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Old 27th May 2012, 12:18 PM   #1
eimis is offline eimis  Australia
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Thumbs up [Solved] How to lower the gain of this headphone amp

Hi.
Currently with these values the gain of this amp is 11:
Click the image to open in full size.

The problem is, I've no idea how to change the gain to, for example, 2? Which resistors to what value should I change..? I hope someone understands this better than I do.

Thank you.

Last edited by eimis; 27th May 2012 at 02:08 PM. Reason: Problem was solved.
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Old 27th May 2012, 12:48 PM   #2
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Hi,

To reduce gain, R3 has to be decreased. With value equal to R2 (10k) you'll have the gain of 2.

To maintain stability, a small (eg. 100pF) cap can be connected between inverting (-) input and output of the OpAmp.

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Old 27th May 2012, 12:54 PM   #3
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Thanks!
So the formula is R3 / R2 + 1.

As I understood, it's better to change R3 and always leave R2 at its original value?

Thanks again!
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Old 27th May 2012, 01:17 PM   #4
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In principle R1 should be equal to R3 to get lower output DC offset
wich i think is even more important parameter with a phone amp
than with a speakers amp.
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Old 27th May 2012, 01:59 PM   #5
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Quote:
Originally Posted by eimis View Post
Thanks!
So the formula is R3 / R2 + 1.

As I understood, it's better to change R3 and always leave R2 at its original value?

Thanks again!
For lower noise, you can change both R2 and R3 to 1k. If you do this, you should increase C2 to something really big like 470uF.

Last edited by dirkwright; 27th May 2012 at 02:03 PM.
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Old 27th May 2012, 02:02 PM   #6
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Quote:
Originally Posted by wahab View Post
In principle R1 should be equal to R3 to get lower output DC offset
wich i think is even more important parameter with a phone amp
than with a speakers amp.
R1 = 220k is really huge.

Is this a circuit board you got from China?

Depending on which op amp you use, you may not need the input coupling cap. If not, then I'd replace the coupling cap with a 1k resistor, and replace R1 with 220pF capacitor. This will form a first order filter to keep RFI out of the op amp.
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Old 27th May 2012, 02:07 PM   #7
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Thank you gurus. I set the gain to 1.91 and am very happy.
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Old 27th May 2012, 02:08 PM   #8
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Since the input impedance is determined largely by VR1 here, you can easily decrease R1 to something like 50k. Then make R3=50k as well for low offset, and adjust R2 for the gain you want.
The noise is determined by the parallel value of R2//R3 and since R3 is largely limited to whatever R1 you use, those are your play limits.
The small cap across R3 is also a good idea.

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Old 27th May 2012, 02:19 PM   #9
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Quote:
Originally Posted by dirkwright View Post
Is this a circuit board you got from China?
It's a DIY clone of a popular headphone amp that I've already built. It had some serious hum issues at first but I solved them by putting capacitors between ground and +/- rails. I also put a cap between ground and smoothing capacitor plate. That shut up all hum. I've no idea what happened there because I'm not an engineer but I'm happy it solved the issue. At the moment the amp has original values, except for R3 which is 9.1K and the amp sounds absolutely fine.
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Old 30th May 2012, 02:12 PM   #10
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Quote:
Originally Posted by janneman View Post
Since the input impedance is determined largely by VR1 here, you can easily decrease R1 to something like 50k. Then make R3=50k as well for low offset, and adjust R2 for the gain you want.
The noise is determined by the parallel value of R2//R3 and since R3 is largely limited to whatever R1 you use, those are your play limits.
The small cap across R3 is also a good idea.

jan didden
The amp uses NE5532. Are these 50k values fine with it?
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