Help with LM386 amp. Vid of the waveform incl... - Page 1
Forum section: electronics / projects

author: regalzack
author_rank: Contributor
timestamp: March 26, 2014, 04:30:00 am
content: All my new gear arrived (oscilloscope, function gen, & power supply)thanks to your many recommendations!!Now I can finally try and diagnose this strange problem.Alright so I've built this whole deal a few times tried new speakers, chips, etc. I stripped it down to bare bones on a breadboard, there is a scratching fuzzy noise that occurs at certain amplitudes, I understand what clipping is, but this seems to be introducing a new harmonic or something that--sounds like poop, even when the input signal is well below clipping.Although the scratchyness is hard to hear well in the video, it's there(only when the extra harmonic/wave is there)!Here is a spec for the amp that I am running, incredibly simple.http://www.beavisaudio.com/library/LM386/Smokey_Amp_Schematic.jpgAnd here is a video I took of the oscilloscope.Thank you all!Zachary Herberholz

author: TMM
author_rank: Frequent Contributor
timestamp: March 26, 2014, 06:15:44 am
content: Exceeding the common-mode input range perhaps? What is the V/div scaling of each channel on the video?Also try putting a capacitor between pins 1 and 8, and add a Zobel filter on the output like this (the 0.05uF cap and 10ohm resistor):http://www.hobby-hour.com/electronics/lm386-power-audio.php

author: icon
author_rank: Regular Contributor
timestamp: March 26, 2014, 08:29:01 am
content: HiI asked a similar question:https://www.eevblog.com/forum/beginners/lm386-amp-on-a-breadboard-oscillation/msg380804/#msg380804The replies helped.John

author: hamster_nz
author_rank: Super Contributor
timestamp: March 26, 2014, 09:10:34 am
content: Why are you connecting 1 + 8? According to the data sheet it should be either a 0.1u Cap or a short to ground.Are you sure that you don't need a cap on the input to uncouple it from DC?

author: fcb
author_rank: Super Contributor
timestamp: March 26, 2014, 09:35:05 am
content: Pin 1 & 8 are either end of a resistor in the LM386. Shorting these two increases the gain from 20 to 200. Using a capacitor to between 1 & 8 increases the AC gain only, and choosing the right value capacitor (to set the LF roll-off) is useful if your driving a small speaker (they tend to be crap at low frequencies).Probably worth AC coupling the input also. A zobel network tends to be necessary with these older type audio IC's also (as someone else has pointed out, 47nF+10R fine).Not sure what you are driving into this, but I would try and avoid running the LM386 at high gains. I've used it a couple of products and avoid using it now, it's quite noisy.

author: regalzack
author_rank: Contributor
timestamp: March 26, 2014, 02:51:02 pm
content: Yeah in the video, I actually have no connection between pins 1 and 8.(decreasing overall gain).I initially had a zobel, a transistor buffer input, etc. I've stripped it down as simple as possible.Wondering if it could actually be some bad capacitors.I appreciate all the advice, I should have some more time to tinker with it today.Thanks again,Zack

author: macboy
author_rank: Super Contributor
timestamp: March 26, 2014, 04:12:21 pm
content: Youneedpower supply decoupling between pin 4 and 6. Use an electrolytic of about 47 µF in parallel with a ceramic of say 0.1 µF. These should be as close as possible to the chip with short leads/traces to those pins.I strongly discourage using the 200 gain setting; this is completely unnecessary and undesirable when using a line level source; it may be useful when using a microphone as a source.

author: redwire
author_rank: Newbie
timestamp: March 26, 2014, 06:50:47 pm
content: The LM386 is a lousy IC that I kinda hate. It is prone to oscillation, on its input, Vcc and its output...On the input add a 100pF to GND to block RF.On the output, have the 10R+0.1uF across the speaker to prevent "bottom side fuzzies" (oscillation due to the lateral PNP output darlington)On power, have a 0.1uF and a large 100uF or more for filtering.Your grounding must be single point.

author: qno
author_rank: Frequent Contributor
timestamp: March 27, 2014, 07:05:45 am
content: I think your input voltage is too high.It looks like the input stage is starting to rectify the signal.Check the datasheet for max input voltage.