellation of 2nd harmonic and
lower (safer) voltages.


I agree.

Also it depends on how accurate the phase splitting section of the circuit is, because if there is phase shift away from 180 degrees even at higher frequencies, there will be a related distortion. no transformers allowed here, eh?


The PS has to be more robust in terms of current and uF's of capacity, but they can be lower voltage. Not a terrible difference in cost. Larger traces and thicker wires, eh, big deal.

I tend to favor the idear of using a common, balanced bridge VAS to drive each OPS instead of two separate VAS circuits. The better CMRR is a plus. In one version of my insane complex bridge amp, I drove the amp to death, deliberately, to see what would happen. One of the N-ch OPT melted short, connecting that speaker lead to the positive rail. The reaction of the DC servos that control the common mode amplifier part then attempts to correct for the CM error by trying to swing that output negative. The result was the other output phase was driven to the positive rail resulting in no DC current flow in the speaker and the circuit locked up in a steady state. This 'auto-protect' feature was an interesting observation I doubt could be achieved easily with a single end output. Hopefully sometime soon I will get some time to play around with it a bit more. I have quite a few experiments and tests I want to do yet.
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Old 8th August 2009, 04:58 AM   #15
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Default Balanced outputs vs. bridged amplifiers

Balanced outputs vs. bridged amplifiers

From Pass Literature:
"The Super-Symmetric X amplifier design exploits the symmetry of a matched balanced amplifier so that distortion and noise are cancelled at the output to the loudspeaker, then uses a small dose of a unique new form of feedback to make that symmetry more perfect.

The topology takes advantage of the character of specially matched balanced amplifiers that are cross-coupled to provide cancellation of distortion and noise. The result provides high performance with very simple linear circuits, better than previous similar efforts by an order of magnitude. It was named Super-Symmetry as an homage to particle physics, but it is popularly known as the X circuit.

Balanced amplifiers improve performance by differentially rejecting distortion and noise. To the extent that distortion and noise are identical, they vanish at the output, typically by a factor of 10 or so for matched single-ended Class A circuits.
Super-Symmetry extends this concept by using feedback only to make the distortion and noise more identical on each half of a balanced circuit, not to eliminate it as such. This gives as much as a 100:1 reduction in unwanted distortion and noise without requiring the equivalent amount of negative feedback. It is simply much easier to tweak the two halves of the circuit into identical symmetry than to eliminate all the distortion in each half of the circuit."

My ears like the sound of bipolar output Super-Symmetric amplifiers. I do not have any balanced drive music sources and use a single ended source input to generate balanced outputs.
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Old 8th August 2009, 08:04 AM   #16
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Quote:
Originally posted by Nelson Pass
You can get reduced power supply noise, better common-mode noise
rejection, twice the slew rate, cancellation of 2nd harmonic and
lower (safer) voltages.

But a major disadvantage should not be unmentioned: The requirement to a small degree of deviation from the two amplifier halves is high and is easiest to realize by CSPP (Circlotron).
Therefore sometimes I observed a lack by quality of power amplifiers in the upper frequency range, when they switched from stereo to mono. I experienced this by certain, individual devices of the NAD power amp 2600 and 2400. The reason was mainly not selected output power devices from Toshiba - so I think
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Old 8th August 2009, 08:35 AM   #17
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Hi,

In theory you can get 4 x the power into the same load, that's if the power supply is beefy enough of course. So that in itself is an attractive proposition.

This also means the voltage rails can be lower, with the benefit of reduced thermal stress, and cooling requirements, and lower voltage output devices.

I think a correctly balanced input and output is hard to beat.
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Old 8th August 2009, 01:00 PM   #18
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Here's a fun circuit which is grounded bridge:

http://media.qscaudio.com/pdfs/disco...s/PL9.0PFC.pdf

schematic:
http://www.qscaudio.com/support/libr...s/pl9.0PFC.pdf

4500W / channel, 2 ohms

The 4 step class H would have needed eight power supply rails and six rail switches if it had been a non-bridged amplifier. Of course, now the power supplies for the two channels need to be separate, but having one channel working in case of PSU failure is probably worth it in this application.
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Old 8th August 2009, 02:15 PM   #19
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One specific benefit of bridging comes with true class-A operation of the halves. Then the total current draw from the supply is constant, the amp cannot modulate the supply with load currents. Still the supply should be low ripple and low and flat impedance of course. Since there are no load currents in the GND, a single supply can be used and a reference GND derived from that in a way that it optimzes PSRR for the individual halves right to start with.

Another point would be, for low/zero FB designs, that one can trade some even harmonics to favor low amounts of odd ones, as the former are cancelled by the bridge.

