Accuphase dont chase THD numbers but also display ULGF in excess of 4Mhz. THD is not class leading but good nevertheless. They use dalington vas and shunt compensation just like I do. Emphasis is placed on SNR and make use of innovative topology where 2 diamond front ends are used in parrallel to this end. They are built to the highest standards and cost ranges from 7000 us $ up. Since around 1985 all their amps have been CFAs.
Originally Posted by jan.didden
"Why do you guys not use a perfect buffer as the output stage, so you eliminate all output stage issues from the comparison? Also use ideal batteries for supply"
Good idea!
"Why do you guys not use a perfect buffer as the output stage, so you eliminate all output stage issues from the comparison? Also use ideal batteries for supply"
Good idea!
That i wanted to lighten...step by step.
The influence of the parasitic capacitances, impedances, distortion cancellation of the LTP etc...
But it seems impossible to make such an exploration here in the middle of such an incomprehensible and stupid noise, insults etc...
And as each time i try to input some non conventional things i had experienced in my life, it is always the same story (look how had turned my thread on impedance correction of loudspeakers). Aggressions from some "non believers" witch had not experienced the subject, ot think academically...
To tired (like LC, and for the same reasons), i just wanted to exchange with gentleman like you...we all have to learn something from others...I have leaved this forum. This time for good.
This said, your work here is fantastic: keep on trucking, man with my affection, admiration and respect.
And i am sure some experienced members, like Elvee will continue better than i those sim explorations.
My best salutation to all my friends here. Have a good life, and enjoy good music... Don't lie too much on numbers...listen with your heart...try to find the essence, the character of each thing...
Last edited:
Esperado,
What you are talking about happens in every subject discussed whether they are audio related or any other subject. There will always be those who only believe what someone else has put into print and others who will believe in black magic if someone tells a good story. I think it is those who go against the wind, the people who are willing to try something new and fail or not who move things forward. If we only go by what someone in authority says we make no progress but to get there we have to listen to all of those who tell us something is impossible or if it is such a good idea someone else who is well know would have already done it. In the plastics world I worked in that happened often, you weren't supposed to try something that the major chemical companies hadn't already explored, you were to stay a follower and not become a leader, that just messed up the order of the universe. Audio has been the same for me since the beginning, how dare I do something that others had not already proved, even if they were wrong in the first place. It is to easy to give up, it is harder to fight the fight, but that is the reward, when you are found to be correct and the naysayers have egg on their faces.
Your buddy,
Steven
What you are talking about happens in every subject discussed whether they are audio related or any other subject. There will always be those who only believe what someone else has put into print and others who will believe in black magic if someone tells a good story. I think it is those who go against the wind, the people who are willing to try something new and fail or not who move things forward. If we only go by what someone in authority says we make no progress but to get there we have to listen to all of those who tell us something is impossible or if it is such a good idea someone else who is well know would have already done it. In the plastics world I worked in that happened often, you weren't supposed to try something that the major chemical companies hadn't already explored, you were to stay a follower and not become a leader, that just messed up the order of the universe. Audio has been the same for me since the beginning, how dare I do something that others had not already proved, even if they were wrong in the first place. It is to easy to give up, it is harder to fight the fight, but that is the reward, when you are found to be correct and the naysayers have egg on their faces.
Your buddy,
Steven
I agree. It all turns into mush. Less teaching and les patience to bring all along for the journey of exploration and understanding. Sooo. I'm outta here. Got places to go and things to do. I'll check in every so often and comment when ever it gets interesting or something unique or special or a nice twist or even some innovative cancellation techniques. JLH circuits of yesteryear?.... I dont think so.
Now gang of cats, go make us a really great Current-Mode Amplifier that DIYAudio will be proud of !
xoxoxo
Thx-RNMarsh
Now gang of cats, go make us a really great Current-Mode Amplifier that DIYAudio will be proud of !
xoxoxo
Thx-RNMarsh
Last edited:
Please don't leave Christophe. You have provided much good stuff to this thread and others.
My own CFA journey is just beginning. I was initially skeptical.
