Mike and all UBITX enthusiasts,
when In looked on the schematic of the new audio amp my first thougt was: why is there no feedback from the output to the input?
Please have a look on the very old schenatics of "old" and discrete audio amps and even tube amps!!!
In all circuitries a feedback is installed to cancel the nonlinearities of the active elements.
Henning Weddig
DK5LV
Am 17.06.2018 um 23:23 schrieb Mike:
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I believe there is an nonlinearity problem on some of the newer V4 uBitx radios (the one with transistors in the final audio amp). Two of my radios had this problem.
The problem is that the two diodes in the final amp doesn't make a voltage drop that is larger than the drop over the BJT B-C junction drops, so there will be a crossover distortion problem.
I did a mod: cut the trace between the diodes. Soldere in a 500 ohm pot paralelled with the a 125 ohm resistor. I adjusted the pot from a low value and increased the resistance until the nonlinearity disappeared by listening to a moderately strong cw signal. I checked the lower transistor so it didnt become too warn from the quiescent current.
73s
