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

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