the perennial LM386 noise

[HuarcheJoe]

Hi Forum-

I'm working with an LM386 right now and struggling a bit with noise - possibly of the ground loop variety.
My LM386 circuit is identical to the attached schematic - but I am attempting to drive it with a bipolar op-amp. I added a 10uf electrolytic between the wiper and the +IN on the LM386 to compensate. But I am getting very disruptive ground-loop-esque noise when the wiper is set to ground and pretty abrasive oversaturation when the wiper is set to the incoming signal.

I'm also curious to know if there is a way to drive a piezo with the LM386 output - the audio transformer I tried at the output stage caused the above problems to be even more pronounced.

Any help would be appreciated!

-Joe

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[bubzy]

to electro-music

you are sharing the ground between the circuits?
are you using a vgnd for the bipolar opamp or "proper" +- rails?

what does the other half of your circuit look like? and sound like(without the amp)

[HuarcheJoe]

I'm using "real" ground for everything, no virtual ground. The op amp is connected to +/- 12v power rails. The LM386 is powered via +12v and ground.

The op amp part of the circuit is a simple buffer attached to an inverting amplifier with a gain of 10. Nothing special and it works fine on its own.

I should add that I removed the 10uf cap across pins 1 and 8 of the LM386 because I didn't require the enormous gain boost.

[HuarcheJoe]

It works a lot better with unity gain in the op amp section - more forthcoming, pending tests. Thanks for the "Welcome", by the way, Bubzy.

[HuarcheJoe]

Ultimately, it's still extremely problematic - the RF noise is quite noticeable and shifts in gain seem to max out the LM386 quickly. I'm assuming there is some sort of DC offset issue that arises from coupling the op amp, since hooking up an external low voltage sound source (iPod) seems to amplify consistently. There is still oscillation without the TL084 op amp, though. I know this is a much discussed issue, but I haven't managed to find a clear solution yet. Any advice would certainly be welcome!

[HuarcheJoe]

Alright - a 1nf cap to ground to the +IN and putting the 10uf Electrolytic before the potentiometer gain stage seems to have done the trick. The only downside now is that Ground sounds like white noise, when the wiper reaches it.
Sorry to be posting a lot!

[new voodoo]

Have you tried switching potentiometers? sometimes its a noisy pot or scrthced section of the copper semicircle in the pot..prob not but i try to check the easiest thing first even if its less likely.

[PHOBoS]

Did you use a power decoupling capacitor (say 100uF) close to the powerpins of the LM386 ? (soldered to them would be ideal)

[HuarcheJoe]

Did some power decoupling - but the thing that "fixed" the pot is a capacitor to ground from the leg that was previous attached directly to ground.

Any ideas about converting LM386 to piezo?

[LFLab]

You didn't mention what you'd like to do with the circuit, but in my "not-so-serious" opinion the best way to handle LM386's is by putting them on a large flat surface, and hit them hard with a not too small hammer.
They will always be noisy.

If you are looking to drive a speaker, choose either a power (op)amp like LM380 or something bigger like LM3886 or LM4780. And if you are looking for a normal opamp, use TL072 or similar.

There's one area where the LM386 excels, which is in a Midfi electronics Clari(not) pedal (circuit is to be found on the web) a really simple overdrive/ delay/ vibrato pedal, really wonderful.