Capsule microphone biasing decoupling?

[alexg]

Hi All, not sure if I worded it correctly but basically I am looking for answer perhaps to one of simplest questions: will placing decoupling capacitor (C18 in the circuit below) at the mic biasing circuit help at all to mitigate a bit of a ripple because its powered from switching regulator? Or it makes no difference or even make things worse?

[RoGeorge]

Maybe, but if you place a resistor or inductance between V33 and the node beteween C18 and the microphone's resistor, then it will certainly help, because it will create a low pass filter for the power noises only.

[alexg]

I have to double check, is this what you advising?

[Zero999]

I have to double check, is this what you advising?

That seems reasonable, although C18 may need to be much bigger for it to be effective.

[Audioguru]

I use the mic's same decoupling for the biasing of the mic preamp like this:

[Zero999]

I use the mic's same decoupling for the biasing of the mic preamp like this:

Yes, that will work but the mic' biasing resistor might have to have a lower value than 10k, if powered off a 3.3V supply.

A bit off topic but increasing R3 to 120k in that op-amp circuit, will help to move the op-amp's bias nearer to half the supply voltage. In fact 150k or 220k may be even better, since the TL072 works better with its inputs nearer +V, than -V.

[Audioguru]

I use the mic's same decoupling for the biasing of the mic preamp like this:

Yes, that will work but the mic' biasing resistor might have to have a lower value than 10k, if powered off a 3.3V supply.

A bit off topic but increasing R3 to 120k in that op-amp circuit, will help to move the op-amp's bias nearer to half the supply voltage. In fact 150k or 220k may be even better, since the TL072 works better with its inputs nearer +V, than -V.

Good points.
An electret mic needs at least 2V so that it does not distort fairly loud sounds and it draws about 0.5mA. Then with a 3.3 supply the resistor powering it should be about (3.3V-2V)/0.5mA= 2.6k. The decoupling resistor also needs some of the 3.3V across it so maybe the decoupling resistor will be 240 ohms and the resistor powering the mic will be 2.4k ohms. Such a low resistor value will reduce the mic output level.

The minimum allowed supply for a TL07x opamp is 7V and it has the "Phase Inversion" problem. If an input voltage gets to within a few volts from its negative supply then its output suddenly goes as high as it can. I have never had an electret mic with a peak output level of a few volts (inside a drum that is played loud?). 

[Zero999]

I use the mic's same decoupling for the biasing of the mic preamp like this:

Yes, that will work but the mic' biasing resistor might have to have a lower value than 10k, if powered off a 3.3V supply.

A bit off topic but increasing R3 to 120k in that op-amp circuit, will help to move the op-amp's bias nearer to half the supply voltage. In fact 150k or 220k may be even better, since the TL072 works better with its inputs nearer +V, than -V.

Good points.
An electret mic needs at least 2V so that it does not distort fairly loud sounds and it draws about 0.5mA. Then with a 3.3 supply the resistor powering it should be about (3.3V-2V)/0.5mA= 2.6k. The decoupling resistor also needs some of the 3.3V across it so maybe the decoupling resistor will be 240 ohms and the resistor powering the mic will be 2.4k ohms. Such a low resistor value will reduce the mic output level.

The minimum allowed supply for a TL07x opamp is 7V and it has the "Phase Inversion" problem. If an input voltage gets to within a few volts from its negative supply then its output suddenly goes as high as it can. I have never had an electret mic with a peak output level of a few volts (inside a drum that is played loud?).

The TL072's output stage works better near the positive supply rail too, so it's still better to bias its output near the positive, than the negative rail. Unfortunately this isn't documented on the data sheet.