Re: uBitxr4 audio distortion fix

Tim Gorman
 

Allison,

This circuit does the same thing I mention, just in a different manner.
This circuit uses feedback to drive the collector of TR1 to the proper
voltage.

My method eliminates the need for the feedback.

Both will work to enhance the fidelity from the amplifier. I have used
my method in several audio amps and had no problem.

tim ab0wr

On Fri, 29 Jun 2018 19:02:34 -0700
"ajparent1/KB1GMX" <kb1gmx@...> wrote:

Tim no.

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.
http://www.deeptronic.com/electronic-circuit-design/small-audio-power-amplifier-using-3-transistors/

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).

Allison

It should be possible to change the V4 circuit to that.


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