I recently got an ADK Vienna large-diaphragm capacitor microphone from a friend. It actually sounded pretty ok as-is, but soon the switch for the built-it 18 dB pad started to crackle. Instead of repairing (or disabling) the switch, I opted to replacing the electronics with the so-called Royer tube modification, or actually, a circuit loosely based on Royer’s idea. I have tested my circuit already a couple of years ago in another mic, which uses a Neumann capsule and a Lundahl output transformer.
Since I don’t have any extra money at the moment, I decided to use the stock transformer. And since the original idea was to use an AKG C12 -style, i.e. edge-terminated capsule, I naturally left the original in place as well. Of course I may later swap both the transformer and the capsule, since – I’ll reveal it already – the mic sounds amazing, actually somehow “better” than the other mic!
This is pretty much everything one needs to build a mic. A tube socket, tube (5654W), transformer, 7-pin XLR connector, couple of resistors (metal film to minimize noise) and two capacitors. The electrolytic capacitor and three resistors are used to filter the capsule polarisation voltage. The remaining two resistors are the plate (anode) and cathode resistors of the tube, which is by the way triode connected. The 2,2 uF plastic capacitor is the coupling capacitor for output. No ‘lytics on the signal path!
After taking the mic apart, I started with replacing the connector. The wiring is teflon insulated and includes some silver as well. Expensive, but good stuff. I nowadays only use this type of wire when I build equipment.
Since the circuit is so simple, I decided to build the mic point-to-point. No problem as long as you remember to insulate the live parts with heat shrink tubing. I used the center tap of the tube socket as the B+ connection and one end of the heater pin as ground.
And as I said, it sounds wonderful! Watch this space later for sound examples.