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Picking up the radio

Discussion in 'Pickups & Electronics [BG]' started by David Jayne, Mar 14, 2014.

  1. I've got two amps that do it, and three that don't. My Thunderfunk 550B and SWR SM900 both pick up the local FM radio station quite loudly and it's annoying as h***.
    Also quite a bit of 'hashy' hiss.
    ONLY happens if an instrument cable is plugged into the amp. I've tried several cables with no change. DOES NOT MATTER if an instrument is plugged in or not.

    Moving/rearranging the instrument cable has a HUGE influence. certain arrangements nearly eliminate the noise. But why just these two amps? My others are silent.

    Pressing MUTE and turning input gain all the way down on the TF reduces but does NOT eliminate the noise. (!?) I think this is a clue but I don't know where to follow it.

    Ground is intact from cable sleeve to third prong on power cord. Outlet tester shows two amber lights (all good).
    Any ideas?
  2. bassbenj


    Aug 11, 2009
    Picking up radio RF in this case is clearly an AMP problem Some amps just do that. It's a fault of the design where the input circuits act as a diode and turn it into a radio receiver.

    Note that RF can travel down the OUTSIDE of a guitar cord as well as on the inside! Thus if the amp has a metal panel it's important that the ground on the input jack be conencted to the metal panel AT THE JACK! Plastic input jacks are especially troublesome for this problem. Typical metal switchcraft style jacks are best because the metal body of the jack screws through the hole in the metal amp face.

    That's the start! The nect thing is to try to short out the radio signals by putting 100 picoFarad caps from all terminals on the jack to panel gound. Also put one ACROSS the jack from tip to ground.

    AT this point it's mostly magic. You just try caps to ground EVERYWHERE until you get the RF shorted out. The idea is that 100 pf caps will short the radio but not change your tone. Higher values are better, but pretty soon they'll be rolling off highs, In serious cases of RF infestation I"ve gone as high as 500 pf as a compromise.

    Usually the problem has to do with a very strong radio station nearby. IN my case I can look out my window dierectly into the antenna of the local loudest major rock and roll FM station. It gets into everything in the house.
  3. Aha, Ok thank you! The jack being not connected to the panel may very well be the problem. As an experiment I ran my bass into the 'quiet' amp and ran a line from its effect loop Out and into the instrument input of the 'noisy' amp and the noise was greatly reduced. I think this supports your theory. I'll have a look at the jack tomorrow. Thanks very much!
  4. dlmaston


    Mar 1, 2007
    Denton, TX
    Massive +1 to BassBenJ.....this is the correct solution. His statement about it being a bad internal design is 100% accurate. My solution is going to the old school, all-metal switchcraft jack as it "grounds out" to the face plate, thus reducing (but usually not eliminating) the RF signal. The only "real" solution is to switch out caps.....which is a major pain in the backside but, it works.