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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
rockinroni's avatar
rockinroni
287 posts
 


Raquel,
You need to check where your phono and your phono section cables are. As well as your tonearm cables.
If they are near any power cords you will pickup the ac line hum. It could be your phono section is to close to your power amp and picking up the field off of its transformer.
You have to play around and see how by moving stuff around this affects the hum.

I had a hum problem, I noticed when I moved my tonearm to is rest I had more hum, Yes it was getting closer to the transformer in my power amp so I switched places, I moved my pre amp next to my table and my power amp further away.
I still had some hum and some RF. I have a outboard phono, it was to close to a powercord I moved It away from the cord and then silence.

It wasn't quite that simple, I had to play around with the position of the phono section. I have a out board motor controller, I cannot have this any where near my preamps or hum again.

You will always have the leeds of your tone arm cables coming off your cartridge the way you do, this should not in and of itself cause hum.

I hope you can use some of these Ideas and get rid of your problem.
raquel OP
841 posts
 
Thank you, but from your response, I see that I was not sufficiently clear about the "loop" of tonearm cable to which I refer. It is not the 1/2 inch or so of wires connected to the cartridge that is at issue, but, as is the case with some linear tracking and VPI arms, a five-to-six inch section of exposed wires arching from the arm to a junction box to which is attached an interconnect (this one shielded) that is the direct connection to the phono stage. And again, the problem only arises when the "electrical link" consisting of the five-to-six inch section of exposed wires (and the cartridge) are connected to the system. The interconnect running from the junction box to the phono stage, the phono stage and everything else in the system has no hum, even at full volume, UNTIL the loop part is connected.

Thanks again.
twl
3,251 posts
 
Sure, it's possible that some hum could be induced there. However, the hum is coming from somewhere, in order for it to be induced on that cable. The wires themselves are not causing the hum, they are just picking it up. Unless there is a wiring problem or dirty connectors.

One thing about your description, is that you seem to state that the loop is somehow separate from the rest of the internal tonearm wiring. This is very unusual. I have never seen a tonearm wired so that the loop is not a continuous section of wire that includes the tonearm wiring. How do you know that the hum is not coming from the internal tonearm wiring? Is there some kind of plug that the loop plugs into at the tonearm, and you can isolate it from the internal tonearm wiring? If so, that may be part of your problem. There should be no breaks in the tonearm wiring from the cartridge clips, all the way to the final RCA plugs. Any breaks cause signal loss, and if they get dirty or have some oxidation can cause this hum.

Something sounds very unusual about this circumstance. What is it? If you have made modifications, tell us so we can properly determine the problem.

Other than that, I would look at trying to eliminate the source of the hum. As Ron said, it is likely a stray hum field from some of your equipment or power cords. If all else fails, get a different tonearm.
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