Two "joe sixpacks" have 20 year old mid-fi ..(hypothetical).
One upgrades to a new CFA marantz , the other to a new H/K VFA. They
plug in their sources and say "wow , this sounds great" (better than what
they had).
After a little "wife swappin' " 😀 , the joe's hear each other's amps - they
still are impressed , as both have recently upgraded.
Having had my own DIY amps for years now , I'm quite satisfied with them.
I really could not conceive of anything better ... short of having the blokes come
into my house to play live for me.
If not for my modular project , I doubt whether I would take the gamble.
VFA is the more common resource.
PS- I'm not "close minded" - at least I will give it ,(CFA) - a try !
OS
One upgrades to a new CFA marantz , the other to a new H/K VFA. They
plug in their sources and say "wow , this sounds great" (better than what
they had).
After a little "wife swappin' " 😀 , the joe's hear each other's amps - they
still are impressed , as both have recently upgraded.
Having had my own DIY amps for years now , I'm quite satisfied with them.
I really could not conceive of anything better ... short of having the blokes come
into my house to play live for me.
If not for my modular project , I doubt whether I would take the gamble.
VFA is the more common resource.
PS- I'm not "close minded" - at least I will give it ,(CFA) - a try !
OS
LOL.
His RIAA was good. I used it to develop a design procedure spread sheet (up on my website).
Reading though his paper, you can see that sorting it out, as he did, was definitely a job for a mathematician 😀
Last edited:
Yep - the most important thing is to have an open mind about it. After you have finished, you may still prefer your VFA's. At least though you would have built both and tried them out.
😎
It is very surprising to read that the CFA lovers should be supposed now to be sort of "closed minded" ! Guilty ?
Near all the amp around us are VFAs. I'm sure all the CFA lovers here have at least, at home, one working good VFA. Very few people in the audio marked had the occasion to listen to any CFA, because they represent a very little percent of the commercial amplifiers .
Are-we guilty to prefer our CFAs ? Are our preferences based on some commercial brainwashing ?
Where is this commercial brainwashing ?
Bring-me a better amp than the one i use, i will immediately adopt-it. Whatever its technology. Class D, or class XYZ.
Near all the amp around us are VFAs. I'm sure all the CFA lovers here have at least, at home, one working good VFA. Very few people in the audio marked had the occasion to listen to any CFA, because they represent a very little percent of the commercial amplifiers .
Are-we guilty to prefer our CFAs ? Are our preferences based on some commercial brainwashing ?
Where is this commercial brainwashing ?
Bring-me a better amp than the one i use, i will immediately adopt-it. Whatever its technology. Class D, or class XYZ.
Mr. Marsh, I hope you don't mean Stan Lipsh*tz. Everything he and/or his partner in crime, John V, writes is worth reading and re-reading and re ....
Maybe not immediately to the unwashed masses .. but certainly when you become a true guru in your old age 🙂
There are ALWAYS practical pearls of wisdom in their work though it might not be obvious.
What was the subject? I've found they always present practical stuff as well as the triple integrals 🙂
Last edited:
I am happy to say that now I have seen your plots I can understand your point and think it valuable.
So the probe placement comment was not a quibble intended to justify a refusal to concede.😉
But the simple VFA Miller compensation is not same as the "CFA" two transistor loop back to the input and I wanted to see if they could be more equivalent.
Also the impedance of the VFA feedback network does not match that of the "CFA" and that could easily be done.
So I would be interested to experiment with the ASC if you post it.
I look forward to it.
Best wishes
David
Last edited:
If translated , you interpreted me wrong. Those who would NOT try a CFA
might be "close minded". Conversely , CFA or D "people" who would not
try or discount other technologies are similar (CM'ed). 😀
OS
I didnt mention name of the presenter because it is not relevant to my point.
[his talk was about dither techniques and the effective noise reduction]
But, did you understand my point?
