An opamp microphone circuit with a transformer.

click to enlarge photo

Documentation in progress - in the meantime ask me if you have questions

email me at parenthetical at this domain

This board has an opamp-driven circuit on a board designed for cheap donor mics the BM700 and BM800.

This is a version of my single opamp circuit, but using a transformer instead of output capacitors.

The input terminal is pin 3 of the opamp where it joins the 1G resistor. The other side of the capsule should go to the polarisation voltage supply (for true condenser capsules) or to ground, (for electret capsules). This circuit is not intended for electret capsules with an internal fet.

I have used an OPA1641 this is a very good sounding low noise modern opamp with relatively low current requirements (1.8ma). The board is made with SOIC8 solder pads - so you can use any SOIC8 single fet input opamp - but if you use one with higher current requirements you may need to lower the values of R3 and R4. You'll also want an opamp with very low noise.

Z1 is a 15v zener on the schem, this gives loads of headroom, probably more than needed.

At time of writing (Aug 22) I have built three and tested two of these boards. They work very well, but some transformers may cause a slight imbalance in the virtual ground, I will add more info on this and what to do about it when I have investigated more.