CFA Topology Audio Amplifiers

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#841
Bob Cordell said:
Well, my last day at work is this Friday. Yes, retiring from the Day Job (undersea fiber optic communications). I should then have more time to brush off the dust from the EKV models and see if I can get them out there. Of course, my wife Angela is busily preparing a "Honey-do" list that is the length of her arm, now that I will have all of this time on my hands.

Cheers,
Bob
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Congratulations! I hope there will be some more books on the way!
 
#843
manso said:
Take a look at audiolabor schematics, here a different method was used to stabilize bias, the same way JLH first did in the 60s. These amps were highly acclaimed by the press in the 1980s. See how elegantly the diamond takes care of this problem.
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Do you mean the Audiolab 8000A? I think this is a complex VFA with QUAD type Triples .. not a CFA at all.
Ive shown my diamond schematic and performance was confirmed by Wahab, some think that the ULGF of 7 MHZ is too high, it may well be lower in practice. The amp has been in production for 2 different manufacturers for over 15 years and no complaints recieved about stability but Ill leave it at that.
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Please, have you a link to this schematic?

Bob Cordell said:
Well, my last day at work is this Friday. Yes, retiring from the Day Job (undersea fiber optic communications).
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Bob, if you intend to follow a beach bum life from now on, I seriously recommend you lay down a stock of Burgundy NOW .. before you start. Burgundy is difficult to obtain for beach bums. I speak from experience. 🙂
 
#844
Bob -- Congratulations on your making it to the 'good life' and off the merry-go-'round of the working world. Keep a new calender near you as you will loose time of the date and day of the week.... because every day is now a holiday 🙂

-Richard Marsh
 
#845
kgrlee said:
When I make a recording, I note the settings of the Soundfield Control Box and the recorder. This gives me an spl calibration to within about 1dB.

I'm one of the crazys who thinks recording he's made with microphones he's designed are the most important to assess reproduction 🙂 I like to play these back at the correct level and often check this with a calibrated B&K mike. But one of the party tricks of good recording engineers is that they can set playback levels to within 1dB of the original for a good recording using simple mike arrangements.

Peter Walker said that for a natural recording, there is only ONE correct playback level. If its played too soft, it doesn't sound further away. It sounds too small.

I concede I'm not typical and I like to sit up front for orchestral concerts.

There are famous Soundfield Mike recordings which overload the mike, Sony PCM-F1 recorder, 1000W/channel amplifier and 90dB/W speakers. But with appropriate speakers & amps, this can still be played at the correct level with no audible distortion.

Google 'The Garage Door'.
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But to put all this semi-liquid BS into perspective ..

For a 50W 8R, I'd pay really close attention to overload & recovery behaviour. This will be important to at least 28% of this august forum ... and at least 50% of people working at Wharfedale in da 80's.

I'd like 200W 8R for my own use at home. Won't quite play 'The Garage Door' properly but OK for music 🙂

For conducting Double Blind Listening Test on speakers, I'd want 1000W 4R and impeccable performance in all other respects. The unit I favour is now Unobtainium. Any suggestions?
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I expected maximal SPL values for a listener seated at a comfortable place in a classical concert.
These are old data but still valid :

An externally hosted image should be here but it was not working when we last tested it.


In HiFi News & Record Review, December 1985, p61, Stan Kelly quoted a maximal SPL of 106 dB obtained in the middle of the hall during an extreme performance.

If obtaining such so-called "realistic" SPLs of classical music in domestic conditions is the aim, it is not that difficult. Use sensitivity of 96 dB/2.83 V at 1 m for your loudspeakers, most amps will never enter clipping with them.

If Peter Walker is the reference, I do not think that the loudspeakers manufactured under his control were able to reach the realistic levels you seem to imply.

Microphones are placed near the instruments to enhance the ratio direct/reverberation of the sound and pick up detailed acoustic images.
The SPLs at which they are submitted are certainly not a good indication of how loud the sound should be reproduced for a listener situated in domestic conditions. What can be the aim is to reproduce at home the SPL as wanted by the sound engineer at the ultimate end of the mixing process.
 
#849
forr said:
I think considerations about SPLs started from the mention of a good behaviour of CFAs on clipping. I defend the idea that clipping should never happen in amps of well calibrated systems. So, the behaviour at clipping is a non-pertinent criterion for amps used in domestic conditions. That's all.
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If I understand what you are saying, then having good clipping behaviour is a necessity if cost is brought into the equation.

To handle an uncompressed (or lightly compressed) recording at decent sound pressure levels then huge amounts of power are required and this is expensive. So people accept clipping as a consequence of this. Having good clipping behaviour is a way of mitigating the issue.
 
