[gjc11028]

From pass labs owners manual on balanced output

The first X amplifier, the X1000 was intended as the premier example of the power of this principle, delivering 1000 watts rms into 8 ohms at low distortion. By itself of course, this is no miracle, but you have to consider that products with comparable performance have complicated circuits with as many as nine consecutive gain stages and lots and lots of negative feedback. The X1000 had only two stages and used only local feedback.
The difference was the unique balanced circuit topology in which circuit errors are replicated at both output terminals so as to cancel and disappear across the loudspeaker terminals. The high quality of the sound reflects both the low distortion and simplicity of the gain path.
The SuperSymmetric circuit consists of two identical matched circuits arranged like the wings of a butterfly, showing symmetry from left to right, and operating balanced to the loudspeaker. The amplified signal appears with opposing phase across the loudspeaker. Most of the distortion and noise appears in phase across the loudspeaker, and is not seen.

 

[NinjaHamster]

And what exactly is the benefit of driving a loudspeaker from a balanced output?

se

 
Actually this is a question I have had recently, but not yet researched.  So there ARE amplifiers that output balanced to speakers (that was my first question). My second question was whether this is of benefit (as it is - arguably - further up the component chain).  It seems it would be of benefit (to a layperson) which is why this question occurred to me. It sounds as though you don't believe there is any real benefit in a balanced output to a speaker - would this be a fair assessment ?

 

[Chris J]

From pass labs owners manual on balanced output

The first X amplifier, the X1000 was intended as the premier example of the power of this principle, delivering 1000 watts rms into 8 ohms at low distortion. By itself of course, this is no miracle, but you have to consider that products with comparable performance have complicated circuits with as many as nine consecutive gain stages and lots and lots of negative feedback. The X1000 had only two stages and used only local feedback.
The difference was the unique balanced circuit topology in which circuit errors are replicated at both output terminals so as to cancel and disappear across the loudspeaker terminals. The high quality of the sound reflects both the low distortion and simplicity of the gain path.
The SuperSymmetric circuit consists of two identical matched circuits arranged like the wings of a butterfly, showing symmetry from left to right, and operating balanced to the loudspeaker. The amplified signal appears with opposing phase across the loudspeaker. Most of the distortion and noise appears in phase across the loudspeaker, and is not seen.

I have an X150, it's a nice sounding amp.

 

[robrob]

Ok, forget about "safety reasons." What exactly should the negative speaker terminals connect to if not ground? Where is the current flowing through the speakers supposed to return to?

And what exactly is the benefit of driving a loudspeaker from a balanced output?

se

 
Steve, see the output transformer secondary in the schematic below? That's a balanced output. The coil, speaker wire and speaker voice coil make up the loop for electron flow. It doesn't need a ground connection. That's how professional audio equipment send out their balanced 3-pin XLR signal, with a transformer made up of a single wire connected to two output terminals (hot, cold and a separate ground wire).
 
A balanced line rejects RFI and radio magnetic interference (RMI) naturally due to each stereo channel's two equal length and gauge wires both picking up the same electrical noise--one wire is positive and the other is negative so when the signal from the two wires is combined at the speaker the added noise cancels itself out. This is called CMR or Common Mode Rejection. Adding a grounded shield to a balanced line will offer even more noise reduction. This is why professional audio gear use balanced inputs and outputs, especially for sensitive microphone wire runs. I realize the benefit for speaker runs is much lower than a microphone run but I still prefer balanced output for my speaker and headphone amps.

 
 
This Tripath 2024 is a solid state transformerless amp with push-pull amplification all the way to the speakers. This is a balanced output too.
 

 
The 'safety reasons' and liability exposure stem from possible electrocution if something goes wrong in the amp and puts high voltage on the speaker wire circuit. Without a ground connection it may remain hot until someone touches the wire or speaker terminals (at the amp or speaker). With a speaker wire connected to ground a short occurs and either a fuse or circuit breaker blows.

 

[camusmuse]

Well, while we are doing this balancing act; I have always been curious about "dynamic speakers without magnets".  Sorry, I cannot recall their proper name; but essentially instead of a rare earth magnet to supply a static magnetic field, it is replaced with another separate voice coil...powered by an amp 180 degrees out of phase of the moving/dynamic coil(one fits inside the other, much like the magnet gap...  So a total of four "balanced" channels to two drivers.  I always thought that would be the ultimate balanced solution.  Magnet strength can decrease over time changing the effectiveness of the driver, the static coil, and the dynamic coil should last as long as the rest of the "soft parts" of a driver, if not abused. 
 
