2. analog
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Peculiar Hum Issue on Jolida JD9 Phono Preamp

I am experiencing hum in my phono preamp after upgrading several of the internal components. The rest of my analog equipment is Teres TT, Basis Vector 4 arm, Zyx Airy 3S SB cartridge, Linar Model 10 amp. I first noticed the hum after one set of upgrades whenever I connected the arm cable to the Jolida. I reduced it to manageable levels by changing the 12AX7 tubes in the preamp to 12AU7 variants (to reduce gain) and using a lower gain setting in the Jolida.
More recently, I upgraded caps in the input section and I purchased the Granite Audio phono burn-in CD. I connected the output of my Oppo 83SE player to the Jolida and now the hum is back. I used 12AX7 tubes for the purposes of burn in as I don't want to reduce the life of my 12AU7s.
If the RCA cables are disconnected from the CD player but still connected to the Jolida, there is no hum. The Oppo does not have a ground in the AC connector.
I know I can probably get the hum down to manageable levels again when I switch to 12AU7 tubes but I would like to try to eliminate this altogether. Any suggestions as to what I can try?
dramaqueendad

Showing 2 responses by jjrenman

jjrenman
279 posts
 
First, a 12AU7 is not a lower gain 12AX7 replacement. A 5751 is although it is still more gain than a 12AU7.

A little more experimenting may help you figure out where your ground loop is. If the JD9 has a ground pin in the AC connector try floating it. Also try leaving the Oppo's RCA cables hooked up but disconnect the video cable. Sometimes the ground loop can be traced back to the TV chassis.

BTW, what specifically was the last upgrade done before you noticed the hum?
jjrenman
279 posts
 
It might not be the TV. Your last experiment just gives us a direction to look into.

We need to be sure it is the TV chassis. Try hooking up the Oppo's HDMI but disconnect any other sources going into the TV. If the hum is present only when the Oppo's HDMI is hooked up than I'd try isolating the ground pin on the TV's A/C cord. If it does not hum with just the HDMI from the Oppo than hook up your other video sources one at a time until you find the culprit.
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