- Klaus
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Old 8th August 2009, 02:15 PM   #20
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"My ears like the sound of bipolar output Super-Symmetric amplifiers. I do not have any balanced drive music sources and use a single ended source input to generate balanced outputs."

Hadley 622 (60's vintage):

Click the image to open in full size.
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Old 8th August 2009, 04:40 PM   #21
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Thanks for all the comments. Some non-obvious ways a bridged amp might sound better have been presented. And there's no question, as megajocke and others pointed out, bridging has some serious benefits for high powered pro amps. But back to high-end home audio with more modest power levels...

FAVORING BRIDGED: When it comes to sound quality, we have in a bridged amp's favor...

Better PSRR - but PSRR is not typically a problem in conventional amps especially with the well filtered stiff power supplies typically found in high-end amps.

Better Slew Rate - but, as above, slew rate is rarely a limiting factor in all but extremely high powered amps.

Lower 2nd harmonic distortion - but, as above, the 2nd harmonic is rarely a problem and also the least objectionable of all harmonic distortion.

Better common mode noise performance - Most of this benefit can be built into the gain stages of a conventional amp and I can't see how it helps an output stage much?

Possibly less distortion from ground currents using a floating power supply - This may have some real benefit but mostly in very high powered amps and only applies to floating designs like Crown's grounded bridge.

Less voltage on output devices for higher SOA - This is certainly true, but it's not clear how much it helps the actual sound quality or measured performance--especially given the current through the output devices is doubled. If the amp can really use fewer output devices for the same performance there might be gains from lower Cbe or gate capacitance on each bank of output devices.

Higher headroom - Due to a bridged amp drawing power on both phases of the signal from each rail, and having twice the current requirement, they're likely to create more power supply sag giving them higher measured headroom. This is controversial as some think "soft" power supplies hurt sound quality.

AGAINST BRIDGING: These issues likely work against a bridged amp's sound quality and measured performance...

Higher distortion from driving half the effective impedance due to beta droop, etc.

Higher distortion from mismatches between the sides of the bridge.

Higher distortion from additive distortions in the mirrored bridge--i.e. phase and thermal distortions.

Higher distortion from whatever circuitry is added to invert the signal.

Twice the current requirements which creates more power supply sag, more I2R losses, may require more output transistors, and possibly increase EMI related distortion.

More difficulty driving low impedance loads due to the load impedance being cut in half. A conventional amp of 200 watts or less, with the same number of output devices as a bridged amp, likely has a significant advantage into low impedance loads.

Given the above I'm still not convinced an ultimate high-end amp should be bridged--at least if it's around 200 watts or less. Clearly bridging has some strong advantages but most of them mainly benefit very high power amps and/or lower cost manufacturing.

I think there is a perception among many that "fully balanced" (and sometimes "fully symmetric") from input to output, all other things being equal, is better. Many companies certainly market their products that way. But speakers are 2 wire devices that don't care if they're floating or grounded. So there's no mismatch between a conventional grounded amplifier and a speaker. The issue seems to come down to mostly the points above.

So perhaps at 600 watts for the Bryston bridging makes sense? But I think Outlaw would be hard pressed to justify how bridging really makes their 200w/ch amp sound better--especially with difficult to drive speakers.
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Old 8th August 2009, 06:35 PM   #22
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Hi Nelson. Correct as usual.

RocketScientist;

It seems you have your mind made up. Have you ever listened to both types of high end amps rated at the same power?

You seem to want this to be about the manufacturers increasing their profit. Some manufacturers think one way to increase profits is to sell what the customer wants. It was the demand for very high powered amps that created the market for them.

All of your points about low impedance loads are addressed in the design stage. Even amplifier designers understand low impedance loads and beta droop. There is also no need for the added inverter for one channel as you suggested.

Quote:
I think there is a perception among many that "fully balanced" (and sometimes "fully symmetric") from input to output, all other things being equal, is better. Many companies certainly market their products that way. But speakers are 2 wire devices that don't care if they're floating or grounded. So there's no mismatch between a conventional grounded amplifier and a speaker. The issue seems to come down to mostly the points above.
The difference is common-mode rejection as Nelson pointed out.
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Old 8th August 2009, 06:39 PM   #23
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Default Diamond Differential Balanced Bridge

Below is the "diamond differential" balanced bridge amplifier published by Takahashi and Tanaka for Sansui in 1984. It's fairly elegant and has some things in common with the Hadley and Pass designs mentioned in this thread.

