A simple Reference Mic
A Simple Reference Mic
Wednesday, December 13, 2017
Need a cheap simple reference microphone? Something that will be within a couple of decibels from 20-20,000 Hz for tuning a speaker or listening room? This will do the job for around $10. It’s not calibrated, but it is close enough to flat for many uses. It’s the simple Mini-mic made with a 6mm Transound capsule from JLI Electronics, the BO6034L-423. Data sheet here:
http://www.jlielectronics.com/content/BO6034L-423.pdf
The mic capsule is at the end of a long probe to reduce reflections off the body and stand. The probe body could be a soda straw or most anything. I used a chopstick with the broad end shaved down covered with shrink tubing. The cap and resistor go inside the XLR connector.
The circuit is dead simple, a 1uF capacitor, a 100KΩ resistor, the capsule, and a male XLR plug. You could make it fancier with an Alice circuit, but this minimum circuit is just as flat, and you probably won’t be using this mic on long cables from your preamp / interface so the slightly high output impedance won’t affect results.
I used a small film cap. If you use an electrolytic, the + side goes to pin 3 of the XLR. For this capsule, which draws <500uA, a 100K resistor is appropriate. I didn’t use shielded wire, just twisted the pair to the capsule. Around my shop there’s not a lot of RF, and my preamp’s common mode rejection is good, so hum isn’t a problem.
The capsule is “hot” at 48VDC, so if it brushes against another mic or DUT, it needs to be insulated. Some clear nail polish takes care of that, and also attaches the capsule to the shrink tubing. The tubing isn’t shrunk around the capsule because the capsule has to be kept from heat.
Keeping the capsule cool while soldering leads to it is very important. I have a pair of pliers with a 6mm hole around the wire cutters, and it made a good heat sink as well as holding the tiny capsule steady. If the capsule gets hot enough to soften the plastic diaphragm, its frequency response will be affected badly.
Not much more to say, so here are pictures.
Heat sink
Chop stick, capsule, and heat shrink tubing
Shrinking the tubing
while keeping capsule cool
Fingernail polish for insulation and securing capsule
Finished mic measures within 1.5dB of my EMM-6 20HZ-20KHz
