Cable construction for phono cables, MM turntable

[noviygera]

Hello,

I am referring to the unbalanced RCA cables + ground wire from the TT to the phono preamp. I have an SL1200 with MM cart and I need to replace the phono cable (old, loose molded plug)
The original's capacitance is 120pF for the 45" = 32pf / ft. Anything around that capacitance is ok. I started thinking about options.. Before ordering and soldering I'd like to find out what are your thought on the actual cable structure, since there are two that make sense to me.

1. Coaxial variety of low capacitance wire AND separate external ground wire, (the common approach)
2. Shielded twisted pair where the shield is used as the ground wire (no separate ground wire)

Can the shield be used as the ground wire?
I am not sure if the 2nd option is going to be too high capacitance due to shield being used as the ground wire?
How to measure capacitance of the shielded twisted pair wire?
Thanks!

 

[thegeton]

I would do this the easy way: contact Blue Jeans Cable and have them make you one of their LC-1 phono cables with a ground wire (12.2 pF/ft).

That's what I use, and it works perfectly.

 

[DVDdoug]

I don't think the separate ground wire matters. It might increase the capacitance since it's physically closer to the signal wire (assuming twisted pair). And it might make it easier to solder because you only need to connect the shield at one end.

Note that lower capacitance isn't necessarily better. There is an optimum, including any input capacitance of the phono preamp.

 

[Speedskater]

a] I don't think that low capacitance coax cable is the best choice for many phono cartridge to pre-amp combinations (although it's total cable capacitance that is of concern).
b] If the separate ground wire is kept in close proximity to the single coax cables, there won't be a ground loop.

 

[noviygera]

Thanks for the advice. However, I am interested in discussing the cable structure rather than brands and model numbers.

particularly these two structures:
1. Coaxial variety of low capacitance wire AND separate external ground wire, (the common approach)
2. Shielded twisted pair where the shield is used as the ground wire (no separate ground wire)

 

[noviygera]

How to test the capacitance of twisted pair with shield cable?

 

[Speedskater]

Doesn't the external ground wire, just connect the tone-arm, turntable motor and chassis to the pre-amp chassis?

 

[Speedskater]

That is one strange paper.

 

[noviygera]

Report back with what your testing reveals.

I can do that. But I'd like to measure cable capacitance to be sure I am on the right track. How do I measure the capacitance in a cable that is twisted pair and a shield?

 

[noviygera]

Standard coax type Technics cable is about 66pF for 47" including RCA connector. I am taking the measurement between two conductors.
But if I try a twisted pair with a shield cable, such as Mogami W2549, I will use the connected shields from BOTH channels (L+R) as the ground wire. The shields will become my ground wire, connected on the pre amp end and also the turntable chassis, just like the regular ground wire gets connected.
How do I measure capacitance now? Do I add the capacitance between "positive" and "neg" conductors and ADD the capacitance between "pos" and the shield and also Neg and the shield?

That is the question

 

[Speedskater]

Each channel of the phono cartridge has two pins. Is there continuity from the shield to either pin of the cartridge?
Shielded Twisted Pair cable? Is this a balanced input pre-amp?

 

[musicforcities]

Din is an inherently better termination for turntables. Or a XLR variant (4 pin, with independent shield or 5 pin with the ground wire).

But if you must use RCA, don’t use the shield for the ground wire. Only negative signal.

Keep the length as short as possible.

Coax or twisted is fine probably.

However Technically the signal from the tonearm to the preamp is inherently balanced (of a sort) , so using twisted pair would make sense. I have used beldon and mogami microphone cable with rca ends and left one wire disconnected for turntables The blue jeans phono cables works great. but I have also used standard rg6 coax with no issues too.

My turntable doesn’t seem to need the ground wire connected. At least I hear no difference. And yes, the ground wire is just tied to the tonearm wand and plinth usually. It is for ground noise/hum and shielding. Therefore using it tied to your negative tonearm signal is a no no.