radio station inteference in my chipamp

[facundonu]

hi, i've built a chipamp based in the typical aplication circuit on the lm1876 datasheet.

this is the amp pcb layout.


this is the preamp pcb layout. the IC is a ne5532.


everything is inside a plastic chassis, but the preamp is sorrounded by a metal shielding connected to signal ground, and the amp has a metal sheet parallel to the board, at a 5mm distance, connected to signal ground as well.

it sounded fine here in the room i build stuff, but when taken to my bed room, where it's finally installed, it plays a radio station!!
it is heard quite low yet annoying in low volume passages of a record.
the volume of the radio heard doesn't vary with the volume pot, wich is connected the standard way after the preamp and before the amp, so i assume the interference is not entering my gear throgh the preamp.
something remarkable is that when the volume pot is set to low, the inteference is heard mainly in one speaker and very little in the other. when the pot is set to full volume the radio is heard in the other speaker, and the one previously playing the radio is almost quiet.

any ideas of the source of this interference?? any advice on how to eliminate it?
thanks a lot!

[poobah]

Can you posy your schem? Do you have a scope?

[facundonu]

don't have a scope.
as soon as i have some time i'll be taking things apart and see if it stops when removing the preamp.
thanks.

[poobah]

Is it the right channel making most of the radio?

Try wrapping the amp in foil and ground the foil.

[facundonu]

i guess it is... but... aren't both channels the same???

i'll try that.

[poobah]

The feedback resistor for the right channel is far from the chip. This is not good... it can pick up noise. Look at the traces, you have a large "loop" (circle for magnetism to get in ).

Try the foil... maybe you just need a metal box.

You could move the 20K resistor to the back of the board and solder it point to point.

The application note shows an inductor||resistor on the output, these can be important. They can help block interence from your speaker wires.

You can prove this maybe by wrapping foil around speaker wires too.

[tiltedhalo]

Bypass the input of both chips with a 220pf capacitor to ground, if this does not work shorten the feed back resistor or bypass with a 5 to 20pf ceramic.

[Greg Erskine]

Hope this helps.

http://sound.westhost.com/amp_design.htm#rf_prot

[frank1]

You are obviously close to an AM transmitter.
Forget the tinfoil.
Use the advice from Rod Elliots site regarding input filtering first.
If that doesn't work, chances are, this is being picked up on the speaker leads.
You need to get some ferrite rings (2" dia).
For AM interferance, tape two together and wrap each speaker lead through the ring as many times as you can. Put these as close to your amp. as possilble.

Frank

[Peter K]

Being a HAM (radio amateur) I agree with Frank1. But before you install the coils, first make the cables between the amp and speakers as short as possible. These cables act like an antenna.

Peter