artto Posted July 6, 2005 Share Posted July 6, 2005 OK guys. I've run into a really strange hum problem on my main turntable. Hopefully some of you more techy guys can shed some light on it. Equipment: Linn LP-12 (Valhalla), SME III arm, Decca London Jubilee pickup, Audio Research SP-6B preamp. Problem: While Im aware the Decca pickups are notorious for hum problems, Ive never really had an issue with them, until now. My older Deccas have a 3-pin out configuration (common ground). The newer Jubilee conforms to the standard 4-pin out configuration (separate grounds each channel). Now heres the weird part. I discovered that the hum is at its maximum, exactly at the radial centerline, half way between the edge of the platter (aluminum) and the spindle, which roughly corresponds to where many LPs playing time ends. HOWEVER, when the platter is removed, I can swing the pickup/arm across its entire arc without any hum. By the time I can hear any hum at all the gain is turned up so high it would be absolutely ear splitting loud (thereby burying any audible hum). Any ideas as to what could be causing hum to focus on this narrow area and be audible at all, only when the platter is on? (The arm is grounded from its post to a ground post on the preamp. All component grounds are floated, and I've tried reversing the power plug polarities) Quote Link to comment Share on other sites More sharing options...
ZAKO Posted July 6, 2005 Share Posted July 6, 2005 I found I also needed to ground the turntable chassis. (Thorens TD 125) Quote Link to comment Share on other sites More sharing options...
artto Posted July 7, 2005 Author Share Posted July 7, 2005 Thanks Maron. I'm pretty sure the Linn has a ground wire "extension from the arm post to the sub-chassis. I haven't had the TT opened up in many years. I guess its time to check if the wire is still intact after the nurmerous VTA and other arm movement adjustments I've made recently. I'm still puzzled how the hum is increased when the platter is on (the Shure V15 is silent, the other Deccas somewhat less so, but no where near what the Jubilee is producing). Quote Link to comment Share on other sites More sharing options...
artto Posted July 7, 2005 Author Share Posted July 7, 2005 Well, I opened her up & checked the sub-chassis. It's grounded at two locations connecting to the arm post as per Linn specifications. Any other suggestions? Quote Link to comment Share on other sites More sharing options...
ZAKO Posted July 7, 2005 Share Posted July 7, 2005 One of my cartridge head shells would not seat tightly against the 4 pins. So I changed head shells. That helped. I guess the tiny springs in the 4 pins gave out or were weak. Probably the ground pin. It was a Orsonic too. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.