THx-RNMarsh
Last edited:
Not looking for you or anyone else to concede Dave - you offer a lot of intersting input that I and others I am sure value. I am not generally argumentative, unless dealing with someone that is obtuse and bloody minded, which you certainly are not.
🙂
Dropping the VFA feedback resistors to the same values as the CFA (47 Ohms and 470) makes virtually no difference to the plots - I tried it. The reason in my specific example is that the VFA feedback network source impedance seen by the -input is already low at 220//2.2k. In all my VFA's, I have settled on ~150 Ohms for the lower leg of the FB network. This is mainly because I run the LTP's rich at c. 5mA per side and need to think about bias current effects and offsets.
My conclusion from the slides I put up is best summarized thus:-
1. The CFA OLG and LG -3 dB BW are wider than corresponding VFA's; Further, their OLG and LG's are lower than VFA's.
2. Thus, the excess phase accumulation in CFA's is less than VFA's at HF, and correspondingly, the phase margins in CFA's greater.
3. This allows the designer to close the loop at higher frequncies than VFA's, and this can offer higher HF loop gains in CFA designs
4. If CFA's are designed with loop gain approaching those of VFA's (e.g. by incorporating high gain TIS techniques), phase accumulation is greater, and the phase margin advantage is lost. In these cases, the loop needs to be closed at a lower frequency and the additional loop gain noted above is not available.
5. Note, both systems are still linear phase.
.asc attached. Hopefully it will run with your models, otherwise I will have to dig mine out and post them up.
Last edited:
From past experience regarding your posts, in truth, Im really not interested in having any sort of dialog with you period, so expect me to answer to your posts as I would a child, ie "technobabble". Firstly you seem to know very little about some fine details involved. To hide the fact that you are inexperieced and have some defenciencies in your knowledge and may not come accross as the know it all you want people to believe you are, you revert to misleading, distorting of the subject and you even dwell as low as putting false words in peoples mouths.
Ive said it months before, I have no interest in your ramblings. Very little to none usefull knowledge is gained from them in contrast to other members who share their knowledge and experience.
Last edited:
Thanks for the ASC.
Yes that is my experience too. Just wanted to make sure the comparison was as indisputable as possible.
I am mainly interested in complementary symmetric IPS so I don't think too much about bias currents because I can trim them out.
Your choice makes sense for your circuits.
It looks to me that the benefits of the CFA can be explained simply by the better open loop performance at elevated frequency.
Of course this permits increased ULGF and/or improved stability.
That seems a simpler way to explain it, in my mental model.
Probably your conceptualisation is different, or do you think there is any other effect?
Did you mean minimum phase?
Best wishes
David
Yes that is my experience too. Just wanted to make sure the comparison was as indisputable as possible.
I am mainly interested in complementary symmetric IPS so I don't think too much about bias currents because I can trim them out.
Your choice makes sense for your circuits.
It looks to me that the benefits of the CFA can be explained simply by the better open loop performance at elevated frequency.
Of course this permits increased ULGF and/or improved stability.
That seems a simpler way to explain it, in my mental model.
Probably your conceptualisation is different, or do you think there is any other effect?
Did you mean minimum phase?
Best wishes
David
Last edited:
Stanley Lipshitz and John Vanderkooy both made exceptional contributions to the field of audio, not just in a confined area, but over a very broad range of topics. I've sat through countless presentations of theirs at AES conventions since the mid-late 70's and enjoyed every one of them, even if some of the math was over my head. Stanley is a Fellow of the AES and a recipient of the Silver Medal for his pioneering work on dither in digital audio.
Cheers,
Bob
And here is the explanation.
In Bonsai's analysis, Uncompensated OLG picture, the first pole (to the left) is determined by the VAS, while the second pole (to the right) is determined by the OPS. It appears that the CFA, when connected to a the same OPS as a VFA, renders the OPS pole in the OLG at a higher frequency. Why?
