Smokey Amp oscillation thingo!?!?

[THOMMO]

I threw this little circuit on my BB tonight. Checked it over a few times. Everything is where it should be.
I then connected a 8ohm 3W speaker and as soon as I hooked up the battery it sounded like a really loud buzzer or something.

When I played my guitar through it, it came through very well, overpowering the buzzing, oscillation sound. Changing the guitars controls
messed a bit with the pitch of the sound when not playing!!

I'm using a homemade Beavis Board(thanks Dano!!) with 2 47uF electros and an LM386N-1. I have 2 other speakers with the same specs. as the first.
And the same sound came from the all. Is it the speakers? Or could it be the 386? I have a few 386's and the result was the same with them all

Has this happened to anyone else? I didn't think such a simple circuit would cause such a problem.

Jason.

[slacker]

I had the same problem. Try using the inverting input instead of the non-inverting one, that solved it for me. Also put a big cap from the powersupply to ground as close to the 386 as possible, that helps.

[R.G.]

I didn't think such a simple circuit would cause such a problem.

The inside of an LM386 is not 'such a simple circuit'. That black epoxy hides a lot.

[CynicalMan]

He's right, you know. The "equivalent circuit" on the datasheet alone has about 20 components.
On your amp, do you have pins one and eight of the 386 connected? If you do then it will amplify everything like crazy, including all the noise you can get, and compress A LOT. If they are connected then just disconnecting them or putting a resistor over them should solve your problem.

[newfish]

Are you using a fresh battery?

You will get oscilation from a 386 with less voltage I think.

Also - the Ruby also uses a 386, and other people have had oscillation issues with it.

http://www.diystompboxes.com/smfforum/index.php?topic=25902.0

Do you have an R/C network from your output signal to ground? (10 ohm, 47nF).  This helped a lot when I was putting my Ruby together.

Just a thought...

[THOMMO]

Here's what I'm going off:http://www.beavisaudio.com/bboard/projects/bbp_CigaretteAmp_Rev1_1.pdf

I removed the jumper between pins 1 n' 8(volume dropped dramatically) but was still buzzing.

Then placed a 100uF cap(didn't really know how big "big" was supposed to be ) between power and ground. All the noise is gone! Will try lower values to see what happens.

I've never heard a Smokey in person but this thing is pretty dirty sounding!! Doesn't clean up well and gets a bit
buzzy/fizzy with the axe volume rolled off. Is there any way to make it cleaner?

Try using the inverting input instead of the non-inverting one

I'll have to search this one!! Don't know anything about inverting/non-inverting inputs!

Thanks for the help guys. I have most of the parts to try the Ruby/Noisey Cricket amps, when I finnish mucking around with the Smokey.

Jason.

[slacker]

Then placed a 100uF cap(didn't really know how big "big" was supposed to be )

100uF will do fine, the value doesn't matter much, by big I just meant 100uF or bigger, what ever you've got handy.

I'll have to search this one!! Don't know anything about inverting/non-inverting inputs!

I can't check the schematic at the minute but you've probably got the input going to pin 3, which is the non-inverting input and pin 2, which is the inverting input grounded. So you just swap those two round so the input goes into pin 2 and pin 3 is grounded.

If you want to clean it up look at the cap and resistor to ground on the output of the Ruby. If you add that it will get rid of a lot of the distortion and the fizz.

[THOMMO]

Thanks slacker! I tried a few different sized caps and ended up using a 152J. Worked the same as the bigger values.

I'll try the input swap tomorrow.

As for the dirt problem, I realised that I had my guitar signal going through a DOD OD 250 circuit, which is on the same BB
Obviously a lot cleaner with just the Smokey circuit. Now all I need is a cigarette packet!

[THOMMO]

Wired up to a board this morning. Input as switch, yellow LED, fired up first go.
A lot of ppl at work smoke, so I should be able to come across a suitable pack.

Jason.

[THOMMO]

Scrap the ciggi pack! I'm getting my son to make me a little cab in wood shop tomorrow!!

Jason.