Capacitance

Paul H wrote: Presumably, the capacitance of the interconnects between pre-amp and amp is of no importance...

It has its own importance but it is nothing to do with cartridge load capacitance, which it cannot "see" through the preamp.

Audio circuitry when properly designed is able to operate stably when driving any reasonable capacitive load, which is what a cable represents.

Around ten to fifteen years ago there was a movement (lunatic fringe) which glorified extra-high capacitance cables because it "improved" the sound. What it did was to alter the sound, especially when used with circuits in products that were not very stable, causing high frequency instability (oscillation and ringing way above the audio band). An unstable circuit will sound different (wrong), and it was such differences which this movement cited as being an improvement.

The simplest filter in audio is called an RC filter. An output has resistance so becomes the resistor. A cable has capacitance so becomes the capacitor. In equipment design I set the upper frequency response as high as it will go without circuit instability. I do that to keep the phase the same at audible frequencies. E.G. a 200kHz filter comes back into phase at 20kHz.

In a system the cumulative responses of each product combine and each one has a 45 degree phase displacement at the turnover point, which adds to the next, etc, and if set too low, the phase displacement will fall well down into the audible spectrum.

The last thing you want is a cable joining in!

The maximum allowable output impedance (driving or source impedance) according to "DIN" is 2.2 kilohms. If that were to be used into a 250pF cable, the turnover frequency would be 300kHz. The phase athough flat to 30kHz, will contribute its influence cumulatively on the system of separates.

Phase flatness isn't truly flat, it is actually -6 degrees or close enough to flat. When lots of -6 degrees are added together they make a much larger phase displacement, pushing further down to frequencies we can hear.

The brain expects phase and amplitude to follow nature's path, so half power in a natural filter is -45 degrees. But if it gets full power at -45 degrees it can be seen that it will confuse the brain... which usually gets a hangover headache from listening many hours (public address being good at that).

If each output impedance were to be nearer 1 kilohm (and most are, or less) and (as a rule of thumb) each interconnect is no more than around 100pf, it is doubtful that any additional phase displacement will cause problems.

You could ask Jon to let you borrow some of our cables whilst loaning the phono preamps??

Interesting as ever.   Accepting cables do make a difference do you use some of your own cables for internal wiring (accepting that internal cabling is modest and would be minus screening)?

Richard