Re: uBitxr4 audio distortion fix
The problem is if the first transistor Q71 is not biased to the half power supply
point the output are both unbalanced and the bias is wrong. IF Q71 collector
is not at about 6V (for 12V supply) full stop. Adjust R72 until it is. Then adjust
the bias if needed to the complementary pair.
The most common variation is to derive the bias from the common point of both
emitters as then it tries to self center at 6V or half the supply. The cost of of doing
that is lower gain but higher fidelity.
See the design from this site (attached) and explanation.
Do not let their published 3V power scare you it works well at 12V too.
The key is how Q1 is biased and the resulting bias to the output string (3904 and 3906).
It should be possible to change the V4 circuit to that.