Shielding amplifier circuits from AM radio noise

[Ratch]

What's the standard practice for shielding small amp circuits (like 2 watt) against AM radio interference?
I built a few amps from downloaded schematics, but when I'm near enough to an AM radio tower, the amp picks up the AM noise. The amp itself is already noisy (see my post a few minutes ago), but since I have an AM tower very close to where I was testing the circuit, it's difficult to find what's AM interference and just plain bad circuitry...

I'm assuming some of the circuit is acting as an antenna. Is there a way to damp it without affecting the audio bandwidth? I'm still very much a noob at electronics, but I'm trying to learn as much as possible.

How can I shield it from this?

Thanks.

 

[Sceadwian]

Put the entire AMP in a metal housing to prevent the circuit from directly picking up RF, and use a small shunt resistor (to ground) at the input to give RF a path to go to, ferrite filters on the input lines would be a good idea as well. A more sophisticated pre-amp stage might solve this as it's not overly difficult to add lowpass charactoristics to an opamp. Setting up a low pass with an upper cuttoff of 200khz should keep your audio pretty clean, even if it's only a single pole filter. I'll let the rest of the audiophiles chime in on this though cause I'm not altoghether that knoweldgeable about this kind of thing.

 

[Ratch]

Ahh, I guess a lowpass is what I'd need to figure out on the component level.

I was hoping to put it in a fiberglass enclosure, but I can probably line the enclosure with aluminum foil and link that to the pcb.

Thanks! Looking forward to the audiophiles input, too...

 

[Sceadwian]

One of the nice things about metal enclosures is they also act as natural heat sinks. Fiberglass is a pretty good insulator. It is only 2 watts, but you'll want to watch out for heat anyways.

 

[Gayan Soyza]

I don't know if it is an IC amp or transistor amp!

But I have experienced some cheapy IC's have that poor effect.One time my original IC got burned, after I replaced with a cheaper IC.It has the same problem you have, oscillating with other frequencies some times You can hear radio channels as well.

*Also you give inputs through screen wires.This is not a problem if you gound it to a metal chasee.

*Also you give the supply from a battery to minimize the unwanted noise, hum etc....

 

[Optikon]

Ahh, I guess a lowpass is what I'd need to figure out on the component level.

I was hoping to put it in a fiberglass enclosure, but I can probably line the enclosure with aluminum foil and link that to the pcb.

Thanks! Looking forward to the audiophiles input, too...

If you go the metal screen route, dont use regular aluminum foil unless you will be securing the connection with a gas-tight ring-terminal or similar. I fyou plan to solder a wire to it, you will have a very unreliable joint (even if the special fluxes are used) I wouldnt do it.

There are screening product available for this type of shielding that can be soldered to. You'll have to google for those.

 

[Sceadwian]

The whole problem of shielding goes away with a metal case, which is why they're so commonly used in amplifiers.

 

[Ratch]

Ok, so I'll look into a screen instead. When it's done, the unit will be mounted in a fiberglass recession on a helmet. I was trying to save space by lining the recession with foil and using some kind of mini plug or board mounting clips to bond to it. I'll look into the screen instead, and see if I can figure out a filter as well.

Thanks for the replies.