Obviously the design didnt really take, mostly because of issues with full size speakers, I believe.  But....and a big butt....anyone thought of making these "speakers" for headphone drivers, smaller might be easier; certainly dont need alot of power....might have to find some dead drivers and start doing autopsies....just call me Dr. Frankenstein....

 
The other version of the "dual" coil magnetless driver, was when the second static coil was supplied with a constant DC voltage instead of the AC audio signal to create the electromagnetic field.  Not as "truly balanced", but being able to adjust the magnetic properties of the driver basically would let you have some control over T/S parameters; that could be pretty handy....in theory of course.....

 

 

[Steve Eddy]

From pass labs owners manual on balanced output

The first X amplifier, the X1000 was intended as the premier example of the power of this principle, delivering 1000 watts rms into 8 ohms at low distortion. By itself of course, this is no miracle, but you have to consider that products with comparable performance have complicated circuits with as many as nine consecutive gain stages and lots and lots of negative feedback. The X1000 had only two stages and used only local feedback.
The difference was the unique balanced circuit topology in which circuit errors are replicated at both output terminals so as to cancel and disappear across the loudspeaker terminals. The high quality of the sound reflects both the low distortion and simplicity of the gain path.
The SuperSymmetric circuit consists of two identical matched circuits arranged like the wings of a butterfly, showing symmetry from left to right, and operating balanced to the loudspeaker. The amplified signal appears with opposing phase across the loudspeaker. Most of the distortion and noise appears in phase across the loudspeaker, and is not seen.

Super Symmetery relates to an amplifier topology, not to any advantages a loudspeaker gains by being driven from a balanced source.

Basically it takes two very simple circuit topologies which would have unacceptable distortion performance on their own, bridges them together and cross-couples them so that the distortion appears common mode at the output terminals and then relies on that distortion being rejected by the loudspeaker. Which would be fine if you're driving a single loudspeaker driver which can be very symmetrical. But once you're driving a loudspeaker with multiple drivers and a crossover, all bets are off as the crossovers in most every loudspeaker are asymmetrical, so you're not going to get the cancellation that's hoped for.

And Nelson really goes beyond the pale saying "products with comparable performance have complicated circuits with as many as nine consecutive gain stages and lots and lots of negative feedback." I'm sure somebody at some time designed a power amp with nine consecutive gain stages, but that's not even close to describing typical amplifier toplogies. Comparable performance can be easily achieved with single-ended circuits no more complicated than Nelson's Super Symmetry.

se

 

[Steve Eddy]

Actually this is a question I have had recently, but not yet researched.  So there ARE amplifiers that output balanced to speakers (that was my first question). My second question was whether this is of benefit (as it is - arguably - further up the component chain).  It seems it would be of benefit (to a layperson) which is why this question occurred to me. It sounds as though you don't believe there is any real benefit in a balanced output to a speaker - would this be a fair assessment ?

Pretty much, yup.

se

 

[camusmuse]

--------------It sounds as though you don't believe there is any real benefit in a balanced output to a speaker - would this be a fair assessment ?
 
Quote:

Pretty much, yup.

se

 
You can run a "speaker amp" on "balanced" headphones(still speakers) without worrying about releasing your amp's magic smoke; regardless of the topography.  Seems like a good reason to benefit this particular thread.  
 
As for real loudspeakers, active crossovers are very common in pro sound.  No passive crossovers, just balanced bliss and a lot better control of the FR curve.  Balanced speakers for "audiophiles"???....well of course...those guys will drink the cool aid....hehe.  Every speaker I build for myself is wired direct, no internal passive crossover, all active crossovers and multi channel amps or "sets" of amps.  I dont know if balanced speaker runs make systems sound better directly, but they certainly dont make them sound worse.  Certainly makes sense if the rest of your gear is balanced as well...

 

[Steve Eddy]

Steve, see the output transformer secondary in the schematic below? That's a balanced output. The coil, speaker wire and speaker voice coil make up the loop for electron flow. It doesn't need a ground connection.

Sure, an ideal transformer as represented by a symbol on a schematic is balanced. But I was talking about real world transformers, and depending on how they are wound and constructed, can have varying degrees of imbalance.

That's how professional audio equipment send out their balanced 3-pin XLR signal, with a transformer made up of a single wire connected to two output terminals (hot, cold and a separate ground wire).

