Brad N8YG has been reading over posts concerning the ssm2167 compression pre-amplifier with quite a bit of interest. So he finally integrated the board into his ubitx v5.
He put a trimmer on the output of the board, and potentiometers on the compression, and noise gates. The problem that he ran into was that on the air, rotating the 100K compression potentiometer had little to no effect. However he was getting compression that was clearly audible on the air.
He re-read the data sheet for the ssm2167 more thoroughly and found that the input voltage need to be quite low at -20 to -30 dBV. A quick check on the electret microphone output using the oscilloscope, showed a much higher output from his microphone.
This led to him adding another pot to trim the input voltage. He still needed the 5V to get to the electret so he wanted the trimmer after the 0.1 uF surface mount capacitor on the board.
Brad modded the board by removingthe 0.1 uf capacitor, and he made connections for the trimmer after the 0.1 uF capacitor.
The net result is a board with three additional potentiometers and one trimmer. The trimmer adjusts the output voltage, and potentiometers control the input voltage to the board, compression and noise gate.
On the air.. with the compression potentiometer turned down the rig sounds like it has no compression… while turning up the potentiometer makes a profound difference in on the air sound. The rig now has a LOT of punch and the power meter goes way up.
Brad believes that what was happening was that the input level was too high forcing the chip into the limiting portion of the transfer curve. Now that the chip is operating back in the linear portion, all is well!
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.
Henning Weddig DK5LV says he is in the process of using an op amp and SSM2166 on his “original” BITX40 as a way of achieving an audio AGC function.
This op amp has a gain of 10 (20 dB) and an audio AGC system. He purchased a pcb from ZL1CVD with the DIL chip via ebay years ago. Unfortunately ZL1CVD does not sell this pcb any more…
The SSM2166 has a dynamic range of 60 dB, the opamp in front of it is intended to replace the first discrete amplifier after the demodulator and will amplifiy the audio into a range the SSM2166 can handle. The SSM2166 has a provision for outputting an AGC voltage (an RMS output) and this may be able to be used for driving an S-meter.
Others are thinking along the same lines. For example, Glenn VK3PE says:
“I’m thinking along the same lines. I built a Mic amp version using the SSM2167 and thought it might work also for the Receiver. It’s essentially the module seen on ebay but I’ve added a level pot on the output side.”
Meanwhile Alex PA1FOX comments:
“I am using the AGC from the original uBitx design, but found the time constant of 1uF and 100k to be far too low. This makes it a very fast AGC, not producing a nice sound with voices. I changed to 1.5 uF and 4M7 and now stronger stations are nice and clear (they ‘push’ the noise level down ) and the gain comes up nice and easy when the QRG is clear. I think I’ll keep it this way.”