Voltage on the condenser microphone capsule.

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vmanj

Well-known member
Joined
Oct 3, 2017
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297
Greetings to all.

I want to measure the voltage on the condenser microphone capsule.

Is it possible to make it an ordinary inexpensive multimeter?
 
Oktavas MK-319, MKL-4000/5000.

Can you please tell me what device can measure the voltage in circuits with such high impedance ?
 
vmanj said:
Can you please tell me what device can measure the voltage in circuits with such high impedance ?

Not really anything 'off the shelf '--- you may have to adapt other equipment to attempt this task.

Have a read through this page for some ideas:

http://www.audioimprov.com/AudioImprov/Mics/Entries/2016/7/4_Bias_Voltage_for_True_Condenser_Capsules.html
 
I used a 1Gohm resistor and I was able to measure the voltage on the capsule.
In my microphone Oktava MK-319 is approximately 28-30V.

The image is a simplified diagram of the MK-319 and I have a question.

- I want to experiment and slightly increase the voltage on the capsule to 40-45V.

If you reduce the value of the resistor R1, then it is possible to raise the voltage on the capsule, or do you need to do something else?
 

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Unfortunately  changing the value of R1 will make no difference  - there is essentially no current flowing in it.. (the capsule and C2 are the only 'load')

There is no voltage mulitplier to provide capsule bias in this circuit, so you are limited to the maximum value you can 'extract' from the phantom power supply....
It would seem that the JFET and associated circuitry are drawing around 5 mA -  that would tie up with a voltage of around 30V you are measuring across the capsule.

(You will need to measure that voltage at the R1/R10 junction for an accurate reading.... If you try and measure at the capsule end of R1 your meter will provide some loading, and you'll be measuring a lower voltage across the capsule. )

The schematic doesn't mention which JFET is fitted, so it's difficult to know what kind of IDSS range is required....
However it seems unlikely you'll be able to reduce the current drain of the circuitry by enough to make  any real difference in the voltage across the capsule, without seriously affecting the performance of the preamp.

I think you are going to need to add a voltage multiplier to make any significant difference to the volts across the capsule...
 
At the point R1 / R10 - 44V.
At the point R1 / C2 / CAPSULE - 28V.
To reduce the measurement load, I use the 1Gohm resistor.

It turns out you can slightly reduce the value of the resistor R1 to slightly raise the voltage on the capsule ?
 
vmanj said:
At the point R1 / R10 - 44V.
At the point R1 / C2 / CAPSULE - 28V.
To reduce the measurement load, I use the 1Gohm resistor.

It turns out you can slightly reduce the value of the resistor R1 to slightly raise the voltage on the capsule ?

Probably not ..... even with a 1Gohm resistor in series  with your measurement probe, you will still be loading the capsule input by a small amount. (1G + the input impedance of the meter)
So making R1 a lower value will certainly let you apparently measure higher volts at the capsule, simply because your meter will be loading the test point slightly less.

When C2 and the capsule are fully charged - and assuming neither 'leak' at all - there is essentially no current flowing in R1, so in theory its value is academic.
In practice there will be some small leakage and there will be a very small current - and hence a small voltage drop - across R1.
But reducing it's value slightly is unlikely to make any real difference.  It might reduce the sensitivity a tiny bit? (the capsule is now essentially working into a slightly  lower impedance).

If you double the voltage across the capsule from - say - 30V to 60V,and maintain the original value of R1, you would increase the sensitivity by 6dB. 

(It's a useful way of creating some 'noise free' gain - assuming the capsule doesn't collapse with 60V of bias!)

But if you only increase the capsule bias volts by 10 V or so, you'd only increase the sensitivity by around 1 or 2dB - probably not worth the trouble?
 
I see, thanks.

This is just an experiment.
I do not aim to raise the sensitivity.
Perhaps this will slightly change the nature of the sound of this capsule?
 
And one more question.
What is the lowest R1 value possible in this scheme (for example, 100-200M) ?
 
vmanj said:
I used a 1Gohm resistor and I was able to measure the voltage on the capsule.
What do you mean exactly?  It's typically the DMM that is the limiting factor in measuring this.

There are special instruments meant for exactly this, typically called 'picoammeters', and are notoriously hard to DIY because even tiny error currents can get grossly magnified.  These meters have buffered probes with guard rings in order to get the accuracy required and are pretty expensive.

You can measure the voltage by finding a node that sources the polarization voltage at a reasonable (e.g. <1M) impedance and measure there, and assume the capsule is at the same voltage (since there is no DC current through a condenser capsule).
 
I'd advise against over polarizing these capsules. Oktava knows what they're doing, and I'm certain these capsules are designed for 30v because Oktava makes quality stuff of their own design, they're not like a lot of budget mic manufacturers using a U87 and/or C414 style circuit without proper voltage. In the long run there's a chance you'll damage a perfectly good capsule.
 
Yes, perhaps I will leave R1 as it is.
In my octave, this is 510M instead of 680M, as indicated in the image.
R2 I also have 510M instead of 680M.

Many user reviews that these microphones have a different sound (within the same model).

Perhaps the octave and knows what it is doing, but I think they have an unstable build quality.

In my microphone, for example, there was a low-frequency rumble.
If I touched the case in the area of ​​the capsule, the hum disappeared.
I checked everything and helped only one thing.
I had to cut a small plate-screen from thin copper, place it between reed switches and magnets.
Probably crosstalk passed through the slotted slots for the switches ...

For this reason, I decided to just experiment with this microphone.
 

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