Why does my headphone amp distort?

insta · 14th November 2009, 04:56 PM

I made a headphone amp with two LM386 chips (one per channel) and a TL082 preamp. I know none of these pieces are that great, but they were what I had on hand and I wanted a basic "headphone get louder" device. When I first power on the amp, it sounds great*, but after a few minutes of listening with it, there is noticeable distortion that slowly creeps in. None of the components get warm to the touch. Schematic and board (made via toner transfer) are posted below. What did I do wrong?

* good enough for my ears

star882 · 14th November 2009, 05:31 PM

The LM386 simply isn't a good chip. Use something more modern.

insta · 14th November 2009, 05:33 PM

Argh. What part of the LM386 causes distortion to set in over time? Did you even look at the attachments?

MJL21193 · 05:47 PM

Where did you get the schematic from? How much gain does it have?
Looking at the spec sheet, the LM386 has internally set gain of 20, without using external parts. I think that a gain of 20 would be more than adequate.
Remove C5,C6 and R3 (C10,C11,R6 other channel) and try again.

Mooly · 14th November 2009, 05:52 PM

Bit hard to make out on the circuit. Is the non inverting input of the LM386 floating... it should have a resistor to ground such as the volume control.

Mooly · 14th November 2009, 05:54 PM

Your PCB shows it floating... try a 10k to ground.

insta · 14th November 2009, 06:06 PM

Quote:

Originally Posted by MJL21193

Where did you get the schematic from? How much gain does it have?
Looking at the spec sheet, the LM386 has internally set gain of 20, without using external parts. I think that a gain of 20 would be more than adequate.
Remove C5,C6 and R3 (C10,C11,R6 other channel) and try again.

I made it. I'll try removing the resistors R3 and R6, which should nerf the feedback. C5 and C10 are bypass caps -- see an explanation on them.

As for the pins being ungrounded, that's a side effect of the ground plane not being drawn. To see the schematic better, click to expand it, then right-click and go to "View Image" (in FireFox).

mjf · 14th November 2009, 06:07 PM

hello.
have a look at the orientation of the input elco's (c3,c8).
what voltreg do you use?
perhaps better to read/look at...........

insta · 06:14 PM

Some more clear images, with the ground plane rendered.

insta · 14th November 2009, 06:11 PM

Quote:

Originally Posted by mjf

hello.
have a look at the orientation of the input elco's (c3,c8).
what voltreg do you use?
perhaps better to read/look at...........

The input electrolytics are the only ones I'm using that are unmarked as far as polarity. They're either AC ones, or backwards

I'm using a 7808 regulator.

14th November 2009, 05:31 PM	#2
star882 diyAudio Member Join Date: Mar 2007	The LM386 simply isn't a good chip. Use something more modern. __________________ "Fully on MOSFET = closed switch, Fully off MOSFET = open switch, Half on MOSFET = poor imitation of Tiffany Yep." - also applies to IGBTs!

14th November 2009, 05:41 PM	#4
MJL21193 Account disabled at member's request Join Date: Mar 2007	Where did you get the schematic from? How much gain does it have? Looking at the spec sheet, the LM386 has internally set gain of 20, without using external parts. I think that a gain of 20 would be more than adequate. Remove C5,C6 and R3 (C10,C11,R6 other channel) and try again. Last edited by MJL21193; 14th November 2009 at 05:47 PM.

14th November 2009, 06:10 PM	#9
insta diyAudio Member Join Date: Apr 2009	Some more clear images, with the ground plane rendered. Last edited by insta; 14th November 2009 at 06:14 PM. Reason: Imageshack is dumb

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14th November 2009, 05:33 PM	#3
insta diyAudio Member Join Date: Apr 2009	Argh. What part of the LM386 causes distortion to set in over time? Did you even look at the attachments?

14th November 2009, 05:52 PM	#5
Mooly diyAudio Member Join Date: Sep 2007	Bit hard to make out on the circuit. Is the non inverting input of the LM386 floating... it should have a resistor to ground such as the volume control.

14th November 2009, 05:54 PM	#6
Mooly diyAudio Member Join Date: Sep 2007	Your PCB shows it floating... try a 10k to ground.

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