I tried to bring his attention on what some call CFA. And had some controversy with him, on what i considered a erroneous analyze of the CFA poles and the supposed distortion cancellation with a differential stage when the inverting one is used for feedback.
When i said to him that, anyway, he cannot leave untold this subject. His response was: "Yes, i can. " !!!
I was very disappointed, as D.S. has some reputation in the audio world.
I believe all the misery about CFA is due to this stupid *C*FA name ! When i was a young engineer, there was not such a name, and it was a very common topology (yet with tubes). I don't know why VFB with differential input stage has took such a monopolistic position on the audio market even with asymmetrical input. Because it looks good on schematics ?
Because some people believe that, adding the same distortion in the feedback path that the one produced by the signal input will cancel distortion, witch is just wrong ?
Everything i have read from Bob Cordel is proper and simple.(clear in his mind, explained in a clear way). I'm not surprised he has too a good knowledge of men and audio history. Don't know for DS, i never bought books from him, and, after his answer, will not.
Anyway, if credits are not so important, as nothing is never totally new in the audio history, good understanding of currents, phases, poles etc. is. As well as common sense.
Like music, it is a too a question of feeling. Never authority.
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TMC appears once only as an alias for Output Inclusive Compensation, repeated from his Linear Audio example. Here, in chapter 13, p334ff he makes reference first to Baxendall, Hitachi's Kunio Seki, Gunderson 1984, TDA7293, Rotel RB1090-3 and in combination with specified 2-pole compensated Yamaha designs and also with his Class G design. Reference is also made to comments made by Bob Widlar regarding opamp design (1981)
Noise?
The "so called" (as Mike would say
) CFAs that I have seen all had noise performance that was inferior to a comparable VFA.That makes them not likeable for my use (compression drivers) at least.
Is there a "CFA" without this weakness?
Best wishes
David
Chapter 8 in his 6th ed. is about p-p Vas circuits.
It has several sections on the Hitachi circuit - there's at least 5 ToC entries with the Hitachi identifier. Followed by a few sections on the Lender circuit and several more Vas configs.
BTW There will be an extensive Book Review on Doug's book in the upcoming Linear Audio Vol 6 by JPV.
jan
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MOSFET Output stages
Hi Ian,
No, it doesn't answer my question. Maybe you could check whether the 6th edition still contains the (erroneous!) picture and text as shown below.
Cheers, E.
Hi Ian,
No, it doesn't answer my question. Maybe you could check whether the 6th edition still contains the (erroneous!) picture and text as shown below.
Cheers, E.
Attachments
Got mine yesterday, ordered (with some Bela Tarr films) last sunday through Amazon french services.
Esperado,
Does-he presents and analyses (with no misunderstanding) current feedback topologies and advantages ?
Drawbacks of so-called CFA are dealt in less than a full page : convincing (p213).
Don't know for DS, i never bought books from him, and, after his answer, will not.
Are you not curious ? You certainly miss something. Even if you are not in agreement with some ideas of the man, you may learn a lot from him.
Everything i have read from Bob Cordel is proper and simple.
I do not like to see Bob Cordell and Douglas Self as rivals. I think they are complementary, each with its own views and approachs, and I enjoy to have two serious authors to read. By the way, have you ever read Peter Baxandall ?
Esperado,
Does-he presents and analyses (with no misunderstanding) current feedback topologies and advantages ?
Drawbacks of so-called CFA are dealt in less than a full page : convincing (p213).
Don't know for DS, i never bought books from him, and, after his answer, will not.
Are you not curious ? You certainly miss something. Even if you are not in agreement with some ideas of the man, you may learn a lot from him.
Everything i have read from Bob Cordel is proper and simple.
I do not like to see Bob Cordell and Douglas Self as rivals. I think they are complementary, each with its own views and approachs, and I enjoy to have two serious authors to read. By the way, have you ever read Peter Baxandall ?
Dave,
I am talking about a power amplifier where you just don't need 1 nV per root Hz performance (which is probably not achievable after LTP degen anyway). And, you have to keep this whole discussion in balance, accepting each has strengths and weaknesses. I for one will design with both. And, I wish VFA proponents would hold back their anti CFA comments until designing and building one.
I am talking about a power amplifier where you just don't need 1 nV per root Hz performance (which is probably not achievable after LTP degen anyway). And, you have to keep this whole discussion in balance, accepting each has strengths and weaknesses. I for one will design with both. And, I wish VFA proponents would hold back their anti CFA comments until designing and building one.
