Tube mic Connecting the capsule 1

  
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Even in the simplest of tube microphone circuits, there are different approaches to connecting the microphone capsule to the tube. Let`s use a single-sided microphone capsule as our starting point. The capsule behaves as a variable capacitor, changing its capacitance in response to changes in air pressure (i. e. sound). In order to generate a signal
Tube mic Connecting the capsule 1 - schematic

, the capsule needs to be polarised by some voltage, creating a difference in potential between the diaphragm and the back plate. This is the first decision that needs to be made - should the polarising voltage be applied to the diaphragm or the capsule backplate In the circuit shown on the left, the backplate of the microphone is polarised at 60V, which is obtained from the B+ supply, via a resistive divider and a small capacitor to stabilise and filter the polarising voltage. The membrane is connected directly to the tube grid, and a high value resistor (Rg, typically 100 k ©to 1000 k ©) connects both the grid and the membrane to ground. We have our potential difference across the membrane, and the sensitivity of the mic may be adjusted by increasing and decreasing the polarisation voltage. As the capacitance of the capsule changes in response to sound, a tiny current will flow through Rg, and this signal is amplified by the tube. In some cases the grid resistor may be omitted. In the circuit below, which appeared in an article in Tape Op magazine by Dave Royer, the capsule diaphragm is grounded by grid leakage rather than a `real` resistor. It works perfectly. This simple arrangement is not possible when the capsule backplate is mechanically (and electrically) connected to the body of the microphone. In this case the diaphragm must be polarised directly. However, having a voltage of around 60V on the tube grid this would...



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