To clarify this, replace the OPS with a synthetic output stage, modeled as a VCVS. When plotting the open loop gain, you will note the standard single pole rollout, and that both amplifiers (VFA and CFA) have more or less the same GBW product. Because the CFA has lower OLG, it also has a higher open loop UGF, as expected.
Now, in the same setup with the synthetic output stage, break the global FB loop by means of a 1G inductance and also connect an AC current source at the VAS output. Run an AC analysis and plot the voltage at the VAS output as a function of frequency. If the AC current is set to 1A, the resulting VAS output voltage is numerically equal to the VAS output impedance.
What will you get? At low frequencies, the VAS output impedance for the VFA is much lower than the VAS output impedance for the CFA. Of course, the output impedance also rolls with the frequency. There is a frequency (here, around 3MHz) at which the two output impedances are equal, from there up, the CFA output impedance becomes lower than the VFA output impedance.
A lower VAS output impedance at HF, combined with a constant input capacitance of the output stage (not discussing yet the bias modulation effect in the OPS) renders a higher output stage pole. Has this anything to do with the global feedback loop? No, because the feedback loop was broken by the 1G inductor all along!
Now, if you would agree so far (I'm not hoping much from others, you seem to be one of the really unbiased member around this thread), I may go one step further and discuss exactly why the VAS output impedance behaves differently for the two topologies. What we can tell so far is that the difference is in the circuit topology, and NOT in some magic properties of the global feedback loop. High level, it has to do with the circuit topology, in particular about how the VAS Cob affects the output impedance of the VAS, in the two cases/topologies. We also already know (and nobody ever debated) that the diamond buffer input stage allows a higher bandwidth compared to the LTP.
.the "phase accumulation" from high gain at low frequency is actually rather weak when you understand the weighting factors
the Bode integral of feedback sums log(gain) over linear frequency
40 dB of "extra" loop gain DC to 20 kHz only "costs" as much as 1 dB of gain over a 1 MHz range
of course there is the practical added cost of the sloping gain over the necessary roll off frequency range - which shows the value of higher order compensation
another way to look at it is the phase from the zero of the open loop gain (the hf pole, veiwed from the right, as a zero cancelling the 90 degree integrating gain) - in the "high bandwidth" CFA example this is < 100 kHz - so the zero is adding < 5.7 degrees to the CFA "phase advantage" by 1 MHz
so the low frequency gain "Excess" of VFA is not really the 1st most important determining factor for stability at loop gain intercepts > 1MHz
the bigger cost in phase shift are excess poles within < 1 decade of frequency
the output Q are usually limiting
for the VFA/CFA difference the places to look are diff pair "added" Q, mirror, tail and VAS for the poles causing the loop gain droop beyond 5 MHz
properly applied small signal Q can easily have ft > 100 MHz, so "extra transistors in the path" aren't per se a big concern with ~ 10 MHz ops Q
the Bode integral of feedback sums log(gain) over linear frequency
40 dB of "extra" loop gain DC to 20 kHz only "costs" as much as 1 dB of gain over a 1 MHz range
of course there is the practical added cost of the sloping gain over the necessary roll off frequency range - which shows the value of higher order compensation
another way to look at it is the phase from the zero of the open loop gain (the hf pole, veiwed from the right, as a zero cancelling the 90 degree integrating gain) - in the "high bandwidth" CFA example this is < 100 kHz - so the zero is adding < 5.7 degrees to the CFA "phase advantage" by 1 MHz
so the low frequency gain "Excess" of VFA is not really the 1st most important determining factor for stability at loop gain intercepts > 1MHz
the bigger cost in phase shift are excess poles within < 1 decade of frequency
the output Q are usually limiting
for the VFA/CFA difference the places to look are diff pair "added" Q, mirror, tail and VAS for the poles causing the loop gain droop beyond 5 MHz
properly applied small signal Q can easily have ft > 100 MHz, so "extra transistors in the path" aren't per se a big concern with ~ 10 MHz ops Q
Last edited: