This is a basic question which I guess many of you will be able to help me with. I've been reading about distortion in classical power amp topologies (Lin 3-stage with complementary pairs at OPS, either EF or CF, ignoring QC for now), and I can't understand why a Class A power amp built in this topology will have its THD curve rising with frequency after being flat till a couple of KHz.
I'll tell you what I've understood. It seems that a well designed Class B amp using the classic Lin type topology (both single-ended and mirror-image) can drive down most other types of distortion almost to the noise floor, but the crossover distortion remains. GNFB linearises this crossover distortion. And since the compensation capacitor reduces global open-loop gain at HF, therefore the GNFB itself keeps reducing with rising frequency. Since the linearising influence of the GNFB reduces, the crossover distortion becomes more and more visible at the output with rising frequency.
And from what I understand, Class A has no crossover distortion.
In that case, why does a Class A amp show a rising THD curve with frequency?
Note that I'm not debating that the Class A amp's distortion may or may not be audible. I'm just asking, why should the distortion curve rise at all if there's no crossover distortion (and other distortions can be driven down into the noise floor anyway)?
I also know that everything I've read (just about two and a half books and lots of NS app-notes and forum threads) does not cover the non-Lin topologies e.g. the ones Nelson Pass designs, etc. So my comments may not apply to those topologies.
I'll tell you what I've understood. It seems that a well designed Class B amp using the classic Lin type topology (both single-ended and mirror-image) can drive down most other types of distortion almost to the noise floor, but the crossover distortion remains. GNFB linearises this crossover distortion. And since the compensation capacitor reduces global open-loop gain at HF, therefore the GNFB itself keeps reducing with rising frequency. Since the linearising influence of the GNFB reduces, the crossover distortion becomes more and more visible at the output with rising frequency.
And from what I understand, Class A has no crossover distortion.
In that case, why does a Class A amp show a rising THD curve with frequency?
Note that I'm not debating that the Class A amp's distortion may or may not be audible. I'm just asking, why should the distortion curve rise at all if there's no crossover distortion (and other distortions can be driven down into the noise floor anyway)?
I also know that everything I've read (just about two and a half books and lots of NS app-notes and forum threads) does not cover the non-Lin topologies e.g. the ones Nelson Pass designs, etc. So my comments may not apply to those topologies.
This is a verry simplistic answer but it may be the capacitences
of the active devices. Take a typical output device with a HFE
of 100 passing 1A well that HFE is at DC. When we move from
DC to AC we have to start charging and discharging the base emiter capacitence and as the frequency rises this requires more current. Well 10ma may turn that device on and off at 2 or 3 hz but at 20khz it will take a lot more current which puts a bigger load on the driver ect.
I said this was a verry simplistic answer and I am sure there
are a lot of other factors. In a mosfet the capacitences are
different but still easier to drive at lower frequencys.
of the active devices. Take a typical output device with a HFE
of 100 passing 1A well that HFE is at DC. When we move from
DC to AC we have to start charging and discharging the base emiter capacitence and as the frequency rises this requires more current. Well 10ma may turn that device on and off at 2 or 3 hz but at 20khz it will take a lot more current which puts a bigger load on the driver ect.
I said this was a verry simplistic answer and I am sure there
are a lot of other factors. In a mosfet the capacitences are
different but still easier to drive at lower frequencys.
For all types of amplifiers using GNFB, you have the problem that above some frequency, the open loop gain of the amplifier starts to drop. Since the feedback strives to maintain the same closed loop gain the result is that the amount of feedback starts to drop when the open loop gain starts to drop. That also means that it becomes less effective in reducing distorsion. This is why many prefer amps with a flat open loop response up to at least 20 kHz, but the (most) important thing is to have enough open loop gain at 20 kHz so the feedback has something to work with.
Then there will also be various effects from capacitances etc. that affect the distorsion of the amp before applying feedback.
Then there will also be various effects from capacitances etc. that affect the distorsion of the amp before applying feedback.
