Turntable hum with tube mono amps

dromunds

Early this year I made a corian armboard and mounted a Grace 707 arm to a Denon DP72L table. I have used it since with no problems. Today I hooked up some used tube mono blocks I recently purchased and my turntable channel suddenly has a very noticeable hum. The CD player is dead quiet. I have a Dodd tube phono stage and also a Dodd battery preamp. The only thing I did different was change amps (previously used a S.A.E. Mark IIIa with no hum on this TT whatsoever) and also changed the physical set up of my gear on my stand. When I used the S.A.E., I previously had the ground wire from the Grace arm going to the Dodd preamp, but when I got the hum I changed the ground wire to connect to the Belkin PF 60, but it made no difference in the hum. The amps have no ground connection for a TT per se. Any thoughts would be greatly appreciated. The sound is awesome but this hum is very frustrating.

polkfarmboy

I got a low hum when I hooked up my dared to my primaluna's but I disconnected everything from the chain and hooked it all ack up again and powered everything up in the chain from the first component to the pre to the amp then to the speakers..... the hum was gone!!!

george daniel

Where are the tube amps connected? Also--try grounding to something else--keep us posted.

Polkersince85

How close is the table/cartridge to the transformers on the amps? Watch where the ICs go also as they could pickup some hum. You may try disconnecting one side at a time to determine the source.

dromunds

Polkersince85, actually the TT was pretty close to one of the amps but I moved it about 24 inches away and still hum. I will try disconnecting one side at a time and see what happens. George, I'm not sure what you mean by "Where are the tube amps connected?" They are plugged into the two amp switches on the Belkin PF 60. That's where I had the S.A.E. amp as well.

dromunds

Disconnected each channel one at a time and still hum in each channel.

Polkersince85

Try plugging the amps directly into the wall socket. If they are two-prong, try them one way and the flip them over. The connector wires at your cartridge may be shorted also, Does the hum change when you touch the RCA from the table?

dromunds

Will do more checking tonight and report back in a.m.

dromunds

Tried plugging directly into wall outlet, same hum. Power cord plugs are three-prong.

schwarcw

You are picking up hum from the transformers in the mono blocks. A **** of a problem to have. Distance and shielding are usually the best choices. You also can mess around with the grounding scheme. What kind of cartridge, phono preamp are you using?

dromunds

schwarcw, I moved the mono amps as far away from the other components as the cables would allow and the hum decreased noticeably but not totally. What shielding could I try? I'm using a Dodd tube phono stage with a Dodd battery preamp. The cart is a Denon 110 right now. I also have a SAE 1000 with a new Soundsmith level 3 retip but have been waiting to switch that cart out until I get a new arm, I am looking at a SME M2-9 when funds allow.

dromunds

Could anyone tell me what works best for shielding, as Carl suggested? I have the amps as far away from the TT as the cables allow and although the hum is very noticeably better it is not gone entirely. I would like to keep at it. Thanks.

[Deleted User]

The best material to use is mu metal, an alloy of nickel, iron and copper. Some phono cartridges use it as well as tape recorder shielding used around playback heads.

dromunds

Thanks much, I'll do some research on mu metal as this is new to me. Any other suggestions as to application methods, etc. or other suggestions would be very much appreciated!

Speedskater

A mu-metal sheet is very, very expensive and very fragile, bending it or machining it can ruin it's magnetic properties. It's seldom seen in audio circuits (the exceptions would be very low level circuits like mic or phono pre-amp input stages)
For airborne (radiated) fields you might try soft or mild steel sheets.

If you moved the T.T. 24 inches and nothing changed then it's a wire borne problem.

[Deleted User]

Here's a listing on eBay for mu metal, I've purchased from this supplier with no problems:
http://www.ebay.com/itm/like/400572008381?lpid=82
You can make fairly gentle bends with no problem, I don't believe machining will be necessary. It is used by guitar builders to solve humming problem with pickups.
How far away are the two amplifiers from your phono preamps?

dromunds

Kenneth, when I get home tonight I'll have to measure the distance from the two amps to the phono stage, I'm guessing its a little over two feet.

dromunds

When I hooked an alligator clip wire from the ground connection on the battery phono stage to the ground connection on the Belkin PF60, the hum decreased to a very slight level. I'll keep working at it but its very much better. A special thanks to Kenneth Swauger for answering my PM questions and guiding me to a favorable result. Thank you everyone else for your help also. Greatly appreciated.