If your Vbe multiplier is decoupled with a decent cap, it's quite ok to run the MIC or TPC network from either side. I've done this in a practical amp and there are no problems. You can also see this in the sx and nx Amps which are CFA.
Re taking the comp off the pre or driver output, yes you will run into problem in a practical amp - I tried this on the e-Amp and the only way to fix it was to add another frequency transition network - which I opted not to do as I deemed distortion performance ok ( c. 10 ppm).
Yes, these came up in Elvee's original thread looking at cfa's. Interesting how they (LT) go out of their way to call it a true voltage feedback amplifier and not a current feedback amplifier. If there is such a cfa topology this one falls into it (please don't tell me its not because the bw isn't dependent on the feedback resistor values, as a buffer and fixed resistor doesn't change the mode of operation).
Thanks
-Antonio
Just find which complete set of formula's the circuit complies with and you have found if it is CMA or VFA.
See #455 for formulas.
-RNMarsh
See #455 for formulas.
-RNMarsh
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New direction in CMA development -
As I started to mention... the next phase in CMA development is via the Current Conveyer or mirrors. They need to be better than what has been put up here. Best would be a 4 transistor cascoded CC.
Go take a look at SWOPA design topology carefully and you will see something similar to this -->
Except Scott chose differential input - better matched devices... readily available.
Thx-RNMarsh
As I started to mention... the next phase in CMA development is via the Current Conveyer or mirrors. They need to be better than what has been put up here. Best would be a 4 transistor cascoded CC.
Go take a look at SWOPA design topology carefully and you will see something similar to this -->
Except Scott chose differential input - better matched devices... readily available.
Thx-RNMarsh
Don't want to restart the nomenclature war, I take Mike and Waly's point but will use whatever communicates best.
But several people, not least Hawksford, have mooted the benefits of speakers optimised for current drive, combined with "real" current feedback amps.
What do we call them?😉
Best wishes
David
Thanks but not my style.
I tend to, time permitting, derive them myself for a deeper understanding.
All I did was a Thevenin equivalent, yes the circuit ends looking like 2 emitter coupled pairs with input biasing as has been done before.
Indeed Scott is special, and there will be bits and pieces of useful information surfacing on this thread as well.
Thanks
-Antonio
This was for everyone... some have other styles besides yours.
If it fits the equations, it is a CMA.
I think Scott would be a great person to explain his circuit and/or Current-Mode to you guys and he is always around. Direct your questions to him if you want to know how the CMA and VFA circuits actually work and what makes them work as they do.
For those who dont know Scott Wurcer... he has been designing analog IC's at Analog Devices for decades and is a Fellow at AD. He is always around DIYAudio giving his advice and insights about circuits and electronics and audio.
Thx-RNMarsh
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Ha! A truly evil combination of 1p Millers & TPC/MIC 😱
Thanks for this Bonsai. 🙂
___________________
IMHO, the Locanthi Triple is both too good & too bad for this purpose.Bob Cordell said:
At LF, a properly biased LTriple is closed to your 'perfect buffer'. But at HF, they introduce 3 poles so disadvantage the comp. schemes which include the OPS like 'pure Cherry' & TMC.
I'd prefer a simple EF2 like in #499, 500 & 502. #499 shows (at least in SPICE world) that its not an obstacle to sub ppm THD.
But for the models, there is no contest. The Cordell models have been proved by many people. I can vouch that they give results close to 'real life' for the examples in Self.
If only we had EKV models for MOSFETs with similar pedigree (hint hint 🙂 )
I shall probably be shouted down in this so I'll ask Bob to suggest which of his evil .. I mean thoroughly tested Triples we should use. Also we limit this to 50W 8R amps to avoid the complications necessary in bigger ones and also to facilitate comparisons with the VFAs in the Self & Cordell books.
_________________
IMHO, how an IPS/VAS topology deals with the evil OPS is a paramount factor in its choice. Advanced comp. schemes on appropriate topologies allow SOA performance with very simple circuits.
Dave Zan said:
This is what I mean when I say, CFA may limit the type of compensation used.Bob Cordell said:
It's not where you take the feedback from which is a problem. The VAS output is usually HiZ (before compensation) It's where you feed it. Symmetrical Amps have 2 paths so 2 inputs to the VASs. This affects plain Miller, TPC, TMC & 'pure Cherry'. Mismatched paths will introduce even harms. at best and they will ALWAYS be mismatched if you are interested in ppm THD.
