oleg87
Addicted to Fun and Learning
Is there a schematic of this thing somewhere? Why do people feel so confident asserting alleged facts about this amp's implementation?
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Class B amplifier with SINAD of 120, how is that possible?
- Thread starter DanielT
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Every amplifier designer is doing this test.
Not an expert by far. But it seems that has been difficulty identifying a pure class B historically. But may be off base here but have huge interest as an owner of a Nakamichi System One.
kemmler3D
Master Contributor
I think the lesson here is that the class of the amp doesn't tell you very much about its ultimate performance.
Hot, pedantic take here, but no analog signal comes "really" or even "kinda" close to infinite bandwidth in practice.
Hot, pedantic take here, but no analog signal comes "really" or even "kinda" close to infinite bandwidth in practice.
Yes but 50kHz or 1MHz makes a big, big difference. And do not forget it is a test signal and you need a fast test signal to test caveats of the circuit design, especially in case of linear amplifiers. There is no excuse for not using the analog square wave generator.
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Good suggestion. Merged threadsMaybe post here. Much to be said and learned.
Class B amplifier with SINAD of 120, how is that possible?
I thought that class B based amplifiers advantage were a lot of power in combination with the disadvantage of high distortion. To solve the negative aspect class A design was integrated, creating the classic combination: Class AB amplifier. BUT why bother with this AB combination if it can be...
I have always favored a 1 kHz and/or 10 kHz square-wave test, bandlimited to 50 or 100 kHz. The harmonics (odd only in an ideal square wave) fall as 1/N for the Nth harmonic so 1/3, 1/5, 1/7, 1/9.... the amplitude means the really high stuff is getting pretty low in amplitude. Five harmonics give a fairly good indication, though I tended to use 10 just for grins (THD measurements use 10 harmonics, so I figured it was good for a square wave test as well), thus a 10 kHz "square wave" limited to 100 kHz was sometimes interesting. For analog systems it was a reasonable, given turntable cartridges can have response to >100 kHz, and these days of higher sampling rate digital sources means to me it might be worthwhile. "Back then" I did two tests, one into a load resistor, and the second in the resistor in parallel with a large capacitor to emulate a worst-case speaker load.
www.audiosciencereview.com
I think @amirm did some tests early on, but for competent amps didn't show anything, so abandoned them as too time-consuming for the reward? Stability is pretty much a given these days.
Composition of a Square Wave (important!)
Author: our resident expert, DonH50 (above title mine :) ) Building a Square Wave Since I used a square wave as an example in another thread, I realized that what is common knowledge for hairy-knuckled engineers such as I and high-brow scientists like some of the other folk here, may still be...
www.audiosciencereview.com
I think @amirm did some tests early on, but for competent amps didn't show anything, so abandoned them as too time-consuming for the reward? Stability is pretty much a given these days.
restorer-john
Grand Contributor
And some of the "giant killer" Protons. Quite a few Korean OEM amplifier/receiver makers too.
oleg87
Addicted to Fun and Learning
I does give me pause personally, with spec-oneupsmanship among these companies compelling engineers to find ways to apply more and more feedback... especially with Topping's history, I'm not sure how thorough their testing processes are.
restorer-john
Grand Contributor
The internal picture shows a power relay, a speaker relay and a bunch of other input/gain switching relays. So, the 9W is reasonable including all the opamps, micro and relays along with amp operational current.
restorer-john
Grand Contributor
I have an interesting Cambridge Azur 840A v2 integrated here. Given to me for parts.
It too has a "patented" amplifier innovation and they call Class XD. This was Cambridge's first amplifier to incorporate this circuit. A Douglas Self design element I believe.
www.cambridgeaudio.com
Patent appears to have lapsed, so I've attached some details for general interest. Including their white paper below (PDF)
Power amp stage schematic attached below. Notice the XD circuit can be switched on and off, like a VW defeat device. LOL.
It too has a "patented" amplifier innovation and they call Class XD. This was Cambridge's first amplifier to incorporate this circuit. A Douglas Self design element I believe.
