AXT Systems

MPM-3A Mic Preamp Module
MPM-3A Mic Preamp Module
The MPM-3 preamp module is an IC based switched-gain preamplifier using a THAT1512 as the main amplifying device, an NE5532 as the +/-6dB trim device and a THAT1646 as the balanced output line driver device.

A pair of connectors may be used to add transformer coupling to the microphone input if desired. Since 48V phantom power may be switched onto the mic input connector, protection diodes and isolation capactors are fitted.

The line input connector may be fitted or omitted, in this case an XLR + 1/4" combi connector was used since only one or the other should be connected at any time.
Construction is fairly straightforward as all components are through-hole types.

Ferrite beads and ceramic capacitors on the inputs and outputs server to provide some EMI protection. In addition to the components on the PCB, 47pF ceramic capacitors should also be fitted between hot/cold and ground on both the mic and line input connectors, at the connector.

The pushbutton switches are C&K PN series switches which may be fitted with a choice of coloured caps.

An equivalent input noise figure of -118dB was measured with a 150 ohm input termination at maximum gain. The harmonic distortion was measuerd as below the distortion of the oscillator used (0.015%).
MPM-3 Mic Preamp with metering fitted
Mic Preamp module with metering module fitted