Bob Cordell Interview: Negative Feedback - Page 51 - diyAudio
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Old 17th March 2007, 12:43 AM   #501
andy_c is offline andy_c  United States
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Quote:
Originally posted by mikeks
...loop transmission at 20KHz is typically 25dB~30dB, while DPC can be persuaded to deliver ~60dB at the same frequency.
Mike,

I just tried a real quick sim of this comp network and saw some distortion improvements. I made C2 and C3 each equal to 2C (don't have any idea if that's right or not BTW - it was just a quick guess). It looks promising. Puts the output stage in the local VAS feedback loop at low freqs but gets it out of the way at high freqs So it prevents the local loop instability that output-stage-inclusive compensation has (Cherry's "world's cheapest oscillator" trick ).
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Old 17th March 2007, 12:46 AM   #502
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Thanks; I'll check it out.

Have you examined major loop transmission at 20KHz for before/after improvements?
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Old 17th March 2007, 01:01 AM   #503
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Quote:
Originally posted by mikeks
Have you examined major loop transmission at 20KHz for before/after improvements?
Yes. It was actually about 4 dB less with the modified configuration above than the original (standard two-pole comp). Unity loop gain frequency was about the same. I think more improvements are possible. I want to understand this better - not just component values, but maybe the topology itself could be improved. Ideally, I'd like the major loop transmission to be unchanged, and have the output stage get out of the minor loop at maybe 1 MHz or so (not sure).

The unity loop gain freq of my VAS is scary high, so there's absolutely no way the Cherry-style output stage-inclusive comp could ever be stable.
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Old 17th March 2007, 02:10 AM   #504
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Hi, Estuart,

Your schematic in post #497, how to determine the value of C1-C2-R1? Is there any paper/article about it?
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Old 17th March 2007, 10:09 AM   #505
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Quote:
Originally posted by mikeks
This accomplishes nothing as far as linearity is concerned: loop transmission at 20KHz is typically 25dB~30dB, while DPC can be persuaded to deliver ~60dB at the same frequency.

Hi Mike

First, do me a favor and revert to a terminology we all can understand, so loop GAIN, not loop transmission.
Second, what do you mean by ‘DPC’? Never heard of it, neither do google or dogpile.
And please, don’t forget that you have already caused much of a trouble on this forum by using your own a terminology.
As far as the transfer function of the global feedback loop concerned, indeed, it’s unaltered and it should be that way. So, what’s your point? Do you suggest that because of this, the thd figures are unaltered too?
Anyhow, my sims show a thd reduction of about 15dB.

And for those who are intending to run a sim, please use a VAS, as depicted below. Don’t use a Darlington, by tying the collectors of Q2 and Q3 together. The Early effect and Cob modulation of Q2 spoils the whole thing.
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File Type: gif vas.gif (3.9 KB, 927 views)
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Old 17th March 2007, 10:40 AM   #506
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Estuart refering to a compensation scheme encompassing the output stage and needing a beta enhancement transistor in the VAS stage makes me to think of the Cherry's NDFL amplifier. The NDFL amplifier described in ETI May 1983 contains a Rush pair (a common collector PNP loading the input stage followed by a common base NPN ahead of a main voltage gain PNP).
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Old 17th March 2007, 10:42 AM   #507
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DPC : Double Pole Compensation ?
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Old 17th March 2007, 11:00 AM   #508
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Quote:
Originally posted by mikeks
Enclosing a power BJT output stage provokes local instability even with a series resistor.

Hi Mike,

Apparently, you doesn’t understand much of my compensation. At least Tim__X does. At one point you are right: Enclosing a power BJT output stage provokes local instability, that is, at fairly high frequencies, 10…20MHz. However, at frequencies that high, my circuit behaves just like a traditional Miller compensated amp and is rock stable. Maybe you should have a closer look at my diagram (it’s clear enough I think) and discover that I don’t use just a single resistor to prevent instabilities, rather an RC network. As a result, below 230kHz, determined by the time constant R1*(C1+C2), my arrangement behaves just like Cherry’s thing, above that frequency, it slowly revert to a conventional and safe scheme, as I said before.

Regards.
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Old 17th March 2007, 11:08 AM   #509
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Quote:
Originally posted by forr
DPC : Double Pole Compensation ?
Thanks forr!
Normaly, it's called TWO pole compensation, hence the confusion.
(just google it)
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Old 17th March 2007, 11:09 AM   #510
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Hi Edmond,

Sometimes NFB arrangements like the one you show can reduce THD at higher audio frequencies but increase 'first cycle' or transient distortion and actually impair output stage control of a dynamic loudspeaker.

Thus reactively tuned THD optimisation can degrade reproduction.

Cheers .......... Graham.
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