LM386 guitar amp = radio?

Thread Starter

daroc26

Joined Apr 4, 2009
30
Hi All,

I built the Little Gem Amp on a breadboard and it plays somewhat as expected. However, now it is picking up a local radio station.

Could anyone explain this?

Thank you!
 

Thread Starter

daroc26

Joined Apr 4, 2009
30
Yes.

Today, I added a SPST On-Off Switch, a 1k resistor and a green LED.

I'm not sure how this would make it pick up a radio signal.
 

hgmjr

Joined Jan 28, 2005
9,027
Where did you place the added components? The presence of the LED could be providing the rectification that when combined with the high gain of the amplifier pulls the radiostation audio out of the noise.

hgmjr
 

hgmjr

Joined Jan 28, 2005
9,027
I believe what is needed is bypass capacitors on the power supply. If you have a large electrolytic capacitor and a 0.1 ufd or 0.01 ufd capacitor in your parts stash then put both of them across the power supply to filter it.

hgmjr
 

Thread Starter

daroc26

Joined Apr 4, 2009
30
I believe what is needed is bypass capacitors on the power supply. If you have a large electrolytic capacitor and a 0.1 ufd or 0.01 ufd capacitor in your parts stash then put both of them across the power supply to filter it.

hgmjr
Okay, that would make sense! I'll give that a try!

Thank you for your help!
 

MikeML

Joined Oct 2, 2009
5,444
If the power supply pin bypasses dont do anything, and I predict that they won't, I may have a fix. Building the circuit into a metal box will go a long way in fixing it. Putting an RF bead on the input wire, and possibly shunting the input pin on the PA with a 100pF or 470pF may help, too.

Question: How close to you is the nearest MW AM broadcast transmitter? You are using a shielded lead between the guitar pick up and the amp, right? Shield is connected to Pin 4, right?
 

Thread Starter

daroc26

Joined Apr 4, 2009
30
If the power supply pin bypasses dont do anything, and I predict that they won't, I may have a fix. Building the circuit into a metal box will go a long way in fixing it. Putting an RF bead on the input wire, and possibly shunting the input pin on the PA with a 100pF or 470pF may help, too.

Question: How close to you is the nearest MW AM broadcast transmitter?
I'm not sure about the nearing AM transmitter...Radio is still coming across, after putting a 350uf cap across the power supply. When I'm ready to build I'll use a metal box for sure.

Thanks for the suggestions!
 

hgmjr

Joined Jan 28, 2005
9,027
I'm not sure about the nearing AM transmitter...Radio is still coming across, after putting a 350uf cap across the power supply. When I'm ready to build I'll use a metal box for sure.

Thanks for the suggestions!
In addition to the 350ufd you will need to put a 0.1 ufd or a 0.01 ufd bypass capacitor across the power supply very close to the power pins on the LM386.

hgmjr
 

flat5

Joined Nov 13, 2008
403
I'd start by putting a 0.001uf cap from the input (pin2) to ground. If not successful, add the rf bead at the input socket. A resistor to ground at the input may be a good idea to protect the IC.
Maybe a 1 megohm or less.

Short leads and a metal box will certainly help,
but you need some input conditioning.
Use a good guitar cord with clean plugs.
 

retched

Joined Dec 5, 2009
5,208
Could you opto-isolate before preamping? It would seem if you isolated the guitar cord from the rest of the circuit, you would reduce possible antenna.
 

flat5

Joined Nov 13, 2008
403
Hi retched. Then you would need a preamp to drive the opto. The radio could sneak in from that preamp. Sneaky RF :)
You would just be adding complexity and distortion. I think.
 

nomurphy

Joined Aug 8, 2005
567
Please see the attached PDF, all the below should help with AM noise to one extent or another.

The power supply inductor value is not critical, but try to find one around 10 uH that has 1/2 ohm to 1 ohm DCR.

The LM386 has a 50K input resistor, the added resistor and cap on the input help match guitar cable capacitance (among other things).

Since you're using this to drive speakers, adding the inductor at the output Zobel network may help (see note in schematic).

I did not show the two pots or pins 1 & 8, but when using pins 1 & 8 then pin-7 should be bypassed as shown.

Your layout and grounding is also critical, but that is another topic entirely.
 

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flat5

Joined Nov 13, 2008
403
Good work but if you want to go to that extent then include an RF bead (or two) on the input.

Edit: Thinking for a minute I realize a musician may play anywhere - including next to an antenna farm or a trucker's party with lots of CB radios.
 
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