First proper build, oscillation/hum problem - please help!

[fbalm]

Hi guys,

I've been reading this forum for a while but this is my first post. Hi!

Anyway, I've been bread-boarding loads of different dirt circuits for a while, and had no issues with any of them. I bread-boarded up a modified bazz fuss into lm386 based amp that I planned to build into a one watt head.

I built the two individual components of the circuits onto a piece of perfboard (whatever the type without copper is called), which I then connected to in, out, pots and ground on a breadboard, so I can confirm that the circuits were built correctly.

When I wired them up inside the enclosure (a lunchbox), I started getting a wierd oscillation that changed pitch as I moved the pots, and no guitar sound. I figured there was a grounding error (I connected all the grounds to the input/output jacks), and the build was very messy so I decided to scrap it.

I made a much neater version of the dirt half of the circuit, and since the lm386 part comes after all the pots, I figured I could add that later and just make sure the dirt part works. I built the circuit minus the in/out, power jack and pots onto a piece of perfboard, then connected that to the rest of the components which were on a breadboard. It worked flawlessly.

I put all the other parts on the enclosure and soldered everything together, using the 'star grounding' technique I had read up on (I am pretty new to electronics). To my dismay when I plugged it in, the same oscillation/hum. At this stage I am figuring there is something I don't understand about grounding. On a breadboard, I just connect all the grounds to the same strip and it works.

Following are pictures. The only other thing I'm not sure about is the last pot, the volume pot - I didn't breadboard this - it is a 10k b pot, I'm not sure what it means but I was told a 100k pot would be too much volume running into the lm386. This time the oscillation does not seem to change pitch (however I will recheck this). The tone and volume knob, which come after the circuit, seem to work as planned, and the input volume knob has no effect.

Please help me troubleshoot, feeling nooby and getting annoyed at all the time I'm sinking and not getting results!

The red and yellow (ran out of red wire) wires are the grounds. Forgive the gross-ness.

[armdnrdy]

I have a sneaking suspicion that if you house this build in a metal enclosure....all of the issues will be solved.

Use a metal lunchbox. 

[GibsonGM]

It's tough, I know!   Like armdnrdy said, try rehousing it.  There isn't any shielding in a plastic box!   You may be picking up radio wave junk in the air and spinning it round and round in there.   Your concept is good, it's just some troubles that over time you'll learn to take care of.  You should make an audio probe to use for debugging ("search").   9 times out of 10, it's just a bad connection somewhere...poorly soldered or a mistake.   Go over everything again with a fine toothed comb...
       
You have the grounds connected to the right part of the jacks, right?  Not grounding out the input signal?   Check that.  BTW, 100K should be fine for your volume pot!  The pot will not add any kind of output, it can only subtract ;o)

It's traditional to use red wire for hot (+) DC, and black for ground.  It doesn't matter except that down the road you may confuse yourself years later when you poke around in something you build, or if someone else digs into it.    Up to you how you do it.   My father in law uses black for hot (from doing AC work), and is confused on every single thing he does to this very day!

[duck_arse]

if your wires were that long and "messy" on the breadboard and it all worked, you can probably get away with the plastic box. you might like to line the inside with heavy aluminium foil to provide at least some sheilding.

as for you squeal, you want to isolate your input stage power supply from your output stage power supply. get yrself over to amz, and look at the hum page. the little RC filter, top left of that page, may well solve your problem.

for the bazz half, you can use a larger value series resistor up to 4k7 maybe, and a smaller cap, like 22uF, and for the lm386 section (there is no point doing it by halves) use a lower value resistor, anything up to 220R, and as big a cap as you have. 470uF is not too much here.

good luck, and where do your sandwiches go now?

[fbalm]

Hi guys, and thanks for all the help. I isolated the problem, it is that I'm an idiot:

It's traditional to use red wire for hot (+) DC, and black for ground.  It doesn't matter except that down the road you may confuse yourself years later when you poke around in something you build, or if someone else digs into it.    Up to you how you do it.   My father in law uses black for hot (from doing AC work), and is confused on every single thing he does to this very day!

This ended up being the problem. I used the correct wiring scheme on the perfboard but not in the box . Switched the two wires around and its working as I expected. There is still some audible oscillation at higher volumes, so I will try the power filtering techniques linked - it gets higher pitched and louder when pedals are powered from the same wall wart. However, it's a cheap probably unfiltered power supply, maybe just using a boss one will fix it. I'll report back, and thanks for the warm welcome!

[GibsonGM]

Glad it worked out and that nothing was damaged!  That's often the case, and it happens to all of us at one time or another )