Topping B100 Amplifier Review

[Old_School_Brad]

Is 'feedforward' the same as positive feedback (regeneration)? I could see using that as a way to fill in the crossover notch inherent in push-pull class B operation.

Who here understands the concept of class B operation?
View attachment 392733
Class B operation was employed in the output stages of radio transmitters and giant PA amplifiers back in the day. The idea is to have the opposite polarity output devices at low or zero current draw with no signal, but start to draw current when signal is applied. You get maximum efficiency that way, but there's a gap at the zero crossing of each sine wave where both devices switch off, causing the notch in the output sine wave that you see in the above illustration.

My question is what magic trick did the Topping engineers employ to get rid of that crossover notch? The classical electronics texts state that simply applying great gobs of negative feedback won't do the trick, because both output devices are momentarily shut off, therefore there is no gain, therefore there is no signal to drive the negative feedback loop. So how did Topping do it?

One problem is that an amplifier with great gobs of negative feedback will need to be carefully optimized so that it doesn't ring or oscillate on signal transients. The classic way to test that was to apply a fairly high frequency square wave (like 10kHz) to the input of the amplifier and look at the amp's output on an oscilloscope. If the amp is ringing, the corners of the square wave will look exaggerated, with a spike coming up from the corner of the wave, instead of a perfect 90 degree angle. Something like this:

View attachment 392738

That's why I asked Amir to show us what the output looks like with a 10kHz square wave input, if possible. It should like as close as possible to this:

View attachment 392739

Instead of assuming they have come up with a method of getting rid of crossover distortion, we should ask if it ever was an audible issue to begin with?
A lot of pro audio PA amplifiers are class B in practice but don't have audible distortion.

Has anyone ever heard crossover distortion? -I haven't. And I've had a lot of pro and consumer gear during the years.

 

[rongon]

Instead of assuming they have come up with a method of getting rid of crossover distortion, we should ask if it ever was an audible issue to begin with?
A lot of pro audio PA amplifiers are class B in practice but don't have audible distortion.

Has anyone ever heard crossover distortion? -I haven't. And I've had a lot of pro and consumer gear during the years.

Oh no, crossover notch distortion is very audible in class B amplifiers. Very audible, and very much measurable. I think you may be thinking of class AB operation, in which the push-pull amplifier stage only goes into class B at very high output levels. At low levels the output stage works in class A, with an area of operation where the output stage gets close to class B operation but there's still just enough current flowing in the output devices that any crossover notch is very slight, and may be unnoticeable. But that isn't true class B operation from zero signal to max signal.

Do you know for a fact that the pro audio PA amplifiers you've owned are designed to run in full class B operation? The ones I'm familiar with (Crown, BGW, etc.) were designed for 'cold' class AB operation. The ones I've heard all sounded like PA amps too. Back in the day, hi-fi amps were usually designed to run in a warmer biased type of class AB, yielding less power but lower distortion (staying in class A longer before transitioning into class B operation).

 

[rongon]

No, not the same at all.


JSmith

Can you explain what "feedforward" is in the context of audio amplifier design, and how it differs from positive feedback?

 

[wwenze]

I'm intrigued by the low noise despite SMPS.

Power conditioners where is your god now

 

[rongon]

Can you explain what "feedforward" is in the context of audio amplifier design, and how it differs from positive feedback?

OK, I found some information on this. Perhaps the best explanation is this one from Keysight support:
_________________________________

A feedforward amplifier is a type of power amplifier that utilizes a feedforward technique to improve linearity.
Here is a step-by-step explanation of how a feedforward amplifier works:

1. The input signal is split into two paths: the main path and the error path.
2. The main path signal is amplified by the power amplifier.
3. The error path signal is delayed to match the propagation delay of the main path signal.
4. The delayed error path signal is then amplified by a separate amplifier called the error amplifier.
5. The amplified error path signal is subtracted from the main path signal.
6. The resulting difference signal, known as the error signal, represents the distortion introduced by the power amplifier.
7. The error signal is then fed back to the input of the power amplifier.
8. By subtracting the error signal from the input signal, the power amplifier is forced to follow the input signal more closely, reducing distortion.
9. The combination of the power amplifier and the error amplifier effectively cancels out the distortion introduced by the power amplifier, resulting in improved linearity.