I bemoan you and LC leaving cos both of you try to do Listening Tests. As a Blind Listening Test guru, I prefer Blind Tests. But IMHO, some Listening Tests is definitely better than none.
______________
The astute among us will have realised that there appears to one type of VFA input; the LTP with emitter degeneration and CM. I include the symmetrical versions in the same category.
However, we have discussed at least 3 different types of CFA input
- the simple type in VSSA which I favour for low noise
- one type of Diamond which Manso & Bonsai favour that has been detailed by Waly. You can consider this using Baxandall pairs to allow direct coupling of the feedback network. Manso & Bonsai use emitter degeneration in both pairs though whether this has any function beyond setting currents is unclear.
- another type of Diamond as in the Revox B750 which Wahab posted. It doesn't appear to use emitter degeneration.
______________
Manso, the 2 obvious ways to use an extra 2 transistors in my #500 simple CFA aremanso said:
- enhanced VAS as you suggest. But the gain of the VAS, whether enhanced or not, is limited by R1/3 vs R6/10. So the advantage is at best less than 6dB
- use evil output triples. More difficult to use my favourite 'pure Cherry' compensation and also takes much longer for the output bias current to stabilize. But nearly the full hfe of the added devices is used provided the compensation scheme allows it.
______________
Andrew, in the spirit of pseudo guru MikeK I attachBonsai said:
[deleted: 33 pgs This is JLH comp .. not Alexander comp ... or MIC ... and yus all idiots & deaf bla bla 🙂 ]
_______________
Why not if they do the job better than new-fangled stuff? I've done my share of new-fangled stuff but NEVER for the sake of 'new'. Always cos I need something that existing stuff can't provide. That's called Engineering. Everything else is Golden Pinnae fashion.RNMarsh said:
Wanna point out a 'new' circuit with similar complexity to my #499 circuit with sub 1ppm 20kHz THD at 50W 8R?
Yus cynics will say it's just ol' Blameless with 2 extra FETs, an extra resistor, 2 caps and one cap moved .. cos that's all it is. Drop the FETs if single digit ppm THD is good enough for you. It takes time and loadsa effort to get the most out of any topology; more than 30 yrs in this case.
But understanding old stuff thoroughly is often more productive than doing new for new's sake. I hope to unnerstan this JLH CFA lookalike properly before the next Millenium.
Last edited:
Its just me. I already know enough about that circuit from decades ago. My goal here was to see if i could point out some characteristics which would motivate people on CMA.
New would be nice if it is better in some way(s) and I am pretty sure it can get better. I'd bet you a whole dollar on that. New for me is more in the realm of cancellation techniques/topology not CMA per se. If or when you get there, i'll jump in. Enjoy! And, take better care of Christophe.... he's a gem.
Thx-RNMarsh
Last edited:
What you say here is very true, but it completely missed my point. I am not talking about any layout, I am not even talking about building an amp. I am suggesting that to do a sensible comparison, one issue is to make sure the output stage does not muddle the comparison, hence the proposed 'ideal buffer'. Just so the comparison between the various input/Vas/small signal stages becomes more meaningful.
jan
Last edited:
For Linsley-Hood, the first reference I have for CFA having CCS in the push-pull input stage is "Symmetry in audio amplifier circuitry" Electronics & Wireless World, January 1985.
The CCS are placed in the collector of each input transistor, in parallel with a resistive load. In fact, they are not really pure CCS as they act as DC feedback to control the voltage drop (0.6-0.7 V) across the emitter resistor of each transistor of the VAS, hence the quiescent current of this stage.
This scheme which was used by JLH in subsequent projects is quite different from the one proposed par Kgrlee where the CCS are located in the emitters of the input stage transistors :
http://www.diyaudio.com/forums/solid-state/240712-cfa-topology-audio-amplifiers-25.html#post3606096
A MIC could be seen in 1969 JLH's class A amp (which was, by the way, a "CFA" as most of its contemporary designs).
Where did you get this one? its exactly the other way around, cascodes do nothing for the PSRR, but help reducing distortions by taming the Early effect in the gain device.
