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Description: The design resembles a combination of circuits from M49 and U47, utilizing the tube from U67. It features a true variable-pattern microphone capable of omnidirectional, cardioid, and figure-of-eight patterns. This is achieved by polarizing the front and back diaphragms of the capsule differently concerning the center electrode. To function as a microphone, a voltage charge is required across the capsule. Changes in capacitance occur as sound pressure alters the distance between the electrodes, affecting the energy stored in the capacitor. Due to the presence of a 1 GigaOhm resistor, the current is minimal, resulting in a varying voltage potential across the capsule. The polarization scheme can be complex. Simple remote switching of polar patterns is accomplished using a single variable voltage that adjusts the charge of the back part of the capsule, while the front electrode remains at ground potential to shield against electrical disturbances and dust accumulation. To maintain a voltage charge across the capsule, two 470K resistors divide a 160V supply voltage, yielding +80V. The front electrode is at -80V relative to the center electrode. If the back electrode is also biased at -80V, the microphone exhibits omnidirectional sensitivity. Polarizing the back electrode at +80V mutes it, resulting in cardioid directionality. When the back capsule is polarized at +160V, a positive sound pressure applied to it produces a signal 180 degrees out of phase with the front capsule, creating a figure-of-eight pattern. The changing voltage potential on the center electrode is amplified by an EF86 Pentode wired in triode mode, similar to the VF14 pentode in the U47. A 1G resistor biases the grid to 0VDC, ensuring high input resistance. The signal is then passed through a 2u2 DC blocking capacitor to a Lundahl LL1538 microphone input transformer, converting the 15K output impedance of the tube to a more manageable 600Ω. The PCB includes an option for a trimmer and capacitor to introduce cathode feedback, which can reduce distortion and output impedance but may compromise the characteristic "tube" sound. This option remains for educational purposes and experimentation, with the recommendation to keep the tube undisturbed for optimal sound quality.

The microphone circuit described utilizes a sophisticated design approach to achieve multiple polar patterns, which is essential in professional audio applications. The variable-pattern capability allows users to adapt the microphone's response to different recording environments and sound sources, enhancing versatility. The polarization technique employed is critical, as it directly influences the microphone's sensitivity and directional characteristics. The use of high-value resistors, such as the 1 GigaOhm resistor, is a notable feature that minimizes loading effects on the capsule, preserving its delicate charge dynamics.

The EF86 Pentode amplifier stage is a significant component of the circuit, chosen for its high gain and low noise characteristics, essential for capturing audio signals with fidelity. Operating in triode mode further optimizes the linearity of the amplification process, ensuring that the sound quality remains pristine. The choice of the Lundahl LL1538 transformer is also pivotal, as it not only transforms impedance but also plays a role in balancing the output signal, making it compatible with various microphone preamps.

The additional feature of cathode feedback allows for tunable characteristics of the amplifier stage, enabling users to tailor the microphone's response to their specific needs. However, it is essential to understand that while feedback can improve certain parameters, it may also detract from the unique tonal qualities that tube microphones are known for. Therefore, careful consideration should be given when deciding to implement this feature.

In conclusion, this microphone design exemplifies a thoughtful integration of classic tube technology with modern engineering principles, resulting in a versatile and high-quality audio capture device suitable for a wide range of applications in the recording industry. The detailed attention to polarization, impedance matching, and amplification ensures that the microphone delivers exceptional performance while preserving the rich tonal characteristics associated with tube microphones.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 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 front capsule.

This sensitivity pattern is then OMNI directional. If we polarize the back electrode at +80 Volts, no voltage difference will exist between this and the center electrode, already offset at +80V. This in effect mutes the back capsule, resulting in the CARDIOD directionality. At last, if we polarize the back capsule at +160 Volts, we`ll have a charge of +80 Volts relative to the center electrode.

Now a positive sound pressure applied to the back electrode will produce an 180 degrees out-of-phase signal compared to the front capsule. When applying a sound pressure from the side of the microphone, so both capsules sees the same sound pressure, the signals coming from the two capsules will be in opposite phases, effectively canceling each other.

This is the FIGURE-OF-EIGHT directionality. The changing voltage potential on the center electrode of the capsule is picked up and amplified by the EF86 Pentode, which is wired in triode mode - just like the VF14 pentode in the U47. It`s input resistance has to be kept VERY high not to disturb the charge/discharge of the capsule, so we use a 1G resistor to bias the grid to 0VDC.

From the anode of the EF86 we take the signal thru` a 2u2 DC blocking capacitor to the output transformer, a Lundahl LL1538 microphone input transformer, here used the "wrong" way around, converting the 15K output impedance of the tube into more useable 600R, and so driving the cable and your microphone preamp with low impedance. On the PCB board there`s an option for adding a trimmer and a capacitor in order to introduce cathode feedback to the tube amplifier stage.

This lowers both distortion figures and output impedance, but cancels out a lot of the "tubey" sound - the reason we bother to make a tube microphone in the first place. The reason I have`nt removed this option, is for purely educational reasons - if you want to do some experimenting yourself.

This is also where you can put in various equalizations if you like that. But for the best sound - at least to me - leave the tube undisturbed by feedback and other dirty tricks. This is an about 420Kbyte