MM cartridge capacitance loading

[stevomaso]

Hi,
I am in in process of building a few MM phono preamps and I would like to know what type of capacitor is best for capacitance loading. I will start with the standard 100pf value but should I use ceramic, mica, polyethylene or polypropylene capacitors. I may use jumpers to switch values for tuning. Thankyou for any suggestions.

[PMA]

Polystyrene and polypropylene would work well. You should know series inductance and series resistance of the cartridge, and cable capacitance. Then, when loaded by 47k you have to tune additional load capacitance for overall flat response. You may find that usual 47k is not necessarily the best value for resonance peak damping.

[stevomaso]

Thank you for your response. Could I add the additional load capacitance via a polycarbonate variable capacitor (5 to 100pf) and the resistive loading with a 100 ohm multi-turn cermet trimming potentiometer. You could adjust both to fine tune the entire setup.

[PMA]

In case you speak about MM, then not 100 ohm, but 100 kohm.

[h_a]

Sure you can.

For a jump start you could search for the cartridge manufacturers recommendation for the capacitance (different for every MM) and begin from there.

Hannes

[stevomaso]

My apologies, I did mean 100 kohm.

[SY]

Value is FAR more important than the plastic film type. PP, PC, PS will all work fine. The issue for some MMs is often that there's already too much capacitance in the interconnects/preamp.

[h_a]

The issue for some MMs is often that there's already too much capacitance in the interconnects/preamp.

Even worse - usually the capacitance of the interconnects coming out of the turntable is completely unknown. Perfectioniados replace them with something better suited and get rid of the regularly crappy connectors as well at the same time.

Hannes

[Bob Richards]

Morgan Jones who recently wrote one of the best books on this subject recommended using a tuning condenser out of an old radio, that might be 300pF when closed, and to adjust it while playing some kind of test record that shows the upper end of the response (pink noise?). Then you'd be taking into account the capacitance of the tonearm wiring and interconnect cables too. This is because of how critical the value is. It's forming a resonance with the self inductance of the particular cartridge, to cause an extension of a frequency response that otherwise rolls off prematurely. So it's important to get it exactly right.

[stevomaso]

I tried measuring the capacitance of my connecting leads. The capacitors I tested were 1.2nf and 47 pf with my DMM with the setting at 2nf. These read at 1.155nf and 0.051nf respectively. My leads were not readable on their own, the DMM gave an open circuit reading. The leads in series with the capacitors did not change the value obtained for the capacitors. Am I doing something wrong as I am unable to read any capacitance values for any of the leads. But then leads are not capacitors.