Actually you'll find relatively few transformers used in professional gear these days. Most balanced outputs are accomplished by opamps or other active circuitry. And that third connection is shield, not ground (see "pin 1 problem")

A balanced line rejects RFI and radio magnetic interference (RMI) naturally due to each stereo channel's two equal length and gauge wires both picking up the same electrical noise--one wire is positive and the other is negative so when the signal from the two wires is combined at the speaker the added noise cancels itself out. This is called CMR or Common Mode Rejection.

Balanced lines are actually very poor at rejecting RFI. They're very good down at power line frequencies, but degrade more and more above that. They're pretty much worthless at radio frequencies, which is why shielding is used to keep RF out in the first place.

I realize the benefit for speaker runs is much lower than a microphone run but I still prefer balanced output for my speaker and headphone amps.

When dealing with loudspeakers and headphones, the more appropriate term is "bridged" rather than "balanced." That's the term used elsewhere in the audio industry, because common-mode noise rejection (which is the whole raison d'être of balanced interfaces) simply isn't an issue with headphones and loudspeakers. It's only the headphone industry that has misappropriated the term and used it to describe headphone outputs. I don't know if this was done out of ignorance, or a marketing decision (balanced is what the pros use so it must be better). In fact the promoters of the first "balanced" headphone amps never said a word about common-mode noise rejection. It was all about "four times the power," "double the slew rate," etc.

This Tripath 2024 is a solid state transformerless amp with push-pull amplification all the way to the speakers. This is a balanced output too.

Except that the rest of the audio industry (including Tripath) calls that a bridged output, not a balanced output. And there's a reason for that.

The 'safety reasons' and liability exposure stem from possible electrocution if something goes wrong in the amp and puts high voltage on the speaker wire circuit. Without a ground connection it may remain hot until someone touches the wire or speaker terminals (at the amp or speaker). With a speaker wire connected to ground a short occurs and either a fuse or circuit breaker blows.

I didn't ask you to explain any safety reasons. I said forget about safety. I asked you where should the loudspeaker negative terminals be connected if not ground? You previously said "these days" most amps connected the negative speaker terminals to ground. So where were they connected before "these days"? And I'm not talking about tube amps with output transformers. I'm talking about solid state amps. Your comment seems to indicate that before "these days" they were connected to something else.

se

 

[Steve Eddy]

Well, while we are doing this balancing act; I have always been curious about "dynamic speakers without magnets".  Sorry, I cannot recall their proper name; but essentially instead of a rare earth magnet to supply a static magnetic field, it is replaced with another separate voice coil...powered by an amp 180 degrees out of phase of the moving/dynamic coil(one fits inside the other, much like the magnet gap...  So a total of four "balanced" channels to two drivers.  I always thought that would be the ultimate balanced solution.  Magnet strength can decrease over time changing the effectiveness of the driver, the static coil, and the dynamic coil should last as long as the rest of the "soft parts" of a driver, if not abused. 

Sure you're not thinking of field coil drivers? Field coil drivers use an electromagnet to provide the magnetic field instead of a fixed magnet. I suspect that's what you're thinking of because if you used two concentric voice coils and drove them 180 degrees apart, their magnetic fields would just cancel each other.

Obviously the design didnt really take, mostly because of issues with full size speakers, I believe.  But....and a big butt....anyone thought of making these "speakers" for headphone drivers, smaller might be easier; certainly dont need alot of power....might have to find some dead drivers and start doing autopsies....just call me Dr. Frankenstein....:rolleyes:

The thought of a field coil headphone driver has crossed my mind. As well as a horn loaded headphone (using balanced armature drivers). But those thoughts were just a bit of daydreaming is all.

The other version of the "dual" coil magnetless driver, was when the second static coil was supplied with a constant DC voltage instead of the AC audio signal to create the electromagnetic field.  Not as "truly balanced", but being able to adjust the magnetic properties of the driver basically would let you have some control over T/S parameters; that could be pretty handy....in theory of course.....:evil:  

Now here you're describing a field coil speaker. And yes, the cool thing about field coils is you can vary the magnetic field strength by varying the current you run through the coil.

se

 

[camusmuse]

Sure you're not thinking of field coil drivers? Field coil drivers use an electromagnet to provide the magnetic field instead of a fixed magnet. I suspect that's what you're thinking of because if you used two concentric voice coils and drove them 180 degrees apart, their magnetic fields would just cancel each other.
The thought of a field coil headphone driver has crossed my mind. As well as a horn loaded headphone (using balanced armature drivers). But those thoughts were just a bit of daydreaming is all.
Now here you're describing a field coil speaker. And yes, the cool thing about field coils is you can vary the magnetic field strength by varying the current you run through the coil.

