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Old 13th October 2014, 05:33 PM   #1
KieranC is offline KieranC  United Kingdom
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Default LM3886 based amp too loud

So I built an amp using a kit from chipamp.com about 6 years ago and it's been working nicely ever since. The main problem was that it was always too loud, I kind of got round the problem by putting a volume pot on the input but it doesn't seem like a great solution. The amp is connected primarily to a raspberry pi and I recently upgraded to a hifiberry DAC, which sounds great but again, way too loud, more so than other devices. I believe it's output is 2v peak to peak, I think that's why it's worse than before.

So, I thought I'd swap R3/Ri for a higher rated resistor to reduce the gain. I started with some 1k trim pots but even wound all the way up, the volume was still too high. I just replaced them with some 1.5k resistors and it's still not much better, so I guess I'm missing something vital. According to everything I can find, Gain =(Ri+Rf)/Ri. Rf is 22k, with the default 680 ohm Ri my gain should have been 33db and with a 1.5k resistor it should be ~15db but it really doesn't seem any quieter.

As it stands, I have to have the volume pot turned pretty much to zero and the output volume from the pi set under 30% or i get nasty noises during loud bits which I assume is the amp clipping.

So, what have I missed? Should reducing the gain not reduce the output volume? Can the lm3886 deal with a 2v peak to peak input? Am I doing something really stupid? (Probably...)
Thanks!

edit: I think (pretty sure) I'm using an 18V+18V 160VA transformer

Last edited by KieranC; 13th October 2014 at 05:46 PM.
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Old 13th October 2014, 07:19 PM   #2
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An input pot should work just fine. What value are you using? I'm building one now and using an 100k pot which is probably over kill.
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Old 13th October 2014, 07:50 PM   #3
KieranC is offline KieranC  United Kingdom
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It's a 50k pot but I'm always working in the bottom 10% of its range, which seems foolish. I'd rather adjust the amp so I can have my output volume fairly high and use the full range rather than limiting the output and having 90% of the range useless.

From a more practical point of view, I can control the output volume from a remote whereas I have to get up to move the pot, which I have to do fairly often since I'm working in a tiny range. I'd rather eliminate the pot and control the volume from the remote, but I can't as the output is too high. Am I right to think that reducing the gain should achieve this? I know the minimum stable gain is 10db, I might try that tomorrow but I get the impression it won't be enough...

edit: i was also under the impression that a 3db drop should half the effective volume, so a 15db drop should be massive, but it doesn't seem to have been. I might try the 680 ohm resistor on one channel and 2k2 on the other to see how they compare...

Last edited by KieranC; 13th October 2014 at 07:57 PM.
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Old 13th October 2014, 08:07 PM   #4
Mihkus is offline Mihkus  Estonia
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You can reduce gain by changing feedback resistor value... It would also decrease noise.
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Old 13th October 2014, 08:09 PM   #5
KieranC is offline KieranC  United Kingdom
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Quote:
Originally Posted by Mihkus View Post
You can reduce gain by changing feedback resistor value... It would also decrease noise.
Rf? currently 22k, presumably I still can't go below 10db gain?
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Old 13th October 2014, 08:09 PM   #6
tomchr is offline tomchr  United States
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A 1 dB change is the auditory change threshold, i.e. the amount of change that's just barely noticeable. A 3 dB change is a 2x change in power, but only a slight (but noticeable) change in perceived volume. You need a 10 dB change (10x) power for something to be perceived as twice/half as loud.

I suggest changing the gain of the LM3886 circuit. The LM3886 is stable down to a gain of 10 V/V. See attached (ripped from the data sheet).

~Tom
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Old 13th October 2014, 08:10 PM   #7
mjf is offline mjf  Austria
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perhaps a part error (wrong part built in, e.g. Rf).........
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Old 15th October 2014, 09:14 AM   #8
KieranC is offline KieranC  United Kingdom
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I'll swap R3 for 2k2 resistors this afternoon and see if it helps, if dropping the gain from 33->15db hasn't achieved much, I don't see that 15->10db is going to work miracles but I'll give it a go.
The rest of the parts are right, the amp's been working very well for 6 years and works fine with other sources, my phone for example, it's just the new soundcard which is tricky.

Would adjusting the value of R1/Rin to increase my input impedance be any use? I don't want to just go swapping random components but I'm struggling to find what exactly I should be doing :/
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Old 15th October 2014, 11:39 AM   #9
DUG is offline DUG  Canada
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Not sure how you are getting your dB numbers:

22K and 680R give a voltage gain of 33.35 or 30.46dB

22K and 1K5 give a voltage gain of 15.67 or 23.9dB

22K and 2K2 give a voltage gain of 11 or 20.83dB

22000
680
33.35
30.46

22000
1500
15.67
23.90

22000
2200
11
20.83
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Old 15th October 2014, 11:50 AM   #10
KieranC is offline KieranC  United Kingdom
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I thought the gain was in dB, if that gives you an idea of my level of knowledge...
My point stands though, my problem exists at a gain of 33 or 23, i 'm pretty sure it will happen at a gain of 20.
Is 2v peak to peak really unusual? Should the lm3886 be able to handle it?
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