Microphone-Parts T-47 Kit
Microphone-Parts T-47 Kit
Thursday, December 31, 2015
One of Microphone-Parts more unusual kits is a large capsule version of the first phantom powered mic, the Neumann KM-84, a small pencil mic. The KMi amplifier body for the Kleine Mikrofon series houses a very simple circuit. The capsule is polarized by the 48V phantom supply without a DC-DC converter, and the audio path consists of 2 capacitors, an FET, and a transformer. The circuit is described here. With a large capsule which can be insulated electrically from the mic body, the input capacitor can be eliminated. Three parts FET, C4, and transformer, is about as simple an audio path as you can get. It also means each part is important and should be carefully selected.
Other parts are necessary, but not directly carrying audio.
The kit contains a circuit board and all the parts to populate it, a mic body, and a capsule. You can choose an RK-47 or RK-12 capsule. I built the RK-47 version. Measurements are taken with a dummy capsule, an 80pF capacitor, so that random room noise won’t interfere with results.
The kit circuit is very similar to my sketch. Parts supplied are “audiophile” grade. 1% precision resistors, film caps, gold contact switches for cardioid / omni and 10dB pad, and selected FET. The transformer is custom wound for M-P modeled after a Neumann original.
So this is a cool kit to experiment with. Want to try out different FETs? Got a transformer in the drawer you’ve been wondering about? This circuit will clearly show how they sound.
It has a couple of quirks as well. FET bias is critical and has to be selected for each individual transistor for lowest distortion. The kit contains a selected FET and bias resistor. If you’re going to replace it, you need a distortion measuring setup of some sort. My software does that, and allows me to watch the harmonics while adjusting bias resistor R3, which I replaced with a 5K pot. In the end, none of the FETs I tried were better than the one in the kit, but this mic is now my FET test bed.
For testing, I temporarily wired in a 20dB pad and capacitor to replace the capsule for signal injection. The body sleeve could be slid into place for hum shielding, tho the base cap had to be left off.
I tried out several transformers I had on hand ranging from 8:1 to 15:1 ratio. One large12:1 was equal to the kit’s 6.5:1 unit. The rest didn’t work as well in this circuit, but I got a good idea of their characteristics.
Great test bed. On to measurements:
Frequency response to white noise at various levels.
Gain & F.R. vs C2 feedback cap
Yel - none
Blue - 4 pF
Green - 19 pF
(pad on)
Harmonic distortion and noise at 100 mV (-20dBV)
output
Yel - T-47 w/
M-P xfmr
Red - T-47 w/
large xfmr
Blue - KM-84i
Neumann
So the T-47 can be a very quiet, clean mic, or if the FET bias is tweaked slightly, it can offer a bit of harmonic enhancement if you want. It measures cleaner than my KM-84s, but has a similar smooth character beyond the obvious differences in capsule and body / headbasket.
