I don't know. That's why I'm asking the question.
For 1ppm 20kHz, you have to get everything good. When you've peeled away the OPS layers (I do this with 'pure Cherry' around OPS & VAS in #499), you have to look at the IPS.
It's complicated cos the Diamond IPS is very linear in itself. Depending on the exact topology, each side can be considered a Baxandall pair where considerable distortion cancelling occurs. Whether you can split this into an emitter follower (outside the FB loop) and a common emitter stage (inside) is part of the question.
I hope the deafening silence is evidence that Diamond fans are furiously preparing definitive answers to this. 🙂 Guru Wurcer?
But 'pure Cherry' & TMC are difficult with the 'symmetrical' topologies that CFAs seem to demand .. or rather my naive efforts don't show any improvement with these 'advanced' compensation methods.
This is essentially where I believe the work is to be done. Pure cherry if I understand it correctly is a capacitor between the amp output and VAS/TIS input. The diamond input is supposed to provide "current on demand". This may mess the cherry comp up as the diamond will fight it. I have done some quick sims on my amp and with a simple cherry cap (with no other comp methods) the ULGF was up at 68MHz. Totally unrealistic.
TMC with some tweaks does work well.
See the thread My new vas topology, I showed the schematic in the first or second page. Performs well but is more complex.
http://www.esperado.fr/temp/VSSA/cfa-vs-vfa.html
One error corrected, enhanced with Noise, PSRR etc...
Files and models available.
One error corrected, enhanced with Noise, PSRR etc...
Files and models available.
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I test on a resistive load connected BEFORE the inductor down to 1 ohm (1-2 V pk to pk) and there should be no overshoot or peaking - clean square wave.
But with capacitive loads, I only do this after the inductor.
I usually use between 0.6 and 1 uH and it's enough to handle really terrible cap loads.
That said, I like 60 deg at least PM and typically 10 dB of GM.
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Generally, in a well designed amp, the class AB OP distortion swamps everything else. Hence the earlier suggestion by Jan to look at the front end in isolation and then as a second step, attach a practical OPS and move forward from there.
Are you suggesting we design amps without an output inductor network?
I know of only 3 commercial amps that are unconditionally stable with load without this. None of them would be considered 'low distortion' today.
There are many Golden Pinnae amps without output inductors. You may like to try my capacitative load test (and more relevant, the big guitar speaker) including mild overload on these.
This was of great concern for me in my previous life as a speaker designer cos dem evil inductors made amps fussy about speakers impedance 😡
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mcd, is this your http://www.diyaudio.com/forums/solid-state/236002-amp-design-attempt-number-2-simpler-24.html#post3608505?mcd99uk said:
My small brain has problems figuring out the compensation. The 82ps C5, 12, 17, 18 & R37, 43 look sorta TMC but from U22, 23 emitters instead of the output.
But C17, R37, C13, R36 look more like TPMIC.
Dunno what the 'FB' node is on the schematic 😕
Can you explain what is the intended scheme please.
http://www.diyaudio.com/forums/soli...-attempt-number-2-simpler-24.html#post3608505
Wow. That is complex. My eyes are rolling around in their sockets!
Anyway, the main thing is you have it working good to see some pics of the finished beast!
🙂
Wow. That is complex. My eyes are rolling around in their sockets!
Anyway, the main thing is you have it working good to see some pics of the finished beast!
🙂
What's needed in this thread is that someone explains how to calculate values of resistors for the given simple CFA template. Then explain/describe the open-loop action of the circuit and what result it causes when the loop is closed. That might help people understand the current-based push-pull action of the circuit.
So far a large part of this discussion thread tends to have a bit of an audiophylic appearance concerning CFA.
So far a large part of this discussion thread tends to have a bit of an audiophylic appearance concerning CFA.
Added ripple rejection comparison at 1MHz.
Some will be surprised:
http://www.esperado.fr/temp/VSSA/cfa-vs-vfa.html
Some will be surprised:
http://www.esperado.fr/temp/VSSA/cfa-vs-vfa.html
Christophe, Thanks for this is very careful and detailed comparison.
My only caveat is that without EKV models for the output MOSFETs, THD is suspect. Bob Cordell deals with this in his book. I hope he finds and publishes his EKV SPICE models so us unwashed masses can have a play too. 🙂
But I think there is no doubt now that simple CFAs like VSSA will equal & beat simple VFAs on purely technical grounds. I would very much like to complete the circle and conduct Double Blind Listening Tests on these but will probably be unable this Millenium 😡
The simple CFA is also simpler than the 'equivalent' VFA. I have a strong belief that 'simple' will sound better than complex for the same technical performance and have some listening test results that support this (alas all with VFA).
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On a purely practical point ..
Do you know what JFETs LC uses for his IPS CCS? Very few JFETs suitable for this are 35V or more Vds. They don't have to be complementary which saves us visiting the Unobtainium FET Shoppe. 😀
Bonsai's qx_Amp_6
MUCH LATER .. I still can't find them. Any hints & links?
Andrew, I give up. I started playing with qx_Amp_6 and stopped saying I'll find the SPICE models later.
MUCH LATER .. I still can't find them. Any hints & links?
Christophe, best to do this by setting one of the power supply voltages to the AC source and sweeping it (I.e AC analysis). This the gives you a plot of PSRR over frequency.