MIC feeds a single point which gets around this. There are yuckier schemes which I've been playing with like compensating just one path. I still haven't seen anything for CFAs as elegant as 'pure Cherry' for simple single path VFAs.
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Don't get me started Mr. Zan!
There are NO speakers optimised for current drive. However, integrating speakers & amps is my true specialty [*] and mostly covered by my patented Powered Integrated Super Sub technology.
Once you start doing this properly, you abandon ppm electronics THD and immediately look at how to reduce the audible distortions of speaker+amp using linear & nonlinear, analogue & digital methods.
* I'm only a pseudo amp & mike guru.
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That's why I wrote mooted - verb p.p. discuss hypothetical idea.
Wondered if this would raise a bite from you - when it would reinforce Mike's contention that most "CFA"s... really aren't😉
Do you have a link, reference or paper on this?
Most interested because, as I have already stated, my own amps are intended for a dedicated multi-way active system, and in particular I intend to optimise each amp for its specific driver. This includes tailored frequency response rather than generic active crossover and wide band amps.
Best wishes
David
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Why not use the fast (low gatestoppers/DG-cap) MOSFET output stage, buffered or not? At least that would add a bit more bandwidth to play with.
Google Powered Integrated Super Sub technology.
As the name implies, the most widespread commercial examples were subs.
The DSP stuff was mainly above 100Hz. I'm not sure its a good fit to modern systems unless you can specify FIR filters at coefficient level. It's used in a commercial mike where this can be done.
Have you got a 'measurement' mike? A Behringer ECM8000 will do. We can talk about this off-list but I warn you, these days, I'm a REAL beach bum so things may take some time to happen.
I posted this on another forum in 2006 when I emerged from the bush .. Excuse the off-topic stuff.
> For what it is worth, I think this is impossible. It seems to violate the cause and effect principle. How could the amplifier "know" what the speaker is up to? Short of sensing the actual motion of the drivers or the actual sound... this has to be sort of by guess and by golly, it seems to me.
It is entirely possible for an "amplifier" to know what the speaker is doing.
It needs to sense the speaker current. An amp which twiddled its Output Z using both current & voltage feedback does this. Speakers act as accurate microphones (sense the actual sound) if operated into Low Z.
See "Loudspeakers as Microphones" - Peter Baxandall special lecture London AES (early 80s, late 70s?)
If operated into High Z, then the voltage at the terminals is a measure of cone velocity.
Both these mechanisms obey superposition & Thevenin so if you're clever, you can look at this while the amp is giving zillion volts and amps to the speaker. But non est tantum facile.
There are several tried & tested methods of using this "controlled output Z" or "current + voltage feedback" or "actual sound & motion feedback" (different descriptions of the same thing) if you incorporate the amplifier design in the speaker. Some of these are in the zanier incarnations of my Powered Integrated Super Sub technology.
The simplest is the negative output R that Fons mentions.
More sophisticated but similar (??!) is ACE technology by Erik Stahl which was used by Audio Pro, Sweden for subs. Unfortunately, since he left, there isn't anyone there who understands it. Anyone have a contact for Erik? Or a clean copy of his original AES preprint?
These methods have the distortion reduction and dynamic overload protection features discussed in Mills & Hawksford. However, they are badly affected by heating of the voice coil.
David Birt (?) did an excellent IoA paper at Windermere where he arranged speaker and amp in a bridge so he could measure and compensate for heating on the fly. Anyone have an email for him?
These are the most elegant methods and they can be analysed from many viewpoints. Some of these viewpoints don't show up the distortion reduction advantages clearly.
I'm contemptous of methods which rely on extra transducers or extra windings (like Mills) or zillion point DSP EQ especially if they don't give ALL the advantages of the elegant methods.
A brute force zillion point approach possible today is measure accurately speaker Z (not that easy) and tailor the output of a High Z amp to suit. This would give some but not all the advantages of the above systems cos it wouldn't "know" what the speaker is up to.
> For what it is worth, I think this is impossible. It seems to violate the cause and effect principle. How could the amplifier "know" what the speaker is up to? Short of sensing the actual motion of the drivers or the actual sound... this has to be sort of by guess and by golly, it seems to me.
It is entirely possible for an "amplifier" to know what the speaker is doing.
It needs to sense the speaker current. An amp which twiddled its Output Z using both current & voltage feedback does this. Speakers act as accurate microphones (sense the actual sound) if operated into Low Z.
