2018-09-02 Mims Mistakes In Audio Amplifier

From FB group Building Transistor Radios, 2018-09-02

I already see one mistake. On p.27, Audio Mixer.
The R6 is only 1k. The voltage divider R5 and R6 will give a voltage less than 0.6 at Q1’s base, which means Q1 will not be forward biased enough and the circuit won’t work or else sound distorted.

R6 needs to be higher to give 1.1 volts at the base. A close value for R6 would be 2.2k. Better choices for R5 and R6 would be 33k and 4.7k respectively.

C1 and C2 are in the wrong place. When the R1 and R2 volume controls are adjusted, the DC bias on Q1 will be changed. The C1 and C2 capacitors should be moved to the right of R3 and R4 to block the DC bias to the base of Q1. It might help if C1 and C2 were larger, 1 uF or 10 uF.

The value of C4 is too low and the low frequencies will sound weak. Change C4 to 100 uF.

Like I said, you will probably find many more errors and omissions in Mims’ books.


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