Ground loop horrible feedback

[Netguru5]

I am getting horrible (ground loop type) feedback in an LM386N audio circuit on a breadboard when I use alligator clips from the power supply either via 9v battery or wall PS. If I use a 9v battery clip, this noise is almost (but not totally) eliminated. When using the alligator clips as leads from the PS , the noise drowns out the audio completely. What could be the problem? What am I picking up that could cause such bad noise? I've even tried turning off lights, equip.
Any help would be greatly appreciated.

 

[ke5frf]

Well, without seeing your exact circuit and schematic it is difficult to pinpoint what might be happening, but certainly there shouldn't be any difference between using alligator clips vs. using the 9 volt battery clip.

It doesn't sound like a ground loop, sounds (possibly) like you have some kind of inductive or capacitive coupling issue between your leads that is minimized by the 9 volt clip. Perhaps you are setting up an oscillation that is being amplified. Just a note from the datasheet, and this is for the AM amplifier application so I don't know exactly what you are doing, but it mentions twisting the power and ground leads. You might even use shielded twisted pair wire for that purpose. Worth a shot.

 

[Mikebits]

LM386 needs short power leads along with proper decoupling near the power leads and again at the Vcc pin of the part. Your long leads are picking up and inducing noise on your 386 supply pin.

 

[Netguru5]

LM386 needs short power leads along with proper decoupling near the power leads and again at the Vcc pin of the part. Your long leads are picking up and inducing noise on your 386 supply pin.

Yes, that was it. Thanks It was the leads/no leads that threw me for a loop!

 

[BrownOut]

Next time you're experimenting with long power leads, twist them together tightly to minimize inductance loop area and differential capacitance pickup, and use an onboard decoupling capacitor at the power connection. A friend of mine spent hours in the lab trying to figure out why his amplifier design was so noise, till the TA came around and twisted his power leads. The niose magically disappeared.

 

[audioguru]

A breadboard should be used only with DC circuits.
Use a compact stripboard or pcd design for audio or radio frequencies.