Cable capacitance

You need to aim for a capacitance of 100pf between cartridge and phono input. Together with the 100pf of the phono input (a nearly universal input capacitance) these two capacitances form a parallel circuit with the sum of 200pf that just about guarantees an exact match for most moving magnet phono cartridges. This balances the cartridge's inductance which is usually in the region of 500mH, and the 47k input resistance (which may also be an impedance) controls the overall resonant Q.

Therefore a half metre length at 64pf/metre is not going to do this.

I visited Clearaudio's website and found it bereft of sufficient technical detail. I couldn't find any mention of capacitance, just the same old hype.

At least AudioQuest are a bit more upfront, but PVC indeed? The capacitance will probably be in the order of 150 to 300 pf per metre.

The cheapest solution would be to buy satellite down lead (not TV aerial down lead) and some metal bodied phono plugs and make your own. The satellite cable is usually called CT100, and you need to ensure it has a cellular polyethylene primary insulation surrounding its solid copper central conductor, and the shield is copper foil and copper braid. Lay it so the signal follows the direction of the jacket printing.

Allowing 25pf for the arm's internal wiring, 1.5 metres will be just about right from the arm to the phono input, and anything between 0.5 to 3 metres will be OK from phono stage output to your amp.

And don't forget to manipulate the full length of the cables to persuade the primary insulation to break its grip on the solid conductor otherwise the timing will sound all wrong. Insulation is simply just a poor form of conductor - there is no such thing as a perfect insulator - the 16th Edition IEEE Wiring Regulations teach us this when they talk about cable heating and protective fuse ratings for different insulation and encapsulation techniques of mains power wiring.

It's not really rocket science and the black art hi-fi paints it - they're just very good at the art of deceit (IMO)....

Either it's me or you but there's some confusion here.

The usual tonearm wiring capacitance (measured) for most tonearms I've ever encountered is around 20-25pf.

Usually tonearms come complete with fixed interconnect or a detachable by means of a 5 pin arm base connector. In the case of the Pro-Ject it looks as if the tonearm is terminated in the turntable with phono sockets.

The usual 100pf or thereabouts (90pf in this case) should refer to the tonearm internal wiring with the usual interconnect hopefully supplied with the turntable.

If the tonearm itself was to be 90pf we would have an impossible situation where (with the exception of ultra short and often impractical ultra low capacity cables as you mention here) by adding the usual length of arm interconnect cable, we would have already used up all the load capacitance before considering that of the phono stage.

The result would be a terrific loss of HF information.

So I'll repeat what I know (about 40 years experience)...

The tonearm itself contributes about 20-25pf. The cable connecting it to the phono input contributes about 70-80pf, and the total arm plus cable capacitance is 100pf plus or minus a bit.

If I have this wrong then I'd better retire now.

Now add the phono stage input capacitance which in reality will be a tad more than the 100pf cap stuck across its input because phono sockets contribute 3-5pf, and you're very close to 200pf.

If you're out by a good percentage in either direction the sound will deteriorate de-facto.

The resonant frequency if things are matched properly will be...

using

Fr = 1/ 2pi x square root of (L x C)

(best I can do here to represent the equation because of this software's limitations - devil in the detail again!)

Fr = resonant frequency, pi = 22/7 (close), L = 500mH on average for a MM, C = 200pf

Fr = 1 / (44/7) x square root of (0.5 x 0.0000000002) = 16kHz (as near as damn it)

Why 16kHz?

because rearranging XL =  2pi x F x L to make F (frequency) the subject

F = XL / 2 x pi x L, where XL is the 47k load and L is the inductance 500mH

F = 47,000 / (44/7) x 0.5 = 15kHz (as near as damn it)

so the approximation for capacitor value is close (within 1kHz)

Therefore the loading is OK.

(Edit: actually 220pf total loading capacitance gives an almost ideal match and therefore 90pf of actual interconnect cable looks like the ideal situation and 1.1 metres of Belden 9907 Coax - Coaxial Cable - Thinnet 10Base2 Ethernet looks tempting. I shall have to try it.)

Now try running it with different capacitances (and even different resistive loads). Too low capacity and it would peak at a higher frequency causing upper midrange losses. Too high capacity and it would peak in the upper midrange causing high frequency losses.

(Edit: And now it can be seen that some phono stage designers talk twaddle when they talk about finding better load impedances for MM cartridges - mentioning no names, and no names of reviewers who support them through their combined ignorance, in one case contradicting the meaning of his very own name...)

As for the phono stage output cable, capacitance still matters but it is not a matching capacitance as in matching a MM cartridge.

The 0.5 - 3 metre lengths of satellite cable I quoted would give 30 - 160 pf. Any more and you run into high frequency attenuation which displaces nth order harmonics, defeating the object of buying the Reflex.

Edited by Graham Slee - 09 Mar 2008 at 8:46am

Jake wrote: Thats some great info there Graham. Im happy to see that my Air Matrix meets your guidelines. Is this the same with MC cartridges, or is the total capacitance TT to Phono Stage that we are aiming for a little different? Cheers, Jake

Hi Jake,

It's different for MC who's inductance is much smaller (in the order of 5mH plus or minus a mile) so the small capacitances discussed above are insignificant, requiring something like 10-20 nano farads to make an audible difference. It is rumoured that SME do a cable that is highly capacitive and many MM owners have been caught off-guard by not realising it's for MC. Whether that's true or not I can't say. However, a high capacitance cable when used with MC into an electronic step-up or high gain phono preamp can result in instability because it makes a resonant circuit at a very high frequency. In which case it needs to be damped by a small value input resistance. It gets complicated so I'd stick with the sort of cable used for MMs.

Mostly it's the load resistance that bends or adjusts the response of an MC. 100 Ohms is just about universal but some cartridges require a bit more or a bit less. People get confused by cartridge manufacturers who say 100 Ohms for use with a step-up transformer or 47k for use with a phono stage direct, simply because phono stages have historically been 47k. 100 Ohms works in both cases.

A final point is the quality of insulation. You find that a good dielectric such as polypropylene is better in film/foil capacitors, well all dielectric means is insulation between plates, so if you can get a good cable dielectric such as polyethylene or PTFE, all the better. "Non-stick" qualities improve skin-effect performance as Sol found by his "cable whirling" process that unstuck the CT100's solid conductor from its primary insulation. See my take on overhead power lines (6 March)