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Old 10th November 2009, 10:36 PM   #1
netbug is offline netbug  United States
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Default A question about the LM3886 (chipamp kit) and Gain…

I have recently added some 98db speakers to my system and have run into a hum issue. My tube preamp has a very minor hum that was not noticeable on my 85db speakers. I’d like to reduce the gain on the ChipAmp to 23 to see if it will help. Then I ran into the following post today…
Quote:
Originally Posted by AndrewT View Post
look at the datasheet again.
At a gain of 10 (+20dB) the amp is just stable.
At a gain of 12 (+21.6dB) it is slightly improved, but the phase margin is still low.
At a gain of 20(+26dB) the phase margin is getting better.
At a gain of 30 (+29.5dB) the phase margin is ~80 to 85degrees.
Many listeners report that the 3886 sounds good when the gain is around 26 to 30 times. I suspect it's due to the improved phase margin avoiding overshoot on fast transients.

Has anyone run a non-inverting lm3886 at 23db gain? What happens to the sound? In what way is the sound “less good”?

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Old 11th November 2009, 07:30 AM   #2
Dxvideo is offline Dxvideo  Turkey
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Do you mean 23dB or x23 gain?
Anyway, I have tested a 3886 amplifier with 10K/680R feedback that makes x15,71 and appx. 23,9dB. I did not hear any difference from x21 configuration.
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Old 11th November 2009, 07:32 AM   #3
ratza is offline ratza  Romania
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The gain on the chip will not help too much, try to remove the hum from the preamp. The problem with the phase margin is that the amp will have the tendency to oscillate when phase tends to decrease to 0 degrees.
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Old 11th November 2009, 03:21 PM   #4
netbug is offline netbug  United States
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Quote:
Originally Posted by Dxvideo View Post
Do you mean 23dB or x23 gain?
Anyway, I have tested a 3886 amplifier with 10K/680R feedback that makes x15,71 and appx. 23,9dB. I did not hear any difference from x21 configuration.
I was referring to 23db. The kit originally had 22k/680 Rf/R3 on the feedback. I swapped out R3 for a 1k. According to the manual gain = 1+Rf/R3 so I'm now at 23db.

Quote:
Originally Posted by ratza View Post
The gain on the chip will not help too much, try to remove the hum from the preamp. The problem with the phase margin is that the amp will have the tendency to oscillate when phase tends to decrease to 0 degrees.
I can still hear the hum when I'm within a few feet of the speaker, but it is greatly reduced with the lower gain. You are right that the best solution is to get ride of the hum. I think it may be an a/c heater issue.
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Old 11th November 2009, 03:24 PM   #5
Dxvideo is offline Dxvideo  Turkey
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Quote:
Originally Posted by netbug View Post
I was referring to 23db. The kit originally had 22k/680 Rf/R3 on the feedback. I swapped out R3 for a 1k. According to the manual gain = 1+Rf/R3 so I'm now at 23db.
No! 22/1 makes x23 gain (~27,5dB) and not 23dB. And this is a good value for NFB line.
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Old 11th November 2009, 03:25 PM   #6
AndrewT is online now AndrewT  Scotland
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Quote:
Originally Posted by netbug View Post
I was referring to 23db. The kit originally had 22k/680 Rf/R3 on the feedback. I swapped out R3 for a 1k. According to the manual gain = 1+Rf/R3 so I'm now at 23db.
gain was = 1+[22000/680]=+30.5dB
gain now = 1+[22000/1000]=+27.2dB
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Old 11th November 2009, 03:35 PM   #7
ratza is offline ratza  Romania
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Quote:
Originally Posted by netbug View Post
I think it may be an a/c heater issue.
What schematic did you use?
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Old 11th November 2009, 07:14 PM   #8
netbug is offline netbug  United States
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Quote:
Originally Posted by Dxvideo View Post
No! 22/1 makes x23 gain (~27,5dB) and not 23dB. And this is a good value for NFB line.
I'm not sure the difference. I'm just going by what it says in http://chipamp.com/docs/lm3886-manual.pdf page 14
I'm a network engineer who's midlife crisis consists of wishing he was an electrical engineer.

Quote:
Originally Posted by ratza View Post
What schematic did you use?
It’s a 6j6 based kit that I stumbled upon. Looked interesting .
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Old 11th November 2009, 08:10 PM   #9
Gopher is offline Gopher  United Kingdom
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Netbug - for info:

Voltage gain (in decibels or dB) = 20 x log G

Where in this case G = 1 + (Rf/R3)

AndrewT's figures are correct

Last edited by Gopher; 11th November 2009 at 08:16 PM.
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Old 12th November 2009, 07:33 AM   #10
Dxvideo is offline Dxvideo  Turkey
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Quote:
Originally Posted by netbug View Post
I'm not sure the difference. I'm just going by what it says in http://chipamp.com/docs/lm3886-manual.pdf page 14
I'm a network engineer who's midlife crisis consists of wishing he was an electrical engineer.
Are you repented from being a network engineer? Why?
I wish I would be a network engineer! Is circumstances different in USA?
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