Simple High Fidelity Microphone

Microphone of extremely high audio sensing precision could be built on a Panasonic WM-61A back electret capsule without patching it with few extra components in a couple of hours. No need to purchase a high end microphone priced over $1000 to record quality vocals, drums, live performances or to measure parameters of an acoustic system and tune movie theater's surround sound. Here you get a microphone of highest fidelity possible at our time for under $4. Let's call it a V.K. Microphone.

Microphone Top Secrets

Original Design by V. Kravchenko, Kiev, 1987

Below are top five secrets of the best microphone amplifier in the world:

Common Base Amplifier topology is the most linear ever known. VT1 is just that.

The main advantage of V.K. microphone is the electret capsule turned into a current source. Sound pressure is transformed exactly into the current, not into voltage, unkike the most microphone amps.

The voltage on a WM-61A always stays around 2 volts. Panasonic specified 2V as a nominal supply voltage for WM-61A because only then microphone's extremely linear frequency responce with lowest distortion and noise are guaranteed.

There is no decoupling capacitor in signal path. Naturally, the capacitor as the non-linear noisy signal distorting part is eliminated.

The output bias voltage could be used by the consequent unipolar preamp to further minimize capacitors in signal path.

The circuit could be viewed as a cascode made of common source JFET built into a microphone capsule and a common-base VT1. Such transistor coupling provides their best mutual compensation of each others's flaws, eliminated Miller effect and excellent thermal stability with lowest noise. But the main point is the top secret number 2 - the microphone capsule as a whole becomes an extremely linear sound pressure to current transformer of highest fidelity achievable nowadays.

With R2 it is desired to tune the output bias voltage to 3+Vs/2. Or just solder the circuit as is and it will work with somewhat lower overload tolerance. Note, that changing R2 changes the gain and the gain also depends on impedance of a particular capsule, so be careful not to go over 26dB. With the gain over 30dB unfiltered, nevermind the A curve, recording the grand piano at forte fortissimo may saturate the amp. The drums will still be recordable, though. As a last resort to decrease the gain, use the RC circuit on the output. For the lovers of SMD components and haters of Soviet Union, feel free to use a widespread yet noisier and not so superlinear 2N3904 (SMD version is MMBT3904, marked as 1AM) instead of state of the art 40-year old gold-plated KT3102E/D made in USSR.

If you plan on using long microphone wires, solder R2 on the end of a receiving preamp for better noise/interference immunity. The rest may be soldered right on a WM-61A capsule with or without PCB depending on your jeweller's skills.

History

My father has invented the V.K. microphone back in 1987 to quickly calibrate his hand-built Hi-Fi speakers without wasting money on Sennheiser. As you may have guessed correctly, V.K. are father's initials. Later that year I made a few more V.K. mikes for quality percussion recording. The original build had the capsule much worse than Panasonic yet it outperformed several professional microphones on distortion and frequency responce. Therefore, I intentionally preserved the Soviet circuit annotation of the time except changing the capsule name and replacing two red LEDs with one white as there were no white LEDs in the 80s.

Only recently I ran into the site of Sigfried Linkwitz and became his secret admirer. Linkwitz Lab is great. We even have similar views on the bridge, although mine is on the other side of the planet. Among other great things, there is a unique design of high fidelity microphone for measurements. The microphone easily handles deafening sound pressure levels unsaturated without much distortion. Patching the tiny capsule had not been pleasant, however. Additionally, the V.K. microphone makes 2dB less noise. To slightly improve performance of the Linkwitz Lab microphone to match the V.K. mic, load it with current source, as shown here, instead of a resistor in the original drawing.

I highly recommend the V.K. microphone or 3TIUM mic, if you prefer, for studio works or for calibrating home/car acoustics. To record a jet engine running 15 feet away in high quality, just shunt the output with the RC gain divider. The V.K. mic design is so simple, natural and genius that I hardly believe audiophilic corporations of the 70s missed on patenting it. Let's assume it had been the main schematic secret in some high end microphones.

Shortcut

If you want to build a professional quality microphone within two minutes, just dismiss all of the above. Instead, buy a penny wise USB audio adapter in Apple style and hook up the capsule to it directly with a broken headphones wire. Adapter's phantom bias is 2.4V, which is close enough to capsule's optimal supply voltage. After slightly pinching the WM-61A into 18-35mm foam rubber, you get the microphone closely comparable to the top models of Sennheiser and Behringer. Only if you intend to record or measure the lowest octaves of audio spectrum with high precision, read from the beginning about the V.K. microphone.

God Bless,
Serge K.
Nov. 1st, 2015