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Theory of Operation

Mixer

The mixer design is a simple inverting summing opamp configuration. I've also included the clipping diodes for an optional distorted output, following Moritz Klein's design (ref. 1).

Attenuverter

Attenuverter

Voltage divider R1-R2 is implemented with a fixed R (R_1') in series with a variable resistor (pot) with max R R_2'.

\begin{aligned} 0 &= \frac{V_p - V_i}{R} + \frac{V_p}{R_2} \\ \to V_p &= \frac{R_2 V_i}{R_2 + R}\quad\mathrm{with}\quad V_p = V_n \\ 0 &= \frac{V_n - V_i}{R} + \frac{V_n}{R_1}+\frac{V_n-V_o}{R_f} \\ \to 0 &= V_n - V_i + \frac{R}{R_1}V_n + \frac{R}{R_f}V_n - \frac{R}{R_f}V_o \\ \to \frac{R}{R_f}V_o &= V_n\left(1 + \frac{R}{R_1} + \frac{R}{R_f}\right) - V_i \\ \to \frac{R}{R_f}V_o &= \left[\frac{R_2}{R_2 + R}\left(1 + \frac{R}{R_1} + \frac{R}{R_f}\right) - 1 \right]V_i \\ \mathrm{let}\ R_f &= R \\ \to V_o &= \left[\frac{R_2}{R_2 + R}\left(\frac{2R_1 + R}{R_1}\right) - 1 \right]V_i \end{aligned}

If R_2=0 (potentiometer wiper is set to the end stop), V_o = -V_i . Choosing the maximum potentiometer value to be R_2'=R and the remaining fixed resistance R_1=R_1'

\begin{aligned} 2 &= \frac{R_2}{R_2+R}\left(\frac{2R_1 + R}{R_1}\right) = \frac{R}{2R}\left(\frac{2R_1' + R}{R_1'}\right) \\ \to 4R_1' &= 2R_1' + R \to R_1' = \frac{1}{2}R \end{aligned}

References

  1. Moritz Klein, "mki x es.edu Mixer" [ericasynths.lv]
  2. "Dual Atenuverter", Befaco [befaco.org]
  3. Horowitz and Hill, "The Art of Electronics", 3rd ed. Cambridge University Press, NY, 2015