Class XD Explained | Cambridge Audio US
It sure sounds fancy, but what exactly is Class XD?
www.cambridgeaudio.com
Patent appears to have lapsed, so I've attached some details for general interest. Including their white paper below (PDF)
Power amp stage schematic attached below. Notice the XD circuit can be switched on and off, like a VW defeat device. LOL.
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More audio output power with less power consumption = higher efficiency and possible cost savings by drawing less current from the psu at all but maximum signal levels.
I see mention of some amazing DIY class B designs with vanishingly low distortion and high power. It would be really nice to see schematics, which should be possible since these are DIY designs, and presumably not commercial designs.
I wanted to correct a mistake I'd made previously. It seems the definition of class B operation does not necessarily include a crossover notch caused by the output devices 'sticking' in a zero-conduction state for any period of time. It is technically possible to get everything lined up with microscopic precision so that the push side of the circuit turns on the instant the pull side of the circuit cuts off.
But how is that achieved in real life, with devices that degrade over time, resistors that drift in value with heat, electrolytic capacitors that dry out with time and heat, etc? That's what would be interesting to know from Topping. My meager understanding is that it's much easier to get and keep a class AB output stage working well than a class B output stage.
Apparently Topping reps were confused at first, calling it class AB but then coming back and correcting themselves, stating it's class B after all. Lots of confusion.
That sales brochure says, "closer to class B than ever before." It doesn't say the output stage is out and out class B. It looks like very cold biased class AB, with at least a tiny amount of idle current drawn by both output devices at the waveform zero crossing.
What is leakage current, please? Where does that appear in this circuit? Thanks.
By definition. Class AB keeps more current going through the output devices, even when no signal is present. The advantage is that with no region where both output devices are in cutoff (not conducting current) there's no crossover notch to fill in. I'd read somewhere (still trying to find where) that negative feedback is powerless to correct crossover notch distortion, but that might be one of those things you read that aren't actually true. Who knows...
One thing I have heard from several engineering types is that with negative feedback in an audio amplifier, it's best to use either very little or none of it (if your amplifier circuit is linear enough that way, very difficult to achieve in real life) or use a LOT of NFB. If you use just a little NFB, higher harmonics are generated in the output waveform, which is not desirable. However, as more NFB is applied, all harmonics are suppressed more, including those higher ones. Pretty soon those higher fundamentals are buried in the noise floor and the main concern becomes stability, freedom from ringing, overshoot or oscillation.
Yes indeed it is! I've asked Amir twice to measure this B100 amp with a 10kHz square wave at a moderate signal level. No reply to that. I wish he would do that. I wonder, do all these wonderful little amplifiers measured here have exemplary output of a 10kHz square wave? No ringing, no overshoot, no slanting, no blunting of the leading edge or the trailing edge, no rounding? Or do we assume that the 10kHz square wave response of all these amplifiers is essentially perfect, so why bother? (I hate assumptions...)
Yes, because a class AB amp goes into class B when playing loud. That's exactly why it's not class A.
Well, there's a controversial statement. I'm not going to argue but I think I'll just quietly disagree. The challenge with applying great gobs of negative feedback to a non-linear circuit (like a class B output stage) has always been stability, overshoot, ringing, oscillation. I'd really like to know how Topping did it, and others would too, it seems. Or did they? How's the 10kHz square wave look?
Is it? Even in a design that uses more NFB than anyone has dared use before? The whole problem with using great gobs of NFB is stability, and a 10kHz square wave is a test of high frequency stability. It seems reasonably to me to check the stability of the latest, greatest wonder amps.
OK, that's all. Thanks for your input, everyone. This Topping B100 amp is one very interesting product. Maybe it is exactly what Topping says it is, maybe it isn't.
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But has true Class B ever physically existed or just slightly overlapping AB, virtual Class B? Does an Class AB amp ever go into true Class B?More audio output power with less power consumption = higher efficiency and possible cost savings by drawing less current from the psu at all but maximum signal levels.
I see mention of some amazing DIY class B designs with vanishingly low distortion and high power. It would be really nice to see schematics, which should be possible since these are DIY designs, and presumably not commercial designs.