It is important to note that feedforward amplifiers require precise component manufacturing and knowledge of the signal path to achieve optimal performance. Additionally, the loop in a feedforward amplifier is not self-stabilizing, so careful monitoring is necessary to maintain stability. Feedforward amplifiers are typically used in high-frequency (HF) and lower very high frequency (VHF) applications due to the requirement of small delays for stability.
________________________________

It looks like Topping have used their experience with radio frequency design concepts to apply them to an audio amplifier. That's very interesting stuff. I can see how -- if implemented successfully -- a feed-forward network could be used to remove the crossover notch from a class B output stage. But this is the kicker...

"...feedforward amplifiers require precise component manufacturing and knowledge of the signal path to achieve optimal performance. Additionally, the loop in a feedforward amplifier is not self-stabilizing, so careful monitoring is necessary to maintain stability."

Is this Topping amplifier unconditionally stable, under all conditions of operation? Yes, it appears to be so with sine waves applied to its input, but what about a 10kHz square wave, or other more difficult tests? Is the 'monitoring' constant enough for the desired results at low frequencies such as those in the audio band?

It could be that this amplifier accomplishes all the goals, including stability under all signal conditions. That would make it a true breakthrough product. The smallish power output is probably a product of the complexity of this kind of design compared to the usual class AB or class D designs.

I wonder how this amp sounds in operation, driving today's relatively difficult speakers with very wavy impedance curves dipping down to under 4 ohms with steep phase reversal angles, etc.

 

[Old_School_Brad]

Oh no, crossover notch distortion is very audible in class B amplifiers. Very audible, and very much measurable. I think you may be thinking of class AB operation, in which the push-pull amplifier stage only goes into class B at very high output levels. At low levels the output stage works in class A, with an area of operation where the output stage gets close to class B operation but there's still just enough current flowing in the output devices that any crossover notch is very slight, and may be unnoticeable. But that isn't true class B operation from zero signal to max signal.

Do you know for a fact that one of the pro audio PA amplifiers you've owned are designed to run in full class B operation? The ones I'm familiar with (Crown, BGW, etc.) were designed for 'cold' class AB operation. The ones I've heard all sounded like PA amps too. Back in the day, hi-fi amps were usually designed to run in a warmer biased type of class AB, yielding less power but lower distortion (staying in class A longer before switching to class B operation).

Well, many are just having the smallest of quiescent current which might be what you call cold class ab. I wouldn't know "the sound of PA amps" as the ones I've heard were indistinguishable to residential ones in blind tests with the exception of residual hiss noise from some of them.

My point was not to claim that crossover distortion does not exist (that would be silly) but rather that it is not an issue I've ever come across or something I've ever heard with even the lowest of biased amps.

 

[restorer-john]

@rongon Just start a separate thread on the subject huh?

 

[rongon]

@rongon Just start a separate thread on the subject huh?

We're talking about a basic design element of this particular 'class B' amp reviewed here. I think that's 100% reasonable.
Or is this a subjective review?

 

[rongon]

Well, many are just having the smallest of quiescent current which might be what you call cold class ab. I wouldn't know "the sound of PA amps" as the ones I've heard were indistinguishable to residential ones in blind tests with the exception of residual hiss noise from some of them.

My point was not to claim that crossover distortion does not exist (that would be silly) but rather that it is not an issue I've ever come across or something I've ever heard with even the lowest of biased amps.

Can you give me an example of a 1000W PA amp running in very cold class AB that sounded indistinguishable from a more hi-fi oriented amplifier like an Adcom GFA-535 or a Hafler PA1000? Can you send me a reference to the blind tests you mentioned?

I know it's de rigeur here to state that all competently designed audio amplifiers sound exactly alike, proven by blind testing that I've never seen, but the idea that those giant near-class B amps like the old Crown DC300A sounded indistinguishable from hi-fi amps from the same time seems to be a hard one for me to agree with. In my experience, those huge old PA amps sounded somehow 'dull but harsh' compared to the more hi-fi stuff of its day, such as the Citation XII or Hafler DH-220. I stopped listening to pro PA amps many years ago, so I may be wrong about this. They're all class D nowadays, so I'm not sure there are any more giant cold class AB PA amps around anymore.

