Is an Electrometer required to properly measure Polarization Voltage?

Paul

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Jun 14, 2013, 2:57:44 AM6/14/13

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Some people are saying any multimeter is too high
a load for the typical DC to DC converters you see
in the Schoeps type mic circuits.

LAB

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Jun 14, 2013, 3:54:27 AM6/14/13

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If you test the voltage beyond the 1GOhm resistor with a standard 10MOhm
multimeter, you will read something about 1% of the rreal value. You have to
test it before the resistor.

Scott Dorsey

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Jun 14, 2013, 8:43:28 AM6/14/13

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It's too a high a load for pretty much any mike polarization supply.
Remember the input impedance of the Fluke 77 is specified only as >10M.
You're dealing with stuff two or three orders of magnitude higher than the
test equipment.

If you think that's bad you should try and measure those Sprague electrostatic
supplies in the Syncron mikes. A 100G electrometer is too low!
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Scott Dorsey

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Jun 14, 2013, 8:44:11 AM6/14/13

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Even before the resistor, the source impedance of a lot of those converters
is very high. They aren't designed to deliver current, they are designed to
be quiet.

Paul

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Jun 14, 2013, 2:40:54 PM6/14/13

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On 6/14/2013 5:44 AM, Scott Dorsey wrote:
> In article <kpei7g$mp3$1...@speranza.aioe.org>, LAB <n...@mail.com> wrote:
>> If you test the voltage beyond the 1GOhm resistor with a standard 10MOhm
>> multimeter, you will read something about 1% of the rreal value. You have to
>> test it before the resistor.
>
> Even before the resistor, the source impedance of a lot of those converters
> is very high. They aren't designed to deliver current, they are designed to
> be quiet.
> --scott
>

Jim William's instructions call for increasing the polarization
voltage from 40 Volts to 57-60 Volts.

I measured about 40 Volts in the stock circuit.

So after converting to fast switching UF4001 and an 8.2V Zener, I
measured 60 Volts at the output of D5, which is before the 1GOhm
resistor. This was done with my LG DM-311 multimeter.

So I suspect Mr. Williams knew most people don't have ultra high
input impedance electrometers, and thus gave measurement instructions
as they would appear using standard DMMs. Therefore, the real unloaded
polarization voltage is actually higher.

is

Don Pearce

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Jun 14, 2013, 2:59:53 PM6/14/13

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No. A normal digital multimeter will certainly load the voltage and it
will read wrong, but if you know the value of the mic's bias resistor
and of the meter, Ohm's law will take you to the true voltage.

d

LAB

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Jun 20, 2013, 7:58:18 AM6/20/13

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What schematic are you reffering to? Something like this?
http://www.xaudia.com/xaudia/Schematics/Pages/Schoeps.html#1

Arny Krueger

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Jun 20, 2013, 9:13:14 AM6/20/13

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"Paul" <Quill...@gmail.com> wrote in message
news:kpeeko$gq3$1...@dont-email.me...

> Some people are saying any multimeter is too high
> a load for the typical DC to DC converters you see
> in the Schoeps type mic circuits.

Probably depends on where you probe them. The actual output of the DC-> DC
converter might be fairly tolerant of loads, but the diaphragm not so much.

You can probably evaluate this by obtaining or building a resistor that is
say 9x or 99x the input impedance of your meter, and doing the measurement
with it and without it. If the measurement with the resistor is very close
to 1/10 or 1/100 that of what you get with the plain meter, you are probably
in good shape.

You can build high value resistors with say 10 or 20 ordinary resistors, a
meg or two each. I'd clean the resistor string up with standard electrical
solvent or rosin cleaner followed by a finishing bath of distilled water,
followed by a good bake at around 150 degrees for a day or so to make sure
that there are no fingerprints and the like on them.