#850
but you could be Klever about it and not let the amp clip - in V anyway - always nice to have the peak current conducting ability in the output Q though

Bob Cordell said:
In all cases in that post I was referring to the amplifier's global feedback loop. The kind of clipper I was referring to as my preferred approach is a soft clipper that precedes the amplifier, so that the amplifier, with its global feedback loop, never clips.

The use of a dynamic soft clip circuit means that the clipping threshold of the soft clip circuit tracks the average available rail voltage so that some degree of dynamic headroom is retained. The Klever Klipper merely uses passive silicon diodes for the clipping action. Those diodes are normally reverse-biased by the clip threshold control voltage.

The Klever Klipper is described in Chapter 17 of my book.

Cheers,
Bob
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#852
clip if you must, but clip early

I've made a few supply-tracking clippers in lieu of compressors and limiters per se when the client had philosophical objections to the latter. In one case they were active-diode-based which made the clipping rather precise and symmetrical, and made it easier to change the thresholds.

But in general, I agree that we ought to be able to avoid power amp interstage or output stage clipping, so that the power amplifier itself isn't pushed into that region to begin with. It is almost always better to do things, roughly speaking, in the small-signal, lower-power domain rather than in the high power one. And it's relatively easy to clip cleanly and recover very quickly in this "signal" domain. It can be done with topologies that avoid virtually all of the static distortion below clipping that might arise from diode voltage-variable capacitances.

When I read about the stupendous overload requirements some propose for vinyl playback, I think all well and good, but wonder what the rest of their signal chain looks like! Do they have multikilowatt power amps, in particular? Of course, often the preamp designer is only concerned with avoiding overload in the preamp. After that, you're evidently on your own!
 
#853
Simple CFA sans Output Stage

Here is a simple CFA (I don't like huge virtual ac gnd caps) that has few parts and good THD (20kHz 0.000018 as shown) performance possibilities.
 

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#855
Belaji said:
Here is a simple CFA (I don't like huge virtual ac gnd caps) that has few parts and good THD (20kHz 0.000018 as shown) performance possibilities.
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This is similar to my VAS, except for that I added degeneration resistors and use a different input bias to mate it to an LTP. With an ideal output I can get the THD meter to say 0% at 20K/50W/8R that way. The pole that the LTP introduces lies high enough up the band to not be bothersome too much - the OPS is the limiting factor really.
 
#857
forr said:
If obtaining such so-called "realistic" SPLs of classical music in domestic conditions is the aim, it is not that difficult. Use sensitivity of 96 dB/2.83 V at 1 m for your loudspeakers, most amps will never enter clipping with them.
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Du.uuh! I have designed many commercial speakers in my previous life, both for domestic as well as Sound Reinforcement .. but my loudest design is only 94dB/2.83 @ 1m 😡 Which ones do you have?

Microphones are placed near the instruments to enhance the ratio direct/reverberation of the sound and pick up detailed acoustic images.
The SPLs at which they are submitted are certainly not a good indication of how loud the sound should be reproduced for a listener situated in domestic conditions.
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I don't place mikes near instruments. Practicallly all my recordings are in 'audience' position. One or two are above & behind the conductor.

Why don't you take an spl meter to an orchestral concert and sit near the front.

Certain types of music, eg VERY GOOD unaccompanied choirs of between 12 - 20 can have peak levels high enough to generate audible intermod in the Mk1 Human Ear at very modest average levels.

I have been privileged to sing in such a choir and the bad luck to try & record them in analogue days. The Sony PCM-F1 and Soundfield Mk4 helped a lot but didn't solve the problem. I used to record a lot of this stuff.

With respect forr, if you don't think the overload performance of an amplifier is an important part of its sound ... 🙂

bcarso said:
But in general, I agree that we ought to be able to avoid power amp interstage or output stage clipping, so that the power amplifier itself isn't pushed into that region to begin with. It is almost always better to do things, roughly speaking, in the small-signal, lower-power domain rather than in the high power one. And it's relatively easy to clip cleanly and recover very quickly in this "signal" domain. It can be done with topologies that avoid virtually all of the static distortion below clipping that might arise from diode voltage-variable capacitances.
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You can go further than that. As really a speaker man, I regard the amp as just another xover component.

If your design the amp+speaker together, you can tailor the small signal stages + amp to overload in a manner that hides speaker overload. A really yucky sound in a subwoofer system is amp clipping so you MUST make sure the amp doesn't clip. This is an important part of my patented Powered Integrated Super Sub technology but I'm surprised not more is done today to use analogue, digital, linear & non-linear stuff to better integrate speakers & amps.

Excuse all this w*nking but I can't let forr get away with claiming overload performance isn't important. IMHO, this may be the MOST important audible difference between 50W 8R amps .. for at least for 28% of this forum according to the poll he quotes. Not everyone has 'louder than 96dB/2.83V @ 1m speakers'. In fact I'd be really interested to know who on this august forum does.
 
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