I wanted to correct a mistake I'd made previously. It seems the definition of class B operation does not necessary include a crossover notch caused by the output devices 'sticking' in a zero-conduction state for any period of time. It is technically possible to get everything lined up with microscopic precision so that the push side of the circuit turns on the instant the pull side of the circuit cuts off.
View attachment 392902
But how is that achieved in real life, with devices that degrade over time, resistors that drift in value with heat, electrolytic capacitors that dry out with time and heat, etc? That's what would be interesting to know from Topping. My meager understanding is that it's much easier to get and keep a class AB output stage working well than a class B output stage.
Apparently Topping reps were confused at first, calling it class AB but then coming back and correcting themselves, stating it's class B after all. Lots of confusion.
That sales brochure says, "closer to class B than ever before." It doesn't say the output stage is out and out class B. It looks like very cold biased class AB, with at least a tiny amount of idle current drawn by both output devices at the waveform zero crossing.
What is leakage current, please? Where does that appear in this circuit? Thanks.
By definition. Class AB keeps more current going through the output devices, even when no signal is present. The advantage is that with no region where both output devices are in cutoff (not conducting current) there's no crossover notch to fill in. I'd read somewhere (still trying to find where) that negative feedback is powerless to correct crossover notch distortion, but that might be one of those things you read that aren't actually true. Who knows...
One thing I have heard from several engineering types is that with negative feedback in an audio amplifier, it's best to use either very little or none of it (if your amplifier circuit is linear enough that way, very difficult to achieve in real life) or use a LOT of NFB. If you use just a little NFB, higher harmonics are generated in the output waveform, which is not desirable. However, as more NFB is applied, all harmonics are suppressed more, including those higher ones. Pretty soon those higher fundamentals are buried in the noise floor and the main concern becomes stability, freedom from ringing, overshoot or oscillation.
Yes indeed it is! I've asked Amir twice to measure this B100 amp with a 10kHz square wave at a moderate signal level. No reply to that. I wish he would do that. I wonder, do all these wonderful little amplifiers measured here have exemplary output of a 10kHz square wave? No ringing, no overshoot, no slanting, no blunting of the leading edge or the trailing edge, no rounding? Or do we assume that the 10kHz square wave response of all these amplifiers is essentially perfect, so why bother? (I hate assumptions...)
Yes, because a class AB amp goes into class B when playing loud. That's exactly why it's not class A.
Well, there's a controversial statement. I'm not going to argue but I think I'll just quietly disagree. The challenge with applying great gobs of negative feedback to a non-linear circuit (like a class B output stage) has always been stability, overshoot, ringing, oscillation. I'd really like to know how Topping did it, and others would too, it seems. Or did they? How's the 10kHz square wave look?
OK, that's all. Thanks for your input, everyone. This Topping B100 amp is one very interesting product. Maybe it is exactly what Topping says it is, maybe it isn't.
Good question! I'm wondering that myself...
Hmmm.... That's a good question. At clipping, does a class AB amp have any period during the output waveform in which both output devices are cut off and not conducting? Or would that be defined as class C and the answer is no?
It could be that the boundaries are fuzzy between 'cold biased' class AB and 'real' class B. Or it could be that real class B operation is incredibly difficult to achieve in real life circuits. But I'm no expert, I really don't know. I am curious, though, because this B100 amp looks pretty remarkable. Maybe it would be worth it to buy one, put a 10kHz square wave to it and look at it on a 'scope. If it passes a nice 10kHz square wave, it's stable at high frequencies, all good -- and with those amazingly low THD measurements, and for not a lot of money. That would rate a sincere "Wow!"
[Sorry, I had to edit this post a lot. Lots of syntax errors on the first try.]
Hmmm.... That's a good question. At clipping, does a class AB amp have any period during the output waveform in which both output devices are cut off and not conducting? Or would that be defined as class C and the answer is no?
It could be that the boundaries are fuzzy between 'cold biased' class AB and 'real' class B. Or it could be that real class B operation is incredibly difficult to achieve in real life circuits. But I'm no expert, I really don't know. I am curious, though, because this B100 amp looks pretty remarkable. Maybe it would be worth it to buy one, put a 10kHz square wave to it and look at it on a 'scope. If it passes a nice 10kHz square wave, it's stable at high frequencies, all good -- and with those amazingly low THD measurements, and for not a lot of money. That would rate a sincere "Wow!"
