RT-EM01 and RT-EM02

RT-EM01 - Circuit for electret microphone capsules with internal FET.

Suitable for Linkwitz-modified capsules.


The RT-EM01 is a pcb which provides a P48-powered circuit for Linkwitz-modified electret microphones.

The circuit is superficially similar to the Schoeps topology, using the FET (of the capsule) to generate the two anti-polar signals (sometimes called concertina mode), and having two emitter-followers as output drivers. However in this slightly more sophisticated design the FET is directly coupled to the emitter followers and the two output transistors develop their output signals from a common load resistor between their emitters, so keeping perfect symmetry despite being of opposite polarity and not necessarily closely matched in their other parameters.

The component count is less than the Schoeps design as it does not require some of the bias resistors or the zener diode and associated components.

The PCB has a position for an LED indicator which can be omitted (replaced with a wire link) if not wanted.
Dimensions 18 x 55 mm.
Price £6.50 with free p&p in UK. £3.50 p&p for ROW.
PCB fitted with optional LED.
Click image for pdf
A prototype built into an old Reslo housing. The cavities were stuffed with cotton wool and a foam cutout made to go inside the grill.
The housing is fitted into a piece of 22mm copper pipe with an XLR connector in the end.
It sounded good so I made a second one. This is a sample of a recording I made using them as a stereo pair spaced about 50cm apart.

RT-EM02 - Circuit for conventional capsules, or electret capsules without internal FET.

Identical backend to the EM01, with an FET front end.

A new development of the circuit, which enables the backend of the EM01 to be used with any capsule.

Click image for pdf
Dimensions 15 x 75 mm.
Price £6.50 with free p&p in UK. £3.50 p&p for ROW.
Connection diagram. Click image for pdf
Prototype build. Here Q1 is a 2N5551, Q2 is a BC212B and Q3 is a J113. R2 is 4.12k, R3 is 1.18k and R1 is a 10k multiturn trimmer.

The gate resistor RG1 is in the right-hand position, connected from gate to ground, and the trimmer R1 in the source of the FET is connected wiper to CW. This is the configuration marked Nuestra Senora in the connection diagram.

After adjustment for symmetry, we get a channel voltage of 5V, giving a drain current of 525 microamps and R3 current of 3.25 milliamps. The voltage across R3 is 3.8V.
Here the gate resistor RG1 is in the left-hand position, connected from gate to the wiper of R1, and R1 has the wiper connection to CW removed. The resistor R2 has been changed to 10k. This is the configuration marked Schoeps-lite in the connection diagram.

After adjusting R1 so that we get 10V at the top of R2, we get a drain current of 240 microamps and the voltage across R3 is 3.9V.