Archived from groups: rec.audio.tech (
More info?)
In <MlGDc.19241$Fo4.251640@typhoon.sonic.net>, on 06/27/04
at 08:45 PM, "Richard Steinfeld" <rgsteinBUTREMOVETHIS@sonic.net>
said:
>I'm using a Luxman C-1010 preamp with a variety of
>Stanton/Pickering phono cartridges. The capacitance load
>requirement for all of these cartridges is 275 pf. Luxman didn't
>specify the capacitance of their input circuit. That's what I need to
>know.
>I have a capacitor test meter, so I can measure the capacitance of all
>cabling; the meter goes bonkers on input circuits. I need to come up
>with a quick and dirty way to determine the
>capacitance of the preamp (or an actual figure!). I'd appreciate any
>help, especially help that's accurate.
Generally, the electronic manufacturers expect cartridges need loading
in this range and design their inputs accordingly. (Unfortunately, they
must guess the turntable wiring capacitance.)
I assume that the C-1010's active components in the input circuit are
frustrating the capacitance meter. Try using the lowest test voltage
that you can. If you don't mind fussing with things, break the
connection between the input jack as physically close to the first
active device as you can.
There will probably be a difference between measuring while the C-1010
is turned on or off, but in general, I feel that the test voltage will
be a bit high and you'll bump into some nonlinearity either way.
There are many methods that could be used to measure capacitance. Pick
one that measures at less than 0.5 volts pp otherwise you could run
into an active component inside the preamp that turns on.
One method that you may be able to manage is to insert a known
capacitor in series with the tester. This may reduce the voltage enough
to avoid the nonlinearity in the preamp and block any DC component from
the tester.
Another technique would be to inject a known single frequency into the
preamp through a resistor and measure the voltage at the RCA jack. Then
start adding parallel capacitors until the voltage falls to 50 percent
of the original. Be careful to use a low signal voltage. Don't forget
to deal with the input capacitance of the voltage measuring device and
your tester.
---
Others have suggested measuring the frequency response of the system
using a test record and tune the result for a flat response. This
sounds like a good idea, but I think you will find enough variation in
different test records to frustrate your sense of accuracy. Also, be
very careful to duplicate the temperature during all test runs, then
operate your turntable at that temperature. Finally, cartridges,
typically, are not very flat. There are peaks, dips, and resonances.
You may have to scratch your head a bit to decide when things are
"flat".
As you measure the frequency response, keep the response of your
measuring device in mind. Most DVM's are not flat enough for audio
work.
---
I think your easiest path is to examine the preamp's input circuit. You
may find series and/or parallel capacitors in front of the first active
device. Calculate the equivalent capacitance of that network and add a
few pF for the wiring.
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