Cabinet Input Jack - HELP

[axekicker]

Hell, all... I'm building a mini amp and cabinet. The amp circuit takes a small 8-ohm speaker. I've created a little cab i.e. cigar box, and wired the speaker with speaker wire to a 1/4 input jack. But when I plug in, I get a wailing noise. I'm guessing I'm using the wrong kind of jack? Do I need a "phone" jack? Switched or unswitched? I though all 1/4 jacks were the same (stereo notwithstanding). Thanks!

[ashcatlt]

Well Hell right back atcha!

Can't imagine how the jack could cause the problem. All 1/4" jacks are "phone" jacks. They were originally used in patchbays run by those nasal operator ladies who had to plug from one hole to another for every call that anybody ever wanted to make.

Did you verify that the amp works with that speaker when wired directly to it? Is it possible that you're using a shielded cable and capacitance is causing oscillation?

[axekicker]

Thanks for the reply... I used a proper speaker cable from another rig. I have no idea why that noise is happening. Might be a bad circuit board? I dunno.

[sumgai]

kicker,

You say you're building the amp..... You also say, on a printed circuit board....

From these statements, I have to guess that a component is mis-wired (incorrect component in Position X), or that one or more parts have gone bad (perhaps they arrived that way?). A printed circuit board generally is laid out so that parasitic oscillations won't become a problem, but then again, not every PCB designer is successful in the art of component layout - you still might have a badly designed board that's the underlying cause.

The clue is, it happens only when you hook up a speaker - that action completes a circuit that contains an oscillator, regardless of what you think you're building. But without some more sophisticated troubleshooting tools, you're left with not much else to do other than start replacing parts, one at a time, and testing - over and over and over and......

Personally, I'd breadboard the circuit first, and see if having all the components spread out eliminates the squeal when the speaker is hooked up. If that's the case, then the PCB was poorly designed.

If it still squeals, then you have a bad component. Not necessarily a kaput one, but a component that shouldn't be present in that value. For instance, small (physically small sized) capacitors are sometimes mistaken for incorrect values - the lettering is tiny! In any amplifier circuit, it's easy to drop in a cap valued at 10 or even 100 times the correct value, and suddenly find yourself holding an oscillator. The reasons for that are well known, but esoteric enough that we needn't go into it here. Suffice it to say, you need to look carefully at every component, not just the caps, and ensure that they are indeed the correct value.

Of course, all the above is assuming that the circuit design itself is correct. Did you get this from a kit-maker, who sold you all the parts at one whack? Or did you see this discussed on another forum, and any problems were brought to light and cured by the forum members?

If all else has failed, post a copy of the circuit schematic here, and I'll vet it for you. Or perhaps jerry will beat me to it.   A picture of your built board might help, but don't go overboard on that, one or two images should be sufficient.

HTH, and good luck!



sumgai