 

[Gremlins]

I'm not knocking the performance achieved here, but as said in a post earlier, I can't help but feel it's been designed for an ASR-based audience with passive speakers on a desktop system, rather than a music lover out there with greater demands on power

No, it has been designed for fidelity lovers .. majority of people want to listen to fidelity tracks ... Not a soup of modified tones ... This item is not for gear lovers that spend 98% time in admiration of the gear and 2% Time listenting music

 

[Gremlins]

On a stand next to me. 65 cm from my head. I feed it with 200W amp and it is just right with uncompressed music.

Death? ... 65cm IS thé lengh of my arm, i need 1 or 2 watts max max max max ...

 

[JSmith]

We're talking about a basic design element of this particular 'class B' amp reviewed here. I think that's 100% reasonable.

There may be some confusion here, as no one actually said the B100 is a feedforward design... someone asked if it was and the discussion went off on a bit of a tangent about the AHB2 and patents. That's why @restorer-john suggested a new thread on the topic.

It's most likely the B100 is using NFB.


JSmith

 

[oleg87]

We're talking about a basic design element of this particular 'class B' amp reviewed here. I think that's 100% reasonable.
Or is this a subjective review?

I mean, do we have as much as a shot of whatever's under the heatsink, much less how anything there is biased? Not sure speculating about how Topping did or didn't do something we have essentially zero information about either way has too much value

 

[Sir Sanders Zingmore]

There may be some confusion here, as no one actually said the B100 is a feedforward design... someone asked if it was and the discussion went off on a bit of a tangent about the AHB2 and patents. That's why @restorer-john suggested a new thread on the topic.

It's most likely the B100 is using NFB.


JSmith

Precisely, and they specifically said “no” when asked by Amir

Topping B100 Amplifier Review

Hope the B200 Amp is coming in soon!~ Im really hoping that Topping speaker amps last longer than they used to though. Their performance is great but they've always had reliability & QC issues. They're performing even better than Purifi amps, would be great to see these used in home theater w...

www.audiosciencereview.com

 

[pma]

Can you explain what "feedforward" is in the context of audio amplifier design, and how it differs from positive feedback?

https://linearaudio.net/sites/linearaudio.nl/files/2022-12/Hawksford_Part2.pdf

You are asking questions related to basics of power amplifier design. I agree with @restorer-john that such debate should not be a part of the review thread.

 

[pma]

My point was not to claim that crossover distortion does not exist (that would be silly) but rather that it is not an issue I've ever come across or something I've ever heard with even the lowest of biased amps

I heard an issue with the crossover distortion, back in the eighties, DIY power amplifier with adjustable idle current. Quite audible at low volume level, and disappeared when idle current set to at least several mA.
It depends on loop gain and real level of spectrum residuals. You will definitely hear them if there is a comb of equivalent height of high order harmonics of some -40dBr.

 

[jomo]

No, it has been designed for fidelity lovers .. majority of people want to listen to fidelity tracks ... Not a soup of modified tones ... This item is not for gear lovers that spend 98% time in admiration of the gear and 2% Time listenting music

Fidelity lovers? Perhaps engineering or measurement lovers. You can't come close to resolving this low level of distortion/noise listening to any music. 120 dB SINAD is a good 40dB beyond necessary for the highest detectable fidelity for a speaker amplifier. Beyond that, noise may be detectable, but only if you get close to the speaker or listen in an anechoic chamber. I don't consider this "listening to music".

What stands out like a sore thumb is that this amp powers down at 70 to 86 PEAK Watts. This is worse than the power nannies on most AVRs and can be a real issue if the amp is used to drive room speakers. This means you would have to treat it like a much smaller amp to protect from a peak trip. Certainly this relegates it to the desktop, folks that have the most efficient speakers, folks that don't play loud or folks that require a perfectly designed, "no-blow", tweeter amplifier.

 

[Mnyb]

can we just cap the discussion with thats its just a bit small on power , find a suitable application . The quietness with very low noise is in favor of a desktop use case , arms length is close enough to hear hiss from some actives .

 

[Sir Sanders Zingmore]

can we just cap the discussion with thats its just a bit small on power ,

Not saying it’s what you mean, but to me that reads a bit like “can we only talk about the good things”

 

[Mnyb]

can it no be tested an the power cube on a lower power level to avoid the protection circuit ? Just to see its stability or not ?

A concern is if the protection circuit needs be this "nanny" or its Topping wanting to avoid to many returns for warranty ?