Hum with CTP2000 pre & Numark turntable

[Drew]

Hi all;
I wasn't sure about starting a new thread on this subject, but here goes.

I am getting a ground hum/buzz from my Numark turntable connected to a CTP2000 that I just received.
I don't get it from the 1000B that I have used for the past year. I also don't get the hum when I connect my old Hitachi to the CTP2000. The hum also becomes louder when I connect the ground wire to the post on the preamp or when I push lightly on the tonearm in it's cradle.

I have been in contact with Curtis at Tracer and we've tried a few things, but to no avail. I've also contacted Numark with this problem, but have not heard back.

Both the turntable and preamp are plugged into the same powerstrip. I just had the thought of swapping out my cart to see if it might make a difference, so I'll try that tonight.

Any ideas as to what may be causing this or a solution?

Thanks....

[Craig Maier]

Is there good ground continuity between the Numark Turntable's grounding strap and the chassis of the turntable? Also, check the grounding resistance between the CTP2000 grounding screw and any of the RCA phono plug commons/shells. You can test that with a multimeter or continiuty tester. The resistance between the ground strap and the chassis of the turntable should be less than 1 Ohm. Assuming that the Groundstrap resistance is low enough, do you get a different audible result by connecting the turntable grounding strap to the computer chassis as opposed to the preamp grounding screw?

keywords: turntable hum, turntable buzz, turntable grounding, record player hum, record player buzz, record player grounding

[Drew]

I'll try that tonight. Both Curtis and I find it interesting that the turntable works fine with the 1000, but not the 2000. I'll let you know....

Thanks....

[Craig Maier]

If I were to make a WAG (Wild Ass Guess), I would tend to think that the CTP2000 grounding screw does not have continuity between itself and the various RCA Jacks of the unit thereby indicating that maybe it has a broken wire inside the preamplifier.

[Drew]

I would agree, except that the preamp works fine with my older Hitachi turntable. So... something in the Numark and the 2000 pre don't want to play together.

[Craig Maier]

Not necessarily. Your Hitachi turntable may produce much less AC leakage current reducing the need for a really good turntable chassis ground connection. The Numark may produce a higher level of AC Leakage current requiring a very good connection to minimize noise injection. And, if the new preamp has an open in its grounding system, that could still explain things.

[Drew]

Maybe because the Numark is a DJ turntable with direct drive as opposed to belt driven, that might have something to do with it needing a more solid contact? It is pretty heavy duty and designed to be taken on the road to clubs and such.

[Craig Maier]

It could be; larger motors translate into higher values of distributed capacitance to the motor housing which results in larger values of AC leakage current which is what the grounding strap is designed to produce a return path for. Less leakage current reduces hum noise injection.

Let us know what you discover as the source of the problem.

[Drew]

Well, I did some more testing last night and found what I think might be interesting.
I was going to test the ground with my computer case and accidentally touched the ground clip to the input jack of the preamp. The buzz all but disappeared. Same with the output jacks.
Thoughts???

[Craig Maier]

That means that the preamplifier has an open ground connection between the grounding screw/preamp chassis and the circuit board's signal common. In other words, there is a broken wire or a missing wire inside the preamplifier.

Note that this conclusion is consistent with my WAG posted above. You can confirm that with a continuity tester. There should be continuity (less than 1 Ohm) between any and all of the RCA jacks and the turntable grounding screw. You can test that with a simple continuity tester or Multimeter operated on the resistance scale.

[Drew]

Yeah, it makes more sense now. I'll crack the case this weekend and see what I can do inside.
Thanks for the help, you've always been great with that on this forum.

[Craig Maier]

If memory serves me correctly, the ground wire is a bare silver colored 20 AWG bus about 6 inches long that runs from the chassis ground to the Printed Circuit Board. I did not keep the prototype and never obtained a production unit for myself, so I am relying on my memory. But that information should be helpful. Let us know what you find wrong inside.

[Drew]

Well, based on your description, it's missing the wire completely. What I found was the ground post, nut & a star washer by themselves, nothing connecting it to the board.

So, I guess it's back to Tracer for repair.

[Craig Maier]

Yup. Sounds like a manufacturing defect. Hopefully, they will get you another one quickly.

[Drew]

Me, too. Thanks for all the help....