Posted on Feb 7, 2014

My design is somewhat like a combination of the circuits in M49 and U47, but mounted with the tube used in U67. There`s really not a lot of complicated electronics here. This is a true variable-pattern microphone, capable of doing omni, cardiod and figure-of-eight. This is acheived by polarizing the front and back diagphram of thecapsule in different

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ways with respect to the center electrode. To act like a microphone, we need a voltage charge across the capsule. When the capsule changes capacitance (that is: the distance between the electrodes are changed by sound pressure) is changed, so does the amount of energy that can be held stored in this capacitor. But as we are charging/discharging with a hell of a small current - the 1GigaOhm resistor - the current really has nowhere to go, and so results in a varying voltage potential across the capsule. The polarization scheme can be a little difficult to understand at first. To acheive simple remote switching of the polar patterns, a single variable voltage is used for this, only changing the charge of the back part of the capsule. We want to keep the front electrode of the capsule at ground potential - 0V - at all times, both to act as a shield for incoming electrical disturbance and to avoid electrostatically attracting too much dust from the environment. So to keep a voltage charge across the capsule, we bias the center electrode by the means of two 470K resistors dividing our 160V supply voltage in half - resulting in +80V. Now we have -80 volts at the front electrode, referred to the center electrode. If we now bias the back electrode with the same (-80V ref. Center = 0V polarization voltage), a positive sound pressure on the back capsule will have the same voltage-potential effect on the center as when applied on the...

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