se

Ha, all I can picture for a horn loaded headphone is Princess Leia with conch shells instead of the hair buns...akin to Spaceballs, I suppose.  Yep, remember the "field coil" term now, its amazing what a few decades can do to your memory.  I still swear I read something about also using a seperate amp to run out of phase AC to the field coil.  Maybe it was "in phase".  It was all conceptual for me, never actually seen one. I did run across a pair of old field coils while trying to pair a 12" Altec Coax 801a(tiny horn as "tweet")full range speaker.  Finally did pair it, but unfortunately neither of the horns work and AFAIK there are no recone kit or diaphragm for the horm for this model....unless I get "custom".  Back to the headphones, I'm game for something different like a FC driver.  But DC run along side AC is notoriously noisy, is it not?
 
I think the jest of the rest the stuff here though, is that you are trying to debunk some of the balanced headphone amp "mojo"...and actually I am not completely in disagreement.  Manufacturers notoriously tout "this or that" special feature, preying on the "innocently ignorant" consumer.  I know, I used to sell that "mojo".  Did a bit of service tech too......enjoyed that MUCH more.  Eventually got out as the internet closed most of my own niche market.  Just trying to play catch up, and of all the gear I had/have...I only had one set of cans....some really nice Sonys that lasted *almost* forever.  I even transferred the cans to a different headband after they were sat on by accident.  Finally one driver gave up the ghost, and they got trashed... wish I could remember the model number... 

 

[Steve Eddy]

Ha, all I can picture for a horn loaded headphone is Princess Leia with conch shells instead of the hair buns...akin to Spaceballs, I suppose.  Yep, remember the "field coil" term now, its amazing what a few decades can do to your memory.  I still swear I read something about also using a seperate amp to run out of phase AC to the field coil.  Maybe it was "in phase".  It was all conceptual for me, never actually seen one. I did run across a pair of old field coils while trying to pair a 12" Altec Coax 801a(tiny horn as "tweet")full range speaker.  Finally did pair it, but unfortunately neither of the horns work and AFAIK there are no recone kit or diaphragm for the horm for this model....unless I get "custom".  Back to the headphones, I'm game for something different like a FC driver.  But DC run along side AC is notoriously noisy, is it not?

Don't laugh. Here's Audeze testing out an early prototype and look where they've ended up!



Well, DC doesn't produce any noise, and if the AC you're referring to is the signal, I don't see where you'd get any noise.

I think the jest of the rest the stuff here though, is that you are trying to debunk some of the balanced headphone amp "mojo"...and actually I am not completely in disagreement.  Manufacturers notoriously tout "this or that" special feature, preying on the "innocently ignorant" consumer.  I know, I used to sell that "mojo".  Did a bit of service tech too......enjoyed that MUCH more.  Eventually got out as the internet closed most of my own niche market.  Just trying to play catch up, and of all the gear I had/have...I only had one set of cans....some really nice Sonys that lasted *almost* forever.  I even transferred the cans to a different headband after they were sat on by accident.  Finally one driver gave up the ghost, and they got trashed... wish I could remember the model number... 

I'm just a little frustrated at how bridged got conflated with balanced. Yes, technically a bridged output is "balanced," but the two serve two different purposes. As I said previously, balanced interfaces are all about common-mode noise rejection. Bridging is handy if you have to deal with low power supply voltages and want to be able to achieve maximum voltage swing, say in a portable unit powered by batteries. But I don't really see anything beyond that.

They say "four times the power." Ok, four times the power compared to what? You only get four times the power that the two amp channels you're bridging can do on their own. But so what? You can build a single ended amp with enough power to melt the voice coils of pretty much any headphone you care to name. So "four times the power" isn't really saying anything meaningful.

They say "double the slew rate." Again, so what? I'm not aware of any amplifiers that are slew limiting, unless someone is making amps using opamps from the late '60s and early '70s. If you're not slew limiting then doubling the slew rate is meaningless.

They say "bypasses ground." No, not really. Although none of the output terminals are ground, the currents flowing through the headphones are also flowing through the circuit's reference ground. It's just that this is happening inside the chassis so it's not readily apparent externally.

