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Old 22nd December 2008, 10:57 PM   #161
andy_c is offline andy_c  United States
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Quote:
Originally posted by MJL21193
I'm feeling postpartum depression here or some such. An anticlimax that has me wondering what I'll do next.
Hi John,

Ever consider trying Scott Wurcer's distortion reduction technique from the AD797 datasheet on this thing? I don't think anybody here has done that with a power amp. Might be interesting to just look at in the simulator. Not sure what effect it has on stability.
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Old 22nd December 2008, 11:18 PM   #162
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Quote:
Originally posted by andy_c


Hi John,

Ever consider trying Scott Wurcer's distortion reduction technique from the AD797 datasheet on this thing? I don't think anybody here has done that with a power amp. Might be interesting to just look at in the simulator. Not sure what effect it has on stability.

Interesting. I hadn't known of this but I just gave it a try. The sim shows a THD drop - from 0.001 to 0.0007 so that's not hurtin'.
Stability seems to be unaffected in the sim.
Worth trying on the actual amp (not that I'm likely to hear the 0.0003 % THD drop) to see if it does alter stability.

Thanks Andy.
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Old 22nd December 2008, 11:23 PM   #163
andy_c is offline andy_c  United States
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Quote:
Originally posted by MJL21193
Interesting. I hadn't known of this but I just gave it a try. The sim shows a THD drop - from 0.001 to 0.0007 so that's not hurtin'.
Stability seems to be unaffected in the sim.
Worth trying on the actual amp (not that I'm likely to hear the 0.0003 % THD drop) to see if it does alter stability.
Not much of a drop - LOL! Just to check, you did use 30p and not 15p for the extra cap, right? (sum of compensation caps).
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Old 22nd December 2008, 11:49 PM   #164
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Quote:
Originally posted by andy_c


Not much of a drop - LOL! Just to check, you did use 30p and not 15p for the extra cap, right? (sum of compensation caps).

Ah, no. I used 15p. I just tried 30p and the drop is an extra 0.0001% but there is a slight overshoot evident, especially on the bottom half of the squarewave (in simulation).
I'll need to try it on the actual amp to see the effect. Back in a bit...
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Old 22nd December 2008, 11:53 PM   #165
andy_c is offline andy_c  United States
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Hmm, doesn't seem worth it. I was just curious to see how this technique might work in a power amp.
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Old 23rd December 2008, 12:10 AM   #166
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Quote:
Originally posted by andy_c
Hmm, doesn't seem worth it. I was just curious to see how this technique might work in a power amp.
Hi Andy,
For the (simulated) reduction in THD, no probably not. I just tried 15p first, then 30p - no effect on stability. No overshoot.
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Old 23rd December 2008, 12:31 AM   #167
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Quote:
I'm feeling postpartum depression here or some such. An anticlimax that has me wondering what I'll do next. I'll recover, maybe
Do an abomination 2 ,or 2b... I am porting the Krill right now,
Mr. dunlap told me to.. It is fast..

Fet's would be the "cats meow" for a fast VAS..
I'm going to aquire some for "playing"around.
OS
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Old 24th December 2008, 03:30 AM   #168
KLe is offline KLe  Australia
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Merry Christmas and well done, John
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Old 29th December 2008, 10:37 AM   #169
AndrewT is online now AndrewT  Scotland
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Quote:
Originally posted by MJL21193



Working with both the simulation and the actual amp, i haven't had any luck removing C4. Stability seems fine, but the DC offset is a real problem. Simulator shows 3.5VDC and the actual amp shows 2.1VDC or so offset. My attempts to bring this down in simulation have ended with worse overall performance.
A servo circuit would be the answer, but I'm not ready for such a complication right now.
[/IMG]
if you remove C4 then the inverting input sees 22k//680r.
The non-inverting input sees 22k.
The input offset current generates a voltage difference due to the difference in source resistances, thus the output offset.
Remove C6 and the non-inverting input sees 22k//2k2. Not quite matching but much closer. Short out both DC blocking caps and measure the output offset. Adjust the offset by changing that 2k2 down towards 680r, or maybe lower.

I keep repeating myself;
Either build a DC coupled amplifier with all the protections required to prevent speaker damage
or
build an AC coupled amplifier.

Do not try to build a mixed AC & DC coupled amplifier.
And if you do build a DC coupled amplifier then the source must not have a DC blocking cap on it's output or the servo will have to offset hard to correct for the input resistance error.
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regards Andrew T.
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Old 29th December 2008, 10:26 PM   #170
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Quote:
Originally posted by AndrewT


I keep repeating myself;
Either build a DC coupled amplifier with all the protections required to prevent speaker damage
or
build an AC coupled amplifier.

Do not try to build a mixed AC & DC coupled amplifier.

Hi Andrew,
It was never my intention to build a DC coupled amp. I have no interest (personally) in such an animal. It was a request by KLe that I try the amp without C4. I gave it a little attention but in the end gave up. One needs to be properly motivated to make these changes and I am not.
The mixed idea you may have is from my first posted schematic where I hadn't attached the input blocking cap yet. The correct and fully up to date schematic is in post 141.
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