LM386 amp projects, help...

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LM386 amp projects, help...

Postby HTH » 02 Sep 2012, 16:13

Hi all, long time lurker, occasional poster - always busy (you know how it is).

Anyway, I've built two different LM386 projects over the past couple of years and both worked, though motorboated with the volume up at any sensible volume.

Adding insane amounts of filtering to the supply helps, but not by much. I've tried 9v and 18v, batteries and mains power supplies - seems to make no difference.

This is just a small little amp for listening to some tunes while I'm in the car (my car stereo is f*cked).

I've added a single-transistor preamp before the LM386 to boost the signal on my most recent version, just a BC184/3 (can't remember which, just an NPN silicon, nothing special).

Anyone else had problems building these things?

Also tried the '1' and '4' suffix LM386s, no difference (the higher suffix just handles more voltage, 20v max).

Anyone? Bueller?

:horsey:
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Re: LM386 amp projects, help...

Postby CodeMonk » 02 Sep 2012, 22:52

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Re: LM386 amp projects, help...

Postby HTH » 02 Sep 2012, 23:42

Thanks for that - I've seen various LM386 projects. It's more the fact that I get motorboating with every LM386 amp build - any ideas?

I'm not a n00b at this by any stretch, been building pedals and amps since the mid 90s, but this one is doing my head in.
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Re: LM386 amp projects, help...

Postby rocklander » 03 Sep 2012, 02:30

it's not altnerator noise is it?
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Re: LM386 amp projects, help...

Postby HTH » 03 Sep 2012, 19:42

rocklander wrote:it's not altnerator noise is it?


'what' noise? :scratch:

I can have the volume pot set very low, but as it goes up above maybe 1 it starts to oscillate and blocks out all of the audio.

I'd make a video but the kittens have killed my digital camera :cry:
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Re: LM386 amp projects, help...

Postby rocklander » 03 Sep 2012, 21:06

oh, I thought you said it was motorboating.... I read that as sounding like alternator noise... perhaps a difference in ideas as to what that term means.. sorry :oops:
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Re: LM386 amp projects, help...

Postby FiveseveN » 03 Sep 2012, 22:04

Are you using a nice fat bypass cap (the 100uF cap in the ROG Ruby project above)?
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Re: LM386 amp projects, help...

Postby HTH » 05 Sep 2012, 17:20

FiveseveN wrote:Are you using a nice fat bypass cap (the 100uF cap in the ROG Ruby project above)?


I'm using four big fat 1000uF caps - more filtering definitely helps, but it's not curing the problem.

Anyone?
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Re: LM386 amp projects, help...

Postby HTH » 06 Sep 2012, 00:07

Thanks for the suggestions guys, I found the problem (whoops), ha ha. The cap (in series with the 10ohm resistor) at the output wasn't grounded. Much better, but I still can't get even half volume on this, weird. A step in the right direction I suppose.
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Re: LM386 amp projects, help...

Postby kkregsg » 06 Sep 2012, 06:33

I had excellent results with Beavis Audio's Noisy Cricket build- no motor boating at all w/ full volume, though it is a 1/2 watt amp. Check his site out. My first one looked like a rat's nest, as it was my very first build. I've made a few more along with other pedals and amps, and lead dress has become much more civilized. I guess practice does facilitate improvement.
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Re: LM386 amp projects, help...

Postby HTH » 06 Sep 2012, 22:14

Here's the schematic I've drawn up if anyone can see cause for concern...

Image
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Re: LM386 amp projects, help...

Postby phatt » 11 Sep 2012, 12:45

My Guess,,,
Remove the 10k resistor at input.
Then connect it between the base of transistor and ground.

Still no go,, try a 100 Ohm resistor between the pwr amp preamp supply.
(supply goes to LM chip then thru a decoupling resistor and over to the Preamp.)
Also hang a 10uF cap across the new decoupled supply.
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