So I just can't help seeing "balanced" as a solution in search of a problem.

se

 

[camusmuse]

Well yeah, bridging an amp essentially makes it balanced(in most cases).  But can't a dual purpose be served here by using the bridged and therefore balanced amp to drive balanced speakers/headphones?  Kinda the cart before the horse, I know.  But I think the spirit of this thread is more about repurposing stereo amps that are cheap and plentiful; as opposed to buying into that "mojo" headphone amp, balanced or not.  For a long time I wanted to build a dedicated HP amp, but after much reading here; along with the ridiculous amount of audio gear I have, I decided to fore go the headphone amps(except for maybe a portable project or two down the road), and just use what I have...lots of amps....LOTS, consumer, hi-fi, pro.....lol...enough for a decent sized venue for sure.
 
Honestly I have never read, or heard, about "four times the power".  I am still pretty new to the HP "scene".  But from what I do know from my own logic...it sounds like BS.  An amp only puts out so much power, regardless if its "balanced" or not.
 
Cannot speak to the slew rate either, I always found it to me more of a "oh by the way" sales technique.  I know its importance, but some people really like to get hung up on specs or any kind.
 
I am by no means an amp designer, I know the basics, and can follow directions, but to my ignorant self; a reference ground sounds like just that, a reference.  Not necessarily "directly" in the audio path maybe?  Really just an uneducated guess...
 
Balanced headphone wiring does solve the problem of speaker level amplifier integration to headphones...so it does "solve" *something*.  Again, might not improve sound, but I really doubt it hurts it.  I have always been of the less is more mentality, hence the lack of passive crossovers.  I know how to do them right, but its expensive and time consuming.  Plus they always seem to add some sort of coloration and for the picky..."matching" components is critical, so if you need two, buy twenty...lol.  I dunno, active crossovers allow fine tuning on the fly and allow say a "midrange" amp to work alot less than a "sub" amp, as each amps power supply is "taxed" differently.  Pretty easy to stay in that first watt if you amp is only pushing horns from 500Hz and up...
 
I am very curious how the pro sound has changed and on what timetable.  I was IATSE for a stint around '04-'05, and had planned on doing pro sound for a living, but life got in the way....

 

[Steve Eddy]

Well yeah, bridging an amp essentially makes it balanced(in most cases).

Well yeah. What "balanced" refers to in the context of balanced interfaces is equal impedances with respect to ground. This is vital for common-mode rejection. Any imbalance in the impedances will result in some amount of conversion from common-mode to normal mode and won't be rejected. At least not with common electronically balanced inputs. Transformers, because they can have incredibly high common-mode input impedances can provide a great degree of common-mode rejection even when driven from a wholly unbalanced source.

But can't a dual purpose be served here by using the bridged and therefore balanced amp to drive balanced speakers/headphones?

I don't know what that other purpose would be. Noise just isn't a problem at headphone/loudspeaker interfaces. No, it wouldn't hurt anything, but you're just doubling up the number of parts for no particular reason. Fine if you can't sleep at night knowing you're only using two amplifier channels instead of four, but other than that...

Kinda the cart before the horse, I know.  But I think the spirit of this thread is more about repurposing stereo amps that are cheap and plentiful; as opposed to buying into that "mojo" headphone amp, balanced or not.

Sure. Nothing wrong with that.

For a long time I wanted to build a dedicated HP amp, but after much reading here; along with the ridiculous amount of audio gear I have, I decided to fore go the headphone amps(except for maybe a portable project or two down the road), and just use what I have...lots of amps....LOTS, consumer, hi-fi, pro.....lol...enough for a decent sized venue for sure.

Honestly I have never read, or heard, about "four times the power".  I am still pretty new to the HP "scene".  But from what I do know from my own logic...it sounds like BS.  An amp only puts out so much power, regardless if its "balanced" or not.

No, not BS. Just Ohm's Law.

If you have an amplifier channel that can swing x amount of voltage, if you take two of those amplifiers, bridge them and drive one channel with a non-inverted signal and the other channel with an inverted signal, for the same level of input signal you'll get twice the voltage. And since power goes up as the square of voltage, you get four times the power. Assuming of course that the power supply is capable of supplying the extra current without loading down.

Cannot speak to the slew rate either, I always found it to me more of a "oh by the way" sales technique.  I know its importance, but some people really like to get hung up on specs or any kind.

Well for decades audio marketing has been little more than a numbers game. If you can come up with a better number than your competitor, then you're perceived as being "better" even if the number was ultimately meaningless. This really heated up in the '70s particularly with the Japanese companies and was particularly bad with power ratings. They were using all sorts of questionable ways to make their gear seem like it had more power. This is what prompted the FTC to finally step in and set forth some rules and regulations as to how power ratings were to be specified.