See "Loudspeakers as Microphones" - Peter Baxandall special lecture London AES (early 80s, late 70s?)
If operated into High Z, then the voltage at the terminals is a measure of cone velocity.
Both these mechanisms obey superposition & Thevenin so if you're clever, you can look at this while the amp is giving zillion volts and amps to the speaker. But non est tantum facile.
There are several tried & tested methods of using this "controlled output Z" or "current + voltage feedback" or "actual sound & motion feedback" (different descriptions of the same thing) if you incorporate the amplifier design in the speaker. Some of these are in the zanier incarnations of my Powered Integrated Super Sub technology.
The simplest is the negative output R that Fons mentions.
More sophisticated but similar (??!) is ACE technology by Erik Stahl which was used by Audio Pro, Sweden for subs. Unfortunately, since he left, there isn't anyone there who understands it. Anyone have a contact for Erik? Or a clean copy of his original AES preprint?
These methods have the distortion reduction and dynamic overload protection features discussed in Mills & Hawksford. However, they are badly affected by heating of the voice coil.
David Birt (?) did an excellent IoA paper at Windermere where he arranged speaker and amp in a bridge so he could measure and compensate for heating on the fly. Anyone have an email for him?
These are the most elegant methods and they can be analysed from many viewpoints. Some of these viewpoints don't show up the distortion reduction advantages clearly.
I'm contemptous of methods which rely on extra transducers or extra windings (like Mills) or zillion point DSP EQ especially if they don't give ALL the advantages of the elegant methods.
A brute force zillion point approach possible today is measure accurately speaker Z (not that easy) and tailor the output of a High Z amp to suit. This would give some but not all the advantages of the above systems cos it wouldn't "know" what the speaker is up to.
Kgrlee wrote "Ha! A truly evil combination of 1p Millers & TPC/MIC "
The 1p is a place holder Richard.
🙂
The idea is you make the Alex comp the value you want and the shunt you set to 1p and vices versa.
If you use the step function on the two caps, you can directly compare shunt and Alex comp.
My the way, in most amps, if you mount the TO-126 devices to the main heatsink, you end up with some pF shunt capacitance anyway. It's something I am planning to measure when I lay my hands on a decent cap meter
The 1p is a place holder Richard.
🙂
The idea is you make the Alex comp the value you want and the shunt you set to 1p and vices versa.
If you use the step function on the two caps, you can directly compare shunt and Alex comp.
My the way, in most amps, if you mount the TO-126 devices to the main heatsink, you end up with some pF shunt capacitance anyway. It's something I am planning to measure when I lay my hands on a decent cap meter
Fair enough VFA:CFA comparizon ?
For the few gentlemen involved in this topic, and those who read it silently, the result of a long process of comparison.
I leave the Bobcats to their usual caterwauling and fights.
I did it as honestly as possible, trying to enhance LTP version each step.
http://www.esperado.fr/temp/VSSA/vssa-vs-vfa.html
It lacks a study on PSRRs, I'll do it later. you can complete it, the files are available.
As Lazy Cat, and for the same reasons:
Bye bye
For the few gentlemen involved in this topic, and those who read it silently, the result of a long process of comparison.
I leave the Bobcats to their usual caterwauling and fights.
I did it as honestly as possible, trying to enhance LTP version each step.
http://www.esperado.fr/temp/VSSA/vssa-vs-vfa.html
It lacks a study on PSRRs, I'll do it later. you can complete it, the files are available.
As Lazy Cat, and for the same reasons:
Bye bye
Please post the results for this. IIRC Edmond was concerned about this in the SuperTIS because the circuit is very sensitive to stray capacitance. I did a few calculations on the area and thickness of a typical kapton/mica insulator and ended up with 1 to 10 pF, also IIRC. Seemed believable but I never measured it so I am still curious.
Best wishes
David
More detailed response to Mr Lee later.
Once you have a clear idea of the CFA/VFA relative merits, you then connect the OPS. If the twonky load then swamps any advantages of one of the topologies, your conclusion is that none has an advantage in real life. If not, you have a clear winner.
In the first case you can also try to eliminate the destructive effect of the OPS by using a buffer between OPS and the CFA/VFA front end.
Divide and conquer.
Else, I predict that this tread will lead nowhere but will take years to get there 🙂.
jan
I think many of Roy Gosser's patents on two stage CFA's have expired. They address some of the issues of the simpler circuits and should be included. Some of the xDSL drivers were based on these principles.