Class-A Opamps - Page 3

jacco vermeulen · 5th April 2005, 11:15 PM

Quote:

Originally posted by joensd
what you do is bias the output stage so probably 2?

imo, there is very little use in running all other stages biased if the last is not, Jens.
I do not like the sound of any (mos)fet in AB mode.
I noticed a more substantial difference with opamps that have fet stages, that was my point.

amplifierguru · 6th April 2005, 02:49 AM

Kanwar,

Sure you're not exceeding the 2N5551's rating on 18V supplies?

BTW what is the 1K and 20K THD of your Workhorse amp?

B.VDBOS · 6th April 2005, 06:54 AM

What about this

amplifierguru · 6th April 2005, 07:32 AM

Yes what is it? It has serious PSRR issues, capacitors in the signal path and no gain.

No gain lots of pain.

B.VDBOS · 7th April 2005, 04:54 AM

Quote:

Originally posted by amplifierguru
Yes what is it? It has serious PSRR issues, capacitors in the signal path and no gain.

No gain lots of pain.

The idea was to have one opamp with +ve offset and the other to have equal but opposite - ve offset (Im not sure if this is the correct way to do this) so current flows between the two opamps forcing them in to class A

Gain is easy, just add resistors

Capacitors could dubble as input caps which are a nessercery evil anyway, thats if you think capacitors are bad in the first place

Anyway just some brian food

theChris · 7th April 2005, 04:57 AM

possibly missing some form of current sharing resistance? as it stands thats probably going to make real opamps very unhappy!

B.VDBOS · 7th April 2005, 05:15 AM

Quote:

Originally posted by theChris
possibly missing some form of current sharing resistance? as it stands thats probably going to make real opamps very unhappy!

The idea is to have current flowing between the opamps on purpose so current sharing resistors may not be nessercery?

My thinking is comming from over biossed AB amps where one transistor is sorcing current and the other is sinking current at the same time
In this case opamps take the place of the transistors and having the bois at the input each opamp would force class A all the way though (again just a thought)

amplifierguru · 7th April 2005, 05:17 AM

BVDBOS,

I see what you're trying to do. The problem is most OPamps have poorer PSRR on the miller comp side - or one side anyway and better NPN side stability so the tendency is to favour the upper half and load it down to the negative rail. Even better to the unreg negative supply!

Having said that the best way is with a distortion meter and residual showing for each different one. That's the way to optimise.

I have to agree with jacco that FET input chips is the more noticeable improvement . Both is excellent. I modified some
Meridian DACs this way with good results. Those damn 4 layer boards.

B.VDBOS · 7th April 2005, 05:30 AM

Quote:

Originally posted by amplifierguru
BVDBOS,

I see what you're trying to do. The problem is most OPamps have poorer PSRR on the miller comp side - or one side anyway and better NPN side stability so the tendency is to favour the upper half and load it down to the negative rail. Even better to the unreg negative supply!

Maby the scamatic I posted would be better for those who want to play with class A power opamps like LMxxxx ect.

It (class A) would add to the majic

amplifierguru · 7th April 2005, 07:29 AM

Thats a cute little drawing package . What is it?

7th April 2005, 04:57 AM	#26
theChris diyAudio Member Join Date: Dec 2001 Location: SIUE, Illinois, USA	possibly missing some form of current sharing resistance? as it stands thats probably going to make real opamps very unhappy! __________________ if only it could be used for good, not evil...

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6th April 2005, 02:49 AM	#22
amplifierguru Account Disabled Join Date: Aug 2004 Location: Earth	Kanwar, Sure you're not exceeding the 2N5551's rating on 18V supplies? BTW what is the 1K and 20K THD of your Workhorse amp?

6th April 2005, 07:32 AM	#24
amplifierguru Account Disabled Join Date: Aug 2004 Location: Earth	Yes what is it? It has serious PSRR issues, capacitors in the signal path and no gain. No gain lots of pain.

7th April 2005, 05:17 AM	#28
amplifierguru Account Disabled Join Date: Aug 2004 Location: Earth	BVDBOS, I see what you're trying to do. The problem is most OPamps have poorer PSRR on the miller comp side - or one side anyway and better NPN side stability so the tendency is to favour the upper half and load it down to the negative rail. Even better to the unreg negative supply! Having said that the best way is with a distortion meter and residual showing for each different one. That's the way to optimise. I have to agree with jacco that FET input chips is the more noticeable improvement . Both is excellent. I modified some Meridian DACs this way with good results. Those damn 4 layer boards.

7th April 2005, 07:29 AM	#30
amplifierguru Account Disabled Join Date: Aug 2004 Location: Earth	Thats a cute little drawing package . What is it?

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