From left: outer shell, screen, aluminized mylar diaphragm glued to a steel ring, spacer (red plastic) to set the distance between backplate and diaphragm, backplate and acoustic delay assembly, solder terminal ring, and screw ring to hold the works together. The backplate is stamped steel, the acoustic delay is a piece of filter paper covered with another stamped steel disk. Except for the outer shell, nothing requires machining, so they can be made with very little labor.

So let’s build a quick test mic with one, much like the Low-Cost Beginner’s Mic.

Start with a $14 BM-700 “Excelvan Studio Mic”, our new larger capsule, a 220 uF / 16V electrolytic cap, and a 22pF NP0 ceramic cap. The larger capsule will sound better and be less noisy, the big cap will filter out noise from the voltage regulator, and the small cap will reduce distortion and raise maximum SPL.

This particular BM-700 had two circuit boards inside. One works, the other has no components on it.

The 220uF cap goes here.

The 22pF cap goes here.

The new capsule is glued to the cut-down capsule holder.

Just for fun, I cut a trace in the voltage regulator circuit and installed a blue LED.

After drilling a small hole in the case, the mic has a power indicator which also serves to identify the front side of the mic.

The result is a much better sounding mic than original. An easy mic mod that results in something any studio can use. Sorry, no measurements for now.

I hope to figure out a way to get useful measurements in this apartment. Space is limited, and there is noise from appliances and air conditioning. Loud chirps late at night aren’t going to please the neighbors. When time permits for experimenting however, something should be possible.

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