question about Little Gem II

[thomas2]

ok just a quick question about this.. i build it yesterday into an old stereo speaker. i couldn't get the sound as clean as i would have liked (always a little distortion when i hit the string a little harder) but it's ok. i thought i could make it into a distortion only amp by installing a distortion circuit before the amp part. then i built the may queen unit from ROG site and tried it. now comes the question part...
There seems to be some pretty nasty oscillation when i turn the volume up a little and i can't get rid of it.. what could cause this? i'm using the same wall wart for both circuits..
i also tried the dod250 before the amp part and it worked fine, but didn't have enough distortion.

thanks in advance :wink:

[thomas2]

an update! the amp thingie works well without any oscillation if i use a battery for the may queen circuit.. now. what can i do to make it work with the same adapter the amp uses? or is it even possible

[thomas2]

anyone? and what does the fet do in the amp circuit? can i leave it out?

[thomas2]

dudes.. i'm pretty sure you know the answers  :lol:  don't keep it a secret.. share your wisdom.

[Mike Burgundy]

hold yer horses, hold yer horses....
It helps to include links to schematics you've used. It takes some time and effiort to find the circuits you discuss, even if one is very familliar with ROG
Obviously the two circuits are coupling through the PS. First, make absolutely sure you have NO wiring errors whatsoever - look for groundloops! Test if the MayQueen squeals on the adapter - without the GemII. Use another amp, or another adapter for the GEMII.
Then see if it helps to install either a series diode and a big cap to ground on the MayQueen's supply line, or a low-value resistor with a big cap in the same spot. Decoupling the MayQueens supply in this way could help.
Always use regulated supplies, too.
Oh, and the FET is a buffer for better compatibility when using it with other effects.

[thomas2]

huge thanks!!!! that'll keep me busy for a while  :lol:

[spongebob]

i couldn't get the sound as clean as i would have liked (always a little distortion when i hit the string a little harder)

You can drastically reduce the gain of the 386 by removing the 10 uF feedback caps if you want to keep the amp "clean".

[thomas2]

i know.. i didn't even use them in the first place. but hey.. what's the purpose of the fet? will there be any ill effects if i don't put that fet there?

[spongebob]

The LM386 by itself would load down the guitar signal too much, its input impedance is very low (have a look at the datasheet) which usually ruins your tone. The FET has a much higher input impedance and simply acts as a buffer (source follower). I wouldn't leave it out, but you could use any other buffer circuit here as well, a "normal" npn transistor for example.

[Steben]

I advice you guys to put a fet stage and not a buffer in front as i already told. The 2k2 in the source and the 6k8 in the drain - thing would do fine.
No decoupling cap.

[thomas2]

oh.. sorry mike! i guess i didn't read your post throughly before i posted the next question the last line i mean .