Compensation in balanced VAS

GEirin · 25th July 2010, 10:45 PM

Hi all.

Problem:
In this circuit, C8 gets the benefit of the Miller Effect because there's voltage swing at the collector of Q5, while C7 does not show the same effect because there was very little voltage swing at the collector of Q4.
Which is the best solution? I have seen different solutions. What is the most effective compensation and best sound?

GEirin · 25th July 2010, 10:46 PM

Different circuits
Kenwood KA89:
- Q13 witch Miller cap C9.
- Q9 cascode transistor, witch collector via R37 to ground. Miller cap C29 in Q9-Q11.
- Phase lead C31.

GEirin · 25th July 2010, 10:47 PM

DenonPOA 2400:
- TR513 witch miller cap C529.
- TR 511 without miller cap.
- TR509 cascode transistor witch capacitor C521 in collector- emisor and collector to ground.
- Phase lead C511.

GEirin · 25th July 2010, 10:48 PM

Mongrel (ostripper)
- Q3 witch miller cap C4
- Q11 cascode transistor witch collector to ground. Miller cap C3 in Q11-Q10.

GEirin · 25th July 2010, 10:49 PM

Supersym (ostripper)
- Q5 witch miller cap C8.
- Q4 witch miller cap C7
- No cascode transistor replace it R48//C6.

GEirin · 25th July 2010, 10:50 PM

Denon PMA 2000
- Q6 witch miller cap
- Q5 without miller cap
- No cascode transistor replace it single R
- R//C Gross compensation.

GEirin · 25th July 2010, 10:52 PM

Any comments are welcome.
Thanks
Guillermo

Bonsai · 26th July 2010, 12:48 AM

Well, if the voltage swing between two nodes is limited, as is the case in a number of the circuits above, there is no point in trying to use a miller cap across those two nodes - you acheive nothing.

For the balanced VAS, I've seen a few examples where only one cap is use from the actual VAS output, back to the associated LTP collector.

Another option here is to forget about miller comp and try some other approaches:-

Cordell Audio: Papers: A MOSFET Power Amplifier with Error Correction

ostripper · 26th July 2010, 03:12 AM

Quote:

Originally Posted by GEirin

Any comments are welcome.
Thanks
Guillermo

Here is the new one. This works the best for the mongrel. The Kenwood / denons are built around older devices. What compensations are used depends on what is driving it (the VAS) and what it is driving (MOSFETs ,EF2 or TRIPLE OPS).

I have seen 20 other techniques (including Cordells).

All of them work , BUT some sound different , are more stable (VERY device dependant) , and some are just CRAZY , like the shunt comp. on the symasym or the Denon 2000 (they sound the best .. ironically).
OS

Bonsai · 26th July 2010, 06:28 AM

ostripper, I think your 5p cap above is going back to the inverting input of the amp, right?

26th July 2010, 12:48 AM	#8
Bonsai diyAudio Member Join Date: Aug 2006 Location: Taipei, Taiwan	Well, if the voltage swing between two nodes is limited, as is the case in a number of the circuits above, there is no point in trying to use a miller cap across those two nodes - you acheive nothing. For the balanced VAS, I've seen a few examples where only one cap is use from the actual VAS output, back to the associated LTP collector. Another option here is to forget about miller comp and try some other approaches:- Cordell Audio: Papers: A MOSFET Power Amplifier with Error Correction __________________ bonsai http://www.hifisonix.com/

26th July 2010, 06:28 AM	#10
Bonsai diyAudio Member Join Date: Aug 2006 Location: Taipei, Taiwan	ostripper, I think your 5p cap above is going back to the inverting input of the amp, right? __________________ bonsai http://www.hifisonix.com/

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25th July 2010, 10:52 PM	#7
GEirin diyAudio Member Join Date: Jun 2004 Location: Mendoza	Any comments are welcome. Thanks Guillermo

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