The authors claim it achieves extremely high CMRR, PSRR, and slew rate while being inherently immune to T.I.M. (which was a major worry back in 1984) without needing complex constant current sources for the "diamond" stage. The lack of current sources also allows higher current drive.
Attached Images
File Type: gif sansui-balanced-bridge.gif (16.4 KB, 302 views)
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Old 8th August 2009, 06:57 PM   #24
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Quote:
Originally posted by Steve Dunlap

RocketScientist;

It seems you have your mind made up. Have you ever listened to both types of high end amps rated at the same power?

You seem to want this to be about the manufacturers increasing their profit. Some manufacturers think one way to increase profits is to sell what the customer wants. It was the demand for very high powered amps that created the market for them.

All of your points about low impedance loads are addressed in the design stage. Even amplifier designers understand low impedance loads and beta droop. There is also no need for the added inverter for one channel as you suggested.

The difference is common-mode rejection as Nelson pointed out.
My mind is anything but made up except that bridging has some obvious advantages for very high powered amps. But for audiophile amps, it's less clear to me.

Like tiefbassuebertr posted in this thread, my listening experiences in comparing bridged designs has consisted of using stereo amps in their bridged mode as mono amps. And, for me, they sounded better when not bridged. But I also realize those designs were likely not optimized for bridged use. It's difficult to do an apples-to-apples listening test of bridged vs non-bridged. And even if I could, I might prefer one amp and you might prefer the other. I'd much rather use more objective data to compare them.

I have no objection against manufactures making money. But I do have a problem when they pitch a given technology as superior, when the real reason for using it is to save money. If Outlaw marketed bridging in their 200w amp as a way to offer more watts per dollar, I'd be fine with that. But to spin it as superior topology, there should be some objective facts to back that up. Sure it might have a faster slew rate, but that doesn't help anything.

I agree about the extra inverter, but there is usually some extra phase splitting circuitry associated with a fully bridged design somewhere in the circuit. The bottom line is the signal goes through more components--with their associated nonlinearities-- before getting to the speaker in a bridged amp. That might not always be a bad thing, but it's true.

And I still don't get the common mode benefit for an output stage. I completely understand it for the gain stages but you can get those benefits with a conventional amp. But I might be missing something in how it benefits a bridged output stage beyond PSRR, and the ground issues, that I acknowledged? What other problematic common mode signals are rejected by a balanced output stage?
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Old 8th August 2009, 07:53 PM   #25
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Quote:
It's difficult to do an apples-to-apples listening test of bridged vs non-bridged. And even if I could, I might prefer one amp and you might prefer the other.
That is true.

Quote:
I'd much rather use more objective data to compare them.
Do you think what Nelson said was only his opinion, or just spin?

Quote:
If Outlaw marketed bridging in their 200w amp as a way to offer more watts per dollar, I'd be fine with that. But to spin it as superior topology, there should be some objective facts to back that up.
I know nothing about Outlaw, but this is what they say about their bridged amp.

Quote:
Okay, so you want big power but you need greater flexibility. Well, the Model 2200's are just that, powerful AND flexible! The Model 2200 ($350) is a single channel 200-watt amplifier. You simply buy one for each speaker. Like our other 200 watt amplifiers, this amp will drive virtually any speaker or (passive) sub you throw at it. So what about space you ask? Each Model 2200 sits just under two inches tall so you won't need to buy a new equipment rack! The Model 2200's advanced circuitry and large pancake style torroidal transformer enable us to deliver a cool yet potent performance. In addition to its sleek design we have provided a special signal sensing circuit that triggers the amplifier to power on whenever an audio signal is present. The Model 2200: small, sleek and powerful-need we say more?
Where is the spin here?

Quote:
I agree about the extra inverter, but there is usually some extra phase splitting circuitry associated with a fully bridged design somewhere in the circuit. The bottom line is the signal goes through more components--with their associated nonlinearities-- before getting to the speaker in a bridged amp. That might not always be a bad thing, but it's true.
It very much depends on how the amp is designed. There is no reason that the signal must pass through any more components.

Quote:
And I still don't get the common mode benefit for an output stage. I completely understand it for the gain stages but you can get those benefits with a conventional amp. But I might be missing something in how it benefits a bridged output stage beyond PSRR, and the ground issues, that I acknowledged? What other problematic common mode signals are rejected by a balanced output stage?
Here I will quote Nelson again.

Quote:
Balanced amplifiers improve performance by differentially rejecting distortion and noise. To the extent that distortion and noise are identical, they vanish at the output, typically by a factor of 10 or so for matched single-ended Class A circuits.
I'm sure he measured that. It may be on his web site. I will not attempt to dig out my own work along these lines, but I measured improvements in noise and distortion also.
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