LM386 motorboating

[AM]

I've built a smokey amp variation. I had it housed in a delay box after the delay circuit and worked like a champ.
Then I took it out and housed it in its own box. It worked fine with a 9v battery and when attaching the leads from a power supply straight on pin 6 (+) and ground (-).
Then I installed a dc jack and it all went south. Motorboating galore!!!
I tested the circuit using 3 different regulated power supplies and always had the same problem.
Using the 9v battery it worked fine. Power supply cables straight to pin 6 and ground, no problem again. Power supply connected to to DC jack, motorboating.
Tried another DC jack, same problem. Using pin 2 for signal input and grounded pin 3, made no difference.
I think I'm totally lost at this point...
Any help would be greatly appreciated!

[Electron Tornado]

Your DC power supplies may not be well filtered so what you may be getting is actually AC noise. Without seeing your schematic, you probably need to add a filter where the power comes in to the circuit, which is simple to do, to get rid of the AC hum. These links might help:

http://www.muzique.com/lab/hum.htm

http://www.diystompboxes.com/smfforum/index.php?topic=17309.0;wap2

This one's even better: http://www.beavisaudio.com/Projects/Huminator/   If you only use R1 and C1 as shown you'll probably be OK. Make sure C1 has a voltage rating greater than 9V. For C1, anything between 100uf and 330uf will be OK.

[Seljer]

Have you tried slapping a big electrolytic capacatior right onto the circuit?
If I recally correctly, mine did the same thing. I had to put at least 220uF to get it to behave.

[AM]

I understand what you say about filtering but the thing is, when I use the power supply and just connect its wires straight to the board everything works fine. Then when
I use the same power supply with a plug attached to it and connected to the dc jack of the box it gives me the motorboating noise.
If the problem was line filtering it would occur in both situations

[PRR]

Either your jack is crap, or there's something about length/distance.

Put a big capacitor NEAR the chip. It really is part of the audio path, and should be "tight" with the power-amp. A couple inches OK, a couple feet can be goofy.

[AM]

Either your jack is crap, or there's something about length/distance.

I tried two different jacks and I got the same results. The same jacks on another circuit work just fine.

Put a big capacitor NEAR the chip. It really is part of the audio path, and should be "tight" with the power-amp. A couple inches OK, a couple feet can be goofy.

A couple of feet? I'm not even talking more than a couple of inches here. It's attaching the power leads straight to the board or wire them to the dc jack and then run about two inches long wires to the circuit board.

[GibsonGM]

try putting the 100R resistor followed by a big cap to ground right at the chip.  Or as PRR says, just pop a cap there.   470u or greater.  Sometimes you need 2 in parallel (or find a 1000u).

You may be able to test this by trying it by hand first.  Don't short your chip, tho. 
 
Doing this works for me about 98% of the time.

[R.G.]

Motorboating is nearly always a problem with high power supply impedance. the fixes you've been told about all address one or more parts of that.

Your description - works with power supply attached directly, not with it through the jack - seems to indicate that the jack has a high resistance for a jack. Maybe something funny about it.

But local decoupling, big electrolytics and a ceramic in parallel, right at the chip, will usually fix that.

[AM]

Thanks gents! I'll try what you suggested and let's hope it will sort out the issues I'm having right now.

[petey twofinger]

fixed this with a lge - ish value electro cap ON the chip .

the build note i used had a 100uf at the power input jack . i moved this to pin 6 and 8 or something like that literally on the chips socket solder points , under the pcb and it went away . i added a bass fuzz though and it squeals ... back to the drawing bord , actually i ordered some 7052a chips and i a m going to try punch amps next ...