There you go, you've nailed it.
Now just play with the values for the best result, like R25, R26 for optimizing TIS gain etc., but you already know the name of the game.
Regards 🙂
There are different level of knowledge about some "things". The highest level is the best, mastering at the "things", or he almost everyday with it, or his job. or he did himself, see and feel at the time.
The lesser knowledge, is hear or receive the knowledge from his trusted source, he doesn't need to do with it, and he know from what information he receive. If the source is trusted, then he has the knowledge.
The worst one is when someone receive the knowledge without caring its source, then his knowledge is just a junk, mixed the right and wrong.
So, there are not always trying to knowing. Someone trying something also not always trusted, for example, someone with color blinded cannot be trusted to when he see color of something.
- Around 1970, i was part of the R&D team of a French hifi manufacturer. We designed an CFA amp (ignoring it was CFA, because the name did not exists at this time). One guy, an audiophile, restored-it recently, it was the amp of his father, and reported it was one of the best sounding amp he had listened to. And he have a collection of prestigious amps restored as well. Thanks to the topology.
- When the Mark Alexander's paper was printed, i began to be aware of CFA. Made a lot of comparisons (blind) in mixing desks with various CF-OPAs. Was convinced of their superiority and perfect transparency (specially in the bus input stages) and even compared with strait wire .
- Listing all the amps we had chosen in the studio i managed, i realized to have chosen quasi all amps as being CFAs (Including Studer) . It was not on purpose, i ignored their topologies and chosen them by listening comparisons.
If you want to get an idea of pro & cons of both VFA & CFA topologies, on theoretical numbers, please, have a look at:
http://www.esperado.fr/temp/VSSA/vssa-vs-vfa.html
But the best is to build and listen CFAs: there is really a difference. Most of the times, you can discover details hidden in your previous VFAs, less fatigue, more dynamic basses, more fluid and natural trebles. Obvious if you make a direct comparizon of a VFA & CFA version of the same amp, everything else the same. That i have done several times before to be a CFA addict.
My preference is not based on books or reviews, neither distortion numbers on data sheets, but long and various listening experiences during my professional life.
Last edited:
Le forum Audio Vintage • Afficher le sujet - Scientelec mach A 30
https://plus.google.com/photos/109296933751482509949/albums/5874381443658096225?banner=pwa
Note the constant impedance volume control (and bass/treble ones) made by 24pos switches and fixed resistances ;-)
Last edited:
The 1979 Hiraga 20W was a CFA.. It was (boosted to 30W) for many many years my best amplifier. Clear dynamic and uncluttered. Still today theres lot to be learned from his simple circuit.
OS, you say that my 200 W CFA amp has to many devices. I have counted active devices in your last CFA(copy of VSSA) and you use 8 transistors and 2 zeners not counting cap multipliers and the OPS, and my amp has 12 transistors (only two more then yours, as I use BJT CCS not zeners). If you call it to much I am lost.
And there is ready PCB layout thank to Chris, and I think he is willing to offer it to the ones who want to built that CFA.
Here is Chris 200 W version and uses four pairs of output MOSFETS.http://www.diyaudio.com/forums/solid-state/243481-200w-mosfet-cfa-amp-35.html#post3718993
and here 100 W version down below in post #346. and uses two pairs of output MOSFETS.
BR Damir
"I think the slew rate thing is a red herring. I do tend to be an advocate of high slew rate, but I do not believe that slew rate achieved by current-on-demand where an early stage may go out of class A is an advantage to sound quality. I like good slew rate margin because it is normally reflective of margin against high-frequency nonlinearity. "
Bob, the early stage does NOT go out of class A!
If you look at my CFA article and the nx-Amp write up, the front ends are solidly in class A and it's easy to arrange the whole front end to do this.
Bob, the early stage does NOT go out of class A!
If you look at my CFA article and the nx-Amp write up, the front ends are solidly in class A and it's easy to arrange the whole front end to do this.
Thanks. One note, even in 1970, and despite we where so young at this time and haven't read any audio books but schools ones, discovering everything by ourselves, we were aware of TIM.
