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How do I eliminate FM interference and low hum?

I just completed a Curcio Premium mod on my Dynaco ST-70 and it sounds great, other than a slight hum and the local mega watt country station bleeding through! I tried unhooking all IC's from my preamp except the ones to the amp and it's still there. Even when I turn off the power to my preamp it's there. When I remove the IC's to the preamp and install rca shorting plugs both the hum and the FM disappear. It seems as though my IC's are acting as an antenna.
My preamp (for now) is the preamp section of a Marantz 1120 integrated amp, I have a Granite Audio tube CD, a Marantz 2110 tuner, a BC music ring and use Buggtussel Amygdalla speakers (91db sensitivity). Prior to this setup I was using a pair of Granite Audio mono blocks and a Granite Audio 760 preamp along with the BC music ring and the Marantz tuner and had neither of these noises coming through my system. Is there some way to modify my IC's or can something be added at the inputs of the amp to filter this noise out? Thanks a lot for any and all suggestions.
nelsress
nelsress OP
5 posts
 
While contemplating this problem it suddenly dawned on me that my Granite Audio CD player has a volume control! So I ran my interconnects directly from my amp to the cd player and voile', no more hum, no more FM interference. This now tells me that the problem is with the preamp, probably the inputs. This now becomes a question of solving the problem of my interim preamp being an unwitting antenna and how can I eliminate that. Any suggestions on this new issue would be greatly appreciated. Any vintage Marantz fans out there who have remedied this situation in the past?
rhyno's avatar
rhyno
649 posts
 
shielded ICs should do the trick.

if it doesn't, you'll need to internally shield your preamp. see percyaudio.com and look into the TI shield he sells for @ $28 for 12*12.

rhyno
nelsress OP
5 posts
 
Thanks Rhyno, I've placed the percyaudio site in my favorites and have read about the TI shield. Very interesting indeed, I'll have to give that a shot as an inexpensive and relatively easy option. A friend of mine also told me to try grounding the preamp to the receptacle and as that's even easier, I'm going to go try that now and see what happens.

nels
nelsress OP
5 posts
 
Just to update, running a wire from the grounding post of the preamp to the receptacle didn't eliminate the problem. It looks like I'll have to try shielding the preamp if I want to eliminate it. Or else I may just use the CD player directly into the amp until I get around to building a preamp.
s23chang
617 posts
 
Put a small capacitor like .47uf/50V from RCA output gound to the body of your preamp chasis.
This the ground loop problem in your preamp. Probably bad grounding point somewhere.
This should do the trick.
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