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Correct way to determine phono input capacitance?


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I refurbished some time ago Ariston RD11s turntable with SME3009S2 tone arm. I have been recently experimenting with Stanton 881S cartridge and tried to get max performance out of this combination. Tonearm cabling capacitance is now a bit high at 200pF. Quad 34/405 combo sounded a bit dull as 34 input capacitance is specified to be 220pF and this means 420 pF with arm cabling. Stanton specified 47Kohm with 275pF to be preferred load.

After this I have used H/K Citation 11 and 17 SS preamps and ARC SP6C tube preamp and ARC SP14 hybrid preamp as they have been specified to have 40pF to 68pf phono inputs. ARC:s without any additional loading at places provided. HF performance with Citations seemed to OK but ARC:s sounded dull. As this did not make any sense, I measured input capacitance of ea amplifier power off and power on and figures were as follows

power off power on

H/K Citation 11 316pF 220pF

H/K Citation 17 170pF 33pF

ARC SP6C 105pF 96pF

ARC SP14 232pF 11500pF

As figures do vary this much, I have begun to think that measuring phono input capacitance at input RCA connector with hand held LCR meter is not correct way to do this...as there is 47K cartridge loading resistor in parallel to capacitance measured.

- Difference in H/K power on and off conditions is probably due input electrolytic having different bias when amps were powered compared to unpowered condition.

- ARC SP14 does have direct coupled FET input... so maybe input capacitance of FET can be seen only after powering the amp.

- Difference between cold and hot input of SP6C was not too much, even I thought that it should be highest due Miller effect (grid to plate capacitance will be multiplied by gain of stage) of input 12AX7A dual triode.

So... I am asking if Pete, Tom, Carl, Roy or anyone out here have thought how phono input capacitance should be measured, if one expects to find correct figure.

Best Regards

Kimmo

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  • 2 weeks later...

I found reply to my question in 1985 Hifi News supplement issue. 47K cartridge load resistor is not the only issue that fouls input capacitance measurement with LCR meter. Feedback in RIAA stage also effects input capacitance. As RIAA curve is usually built in feedback loop, amount of feedback varies with frequency... so input capacitance is not constant over audio band. Most useful figure for input capacitance should be measured at around 10 000 hz with signal generator and oscilloscope, as this is the frequency band where input capacitance does effect most. Some math is also needed to determine resistive component of input resistor and reactive component of input capacitance at 10 000 Hz.

There was note in same article about proposed standard for 220pF input capacitance in MM phono stages (this would explain 220pF Quad 34 input capacitance). If this had become true... it would have helped many MM cartridge designers a lot, as they had been aware how their designs are loaded when used in real world.

Best Regards

Kimmo

PS any other thoughts???

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