Lm386 audio amp troubleshoot. - Page 1
Forum section: electronics / projects

author: regalzack
author_rank: Contributor
timestamp: January 22, 2014, 02:51:20 pm
content: Hello,I have a Lm386n-4 guitar/audio amp I built based off of <http://www.runoffgroove.com/ruby.html> with the bassman mod(listed towards bottom of page) but something is strange, any time I have the volume above ~70% I get what I think may be some nasty even order harmonics or something. It sounds like scratching on the speaker cone, or a rattle in the cab.What I've tried:1. I've rebuilt the circuit two times with all new components(other than resistors) to no avail.2. Removing the input buffer circuit (mpf102).3. Increasing capacitance values4. Adding input resistance5. Changing gain values (resistance on pins 1-8)6. Different speakers7. Bypassing the potentiometerThe only thing I have not tried yet, that I can think of is the actual resistors themselves... Is there any way they could be the culprit?Thank you so much!Zachary Herberholz

author: grumpydoc
author_rank: Super Contributor
timestamp: January 22, 2014, 03:03:21 pm
content: Well, the intent is to drive the 386 into clipping since that's what causes the "nice breakup" and "natural overdrive" so it might be functioning as it was designed to do.

author: regalzack
author_rank: Contributor
timestamp: January 22, 2014, 03:30:10 pm
content: Yeah, I am pretty sure something is awry beyond the natural clipping of the chip.It sounds almost like a ghost note, totally ruins chords, but only above 60-70% volume.I sure with I had an oscilloscope--maybe with the next student loan check...

author: fcb
author_rank: Super Contributor
timestamp: January 22, 2014, 05:22:06 pm
content: LM386 runs at 10% THD at 0.5W - and it sounds nasty at this. It's not some nice tube-like 2nd harmonic.Also, check your supply rails aren't collapsing.

author: regalzack
author_rank: Contributor
timestamp: January 22, 2014, 07:17:56 pm
content: urg, so much to learn!So what do you mean by collapsing supply rails? (I apologize for my lack of knowledge)I am powering with a 9v battery.

author: fcb
author_rank: Super Contributor
timestamp: January 22, 2014, 10:46:15 pm
content: Scope your 9v rail when your playing some music. The ESR (equivalent series resistance) of a *typical* 9v battery is pretty high.

author: calexanian
author_rank: Super Contributor
timestamp: January 23, 2014, 02:09:20 am
content: The 9 volts may be failing and dropping. Thats another way of saying collapse. Also depending on how the wiring is it may be oscillating.

author: anvyo92
author_rank: Newbie
timestamp: February 18, 2016, 12:15:20 pm
content: Hey regalzack, Did you find the solution for scratching sound above 70% volume?I have also built this circuit using some tutorials (lm386 audio amplifierandinstructable), but facing the same problem, that sound get ruptured on slightly high volume. I have lower down the power source to 5v (as suggested above) and also used MPF102 as in your link, but all in vain.Please guide if you find the solution and reply in a "Beginners language".Getting a warning of "120 days old thread", but my question is exactly same, so no point creating new thread.Thanks.

author: meerweten
author_rank: Contributor
timestamp: February 18, 2016, 07:19:50 pm
content: as suggested above, it might be the voltage rail.try using an adapter of some sort that can supply more curren with a lower ESR.i'm also missing some caps on the voltage rail, there sould be atleast a 100nf and a 10µF cap on it. maybe even 100µ or more.hope it was beginners language

author: Audioguru
author_rank: Super Contributor
timestamp: February 18, 2016, 07:43:39 pm
content: I agree the 9V from a battery that might be made "over there" and maybe spent a few years on a store shelf or at ebay is probably no good. Can't you measure its voltage when the circuit is making the bad sounds?The LM386 with a low supply voltage provides a puny amount of output power before its output begins to clip like crazy (acid rock sounds?) if you try to make it louder. What do you expect?The datasheet always shows a capacitor in series with the resistor to increase the gain, to less gain than with only a capacitor between pin 1 and pin 8.

author: anvyo92
author_rank: Newbie
timestamp: February 19, 2016, 12:47:26 pm
content: Thanks for Reply,I am now giving power from USB of my PC (smooth and high current) and also used the capacitors as suggested, it definitively reduced that crazy sound. But it still creating noises on 85% plus volume.But anyway thanksmeerwetenandAudioguru, I am still experimenting and keep posting my findings.

author: Zero999
author_rank: Super Contributor
timestamp: February 19, 2016, 01:15:43 pm
content: Quote from: anvyo92 on February 18, 2016, 12:15:20 pmThe instructable is wrong. The volume control should be on the input, not the output. The tutorial pointed to by the first link is the correct way to do it.As mentioned above, the LM386 is only a small power amplifier and will clip, if the output signal is too high and will sound horrible.If it needs to be louder, the TDA7052 is a better choice of IC.http://www.nxp.com/documents/data_sheet/TDA7052.pdf

author: GamerAndds
author_rank: Regular Contributor
timestamp: February 19, 2016, 05:30:45 pm
content: What kind of cabinet are you driving with that thing? I have a similar chip I use for my headphones and it works perfetley up to supply voltage at which point it clips- Im using 9V battery's aswell.