I think that is the most sensible way. It is very, very rare for one author, whatever the field, to have the patent on THE TRUTH (tm).
Learn from different views, and develop your own view.
jan
Noise?
The "so called" (as Mike would say
That makes them not likeable for my use (compression drivers) at least.
Is there a "CFA" without this weakness?
Best wishes
David
This is not a weakness, I think you should give it some thought or calculation. While ultimate low noise achievable with specially designed for low noise LTPs cant be attained, it can achieve the same noise figures that are obtainable with a LTP based amp. The use of degeneration and current mirror with LTPs equals matters from the onset and if your preamp or source has highish output impedance it wont help either. A purposely designed LTP for low noise does not have very good THD performance especially at high frequencies for the lack of techniques that make LTPs linear.
In all, the noise figures obtained in audio power amp designs using the 2 varients are very much the same.
BTW to lower the noise of CFA design is a simple matter of upping the current of the input configuration which is usually a diamond buffer. Another technique is to use two diamond buffers in tandem to lower noise, its more complex but then again it has other benefits as well.
Funny you should say that as I was drawing an idea for something I must do pronto . I have a Quad 405 I can use . I would have to modify it a bit so would rather build this in an old PA amp chassis I have . This is the old Hitachi double VAS degenerated into a Self design . I need an experimental amp for a 15 inch bass unit . This is at the musing stage . The input bootstrapping is a waste of time if I filter the output as the bootstrapping dies with the HF . Thus the ideal input load is lost . However if used as a general purpose amp it might have virtue . Takes a second to remove it so where is the harm to try it ? If I had some Hitachi PCB's ( Like Maplin LP56 ) I wouldn't have thought of this . Now out of the need to press on I am going to build this as a Gain clone . I have the parts and have done it before .
Output stage compensation is a possibility ( two pole ? ) . Again I have done it in the past and had no real problems . HF distrotion is lower at 50 kHz .
This design works well with VAS bootstrap ( 2 x 3K6 = 7 mA @ 50 V ) . Even though FET gain is far less than 1 the gain is enough to drive a bootstrap . In fact a gain of 0.5 is enough to do something .
This circuit is the good and the bad of Self . The Hitachi is better , this one is good enough . The CRD are ones I have . Vishay are too expensive now . 3 resistors and a bootstrap cap will do about the same . I have degenerated the LTP to bring it back to Hitachi re of 50 R .
BTW . Many people refer to their understanding of electronics to know an amp like this is not worth building . Funny thing is my spectrum analyzer wasn't told that and liked the design ( distortion about - 80 dB I seem to remember , alas I lost the notes ) . Ignore the quirky filtering . Doubtless that will be big trouble . I will move slowly on that . You are right if asking . 60 Hz - 3dB ! . Far cheaper than 30 mH air cored .
Yes, yes.yes.
I imagine every body has his own mental representation of the flow of electrons in electronic circuits, and the way components deal with them. I imagine everybody has his own feeling about elementary parts, and their sonic character.
Well, i suppose the experience of old men like me, witch where obliged to design, build, tune and listen our prototypes with real parts can be very different in term of mental landscape than younger, witch have more deep reflexes and spend more time with computers simulations...
Two different ways to explore each details of a complex assembly. One more 'sensitive', one more intellectual ?
Same kind of difference between a guitarist improvising and a composer in front of a blank partition ?
Happy for us, (what the hell is doing Murphy ?) low signals amplification does not request high slew rates, while noise factors are no more an issue when signals are high enough to take some advantages of high speed and increased bandwidths. Comparing the two topologies in the same power amps or line level preamps, i never noticed any real differences or impact in noise factors between the two.Noise?
The "so called" (as Mike would say
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Either you have not at all understood my post, or you chose to deliberately misrepresent it.
jan
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As English is not my native language, your first supposition can be explained, but i feel the second one like unfair and near injurious: i pretend and try to be absolutely honest.
May-be, too, my clumsiness in English make you misunderstood what i tried to express ?
Anything so strange or disagreeable in my post to deserve such an aggressive answer ?
Sometimes, i don't understand reactions of some people in this forum.
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Beautifully put . Thank you . Your English far better than mine . ( Jan also . I visited your town at about 2 o'clock one morning ( wrong Lillle ) , two guys who might have been drugs dealers for all I knew guided me to Tournai { Dornik } , we went across old farm type roads so was a little worried ) .
