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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….
- Proof of concept – testing various microphones with the uBitx using a mini sound board as a preamp. Adding some treble and cutting the bass a little had the uBitx sounding as good as the Yaesu. Three microphones are shown – a Peavey dynamic mic, an electret element (the white wire) and a PZM mic (the square plate).
- Isolated microphone power supply and PTT optical isolators on circuit board, preamp on breadboard, connected to a Yaesu MD-100 microphone. The MD-100 has an active filter to add high emphasis and a some low cut, which really makes the uBitx sound good, hitting a little over 15 watts pep (running on a 13.8 VDC power supply). The MD-100 PTT paddle seems to couple a lot of creaks and groans into the mic (even when trying to hole it motionlessly), so I built a hand-held PTT from PVC and momentary plunger switch (shown here with a non-chrome cap).
- Final testing of preamp, PTT, and switching relays. I pirated the panel mount microphone connector from the MD-100, which apparently has a panel mount connector built into it’s base, apparently in case you want to connect another microphone to your microphone – but it was just what I needed.
- Close up of board with all components and in place except speaker and computer cables that connect to empty terminals
- Circuit board and switches mounted in box, all cables ready to plug into rigs, computer, and power. Small audio transformers are built into the computer audio cables to provide ground loop isolation (otherwise there is extreme noise).
- Box mounted in place under shelf, cables tidied up. Knob is uBitx mic gain, left switch selects Yaesu or uBitx rig (mic, PTT, speaker) and right switch selects mic or computer audio (FT8, FSK, bluetooth headphones, white noise tests, etc). Clark and his golden ears were a big help in setting gains, so I replaced the black plastic auxiliary PTT plunger cap with a metal chrome plated cap in recognition.