Distortion in stereo LM3886 amp

[umut1001]

hi..i made a stereo lm3886..it had no distorsion when not connected in the past ..but we moved another house and it has distorsion now..why?

[Nordic]

UMUT, open a seperate thread so the members can help you solve your problem... explain, the type of distortion or noise etc... I'm sure more questions will follow to narrow it down.

[gootee]

Originally posted by umut1001
hi..i made a stereo lm3886..it had no distorsion when not connected in the past ..but we moved another house and it has distorsion now..why?

Hi umut1001,

(This should be in a separate thread.)

If you don't already have RF (Radio Frequency) filters on the inputs, you should probably try that, first.

Without seeing your schematic, it's difficult to suggest component values. But there are threads that you can find, here, where it has been discussed.

For a non-inverting amplifier channel, you can place a small capacitor to ground, just before the + input pin of the chipamp, to form a low-pass filter with the upstream resistance. Use the smallest capacitor value that solves the problem. The filter's -3dB "cutoff" frequency will be f= 1/(2*Pi*R*C). So, just as an example, if your resistance ahead of the + input was 10K, then to get a -3dB low-pass cutoff frequency of, say, f=200kHz you would need a capacitance of C = 1/(2*pi*R*f) = 1/(2 * 3.14 * 10000 * 200000) = 79.6 X 10e-12 = 79.6 pF, and you could use a standard 82pF ceramic capacitor. To get a higher cutoff frequency with a given upstream resistance, you would use a smaller capacitance value. And a higher resistance upstream requires a lower capacitance value, for a given cutoff frequency, and vice-versa.

At the negative input of a chipamp or opamp (such as in an inverting amplifier configuration), connecting a capacitance directly from the input to ground usually causes oscillation. So, for the neg input in an inverting amp, you can split the existing upstream resistance into two series resistances, each with 1/2 the original value, and put the capacitor to ground from in-between them. In that case, you would use only the R portion ahead of the cap, in the equations above.

You could also, or instead, try placing a small capacitor (maybe up to 220 pF or so) directly across the + and - input pins of the chipamp, possibly with a small resistor in series with the cap.

Good luck.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html