LM386 Stress , Please HELP

[Happy_Jock]

Hi folks.

Lately I have been having trouble trying to make this Stereo Amp using 2 LM386 IC's and I can say that no circuit has ever caused me more stress
Here is the schematic from a book called 101 spy gadgets for the evil genius.. I had made another mono preamp from this book using an LM358 it worked fine.


I also found this video on youtube of a guy refering to the same circuit saying it works very impressively. https://www.youtube.com/watch?v=JaXwVkgDxXA

I constructed this on a pcb i designed, only to find when I powered it up all I heard was a loud screech like scratching unbearable sound coming from the headphones, no matter how much I adjusted the 2 pots all I got was ridiculously loud screeches and whines, I also found touching the metal of the pots caused a lot of effect to these sounds.. I thought I may have made a mistake on the PCB so moved everything to the breadboard again with new components, I have rearranged it around 3 times now on the breadboard but still keep getting this horrible noise and no clear sound what soever.

Has anybody any idea what I could be doing wrong, as its really irritating seens it seems to work for this guy in the video. Like most circuits that go wrong for me annoyingly it makes me more obsessed with wanting to get it working.

any advice would be really helpful and appreciated

cheers

[pebe]

Hi again, my friend,

Are you using the same 9V battery for the LM386 supply, and to polarize the mics?

[Happy_Jock]

Hi again, my friend,

Are you using the same 9V battery for the LM386 supply, and to polarize the mics?

hi pebe, yes, a assumed the 2 separate 9v points in the schematic meant the same supply, the schematics in this book are rather unusual to the ones i'm used of lol. do you reckon this could be an issue?

[pebe]

Yes, that is almost certain to be the problem. The varying current through the LM386s due to outputs changing with audio levels will cause voltage changes at the battery terminals. Those changes are being fed back into the mic supply, where they go into the LM386 which amplifies them. That's a vicious circle which is causing the noises.

The solution is simple - decouple the feed to the mics as shown in red on the circuit

[Happy_Jock]

hi there pebe, thanks for your advice, im in the process of bread boarding this again now, still getting the same nightmare, though a wee bit milder, I'v all but given up, im beginning to wonder if the pots I am using could be contributing, I salvaged them from an old radio scanner, in the circuit they don't seem to adjust the volume only create a selection of unusual noises when touched. I don't seem to have much luck building amps with the LM386, I had tried 1 or 2 other mono circuits with similar results. Unusually I have had far more luck with the LM358, using this schematic from the same book.



on the page this is on there is a twin schematic tagged "left" using different pins of the LM358, ? do you suppose If I had 2 of these amps left and right outputting to 1 stereo Jack I would achieve a similar stereo result to the lm386 circuit (obviously realizing the output of the LM358 is less powerful)

I am currently using the LM358 amp for auio along with the Door Camera circuits in our past discussions, I had planned on upgrading it to the LM386 circuit, and activating it with the same relay as the one for the cam. ??

[pebe]

Regarding your 386 circuit, it’s difficult to say for sure now what the problem is. As your circuit is bread-boarded it may be the wiring to pin3 may be picking up stray signals from the outputs. Disconnect one of the 386 chips and just try a single amp first. You could try these steps:
1. If you are using a battery that is not new, then its internal resistance may have increased. That would give you a degree of common coupling from output to input. Try fitting a 470µF cap across the battery.
2. If that doesn’t help, disconnect the input cap from pin3. if that effects a cure, then it is pickup on the pots or input wiring. Make the wiring as short and direct as possible, and the ground of the mic goes direct to pin4.
3. The 10µF cap fitted between pins 1 and 8 increases the gain from 20 to 200. Disconnect the cap to reduce the gain. If that cures the problem then you may not need all that gain. Look at the data sheet to see how you can fit a resistor to reduce the gain.

If all fails, could you give us a photo of your layout?

Re the 348 circuit. If you only want headphone reception then it should be OK. The output can source 20mA so connect phones between the output cap and ground. With the resistor values given, the gain can be anything between unity and x100.
Again, if you are using a battery, fit a 470µF across it.