[Sorry, I had to edit this post a lot. Lots of syntax errors on the first try.]
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It's the current from collector to emitter when the base is not at forward bias voltage, i.e., the transistor isn't turned on yet.
Yes, class B amplifiers existed and still exist today, though generally not for audio applications. @pma posted a true class B audio amplifier circuit earlier. Class AB means an amplifier that "moves" from pure class A to class B as the signal gets larger so the "other half" of the circuit is no longer involved but, as a class AB design, the "off" half still has bias current. You could argue it never goes into "pure" class B because there is always standing bias current in the "other" half of the output stage; that is what makes it class AB and not class B. Leakage current is usually very tiny and neglected as it is not enough to actually cause the devices to enter normal operational mode.
This is my ancient copy-and-paste on amplifier classes FWIW:
Amplifier Classes
Here is a summary from memory so don't hold me to any mistakes:
Class A = bias current flows through the output devices all of the time. Most wasted energy and heat, max theoretical efficiency ~50% for a push-pull design (only ~27% for a single-ended design IIRC). Commonly used for low-level circuits like preamps and power amp input and driver stages, rarely for output stages since it is so inefficient. More common in tube amps these days, I think.
Class B = bias current flows half the time, so in a push-pull design one device is on and the other is off. Typically one device amplifies the (+) half of the signal and the other the (-) half as it swings around ground (0 V, or a common bias voltage). Can achieve ~67% (SE) to ~78% (push-pull) efficiency in theory. In practice there is crossover distortion around the crossing point as one device is switched off and the other turned on since it does not happen instantaneously. Used for some power amplifiers in the past (do not know about today), with feedback used to reduce crossover (and other) distortion.
Class AB = biased in class A for small signals then moves to class B. This lets small signals around the crossing point stay in class A for lower distortion, then as the signal increases and moves out of the small signal region transitions to class B to save power.
Class C = bias current flows less than half the waveform cycle. The "missing" energy is usually generated by a resonant circuit (e.g. inductor/capacitor (LC) tank). Common in RF circuits where high power is needed and distortion less an issue, and oscillators which are narrow-band (audio is very wideband, spanning multiple decades) and incorporate a resonant circuit by design.
Class D = bias current flows only as output devices switch states, in a form of pulse modulation (pulse width, frequency, or both). Can achieve >90% efficiency. The high switching frequency is provided by a clock source or (for most audio amps) is self-generated by the circuit. The output pulse train is filtered so only the fundamental signal remains. See https://www.audiosciencereview.com/forum/index.php?threads/class-d-amplifiers-101.7355/
Class E, F = utilize switching as well but constrain the switching to certain points in the signal cycle (e.g. at voltage or current zero crossings) for higher efficiency since less power is dissipated in the switching transistors. These are used exclusively in RF circuits AFAIK. Class E is used in tuned amplifiers (narrowband, again) and class F is used for generating harmonics of the fundamental so you can say build a high-frequency oscillator output from a lower-frequency circuit.
Class G, H = wrap a varying power supply around the core (typically AB) amplifier to improve efficiency. By changing the power supply voltages it uses (wastes) less energy for small signals by applying low supply voltage, then increases the voltage as required as the signal gets larger. Class G uses discrete rails so the power supply switches between two or more (high/low) voltages. Class H uses a tracking supply that varies continuously with the signal level.
There are some more esoteric classes I am not familiar with. I have only designed and worked with the classes above.
HTH - Don
Edit: Found a Wiki page that probably does a better job than I but I didn't read it: https://en.wikipedia.org/wiki/Power_amplifier_classes
Thanks Don, I kind of knew much what you posted. The thread and many posts seem to point to a true high power low noise/distortion class B is a unicorn, and Topping has mischaracterized what they are selling. So my current vision of class B is that without some initial class A pure class B mode is rarely occurs and historically so are stand alone class B circuits. I'm a ME and audio enthusiast and not an EE, so a lot of this is over my head.
