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Old 22nd February 2012, 04:07 AM   #1
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Default phono cable capacitance

I have a set of cables on my SL1800MK2 which are not original and I have no clue what their capactiance is.

Where I work we have a pair of "smart tweezers" which claim the following :

Capacitance:

Range:
0.5 pF to 4999 μF

Precision:
up to 0.2% (see datasheet for deatils)

LCR Meter - Smart Tweezers - Digital RCL, Impedance and ESR Meter, SMD Component Tester

If I pull my headshell off and place these tweezers across each RCA connector center to ground, it should show me the capactiance of the entire circuit, correct?

I have no idea how accurate this will end up being.

Thanks,

Chris J
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Old 22nd February 2012, 01:28 PM   #2
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hello,
just make sure there is no DC path to ground either through the cartridge coils or via a resistance network. So, whether you measure the external interconnecting phono-to-preamp cables or the internal tonearm cables the rule applies. For the internal arm cabling use an ohm-meter (with the cartridge removed!) and make sure there is an open circuit from signal conductor to ground. Then use the capacitance meter. good luck!
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Old 22nd February 2012, 01:37 PM   #3
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Thank you for your response.

I'm a bit concerned as I just tested some audio coax with these tweezers and came up with around 30pf per foot. Does this sound plausible?

When I go home tonight I'll test the turntables cabling and see what I come up with.
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Old 22nd February 2012, 01:43 PM   #4
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Quote:
Originally Posted by thetube0a3 View Post
I'm a bit concerned as I just tested some audio coax with these tweezers and came up with around 30pf per foot. Does this sound plausible?
Very plausible.
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Old 22nd February 2012, 01:50 PM   #5
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yes, depending on the dielectric and particular geometry the distributed capacitance may vary from 10 pf/foot (rare and special low-capacitance stuff) to 50 pf/foot to well over that.
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Old 23rd February 2012, 12:42 AM   #6
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Sigh,

So I measure 320pf with this coax and the tonearm combined. I'm guessing the tonearm is probably what, 20pf alone? That would mean this coax has 100pf per foot.


What I'm not sure of is my V-15 III calls for 47K load and 400-500pf capactiance. My APT Holman preamp allows me to adjust capactiance, so I have it set to 100pf. Surprising to me, this is where I had it set by ear.

Being this gives me the necessary capactiance should I bother swapping the cables out?

I'm considering either stock Technics cables for an SL-1200 (same table really) or just buying some RG59 and making my own cables.

Opinions?
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Old 23rd February 2012, 05:24 AM   #7
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hi,
the cable in the tonearm is much thinner, meaning there is a thinner dielectric thickness between the signal and ground conductors. I hate to think of the case where there is a twin (stereo) signal sharing a common shield which would only make things worse. Having said that, the capacitance of the cable in the tonearm is usually higher per foot than with thick (audio/video 50/75 Ohm) cable. So, i wouldn't be so sure about the 20pf for tonearm and 100pf/foot for interconnect. Other than that your setting sounds reasonable.
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Old 24th February 2012, 03:49 PM   #8
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Quote:
What I'm not sure of is my V-15 III calls for 47K load and 400-500pf capactiance. My APT Holman preamp allows me to adjust capactiance, so I have it set to 100pf. Surprising to me, this is where I had it set by ear.
Perfect - well done

Quote:
So, i wouldn't be so sure about the 20pf for tonearm
From what I know this value is typical.

Hannes
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Old 24th February 2012, 07:21 PM   #9
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Well for the fact that I may want to switch cartridges in the future I think I'm going to swap in some good coax when I get some time.


A friend is giving me some Belden 1505A which is RG59 and 16.3pf per foot. Only drawback is its a solid center conductor so not too sure on how flexable its going to be.

I just hope I can fit the RG59 into the turntable, not sure how big the opening for the cable is and I refuse to do any physical mods to the table.

Any opinions on this approach?
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Old 24th February 2012, 07:28 PM   #10
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You put your finger right on the problem- the mechanics. You don't want your turntable's isolation compromised by a stiff cable. There are workarounds (e.g., using thin wired inside the TT to RCA females mounted on the base, then the stiff cables connected there), but you have to be careful that the medicine isn't worse than the disease.
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