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Old 20th April 2015, 02:40 AM   #1
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Default Noisy trough

I have an 8ohm speaker driven by an LM386, function generator, and lab power supply. Everything is carefully bypassed. For most frequencies when I apply a sin wav or sawtooth wav the input is clean, but the output has the lower part of the trough filled in with noise (on the oscilloscope). I can't figure out what this could be. Speaker/mechanical resonance that needs damping? Any suggestions for solving this? Thanks!
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Old 20th April 2015, 03:16 AM   #2
DUG is offline DUG  Canada
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Do you have the 10R-0.05uF network on the output, it is critical for stability.

I used this amp once for a metal detector AF amp, had stability issues without it.

See page 5 of schematic...all amps on this page have it on.

http://www.ti.com/lit/ds/symlink/lm386.pdf
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Old 20th April 2015, 03:20 AM   #3
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I don't. I can't believe I missed that. I will try it Tuesday when my lab reopens and I have an oscilloscope on hand. Thanks.

Out of interest, would you know why it is required, given that the power supply and input are already bypassed?
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Old 20th April 2015, 08:56 AM   #4
DF96 is offline DF96  England
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The Zobel network at the output keeps the feedback loop stable at ultrasonic/RF frequencies. Most solid-state amplifiers need this. Nothing at all to do with bypassing - that deals with other sources of instability.
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Old 21st April 2015, 06:10 PM   #5
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Thanks guys. That solved the noisy trough problem. Clean as a whistle now.
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Old 21st April 2015, 10:16 PM   #6
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Quote:
Originally Posted by slobberchopz View Post
I have an 8ohm speaker driven by an LM386, function generator, and lab power supply. Everything is carefully bypassed. For most frequencies when I apply a sin wav or sawtooth wav the input is clean, but the output has the lower part of the trough filled in with noise (on the oscilloscope). I can't figure out what this could be. Speaker/mechanical resonance that needs damping? Any suggestions for solving this? Thanks!
Move deep into rural area's , use incandescent lamps and have no digital
"toys" (TV , pC's ...).
really .. your power line is "dirty" from all the cheap walmart 21'st century
garbage.
added a ground to a old house mains box. the only place my scope could
NOT see noise was at the ground rod itself and at short run circuits used
for the outside outlets.

Filters , using a separate house circuit (one close to the mains box).
For the actual AC , even the utilities line will pick up a close RF source.
remember the "whole house antenna" back in 1980 - it works !


OS
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Old 22nd April 2015, 09:35 AM   #7
DF96 is offline DF96  England
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The "noise" in the trough was not noise but parasitic oscillation.
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