Pre-amplifying an electret mic already in use

Question

I want to remove the DC component and boost to a line in level the mic signal of a regular headset plugged into a mobile phone while still allowing the mic to be used by the phone at the same time:

^{simulate this circuit – Schematic created using CircuitLab}

I need this to work with any standard headset and mobile phone; i.e. I want the preamp box on the diagram to act transparently, not affecting the interaction between the mic and the phone. For example, it shouldn't modify the impedance since iphones measure that impedance to detect whether a microphone is connected or not.

What is the simplest way to achieve this bias removal and amplification so that I get a low-noise line in version of the mic signal that I can use for other means (e.g. feed to a computer for monitoring or recording)?

Answer 1

I have used electret microphones for a few different projects and I use the following circuit (from here):

It works very well and provides a voltage gain of about 100, while also serving as a bandpass filter. It should solve your impedance requirements as well!

Answer 2

The DC component from the phone will continue to supply the electret mic capsule. As you show, the connection between the mic capsule and the phone remains intact.

It is trivial to block the DC component with a simple capacitor. A value between 1 and 10 uF would be more than adequate for the expected impedances and low frequency response needed. I would use a capacitor rated at least 5~10 V.

Then you simply need a "mic preamp" which is just an amplifier stage of perhaps 40dB gain (depending on the signal level from the mic and what your alternate destination needs. There are scores or hundreds of examples online of op-amp circuits which provide varying amounts of audio gain. Many are specifically designed for low-voltage battery operation.

Note, however, that computers and recorders typically also have mic-level inputs, so you may not actually need any gain here. Simply using a capacitor to block DC (in either direction) would be something I would try first, before going to the trouble of making a gain stage.