What is my input capacitance, can I measure it?

[Balle Clorin]

I got an old lovely Technics SU-A900mk2 I bought in 1994.
I got curious about using the RIAA to do cartridge frequency measurements, but there is no information about input capacitance for MM.
Can I tell from this schematic what is is?, can I put my LCR meter on the Phono input and measure it safely?

http://www.philipsradios.nl/Schemas/overig/Technics%20SU-A900%20MK2.pdf

 

[JP]

#160

 

[solderdude]

Around 250pF.
You cannot use a capacitance meter.

The best way would be to use a tone generator and a resistor and determine the -3dB point and calculate the capacitance.

 

[Balle Clorin]

Maybe simulate it in LTspice then ?at A given Source resistance

 

[SSS]

I got an old lovely Technics SU-A900mk2 I bought in 1994.
I got curious about using the RIAA to do cartridge frequency measurements, but there is no information about input capacitance for MM.
Can I tell from this schematic what is is?, can I put my LCR meter on the Phono input and measure it safely?

http://www.philipsradios.nl/Schemas/overig/Technics%20SU-A900%20MK2.pdf

View attachment 300513

It depnds on the test voltage and frequency of your LCR meter. If voltage is around 100 mV and 1 kHz then it should work. But you may add the values of the capacitors C126 128 102. How much C106 counts is questionable due to R110 and it is frequency dependent. So around 227 pF may be the value. Don't forget the capacitance of the cable from pickup to phono input. Needs to be added too.

 

[MC_RME]

Around 250pF.
You cannot use a capacitance meter.

He wrote LCR meter, and he can use that. I do this all the time, either with single spots at 100/120/1k/10k/40K100k Hz (handheld LCR meter, ET432), or even a full sweep using the Analog Discovery 2 with Waveforms software, or the APx555 (with limits, and only on outputs). The first two work perfectly on both inputs and outputs with the ET432 having a fully galvanically isolated and unbal/bal capability advantage.

 

[MC_RME]

#160

A DMM is not a LCR meter. Using the latter would work, but still requires to remove the cartridge and connect to the loose cable ends hanging out of the tone arm, as @pkane mentioned there.

 

[JP]

A DMM is not a LCR meter. Using the latter would work, but still requires to remove the cartridge and connect to the loose cable ends hanging out of the tone arm, as @pkane mentioned there.

I never said it was. LCR meter can work as long as it doesn't overload the input. The linked method always works.

 

[Avp1]

I got an old lovely Technics SU-A900mk2 I bought in 1994.
I got curious about using the RIAA to do cartridge frequency measurements, but there is no information about input capacitance for MM.
Can I tell from this schematic what is is?, can I put my LCR meter on the Phono input and measure it safely?

http://www.philipsradios.nl/Schemas/overig/Technics%20SU-A900%20MK2.pdf

View attachment 300513

In MM mode - 47K ohm and 300 pF
In MC mode - 220 ohm and 1300 pF

 

[Jim Hagerman]

Around 250pF.
You cannot use a capacitance meter.

Wouldn't it be a bit more than that? I see 27 + 100 + 100 + 82 + whatever you get from the JFET * Miller effect. So at least 300pF+?

I've been using this cheap LCR meter (is it really different from a C meter?) that inputs a frequency (I haven't measured yet) that then detects either phase or resulting impedance and converts to F. I just make sure that the phonostage remains linear and that the sinewave shows up at output with no distortion. Seems to give me reasonable and expected numbers. Except for my Cornet MM, which came out at 7pF. I know it is low, but in my head seems maybe it should not be quite that low. If the frequency is one the order of 10kHz, then it may be correct, because I load the first tube with EQ, eliminating Miller effect multiplication.

 

[Balle Clorin]

Thanks @Jim Hagerman . So I do this with the unit ON?

 

[solderdude]

Some LCR meters might give the correct values some may not.
When it reads somewhere between 250F and 300pF it will be a reliable measurement.

 

[Jim Hagerman]

Thanks @Jim Hagerman . So I do this with the unit ON?

Yes, input stage needs to be biased properly for proper operating conditions. Make sure you are not saturating amplifier with signal level. If unit is off, you will run into PN junction clipping or something else, that is not truly representative.

 

[morillon]

approach the problem in a roundabout way...
you observe the response fr above 5khz of a cartridge and compare it with other measurements made with your external phono riaa stages which are rather precise on this point... will give you a good order of magnitude...

 

[SSS]

View attachment 301601Thanks @Jim Hagerman . So I do this with the unit ON?

Viewing the manual the test voltage is around 500 mV which is much more than the max voltage for the MM input. So I would not recommend it.
But I wonder why the capacitance needs to be measured exactly? Take just the values from the schematic. Of course the capacitors built in have tolerances from the stated value.