How to insert -10dB Pad circuit

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Bernhard

New member
Joined
Dec 9, 2017
Messages
3
Hi all!

First newbie-post..
I’m planning to diy a schoeps like circuit with electret capsule I found here:
https://www.instructables.com/id/Modify-a-cheap-LDC-Condenser-microphone/

..for 2 reasons: I want some further training as it is my 2nd diy mic project and I plan to do a pair of D87 next. And also I for sure will find uses for a this cheap LDC as utility mic anyways.

As for this build I got a question:
I’ve seen microphone parts com sells it with a pad circuit. Is there any way I could implement that in my diy project. (Sure there is but could someone more experienced please tell me how to do?)

Thanks a lot and cheers,
Bernhard.
 
With a 'real' condenser capsule there are two options: lower the polarisation voltage, or shunt the capacity of the microphone capsule with an extra capacitor. (To ground or as a feedback capacitor)
For 10 dB select a capacitor to ground with the double value of the capsule capacity. (That will give ~ 10 dB.)
 
Thanks Jakob and RuudNL for your fast responses!

I will try when I have the capsule. Still on its way..

Bernhard
 
If you follow the AliceRF circuit diagram, there is no guarantee that the FET is properly biased.
I would connect a trimpot (500 K or 1 M.ohm) over the source resistor and connect the 1 G.ohm resistor to the wiper of the trimpot (instead of ground). This way you can adjust the correct bias for the FET.
 
RuudNL said:
If you follow the AliceRF circuit diagram, there is no guarantee that the FET is properly biased.
I would connect a trimpot (500 K or 1 M.ohm) over the source resistor and connect the 1 G.ohm resistor to the wiper of the trimpot (instead of ground). This way you can adjust the correct bias for the FET.
Actually, the circuit shown in the originallink describes that.
 
With a 'real' condenser capsule there are two options: lower the polarisation voltage, or shunt the capacity of the microphone capsule with an extra capacitor. (To ground or as a feedback capacitor)
For 10 dB select a capacitor to ground with the double value of the capsule capacity. (That will give ~ 10 dB.)
Hi RuudNL,
Do you have the scientific explanation why add a capacitor makes a PAD? Do you have any formulas to calculate the attenuation? I've got the same trouble with the KM84!
Many thanks
 
Hi RuudNL,
Do you have the scientific explanation why add a capacitor makes a PAD?
Roughly, the voltage depends on the ratio of the capacitance variation to the nominal capacitance (dC/C). Increasing nominal capacitance by simply adding a fixed cap in parallels lowers ratio.
Do you have any formulas to calculate the attenuation?
Again roughly, adding a similar fixed cap, i.e. doubling nominal capacitance, reduces level by half or 6dB. Adding twice reduces sensitivity to 1/3rd, or about -10dB. Adding 9 times reduces sensitivity by 10 or -20dB.
 
Roughly, the voltage depends on the ratio of the capacitance variation to the nominal capacitance (dC/C). Increasing nominal capacitance by simply adding a fixed cap in parallels lowers ratio.

Again roughly, adding a similar fixed cap, i.e. doubling nominal capacitance, reduces level by half or 6dB. Adding twice reduces sensitivity to 1/3rd, or about -10dB. Adding 9 times reduces sensitivity by 10 or -20dB.
Many thanks. You put me on the right track

Is that correct if I reword like this?

U = Q / C = Q / (Ccapsule + Cfixed)

In case doubling nominal capacitance: U = Q / 2C, that means the voltage is divided by half or 6dB.

In case tripling nominal capacitance: U = Q / 3C, that means the voltage is divided by 3 or 10dB.
 
This exactly how pads are implemented on cheap condenser microphones, usually with a swich (to enable -10dB) and a 100 pF (101) capacitor from capsule to ground.

Can you apply this -10dB PAD via capacitor on an electret microphone or we should go with the resistor route?

Thanks!
 
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