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turntable issue - faint hum on one channel

Just got a phono board installed in my Arcam AV8. Picked it up yesterday, and have been playing vinyl all day today. Sounds great, even better than the Rotel RQ-970BX that I just removed from my cabinet.

I was removing the Rotel unit from the cabinet, and started adjusting the rat’s nest of cables and wires. As an experiment, I set the input on the AV8 to turntable, turned the volume way up on, and listened as I moved cables. Sure enough, a bit of low level hum came and went. It seemed to track the turntable power cord at first, but then I learned a hum was present even if that cord was completely disconnected. After an hour of work, I tracked the hum to the wire leads on the turntable wires which connect to the phono cartridge. My turntable is an 15 year old Thorens TD-280, with a newer Shure V15VxMR cartridge. The wire connectors had some corrosion. I pulled the leads, and then used some naval jelly and some 400 grit sandpaper to rough up the leads. The pins on the cartidge looked fine, so I did nothing to those. I re-connected everything, and turned the volume way up again.

The hum is reduced susbtantially, but still present. It is higher on the right channel. Left channel is almost dead silent at high volume. Right channel hums, and when I touch the red wire on the cartridge the hum gets louder. This is only present at high volume though. It is just barely audible at normal listening volume when I am a few feet away. Still, I’d like to eliminate it, you know?

Any insights on eliminating this hum completely?
designdude

Showing 2 responses by designdude

designdude OP
23 posts
 
Some great ideas here. I should have mentioned the hum is present even with the power supply disconnected. I looked at the inside of the RCA plugs, and since they looked tarnished I rolled up some sandpaper and sanded the interior contact surfaces, and then reconnected them. I took care to bend them inwards a little so they fit tightly. Better, but still a bit of hum. I then switched various power cords, no change. But when I removed the ground wire, dead silence. Interesting! But when I connect the power adapter (which is an 120V AC to 16V AC) the hum went through the roof. The ground wire then reduces the hum to what is was before.

So I am stumped. I am going to look for a new AC adapter. Fortunately 16V is a standard size for home alarm systems.

designdude OP
23 posts
 
It's the d@™ phono board!

After hours of research, countless power cord and interconnect changes, replacing the AC transformer power supply, etc, I tracked down the source of the hum.

It's the board, or at least the circuit the board is connected to. There is hum only when the RCA cords are connected directly from turntable to these jacks. When I coonect my turntable through another input jack using my Rotel RQ-970BX pre, the system is dead silent, as are all other input sources.

Is the Arcam phono board a POS, or is it installed wrong?
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