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Old 25th June 2013, 08:57 PM   #1
dadod is offline dadod  Croatia
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Default ODNF or no GNFB power amp

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.
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Old 25th June 2013, 08:59 PM   #2
mcd99uk is online now mcd99uk  United Kingdom
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In LTspice at least the choice of op amp can have a massive affect. Try LT1128.
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Old 25th June 2013, 09:05 PM   #3
dadod is offline dadod  Croatia
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Quote:
Originally Posted by mcd99uk View Post
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.
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Old 25th June 2013, 09:09 PM   #4
mcd99uk is online now mcd99uk  United Kingdom
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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.
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Old 25th June 2013, 09:18 PM   #5
dadod is offline dadod  Croatia
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Quote:
Originally Posted by mcd99uk View Post
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.
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Old 25th June 2013, 09:23 PM   #6
mcd99uk is online now mcd99uk  United Kingdom
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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/
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Old 26th June 2013, 12:54 AM   #7
Struth is offline Struth  Canada
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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
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Old 26th June 2013, 05:37 AM   #8
dadod is offline dadod  Croatia
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Quote:
Originally Posted by mcd99uk View Post
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.
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Old 26th June 2013, 05:43 AM   #9
dadod is offline dadod  Croatia
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Quote:
Originally Posted by Struth View Post
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
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Old 26th June 2013, 08:09 AM   #10
mcd99uk is online now mcd99uk  United Kingdom
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Quote:
Originally Posted by Struth View Post
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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