A long time ago I posted about the STC 4136 condenser microphone and how to modify it to work on standard 48V phantom power, using a small circuit built on perfboard.
STC 4136 microphone, in pieces.
I had another opportunity to work on one of these microphones. This one was a challenge as it had no circuit inside at all, just the capsule and bodywork, although that also gave free scope to start from scratch.
STC 4136 microphone capsule.
Space is tight in this microphone, but with a careful layout and small components everything will fit neatly. My solution for this one was to fit a small KM84 circuit and Neutrik NTE10/3 transformer. This transformer performs well, does not break the bank, and can be squeezed into tiny spaces where nothing else will go.
New circuit board and transformer
This time I commissioned a printed circuit board rather than working on perfboard, which does save time and look more professional. The board will be useful for other projects too – I have a handful of other small microphones which would benefit from updated, lower noise circuits.
Made in England
Thanks to Robert at Russell Technologies for the board layout and advice.
Here is a little curiosity. STC are mostly known for making high quality ribbon and dynamic mics, inclduing the much loved 4033 and 4038 models. But they also made condenser mics, and this nice little STC condenser mic came in for service recently….
It is model number 4136, and according to its specification sheet would originally have had its own power supply, which ran on either mains voltage or five 9 Volt batteries. Unfortunately the mic was bought without the power supply. The power would have been supplied to one pin of the XLR connector, ground to another, and (unbalanced) audio to the third. The capsule has a sintered glass, metallised backplate.
The battery supply suggests that 45V should be sufficient to power the mic. There was some corrosion on the circuit board from leaky capacitors, and without the original PSU, or knowledge of the operating voltage, it seemed sensible to create a balanced phantom powered circuit, loosely based on the Neumann KM84 – this is shown below along with the original circuit.
In practice, the conversion worked nicely, and despite its age and rough looks, the capsule gave a relatively even response, with a broadly cardioid pattern.
In that post I had sketched out the schematic. I have since converted one for a customer to run on phantom power, and spotted a glaring error in the schematic. Here is the revised version…
The transistor is of course a P-channel JFET, and the battery polarity is reversed, giving a positive ground. The batteries are switched off when the plug is disconnected, and the routing through the plug makes tracing a little tricky – that was my excuse anyway.
All of this means that some small modifications are needed for phantom power use, because negative ground is by far easier to implement. Using an N-channel JFET makes things much more straightforward – something like this…
The ‘adjust’ resistor is tweaked for best response to a sine wave applied across the head amplifier, and in this case the result was around 1kΩ. JFETs can vary quite a lot, and it is sensible to adjust this individually for each mic.
I built a small breakout board to supply the required voltages from the phantom power. The board fits neatly in the battery compartment.
The “110K” is again adjusted on the bench to ensure that the voltage is correct under load.
There is one more thing to note – now we have switched to negative-ground and an n-channel device, the output cap needs to be flipped round.
Here’s a measured frequency response plot for the modified mic (the dips at around 150 Hz and 600 Hz are likely to be room modes)…
The microphone works perfectly, and it is nice to hear one brought back to life after all these years!