We used, for the first time in audio industry, brand new fast planar epitaxial power transistors BDY56. As it was not possible to find PNP fast power devices, we were obliged to use a semi symmetrical topology (Quasi complementary).
This amp measure 0.025% HD at 50W, exceptional at this time.
Regulated power supply for PSRR, hehe 🙂
+1 While it is not so important, as long input stage is still class A at the moment output stage turn in class B.
Last edited:
For SR I believe 1 V/us per peak output volt is the minimum. So for an amp that outputs 100 V pk to pk, I would shoot for 100 V/us minimum. But, I would also say that going overboard adds no value either. 🙂
The goal has been to find out if there is any fundamental advantage and any subtleties -- more about how it works than a DIY build something topic.
There are no more issues with matching than a VFA has. For myself, I match with either topology to get the lowest distortion possible... but that isn't everyone's goal. I find matching to be another tool in fine tuning the amp for best possible performance. How much of a perfectionist are you.. determines if you want but don't need to match. It only become important issue if the gnfb is very low and how low you want the distortion(s) to be.
I'll bet a whole dollar that with your keenly developed insights and abilities, that if you designed and built a CFA, you could tell us all what you find are its finer points, if any. Even if it were just a SIM and never listened to it.
Please. We need and want you to try it. And, detailed writing about the variations and their pro-con/trade-offs in your book. Because it doesn't look like the subject is going away or about to die anytime soon .... not here nor in the market place. We have made a good beginning here on our own.
Thx-RNMarsh
Last edited:
This is very close to my rule of thumb. I like to see 10 times the slew rate of a full-power 20kHz sine wave. Such a sine wave has a slew rate of 0.125 V/us per volt peak. So this comes out to 1.25V/us per volt peak of output. This compares to Bonsai's 1V/us per peak output volt (Bonsai's second sentence implies half that, so I'm uncertain of the apparent contradiction).
Cheers,
Bob
The JBL SA600 (1967) was given for better than that.
A reference still today for its full complementary output stage designed by Bart Locanthi.
The first important thing is to understand the circuit. If you are a regular observer of amp schematics, you can easily detect if there are defaults in its conception. No real need for maths. Have a look at the two major books currently available on power amps, concepts need very little maths to be grasped.
Personally for me, setting the slew rate of the amplifier does not say anything without supply voltage. Some authors have even introduced the concept of reduced rate of rise of 1 V supply voltage. But this should not be done if we use the concept of bandwidth at full power. This concept includes two concepts: the slew rate and the reduced rate of rise of 1 V supply voltage.
regards
Petr
regards
Petr
It's just a point to keep things in balance. As a designer I think I you can stress one parameter over another needlessly. I would not for example target 1000 V/us, and I don't target 50 ppb distortion because in both cases it's wasted effort and drives uneccessarily complexity.
Last edited:
Member
Joined 2009
Paid Member
If we think its necessary to test for thd and IM using high freqs (10Khz-20KHz), then it implies a linear BW much greater than 20KHz.
The SR might be more in line with the other tests we do..... maybe a SR of 40Khz signal or higher. Minimum of 2.5 - 5v/usec /peak V is what I use.
Push yourself a little more -- If you want a yard stick; My own amp designs will do full/high power at 100KHz at low thd (not SIM). Why not? Isn't that hard to do. Push the envelope to see what is possible. We don't have to sell them and make money here. Its R&D.
But, If you want to just do 1.25v/usec SR, it wont be a disaster/failure either......
Thx-RNMarsh
The SR might be more in line with the other tests we do..... maybe a SR of 40Khz signal or higher. Minimum of 2.5 - 5v/usec /peak V is what I use.
Push yourself a little more -- If you want a yard stick; My own amp designs will do full/high power at 100KHz at low thd (not SIM). Why not? Isn't that hard to do. Push the envelope to see what is possible. We don't have to sell them and make money here. Its R&D.
But, If you want to just do 1.25v/usec SR, it wont be a disaster/failure either......
Thx-RNMarsh
Last edited: