Oh alright, it was an old radio shack DMM. That crossed my mind that I'm trying to measure what ought to be an extremely high impedance circuit with a meter that is likely multiple times lower impedance than what I am measuring. And I'm still reading over 100 volts across the 2 capsule feeds from the circuit board. I ought to take the mic into my work where we have some very ex;pensive measuring devices along some very brilliant engineers that might offer some insight. I'm just the dumb R&D tech.
My best guess:
The 125.3 V side is connected to a relatively low impedance supply.
The 12.17 V side is connected to a very high input impedance, very low input noise current preamplifier.
When you connect a meter between the 125.3 V side and the 12.17 V side or from the 12.17 V side to ground, the current through the meter brings some junction in the preamplifier input stage into conduction, which is why you can measure something with a normal meter. Example: gate-source forward conduction of the input JFET or conduction of a reverse protection diode.
It would still be interesting to know if you get a loud hum when you touch the 12.17 V side with a meter lead (other side open) and if you don't get that when you touch the 125.3 V side.
If so, do you get a reasonably flat response/reasonably normal sound when you inject a signal into the 12.17 V side via a 47 pF coupling capacitor?
You're right that a condensor microphone capsule should behave like a capacitor, not like a 2 ohm resistor.
The 125.3 V side is connected to a relatively low impedance supply.
The 12.17 V side is connected to a very high input impedance, very low input noise current preamplifier.
When you connect a meter between the 125.3 V side and the 12.17 V side or from the 12.17 V side to ground, the current through the meter brings some junction in the preamplifier input stage into conduction, which is why you can measure something with a normal meter. Example: gate-source forward conduction of the input JFET or conduction of a reverse protection diode.
It would still be interesting to know if you get a loud hum when you touch the 12.17 V side with a meter lead (other side open) and if you don't get that when you touch the 125.3 V side.
If so, do you get a reasonably flat response/reasonably normal sound when you inject a signal into the 12.17 V side via a 47 pF coupling capacitor?
You're right that a condensor microphone capsule should behave like a capacitor, not like a 2 ohm resistor.
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I havent tried this yet but, I believe what you are getting at is that the voltage from the meter itself is biasing some input to an "ON" state of some sort, allowing a conduction at some level to take place. is that right?
if that is so, I wonder if I could expect getting different measurements of voltage if I reversed the polarity of the meter leads and referenced the meter lead to a ground - the 12.1 volt pin to ground - then reverse the leads see what it is, then do the same thing with the 125.3 volts pin, reverse the polarity of the leads, see if the reading changes based on polarity. I just may have to try that tonight.
Does that sound like a worthy measurement experiment?
Phil D
if that is so, I wonder if I could expect getting different measurements of voltage if I reversed the polarity of the meter leads and referenced the meter lead to a ground - the 12.1 volt pin to ground - then reverse the leads see what it is, then do the same thing with the 125.3 volts pin, reverse the polarity of the leads, see if the reading changes based on polarity. I just may have to try that tonight.
Does that sound like a worthy measurement experiment?
Phil D
Two things:
1. I suspect that when you are measuring voltages, the current through the meter is forward-biasing some junction; like you wrote except that voltage from the meter should be current through the meter.
2. The proposed experiment with just one meter lead that is not connected to any meter is just a quick and dirty way to capacitively inject a signal, namely the mains hum picked up by the meter lead.
You can always try, but I would expect you to get the same result as with the leads not swapped (except for the sign, of course). Digital multimeters set to measure voltage usually behave more or less like a 1 Mohm or 10 Mohm resistor, so you are swapping the leads of a resistor.