AM Radio Interference-How to cure?

[thousandshirts]

Also, some house wiring systems (particularly older houses with aluminum
wiring) can act as a massive antenna. I always use IEC sockets in my junk,
and can always swap in an IEC socket with a filter on it, such as one of
these:

[img:239:200]http://www.newark.com/productimages/far ... 405-40.jpg[/img]

There are a pile of different filtered IEC sockets available, have a boo and
you will probably find one relatively close-by.

[jjman]

You mentioned that the problem is present with nothing plugged in to the input.

Assuming that the input jack(s) is/are wired in the normal "shorting" fashion, and are "clean" and functioning properly, the problem is not related to the input area. This is because a properly grounded wire is not going to behave like an antenna.

Does the amp's volume knob change the volume of the radio sound?

[drz400]

Many of my amp builds have a problem with picking up AM radio signals. It isn't just in this house-it happens elsewhere
as well.

Is there a quick fix for this problem without drastically altering the tone of the amps?

If your input jack is not going to chassis ground right at the jack then run a 10n in series with a 51ohm resistor direct to chassis
Or
Make sure your audio ground path terminates at the input jack and goes directly to chassis, this way the garbage is dumped on to the AC wall ground, keep the Line third pin ground at least 2 inches away from the Audio ground and do not use the chassis as a ground path. I promise that if you keep the ground path correct and ground the input jack very close to chassis and terminate the circuit ground there you will not have any RF

[Cygnus X1]

Thank you, dr, and all.
I do typically run a star ground system. The two shortest paths to ground are at the inputs, and earth for the power cord on the other end. I lift everything from the chassis. My builds are typically extremely quiet concerning hum, hiss, or buzz. Just AM radio...

I found the "cap" solution at another site that I'll try first though.
Here is the qoute:

"This a good one, and props to Guy Hedrick (who is brilliant) for hipping me to it.

Let's start out with the understanding that: 1. A good portion (if not all) of the radio interference that guitar players tune in, rides freely into the amp aboard the guitar cable's negative shield, and: 2. The input jack(s) at the amp need two distinct grounds, an AC ground, and a DC ground.

The metal Switchcraft type jacks (such as used on classic Fenders) get a tight, solid AC & DC ground connection (when properly tightened that is) to the chassis. This is desirable for shunting radio interference to ground, preventing it from getting into the amp, but not always desirable for controlling hum due to the potential formation of a ground loop.

The insulated plastic input jacks (typical of Marshall) have no connection to the chassis aside from what they're given, via a lead. This is desirable for controlling the DC return path (DC ground) for prevention of ground loop induced hum, but not effective at giving the incoming RF a swift, solid shunt to ground, and therein lies the problem. But alas, there is an elegant solution! A .01 ceramic cap soldered from the ground lug of the input jack directly to the chassis (with the leads kept as short as possible) will effectively shunt incoming RF straight to ground before it gets far enough into the chassis to make "music". /End Quote

I suppose my only concern will be now there will be a chassis connection than I avoided in the past, albeit through that cap.

[Andy Le Blanc]

so this approach is to bypass the RF to ground from the cable shield through
a cap to ground, even though its already grounded?

[Cygnus X1]

Not sure, Andy.

I guess my point is I don't go "straight to chassis" with my grounds, much less the input...so a straight to chassis ground with the cap is something to think about.

I guess.

I have ten other things on my bench all of a sudden.
Which is good!

[paulster]

At the risk of quoting myself:

Are you using insulated input jacks, e.g. Cliff jacks?

If so then solder a 0.01uF cap between the ground connection (not the signal connection, despite what some may mistakenly say, or you'll lose all highs) and the chassis right at the jack, keeping the leads as short as possible.

It makes a world of difference as it dumps any of RF riding on the cable right as it enters the chassis and doesn't give it a chance to use your ground connection back to your preamp ground point or star ground point.

[Cygnus X1]

Thank you, Paul.
Somehow I missed that one.