Hi,
Crossover distortion is only part of the problem.
Class A = no crossover distortion, but the input stage and voltage amplifier stages all contribute to the overall distortion figure.
The topology used, LTP or single ended input determines the "characteristic" of the distortion, for e.g. the rate of rise with increasing frequency and it's harmonic stucture.
Crossover distortion is only part of the problem.
Class A = no crossover distortion, but the input stage and voltage amplifier stages all contribute to the overall distortion figure.
The topology used, LTP or single ended input determines the "characteristic" of the distortion, for e.g. the rate of rise with increasing frequency and it's harmonic stucture.
This far, I too have been able to understand. The Miller cap or compensation cap is responsible for providing local feedback to the VAS and this reduces the open-loop gain.janneman said:
My point was: you need OL gain to generate GNFB so that you can linearise the distortion. But if, as Self says, a well-designed Lin 3-stage amp has pretty much no other distortion left other than crossover distortion, and I operate the amp in Class A, then there should be no distortion left to linearise. If I've understood Self right, then the dropping GNFB level should not result in increase of distortion, because there should be no distortion left to linearise.
That's really where my confusion started. What am I missing?
tcpip said:
I think Self means that after you have applied feedback, there isn't much distorsion left other than crossover distorsion. Crossover distorsion is harder to remove by GNFB. That means a class A amp should be very linear with GNFB since it has no crossover distorsion.
If Selfs amps were so free of distorsion already in open loop, why would he even bother with a lot of GNFB?
BTW, I have noted that Self is nowadays a member on this forum, so perhaps he reads this thread and can comment himSelf on what he means.
Yes, this is what I'd understood.Christer said:
This is not how I'd read his text. I'd gotten the impression that he means crossover distortion could be removed if there was enough GNFB available, but there isn't enough at high frequencies.
Exactly. Then why does the distortion rise at HF?
In fact, I never got the impression that he is suggesting that his amps are low-distortion without GNFB. Well, he does talk about their open-loop distortion, and he does say that a good design should have open-loop distortion lower than a bad design (my words, not his), but he never says that his amps have acceptably low distortion in open-loop mode, IMHO.If Selfs amps were so free of distorsion already in open loop, why would he even bother with a lot of GNFB?
Wow, not bad. Hope he or some of the others chip in.
I wonder whether Nelson Pass' super-symmetrical Class A designs also have THD rising with HF?
No amp is distortion free. In a well designed class-b amp crossover distortion is not the only source of distortion, it is often the dominant source, but not the only.
All amplifiers distort, the open loop gain of all amplifiers (eventually) falls with increasing frequency, the ability of nfb to reduce errors falls with falling open loop gain.
Your primary failure of logic is to assume class-a produces "pretty much no distortion", there are plenty of ways for an amplifier to distort besides crossover distortion.
All amplifiers distort, the open loop gain of all amplifiers (eventually) falls with increasing frequency, the ability of nfb to reduce errors falls with falling open loop gain.
Your primary failure of logic is to assume class-a produces "pretty much no distortion", there are plenty of ways for an amplifier to distort besides crossover distortion.
I was only going by Self's comments about good Class B amps, where he said that other than crossover distortion, the other sources of distortion can be brought down into the noise floor. I think I'd explained all this in my opening post.Tim__x said:
In view of this, why should a Class A amp (which has no crossover distortion) show a rising distortion with HF? Why should GNFB be needed to remove distortion in a Class A amp, which has no crossover distortion and where (if well designed) other distortions are below the noise floor?
I know I'm sounding repetitive, but I think that's the only way I could explain my perspective to you.
You could of course always contradict me by saying Self wasn't right when he said that other forms of distortion can be driven below the noise floor. I was just quoting Self.
tcpip said:
Crossover distortion is not the only form of distortion. As a rule,
Class A concentrates distortion into lower orders, such as 2nd and
3rd harmonic and also lowers distortion, but it doesn't make it
go away entirely. Single-ended Class A amplifiers distort too.