I am by no means an amp designer, I know the basics, and can follow directions, but to my ignorant self; a reference ground sounds like just that, a reference.  Not necessarily "directly" in the audio path maybe?  Really just an uneducated guess...

Reference ground is just the particular node in the circuit that everything else is referenced to. Not to be mistaken for that third pin on the IEC power recepticle. A lot of people refer to that as ground, but it's not. It's just for safety, a return path back to the AC mains neutral in the event that a failure on the hot side comes into contact with the chassis.

Balanced headphone wiring does solve the problem of speaker level amplifier integration to headphones...so it does "solve" *something*.

I'm afraid I'm not seeing how that solves the problem. I'm not really sure what problem you're talking about. If you need to reduce the amp's output, the solution to that would be an attenuator, which doesn't require balanced headphone wiring. Or are talking about some other problem?

Again, might not improve sound, but I really doubt it hurts it.  I have always been of the less is more mentality, hence the lack of passive crossovers.  I know how to do them right, but its expensive and time consuming.  Plus they always seem to add some sort of coloration and for the picky..."matching" components is critical, so if you need two, buy twenty...lol.  I dunno, active crossovers allow fine tuning on the fly and allow say a "midrange" amp to work alot less than a "sub" amp, as each amps power supply is "taxed" differently.  Pretty easy to stay in that first watt if you amp is only pushing horns from 500Hz and up...

I am very curious how the pro sound has changed and on what timetable.  I was IATSE for a stint around '04-'05, and had planned on doing pro sound for a living, but life got in the way....:confused_face:

Oh yeah, active crossovers are the way to go if you have the expertise.

se

 

[Chris J]

  Well yeah, bridging an amp essentially makes it balanced(in most cases).  But can't a dual purpose be served here by using the bridged and therefore balanced amp to drive balanced speakers/headphones?  Kinda the cart before the horse, I know.  But I think the spirit of this thread is more about repurposing stereo amps that are cheap and plentiful; as opposed to buying into that "mojo" headphone amp, balanced or not.  For a long time I wanted to build a dedicated HP amp, but after much reading here; along with the ridiculous amount of audio gear I have, I decided to fore go the headphone amps(except for maybe a portable project or two down the road), and just use what I have...lots of amps....LOTS, consumer, hi-fi, pro.....lol...enough for a decent sized venue for sure.
 
Honestly I have never read, or heard, about "four times the power".  I am still pretty new to the HP "scene".  But from what I do know from my own logic...it sounds like BS.  An amp only puts out so much power, regardless if its "balanced" or not.
 
Cannot speak to the slew rate either, I always found it to me more of a "oh by the way" sales technique.  I know its importance, but some people really like to get hung up on specs or any kind.

 
Here's a practical example of a bridged output audio amp:
I have a Bryston 2B-LP stereo power amp.
60 Watts/ch @ 8 Ohms
100 Watts/ch @ 4 Ohms
Manufacturer does not recommend using this amp to drive 2 Ohm loads.
 
A few years ago I needed to press it into service to drive the centre channel in my home theatre system.
There is a stereo/mono switch on the rear of the amp.
This bridges the two channels together, so now I have 200 Watts @ 8 Ohms.
Approx. twice as much voltage, approx. twice as much current into 8 Ohms and twice the slew rate! And approx. 4 times as much power! Yippee!
Arguably I needed twice the slew rate as I now have twice the output voltage!
The downside is the mono (i.e. bridged) configuration is not recommended for driving 4 Ohm loads. Yes, there's a downside.
BTW, the manufacturer makes no claim to this bridged (i.e. mono) configuration reducing noise or distortion, just more power into an 8 Ohm load.
 
So the "twice as much voltage, current and slew rate, and four times as much power" statement is true, but is rather misleading too.
 
Oh yeah.....
Guess what? My centre channel is a 6 Ohm load with a 4 Ohm minimum. Mind you, I never had any real problems with that amp in mono mode but eventually I got a Bryston mono bloc for the centre channel, which IS rated to drive 4 Ohm loads.
So basically I bought a purpose built mono bloc power amp.
 
Furthermore, this amp (whether used in stereo or as a mono bloc) has balanced inputs (i.e. differential inputs) so, assuming it's receiving a signal from a balanced output pre-amp, and you are using a balanced interconnect, the input will reject common mode noise (within a limited bandwidth, of course).
BTW, the input stage converts the signal into a Single Ended signal, which then drives the power amp stage.
 
Other amps may have different configurations, YMMV.