Building a Pair of High-Quality OPA Microphones
These two true condenser microphones use an OPA impedance converter circuit and a hex inverter voltage multiplier to produce bias voltage.
DJJules has built a pair of true condenser microphones that use an OPA-based impedance converter circuit and a hex inverter voltage multiplier to produce bias voltage. Both OPA circuits are on a single board with a common ground, allowing the two signals to come out of the microphone with a 5-pin XLR. One microphone is a cardioid, while the other employs a double diaphragm, enabling a user to select the response pattern. The microphones also have TSC-1 and TSC-2 transound capsules, which are center terminated. There is a backplate that serves as the other electrical connection in the middle of the microphone capsule.
An external voltage supplies a charge to the bias voltage generator, which turns the capsules into working microphones. The hex inverter multiplier converts the 12 VDC from the OPA boards into 80 VDC, making the microphones more sensitive. These microphones feature CMOS inverters to drive the voltage multiplier circuit, which is achieved with a Schmitt-Trigger hex inverter using a 4584 or 40106. They work with 12VDC or higher.
The impedance converter has two functions that the electronics must perform. First, the circuit needs very high impedance, so it doesn’t load down the capsule. Second, it needs to drive a long microphone cable without affecting the signal. The OPA swings from rail to rail, allowing the capsule to distort before the internal electronics, which have a dynamic range of 130db.
The OPA circuit is a low impedance to function as a signal return path, but it’s only 5.5-6V, depending on the zener diodes voltage. It also uses wima polyester 0.1uF capacitors. Additionally, the OPA board and the area around the 1Gig resistor needs to be very clean.
Finally, the two mini U87 donor bodies for the microphones are made of copper/bronze alloy, which was hand polished, giving it a steampunk retro appearance. The third body is made of a zinc-based alloy.
The surfaces of the body parts need to be sanded if there is any buzz or handling noise, which is usually caused by the ends of the body cylinder. It won’t conduct if there is paint in the end.