Bass-Boost combined with DC-servo problem - diyAudio
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Old 30th September 2013, 10:20 AM   #1
Roushon is offline Roushon  India
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Default Bass-Boost combined with DC-servo problem

Hi,
I built a simple bass-boost circuit (using OPA627) sometime back based on
Linkwitz page for my LM3886 amp and open baffle speaker. That solved the
problem to a certain extent. Recently, I wanted to experiment with AD8620
the same bass-boost circuit. As 1/2 of the opamp was idle I converted it into
a dc-servo circuit. There is another reason to it: I wanted to remove the
capacitor on the signal line in the cd-player.

The circuit is attached.

The problem: The OPA627 bass-boost was alright. Now the AD8620
based bass-boost together with the dc-servo gives a strange booming sound
even through the open baffle. It is hurting my ears. Also the amp is heating
up more than it used to. Where could be the problem?

Thanks in advance
Roushon.
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Last edited by Roushon; 30th September 2013 at 10:43 AM.
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Old 30th September 2013, 11:07 AM   #2
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The amp heating more makes me think that you may have an oscillation issue. Power supply bypassing issues could cause a low frequency oscillation sometimes called "motorboating." Does the booming sound like an old outboard motor putting along?

The AD8620 is a bit faster than the OPA627, requiring even more attention to power supply bypassing. At the speed of the AD8620 you probably want to use 100nf SMD caps right at the rail pins directly to the ground plane. A 100nf cap rail to rail may help, if you can't make a new board.
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Old 30th September 2013, 11:18 AM   #3
FoMoCo is offline FoMoCo  United States
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It looks like your DC servo has WAY too much gain. I'd make R10 closer to 100K. Remember that you really only have to servo out millivolts for a DC servo. Because you have two poles in the feedback loop, coupled with more gain, you're probably pretty close to oscillation. It might not hurt to put a resistor in series with C8 and C9 to add a zero to the loop. Maybe 1 to 10K? Regardless, I strongly suggest trying a 100K for R10 as your first step.

Edit: Bob's bypassing suggestions are great.
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Last edited by FoMoCo; 30th September 2013 at 11:20 AM.
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Old 30th September 2013, 11:50 AM   #4
Roushon is offline Roushon  India
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Thanks BobEllis and FoMoCo for your replies.

The sound is not motorboating, but smooth. There are 330uF+0.1uF within 1cm of the power pin of the IC. I will put another 0.1 on the pin and see.

I had a suspicion that the dc servo is contributing to the bass-boost or oscillation. Will make the change.

Regards
Roushon.
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Old 1st October 2013, 12:56 AM   #5
Roushon is offline Roushon  India
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Before I get down to implementing the suggestions, I am curious if size of the 1uF caps matter? The ones I used are pretty big 250v type, that made them
connected a little far from the IC pins.

Also I made a mistake in by previous post, the 330uF+0.1uF caps are about 2 cm away from the pins.

Roushon
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Old 1st October 2013, 01:11 PM   #6
FoMoCo is offline FoMoCo  United States
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Quote:
Originally Posted by Roushon View Post
Before I get down to implementing the suggestions, I am curious if size of the 1uF caps matter? The ones I used are pretty big 250v type, that made them
connected a little far from the IC pins.

Also I made a mistake in by previous post, the 330uF+0.1uF caps are about 2 cm away from the pins.

Roushon
At that frequency, it probably doesn't matter that they are 2cm away. But, it wouldn't hurt to put the 0.1uF right on the power pins.

I'm still thinking it has more to do with loop gain (see my previous post), but it doesn't hurt to rule out bypassing before doing anything else.

Stupid question: Why not ditch the DC servo? You really shouldn't need it. An input capacitor isn't as evil as some make it out to be.
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Old 1st October 2013, 01:27 PM   #7
Roushon is offline Roushon  India
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Thanks again. Actually I have heard myself the effect of
capacitor on the signal path, that is the reason.

Regards
Roushon.
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Old 2nd October 2013, 12:23 PM   #8
Roushon is offline Roushon  India
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Replaced R10 by 100K and added 0.1uF+100uF(Nichicon KZ) directly on the power pins and the booming seems to have gone. Will check with some more music...
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Old 2nd October 2013, 03:53 PM   #9
Roushon is offline Roushon  India
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Default Problem solved!

Thanks again a lot. The problem is solved. No booming. Smooth raised bass....

Regards
Roushon.
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Old 28th October 2013, 11:49 AM   #10
Roushon is offline Roushon  India
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Huh! It needed a (minor) accident to realize that the DC-servo in fact not working after I changed R10 to 100K. Should have had checked. Now I have to start reducing the value of R10 till the point it works.

Roushon.
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