Replacement to Q70 for more audio gain, less overload

Tom W1EAT  has noted that Q70, the audio pre-amp, has a really big job in the uBITX and sometimes gets overdriven by very strong signals.   Tom also replaced the 2822 audio amp with an LM380 and lost around 8db in audio gain.

So he though he would try another transistor to try and improve this situaiton.

He had some 2SC945 transistors marked with the highest “beta”(AKA hfe) rating, so I took one with a 360 hfe and replaced Q70.   He made no other component changes. The 2SC945 is supposed to be specially designed for audio pre-amps, so it ought to be a good replacement.

With RX voltage of 13.6, the Q70 collector voltage is 4.67 on the 2sc945.

I should mention that I added a 16K resistor from the base to ground when Q70 was a 2n3904, which was suggested by W3JDR to reduce distortion. I did not remove that resistor.

The results were totally positive. Tom has more AF gain, and none of the signals in tje IARU HF contest overloaded it.

Something for experimenters to try out …

Reference

Variable IF bandwidth from W1EAT

Tom W1EAT provides a  schematic for his mod of a variable IF bandwidth circuit for the µBITx.  The RX and TX voltages are about 12V,  so he used a resistor network to keep the applied voltage to about 1 to 9V because that is the usable range for the varicap diodes he uses.  As the voltage on the caps is reduced the higher audio frequencies are reduced until the bandwidth is probably 100Hz or so at 1 volt. The filter centre frequency is very low, 200 Hz or so with Tom’s BFO setting.

The BB112 has about 500pf capacity at 1 volt applied.

The 4.7K resistors that feed the voltage to the VVC diodes could be 10 or 20 K, or what have you, as there is very little current needed.

Reference