Referring to my amplifier above . I felt I had been horribly lied to by " experts " when I listened to it . It was as excellent as most will ever need ( measures at least 20 db better than hearing as we understand it , that includes crossover distortion , below 1 watt and up to 50 kHz ) . It reminds me of the Mosquito aircraft . Because it was wood it had no rivets . That was worth + 35 KPH . It was the fastest operational aircraft in the world in 1942 . Above that the propeller has difficulties and would need to be a jet . The ME 109 a fighter could not catch this bomber ( 4000 Kg load ) ! It's designer was told very often he was an idiot . He from his own money developed it in secret . From what I know it was our most successful bomber ( more on target certainly ) . Many pilots lives would have been saved if only using them , it also survived bullets better . It was available in 1939 . You might say people are not so stupid now and would have supported him . If so we have evolved more in 50 years than in the last 1 million .
The 100 R bootstrap resistor is not required if MOS FET output on my sketch amp . Exicon are made in Scotland ! Tune VAS re for preferred sound ( 0R = punch 16 R = nice compromise in IM distortion ) . If unsure make it 0R . The transistor needs to be high gain if 0R . If 2N5551 I would cascode with something . 100 mA FET current seems ideal ( positive tipping point ) . These FETs can replace Germanium's . They come on very early at similar current ( 20 mA circa 0.2 V ) . 100 mA is about 1 to 1.2 V across the 2 devices . MOSFET's never perfectly define a class B point ? Perhaps . They very rarely have a bad one . They are not great in class A as Ron is too high . You might find under biasing gives a sound you prefer . Even zero bias is not the worse sound I ever heard for a measures well hi fi amp ( Radford HD 250 ? ) . Mr ESP says the same . 0 bias = circa 3 mA . Use non polar caps where required . They are better than you would think ( keep them under 0.4 V bias if you can , 0. 3 Vrms ) . My bootstrap would use a larger non polar .
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Sorry Christophe, you are right, I overreacted not fair to you.
Apologies.
I do think though that my meaning did not come through.
jan
When writing a book like this the author must choose some criteria. In Doug's case it is technical correctness. But technical correctness is like political correctness, it deliberately excludes relevant things.
Esperado wrote an excellent post in this thread about his experiences with VFB and CFB topology. His experiences are very relevant because his is the man that spends hours in studio listening amps. I must repeat what he said and is relevant for listeners. VFB amps have some irritating quality that is always present. Sometimes it is present more and sometimes less, but is always present. It does not help to put many enhancements in the circuit because it always stays there. It is obviously topology related. After some time listener has urge to stop listening. With CFB topology irritating quality is gone, you don't skip songs on CD, you don't feel the need to frequently change the loudness level, as Esperado said the sound is relaxed yet detailed and fast. With classical music you can follow the lines more easily. I like to play disc 4 from Stan Getz "The Girl from Ipanema - The Bossa Nova Years". With CFB amps the sax is less shouty, and the mass of percussion are revealed with greater precision. There is nothing you can do with VFB amp that will force it to sound that good in home environment and lose it's irritating quality.
All I said is relevant for high quality home listening. But when you need amp for sound reinforcement, where the sound will anyway be highly manipulated with processors, mixers, etc. VFB amps rule because of their practical technical qualities.
Esperado wrote an excellent post in this thread about his experiences with VFB and CFB topology. His experiences are very relevant because his is the man that spends hours in studio listening amps. I must repeat what he said and is relevant for listeners. VFB amps have some irritating quality that is always present. Sometimes it is present more and sometimes less, but is always present. It does not help to put many enhancements in the circuit because it always stays there. It is obviously topology related. After some time listener has urge to stop listening. With CFB topology irritating quality is gone, you don't skip songs on CD, you don't feel the need to frequently change the loudness level, as Esperado said the sound is relaxed yet detailed and fast. With classical music you can follow the lines more easily. I like to play disc 4 from Stan Getz "The Girl from Ipanema - The Bossa Nova Years". With CFB amps the sax is less shouty, and the mass of percussion are revealed with greater precision. There is nothing you can do with VFB amp that will force it to sound that good in home environment and lose it's irritating quality.
All I said is relevant for high quality home listening. But when you need amp for sound reinforcement, where the sound will anyway be highly manipulated with processors, mixers, etc. VFB amps rule because of their practical technical qualities.
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