John, VK2ETA, has used the small circuit board “SSM2167 Microphone Preamplifier Board Preamp COMP Compression Module DC 3V-5V”available on eBay or Aliexpress as a compression and mic pre-amplifier.
He simply connected the input to the mic, added a 4.7K ohm resistor between the mic input and the 5VDC (taken from the Raduino) for biasing the electret and put a 10K ohms potentiometer in the output to adjust the power level to the mic preamp stage.
He didn’t modify his uBitx board, but simply inserted the board prior to the mic input. The gain of 20dB is reduced back with the output potentiometer. John removed the “R1” resistor and replaced it with a 51K Ohms resistor to get a 4:1 compression factor, up from the 2:1 as delivered, but this change has yet to be tested “on air”.
John hasn’t received any negative feedback about the compressor except when I pushed the output potentiometer too high.
Simon VK3ELH used the same board and a similar scheme for powering the module from the regulated 5v line on the Raduino. It is also installed separate to the main board and inline with the mic input.
Simon used a 75k ohm resistor for compression and 1k ohm resistor for the noise gate and a 100k pot on output. At full output, his audio was readable but distorted based on an audio check QSO, so the output has been turned down.
He put a larger heatsink on the IRF510 to cater for the higher average output, as the stock one was getting warm!
A side effect of the mic being on all the time is that there is leakage through to the speaker and it causes some feedback if the mic is within 2 inches or so of the speaker.
Anders SM5NNO has drawn attention to a published fix for low levels of drive on SSB with BITx transceivers from Mr K P S Kang (VU2KR / VU2OWF) on this blog. While the mic gain fix relates to the BITx40, it translates readily to the uBITx.
Dennis Yancey asked the group “Has anyone used a dynamic mic on the uBITx?” Dave WI6R responded with a guide for modifying the uBITx to use a Dynamic Mic:
Referring to the uBITX Schematic. The Gain of the Mic PreAmp is controlled by R63 in the Emitter of Q6 to C62/R64. It basically decouples the audio bypass of R64 by C62 to limit gain for the high output Electret Mics.
Substituting a Dynamic Mic
Replacing R63 with a Zero Ohm resistor and putting a 10K potentiometer at the Mic input should allow a Dynamic Mic to work now with a Mic Gain Control. Also, R60 that supplies Mic BIAS to the Electret-Condenser Mic needs to be removed. If there is not enough gain after this mod, you might have to reduce the value of R64 to maybe 470 Ohms or so.
If you cannot look at your transmitted signal on a scope at least listen to your audio on another receiver to verify you are not causing distortion.
More Gain for an Electret Mic
To simply get more gain with an Electret Mic you might try dropping another 47 Ohm resistor on top of the existing R63. You can solder another chip resistor on top by soldering one end at a time.
[EDITOR: you can also bridge the surface mount resistor with a standard through hole resistor. Shorten the leads and bend over about 1/8″ or 3mm for soldering to the ends of the surface mount component]
David N8DAH has been testing the MAX9812L Mic Pre-amp module on his BITx40. In theory this should improve the gain and signal quality.
David says “So far its working ok at best I sound a bit robotish.”
“I am TXing at around 20w with my amp. I took the audio out through a 10uf dc blocking cap to the bitx40 mic in. I powered the board from a 9v just for testing. R136 is about 1/4. If you use a pre-amp you should adjust this lower or you will cause one heck of a noise on tx. I am not yelling or shouting to get audio out now but not sure I like the audio in any case.
This is without the pre-amp … with the audio files from Michigan to Milford PA websdr
This is with the pre-amp …
David has decided he “might just stick with the slight yell to get the audio out. I like the idea of not having to shout but do not like the audio from this version of preamp”.
Others may think differently. Mike ZL1AXG thought his “more robot-like” voice was more intelligible because it was more “punchy”.
Jeff AD6RH says:
“I used another mic housing with a DPDT switch and wired it so voltage is supplied only when PTT is engaged. I am using a CR2032 3v button cell. It seems to work fine, but I have not compared the stock vs. preamp mic with anyone on the air yet. It definitely has more average power on the watt meter. I can hear some peaks come thru the speaker when transmitting. I may try installing a pot to dial back the gain.”
Dave WI6R thinks that the rig only needs slightly more gain than the existing Mic PreAmp and that adjusting values to add gain is really all that is needed. He doesn’t think it needs 40 dB of extra gain.
He has resurrected a Mic Pre-amp design used 50 years ago in the SBE SB-33 SF-1 solid state rig that first used bilateral amplifiers. Bilateral amplifiers are used, of course, in the BITx transceivers.
This design had plenty of gain and worked with a Dynamic Mic with significantly lower output than any of the typical Electret-Condenser Microphones used today. Also note that the Mic PreAmp was powered only when the rig was in TX and a simple diode was used to shut off the Mic PreAmp in RX with the same TX voltage.
Dave used this rig in the ’60s and doesn’t recall any “pop” when going from RX to TX or vice-versa. The “Signal Splitter” was used to isolate the TX and RX Audio.
Raj, VU2ZAP responded suggesting that simply decreasing the value of R63 would give you more gain!