Tonearm cables

Please educate me. I am planning to buy an Audiomods Series 6 tonearm for my PTP Lenco. It will, replace a Rega RB301.

I have three turntables all connected to a Dodocus switch box which then connects to my Accession M via CuSat50. Positioning the TTs isn’t easy; it’s a bit of a stretch for all the tonearm cables. The Audiomods cable is 0.9m, which I think (but not yet checked) is shorter than the Rega.

Jeff Spall has said

“It is possible to do a custom version at about 1.1m but I don't advise longer because the loom uses the fine silver tonearm wire throughout, without any joins. It's always best to keep the cable before the phono stage as short as possible and use a longer line-level interconnect. This is especially important with MM cartridges because capacitance will increase with length.  If you need a longer cable it's best to have the arm with a 5-pin plug, L*nn/SME style in the base and use a dedicated tonearm cable”. 

 

So, what do I need to look for when buying a tonearm cable with a 5-pin plug?

Graham Slee wrote:  Have you considered a round table?

No!

Graham Slee wrote: Jeff is right about capacitance, and so if too long, MM's won't sound very good

What’s too long? Remember I have the switch box and some CuSat upstream of the Accession.

Just my opinion, but the cartridge wire loom should transition to a pair of coaxial's as early as possible. See "RF Issue, Hana EL Based System" to get the gist of why. There is no shield by extending the loom "one-piece" for up to 1.1m - just a very effective receiving antenna.

(It looks like the five-pin plug & socket is inevitable)

The cartridge wires inside the arm contribute about 25pF of capacitance. Unfortunately, this is not only signal "pos" to signal "minus," but to the other channel too. Therefore, an "extended loom" will sound a little different to coax.

Because I can't quickly post scientific notation, I will say that cartridge inductance parallel with wiring capacitance results in a tuned resonant circuit.

That approximately 250pF and 500mH tunes to 15kHz, where there is a spike, and a damping resistance of 47k ohms brings that spike down to an acceptable bump of near to one decibel.

The above is why the MM input is 47k and the "suggested" capacitance being in the 200-300pF region in the cartridge specs.

The cable contributes to a good half of that 250pF. It is a good idea to keep it around 100pF, such that a phono stage with input capacitance at 100pF totals 200pF. The phono plug/socket capacitance might contribute an additional 10 - 20pF etc.

If the arm wiring contributes 25pF, then the arm cable should be around 75pF, which makes 100pF.

If the stock cable used to make the arm cable is 50pF per meter, then 1.5 metres is 75pF. Therefore, 1.5 metres should be your maximum.

Don't forget that your switch-box itself contributes some capacitance. We don't know the amount, so we have to guess, and I suggest 50pF tops. This is why I was not enthusiastic about your suggestion of a switch box.

So, 50pF plus the 30pF of your short CuSat's totals 80pF, and to keep the cable to 75pF means the cable length has to be minus 0.1 metres!

But not to worry, as I'm damn sure another 50 or 60 pF won't make such a profoundly audible difference as the purists imagine - but I'll do the maths if you insist.

Therefore, in my honest opinion, the arm cable length should be 1 metre.

It will be impossible to place three turntables in usable locations and have the switch-box neatly positioned without abstract thinking, hence my suggestion of a round table!