|Thiele M4 microphone|
The Thiele M1 (cardioid) and M4 (cardioid and omni) are great looking microphones but feature some frankly bizarre design decisions, most notably the placing of the tube directly behind the capsule, which interferes with the rear pickup whilst simultaneously cooking the PVC on the rear capsule. Of course the manufacturers would never have expected that we would be picking up these mics sixty years on and trying to record with them.
|Inside a Thiele M4|
In addition, the power supply was built into the base of the mic which can lead to hum issues despite the on-board potentiometer that acts as a 'null'. This also means that the mic has two cables running from it - one for power and one for audio. Both mics use two-stage head amplifiers based around an ECC83 tube, and both have unbalanced outputs. Here are the official schematics for the M1 and M4 mics.
|Thiele M4 microphone circuit|
|Thiele M1c tube microphone circuit|
The M5 seems to have been the 'Studio' version, with an external power supply and output transformer for balanced low impedance operation. I haven't seen a factory schematic for the M5, but this is a drawing that I traced out from a specimen on the bench.
|Thiele M5 microphone, inside and out.|
Thiele sales brochure for M1 and M4
Elektro-Medizin M4 technical document
Elektro-Medizin M4 & M5 product sheet
Many thanks to the Microphone Online Museum for kindly sharing the documents and schematics.