OT - LM386 Stereo Amp Problem/Question

[orangetones]

I was trying to fixup some old computer speakers for my wife and decided to have a go at these LM386's to build a stereo amp.  I used this layout here (that I did) and I get wicked hum on one channel, with some distorder signal and then I get no signal on the other channel.  Any thoughts on why this might be happening?

I have some part subs in here though too.  I used a 12R and a 10R, as I didn;t have 2 of the same value.  As well, I used a 220uF and a 330uF, but othe than that everything is as shown here.



This little amp is based off the sample applications from the datasheet, which are very similar to the Ruby.

I really think this should work as laid out, but maybe I am missing something here. 

[waky]

im probably wrong here, but arent pins  1 - 8 suposed to be the gain control?,  i THINK thats their only function so why do you get pin 1 connected to that 15k resistor?...  maybe thats what causing a weird effect?

[orangetones]

Here is the datasheet.  In the typical applications section you can see that running a resistor and capacitor from pin 1 ro the output will allow you to do some frequency shaping.

http://www.national.com/ds/LM/LM386.pdf

So anyone have thoughts on this? 

I breadboarded this application and it worked, but I only breadboarded half of the circuit (for one channel), not for both channels.

[waky]

    If the hum on one of the channels is a high pitched squeal, you could try shielding the input or making it shorter(i've had this problem with some builds, especially high gain circuits), also check for solder bridges etc, if you breadborded the circuit and it worked before there must be something wrong with the build, as for the stereo think... those circuits are sharing nothing but the power supply so i dont think it should be a problem.. You can also try using one battery for each channel to see if its some kind of ground loop?, are the output caps polarized?. if so are they polarized correctly?.

    By the way, whats that "bypass cap" on the schematic for?

     Keep in mind that im just starting on the whole diy stuff, so alot of my suggestions may be wrong. But it doesnt hur to try right?  icon_razz:

[orangetones]

WOrks ok with batteries, not with this 9v DC 1000mA wallwart run into a 7809 reg.

Hmm...  Filtering needed?

[waky]

ok, so we've isolated the problem xD, hmm if you didnt built it yourself its posibble that it doesnt supply 9v, most of my 9v wall wart put out like 14 v without load, or maybe the current requiriments of your amp are too hight (though i doubt it if your supply gives 1 amp =/)