Audio Technica ATM33a Microphone Repair

The replacement model, the AT8033, is presumably essentially the same as this one.

SoS Article

These little mics offer quite good performance so I was saddened when one of mine started giving a low-level of hiss that rendered it essentially useless for anything professional. I thought I'd have a go at fixing it ...

Disassemble It

To open the microphone, first unscrew the lower barrel and slide down to reveal the battery compartment. Then, insert a sharp knife blade between the top of the decorative ring and the upper barrel so that this ring can be slid down to reveal the small screw that holds the two halves together. Unscrew that screw. Carefully pull out the transformer and PCB from the upper barrel and remove the insulation from the back of the PCB.

The ring is not threaded so does not need to be rotated before it can be moved down, but it may be lightly tacked in place with some varnish and might need a bit of force to free it.

Circuitry

Audio Technica ATM33a microphone schematic

atm33a sketch of PCB layout atm33a pcb photo

I reverse engineered the circuit. The only gain stage is an NPN emitter-follower stage that buffers the high-Z signal from the FET in the electret to drive the output transformer primary.

You can see there is an additional FET that acts as a switch to alter the bias condition and hence the dynamic range depending on whether fantom power is available. When the 5 volt supply from the fantom is present, the P-FET switches off to increase the capsule's load resistance.

Operating points table (volts)

Battery Feed Fantom Feed
White Capusle 0.67 2.2
NPN Base 1.3 3.0
NPN Collector 1.4 4.4
NPN Emitter 0.78 2.4
Battery 1.6 -
Xformer-CT - 41.0
Fet Drain 0.003 1.5
Fet Gate - 5.0

Fixing it

I'll write up the fix procedure as a blog article ... TNDJG:0007 Repair of Audio Technica ATM33a Microphone.
DJ Greaves - December 2014.