PO'ed at RFI

[korneluk]

I have been dealing with RFI issues in my tube gear for the past year since moving into a new house and discovering that there are three thre-hundred foot radio towers within a 5 mile radius - including one I can clearly see from the listening room.

I got a step-up transformer for my Koetsu Rosewood cartridge and thought I had licked the problem... until I clearly heard a radio station coming in loud and clear thru my Lowther speakers today!

After a few PO'ed moments and much @#$%!^&, I found the interference is being picked up thru the long interconnect run between preamp and the Moth S2A3 power amp because when I short the power amp inputs the radio station is no longer playing thru the speakers. Next step I guess is to get shielded interconnects or go balanced.

Is this ever going to end???

-- josé k.

[HDTVman]

I don't know how you feel about this but I think I would try 47pf ceramic cap as RF bypass on the input of the amp, if shorting the interconnects helps.

It could be the preamp output that is the issue and acting as a detector. In that case the bypass should go at the preamp output.

Short interconnects could help too. The long ones are the antenna that's receiving the radio signals and delivering it to your equipment.

Better shielding of the interconnects may not be the whole answer. You may need your shield the equipment also.

Just a few sugestions of things to try before you go out and spend a load of cash on cable and still have the problem.

Good luck BZ

[CaliforniaBob]

I had this problem once after I modified a preamp. Started to hear radio stations (better than voices in your head).

Try some ferrite core clamps around all the critical cables. Start with one end and add to the opposite ends if needed.

good luck

[richwalters]

Obviously, the RF volts/meter in your vicinity is far higher than most are used to.
As others mentioned this has to be approached systematically.A tube in your preamp is either oscillating on the quiet and reacting with a strong signal (i.e screening cans not fitted->easily happen without being aware of it, Or the first preamp tube has a too wide b/w and acts as a simple diode detector demodulator for RF signals. A small 47pF from g1 to chassis (shortest leads) might sort it out. Any more might effect treble. Disclosing your tube line up might be helpful. Pay attention to input wiring screening....

If the main amp is suspect for RF pickup, disconnect signal inputs from preamp, to ascertain which lead(s) stray RF is actually coming back. In my case the culprit was the LS cable. I then s/circuited inputs and again may be optimised for s/n ratio, but may not be for RF rejection. It's a case of elimination but even though you have local emissions, also watchout for those AM signals that fade in and out !

So much depends on construction and earthing.

richj