Building a uBitx microphone preamplifier and Yaesu/uBitx to microphone/computer switch-box with parts on hand while riding out a pandemic

Yadda yadda about building a simple preamp for using a dynamic microphone with the uBitx, plus notes about options, trials and errors building a universal-esque two-rig/mic-or-computer audio switch-box from parts at hand.  The preamp provides the gain needed for using the MD-100 with the uBitx (my uBitx is puttin out 35 watts of clean audio with the finals running at 18V). Audio from the computer runs through a buffer amp to approximate the MD-100 audio level.

The uBitx/Yaesu switch would have been much simpler if I had been able to find the 4-pole/double-throw mini toggle I used to have, or if I didn’t need to provide ground loop isolation due to running everything off DC/DC converters connected to the house battery, which varies from 20 to 30 volts. The switch-box switches the mic, mic ground, PTT, and speaker when switching between rigs.  There is an extra set of contacts to control an antenna relay when I find or build one.

I got it into my head that I should be able build it with vintage parts on hand as a sort of coronavirus challenge, so I used opto-isolaters for the PTT keying and six SIP (single inline package) single-throw/single-pole relays to simulate a three pole double throw switch (for mic, mic-ground, and speaker).  

The Yaesu FT-990 doesn’t provide power to the active filters in the MD-100, so that was another reason for building this switch-box, since more than one person has suggested that the MD-100’s high-emphasis/low-cut filter would help my particular voice (which appears to be true).  I used a 24V to 9V isolated DC/DC converter for the preamp power, but the MD-100 didn’t like 9V so I had to add a 5V regulator to make the MD-100 happy (I would have just used a 24V to 5V regulator if I had one).

I need to produce a legible schematic before actually publishing this….

Updated: May 26, 2020 — 10:56 am

The Author

Don Alejandro

Don is a renegade physicist who manages websites for several WV groups from an undisclosed location in the woods near Spencer, WV...

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