A typical Mosfet Class A follower output stage still has on the order
of .01% to .1% at a few watts. If you want to see less, then you
will be applying some form of feedback.
In addition, due to non-linear capacitances in the devices you will
see more distortion up at 20 KHz, for example, even if the open
loop bandwidth of the feedback is constant.
Good to hear from you, considering the amount of effort you've devoted to exploring Class A.Nelson Pass said:
Would you agree with Self that a well-designed amp with GNFB can drive down most distortions practically below the noise floor? His comments were of course related to his own Class B circuit, and he readily agreed that crossover distortion can't be dealt with this way because there's not enough GNFB at HF. We are also keeping the how-it-sounds question out for now. Let's assume we are "designing by numbers". In that case, do you think this is possible with GNFB? That was really the starting point of all my questioning.
Basically, my question is: is crossover distortion really the biggest source of distortion by far in a well-designed Class B amp? If yes, then how much does the distortion drop if we switch it to Class A by simply increasing the bias voltage? I can see from Self's book that it drops, but I can't understand why the curve still continues to rise with HF. Do the other sources of distortion too add up to a sharply rising pattern like xo distortion?
Does this distortion of a Class A MOSFET stage rise with HF? How much does it rise?
tcpip said:
Five questions, five answers.
1) Possibly - I haven't thought about it that much because it's not
particularly important to me. My definition of a well designed amp
is a little different than Self's.
2) If you mean Class B (with no bias at all), generally yes.
3) Take a look at the article "Leaving Class A" at www.passlabs.com.
4) Yes, as I mentioned previously.
5) Yes, but it varies depending on the design. Sometimes it barely
rises at all in the audio band, sometimes it rises dramatically.
Why does Class A distortion increase with frequency ?
quote:
Originally posted by Lumba Ogir
all ends meet with blameless precision, I can only see one minor yet bothering detail waiting to be explained: as there is almost no distortion in Self`s amplifiers, what makes them sound so awful?
Have you ever listened to a Douglas Self Class A amplifier, or constructed kits that were developed using Douglas Self's design guidelines?
In my experience they sound fine,but unexciting, with a relatively small soundstage. They are very clean sounding, which some people may not like. Neither do they have an artificial warmth.
Like most amplifiers, they can be further improved by extra attention to the power supply feeding the front end. A high quality, VERY low impedance regulated front end PSU will further improve their performance, in both revealing low level ambience and improving channel separation. (I think that most amplifiers will benefit from this approach.) They are capable of further improvement with attention to detail,such as matching front end devices etc. They make damn fine building blocks, as Mr. Self has already taken care of most of the problems that afflict most solid state amplifiers. Douglas Self did indeed respond to a post I made recently, and gave me the opportunity to reply to his email. Unfortunately, I do not know if he received my reply, as it has not been achkowledged.
SandyK
quote:
Originally posted by Lumba Ogir
all ends meet with blameless precision, I can only see one minor yet bothering detail waiting to be explained: as there is almost no distortion in Self`s amplifiers, what makes them sound so awful?
Have you ever listened to a Douglas Self Class A amplifier, or constructed kits that were developed using Douglas Self's design guidelines?
In my experience they sound fine,but unexciting, with a relatively small soundstage. They are very clean sounding, which some people may not like. Neither do they have an artificial warmth.
Like most amplifiers, they can be further improved by extra attention to the power supply feeding the front end. A high quality, VERY low impedance regulated front end PSU will further improve their performance, in both revealing low level ambience and improving channel separation. (I think that most amplifiers will benefit from this approach.) They are capable of further improvement with attention to detail,such as matching front end devices etc. They make damn fine building blocks, as Mr. Self has already taken care of most of the problems that afflict most solid state amplifiers. Douglas Self did indeed respond to a post I made recently, and gave me the opportunity to reply to his email. Unfortunately, I do not know if he received my reply, as it has not been achkowledged.
SandyK
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