2. analog
  3. 1054175497

Exposed tonearm cable source of hum??

I run a 0.65 mv. output van den Hul Frog through a tonearm that features a looping, exposed (in the sense of lacking any real shielding) tonearm cable such as are found on some linear tracking arms, VPI arms, etc. (brand of tonearm to remain nameless so as to keep passions to a minimum). The tonearm cable becomes shielded after the "loop" and runs to a Rowland Cadence phono stage that features a built-in step-up transformer, i.e., I have a lot of gain. I have had a consistent hum with this set up in two different apartments. After consulting with the manufacturers and trying nearly every conceivable fix, I narrowed the problem down to the cartridge / exposed-loop portion of the tonearm cable which is the sole source of the hum (everything beyond the "loop" going in the direction of the speakers is dead quiet). Because the cartridge leads are connected properly, my guess is that it is the five or so inches of looping tonearm cable that is introducing what is a quite audible hum.

Does anyone out there know if such exposed tonearm cable "loops" are indeed known to be sources of hum? If yes, is there a fix (I can't imagine what that would be) beyond switching to a different tonearm cable design (which would mean, practically speaking, going to a different arm)? I have been listening to a lot of chamber music recently and the hum is starting to really bother me.

Thanks in advance.
raquel
raquel OP
841 posts
 
One more thing ... the problem exists whether the turntable motor is on or off, and regardless of the position of the tonearm -- these variables do not change the nature or quantity of the hum.

Thanks.
jameswei
883 posts
 
How do you know the hum isn't from upstream of the exposed cable, like from the cartridge or in the arm?
raquel OP
841 posts
 
The previous post makes a good point. I had reconnected the leads to the cartridge with the thought that these connections may be the problem, but it changd nothing. What I did not do until yesterday, however, was try switching the leads around a bit. It would seem that the Blue Negative tonearm cable lead is the source of the problem inasmuch as the system has no hum until that lead is connected to the cartridge, regardless of which post on the cartridge it is connected to (I switched the white positive / blue negative to the other channel and also wired the cartridge out of phase with the negatives connected to the positives). As soon as the blue lead hits a post on the cartridge, I get hum. I also connected the cartridge leads to a different cartridge, and the hum is still there (albeit of a slightly different noise signature). I would be prepared, now, to conclude that I have a bad blue lead, but have to, at this point, get back to the manufacturers with this new information.

Thanks to all of you for your help.
zaikesman
3,017 posts
 
Do this diagnostic test, if you haven't already, just to confirm your suspicions: Try manually introducing a temporary shield around each of A) The exposed wire, B) The tonearm, and C) The cartridge/headshell, simply by surrounding the part in question within your cupped hands while monitoring through the speakers at a revealing volume (or maybe even better yet through headphones, if possible). If the hum is being induced from outside interference in any of these parts, the level should be noticeably attenuated when you cover it up in this way. On the other hand, if there's no changes at all, I would move on to suspecting something internal...
raquel OP
841 posts
 
Thanks for the previous post. I tried that already and it yields no change.

The plot thickens, however. This morning, I was fooling around with the cartridge leads again, as well as the interconnect running from the junction box to the phono pre-amp, when I discovered that, while the blue ground is indeed the "trigger" for the hum, the hum only comes out of the left speaker, regardless of whether the blue lead is on the left or right channel, and most bizarrely, regardless of which channels the junction box interconnects are connected to. This would suggest that there is something in the system which "inhabits" the left signal path that does not like the blue ground lead. I have not yet had time to start going back through the system, component by component, to find out which component has a left channel compatibility problem with the blue lead -- it is a royal pain for me to switch phono preamp or preamp interconnects (two hour job) because of the component shelf I had made, so it may be awhile before I continue the fight.

Thanks to all.
More to discover