No Jan, i did get it.
And it make sense, and i do it myself. But instead on focus on the optimum VAS and Frontend into a perfect buffer, i would not reject any of them before i have tested with a output stage with load.
The "perfect buffer" is a perfect load for the previous stage. It is true that you can get really close with and linestage with an good buffer.
But as you go up in power level devices in the output stage gets more and more unlinear and the loading of the vas stage gets a more unlinear and complex load.
You could easily end up with an less perfect VAS stage performing better.
- Sonny
It helps in distorsion reduction but only at lower frequencies ,
much less or even not at all at the upper extremity of the audio band.
Edit : on second thoughts , since the early effect is reduced ,
that is , the resistive emitter/collector coupling , shouldnt
this isolate the VAS output from the supply rails and hence
reduce PSU noises.?..
Last edited:
Draw yourself a cascode stage and look closely. The cascoded device is usually a low Vce device with high beta and low Early voltage. It would not support the collector voltage, and the distortions would be rather high (because of the low Early voltage which is, BTW, rather frequency independent).
The cascoding device is a high voltage, high Early voltage, but with rather low Beta, device (remember that the Beta*Early voltage is a process dependent constant, kinda "process figure of merit"; you can't have the cake and eat it too, that is both high Early voltage and high Beta).
The cascoding device keeps the cascoded device Vce constant within the datasheet limits, and obviously the Early effects are tamed. The large voltage excursion is now moved to the cascoding device collector, but as this one has large Early voltage, the distortions are lowered.
Shortly, a cascode stage is is in fact building an optimized device, with both high beta and large Early voltage, by taking the best out of two classes of devices. If you are using modern devices like the KSA1381/KSC3503 which are both high voltage and very high Early voltage, using a cascode is pretty much moot.
Regarding PSRR, look at the effect of PS variations at the output. There isn't any difference from a usual common emitter gain stage. Actually the overall cascode stage gain is the same as the common emitter stage of the cascoded device, in the cascoding device load.
Waly
Beat me to it, totally agree that cascoding does open the possibilities in transistor selections optimized for each stage. The same is true for the more typical cfa current mirrors (where high beta is not as much as requirement but matching is, and the higher open loop gain is welcome also).
Thanks
-Antonio
Beat me to it, totally agree that cascoding does open the possibilities in transistor selections optimized for each stage. The same is true for the more typical cfa current mirrors (where high beta is not as much as requirement but matching is, and the higher open loop gain is welcome also).
Thanks
-Antonio
Last edited:
What early effect, perhaps you havent noticed that the gain devices are bootstrapped. PSRR is a little bettered, sims show. The cascodes increase the impedance at the summing node which should be as low as possible as theory suggests, hence a slight increase in THD. Even the use of hawksford type cascodes doesnt improve matters and could potentially make them worse. For those not using bootstrap it may be required for the use of low voltage transistors. Bootstrapping also doesnt bring significant improvement in THD as the Vce of the summing node transistors is lowered and results in higher capacitance between base collector which means higher THD at high frequencies although the reduction in early effect depending on the transistors used, improve matters marginally at frequencies below 1Khz thereabout.
In the actual amp low rb rf transistors are used for the diamond buffer for low noise and lowest high frequency THD, this to keep a uniform THD pattern from 20 hz to 20Khz. These transistors have very low Cob figures although in detriment of input impedance because of low Hfe, a slightly lower impedance can be tolerated. Matching and hfe selection is a nightmare but possible.
Last edited:
I would reckon that BC5x0 devices would be well suited to a cascoded diamond front end. The low Vces would suggest that the so called "quasi saturation" effect is present.
The use of the ccs doesnt change the topology, the resistors do the function of the ccs. Indeed the use of ccs improves PSRR but sadly doesnt do anything to improve THD. Study the SSA thread, two members there namely homemodder and Elvee seem to have the topololgy well studied and show sims proving this point. Elvee also shows via sims how cascoding is actually detrimental to THD.
Yes, but the higher hfe makes for easier matching especially when using discretes.