My ARC preamp has a built-in AM tuner!

Russtafarian · « **on:** 31 Jul 2009, 12:35 am »

Last night I noticed something strange on my ARC SP-14 preamp. When I touch the volume control knob, I hear AM 640 (KFI talk radio here in L.A.) through the system. This only happens with the volume control knob. No other knob, switch, or chassis part does this.

What?s going on?

Can I change the frequency to AM 830 to hear the Angels game in high-end vaccum tube glory?

Russ

mgalusha · « **Reply #1 on:** 31 Jul 2009, 12:48 am »

The joys of high bandwidth circuits.

Probably a combination of a very wide band circuit and the addition of capacitance from your body is working nicely as a radio tuner. Some folks incorporate a low pass filter on the input to prevent or at least reduce the possibility of RFI coming in via the interconnects.

If you're not running shielded interconnects you might want to try a set and see if that helps.

TomS · « **Reply #2 on:** 31 Jul 2009, 01:46 am »

Don't tell William Zane J or he'll want to charge you extra for that "feature"

Russtafarian · « **Reply #3 on:** 31 Jul 2009, 07:33 pm »

So if I add some capacitance to my body I can change the station? Is pizza a good dialectric?

Seriously, I'm curious why only touching the volume control causes this to happen. The VC is pretty noisy when adjusted, no surprise since the unit is nearly 20 years old. I wonder if the alps pot needs to be replaced.

Russ

TomS · « **Reply #4 on:** 31 Jul 2009, 07:39 pm »

Quote from: Russtafarian on 31 Jul 2009, 07:33 pm

So if I add some capacitance to my body I can change the station? Is pizza a good dialectric?

Seriously, I'm curious why only touching the volume control causes this to happen. The VC is pretty noisy when adjusted, no surprise since the unit is nearly 20 years old. I wonder if the alps pot needs to be replaced.

Russ

Not sure what type of attenuator is in there, but if constructed with a wiper arm to another piece of metal (stator) it can accumulate crud between the the two metals and make a decent diode (could detect AM). Stepped attenuator with individual resistors would have the same thing between the switch contacts.

Audio-den · « **Reply #5 on:** 31 Jul 2009, 08:57 pm »

Try to cover open rca inputs with covers Cardas sells them Enjoy!!!!

Speedskater · « **Reply #6 on:** 31 Jul 2009, 08:58 pm »

Jim Brown at Audio Systems Group has some good papers on the subject!

http://www.audiosystemsgroup.com/publish.htm

Jim Brown is a noted expert on audio wiring and grounding.