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Unknown German Prototype Tube Microphone - 'The Unbekannt"

Unknown German tube microphone
Here is a recent Ebay find. It's an unbranded tube microphone, and judging from the components probably from the 1960s, in what was formerly West Germany. We've called it the 'Unbekannt', which is simply German for 'unknown'. The amplifier circuit is a 3 stage unbalanced transformerless design, using EF40 pentode and an ECC81 twin triode. The final stage is a cathode follower.

The schematic is here.

As is so often the case, the microphone has been separated from the original power supply, so it is not possible to say what the exact operating voltages would have been. However, the voltage divider for the capsule polarisation may give us a clue - 2 Meg and 400K would be a simple way of using a 240V supply to put 40V on the capsule.

The metalwork is nicely done, and is comparable to Reissmann, Thiele and Teladi microphones of similar age. It seems too well constructed to be a DIY mic, but the oddball range of parts makes us think that it is some prototype from one of the microphone makers of the era.

The capsule is quite unusual, but sadly is missing a diaphragm at the moment. It uses springs and screws to adjust the tension and space to the back plate, so this can be adjusted after installation. We'll try to get that up and running very soon so we can see what it sounds like!