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-   -   ODNF or no GNFB power amp (http://www.diyaudio.com/forums/solid-state/238252-odnf-no-gnfb-power-amp.html)

dadod 25th June 2013 08:57 PM

ODNF or no GNFB power amp
 
2 Attachment(s)
This is no GNFB amp, the gain part is a current conveyor and output stage use Only Distortion Negative Feedback (ODNF).
The gain is set by the P1/R9 ratio.
Distortion was lowered at 1 kHz with ODNF from 0.036191% to 0.000323% (40dB) and at 20 kHz from 0.037325% to 0.006647% (15dB). I am not sure what is the reason for that low improvement at 20 kHz, could be the OP amp used in ODNF or I do something wrong here. Comments and suggestion welcome.

mcd99uk 25th June 2013 08:59 PM

In LTspice at least the choice of op amp can have a massive affect. Try LT1128.

dadod 25th June 2013 09:05 PM

Quote:

Originally Posted by mcd99uk (Post 3539338)
In LTspice at least the choice of op amp can have a massive affect. Try LT1128.

Simple changing from LT1056 to LT1128 I've got oscilation, so simple swap does nor work, need more simulation.

mcd99uk 25th June 2013 09:09 PM

Try putting a cap across the opamp from output to inverting input. Start at something stupidly high like 1n.

This is based on what I have found using a similar system in a global feedback amp. Remember, it is a feedback loop and may need some compensation.

dadod 25th June 2013 09:18 PM

Quote:

Originally Posted by mcd99uk (Post 3539354)
Try putting a cap across the opamp from output to inverting input. Start at something stupidly high like 1n.

This is based on what I have found using a similar system in a global feedback amp. Remember, it is a feedback loop and may need some compensation.

With 5 pF it still have small oscilation, with 7 pF is table and distortion at 20kHz drops fro 0.037325% to 0.003628% and that is 20 dB still to little.

mcd99uk 25th June 2013 09:23 PM

Try reducing P1 in value.

Also, watch out for capacitive loads on the output.

Have a read of this... this is for global feedback amps but may still be of use.

http://http://hifisonix.com/error-correction/

Struth 26th June 2013 12:54 AM

Hi Guys

Is there the opportunity to use opamps other than LT? I think their library is the problem.

Your implementation of ODNFB is interesting. Do you have a sim for just the section enclosed by ODNFB? This would let you know how much THD is from that section and how much from the input section.

Luxman runs the output stage openloop and compensates for the voltage gain stage errors (using ODNFB), so different from what you are doing.

Have fun
Kevin O'Connor

dadod 26th June 2013 05:37 AM

Quote:

Originally Posted by mcd99uk (Post 3539377)
Try reducing P1 in value.

Also, watch out for capacitive loads on the output.

Have a read of this... this is for global feedback amps but may still be of use.

http://http://hifisonix.com/error-correction/

If I reduce P1 I have to reduce R9 too as the ration P1/R9 defines the gain, and in this case distortion of the gain stage is going up.
Thanks for the link, yes I follow Bonsai nice amps designs and build.

dadod 26th June 2013 05:43 AM

Quote:

Originally Posted by Struth (Post 3539573)
Hi Guys

Is there the opportunity to use opamps other than LT? I think their library is the problem.

Your implementation of ODNFB is interesting. Do you have a sim for just the section enclosed by ODNFB? This would let you know how much THD is from that section and how much from the input section.

Luxman runs the output stage openloop and compensates for the voltage gain stage errors (using ODNFB), so different from what you are doing.

Have fun
Kevin O'Connor

Hi Struth,
Yes I simulated separately the gain stage and output stage. The gain stage has very low distortion in whole audio band, almost all distortion was generated in the output stage, that's why ODNF was used on output stage.
BR Damir

mcd99uk 26th June 2013 08:09 AM

Quote:

Originally Posted by Struth (Post 3539573)
Hi Guys

Is there the opportunity to use opamps other than LT? I think their library is the problem.

Yes, but adding other op amp models is far from easy / obvious. I'm struggling to add the lm4562 to LTspice. Can add simple models (sub circuits) eg diodes and FETs but so far op amps have eluded me.


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