A Bandpass Twin-T Active Filter Used in the Buchla 200 Electric Music Box Synthesizer

Abstract

This paper analyzes an unusual active bandpass filter employed in the Buchla Model 295 10 Channel Comb Filter, a synthesizer module developed as part of the Buchla 200 Electric Music Box by Donald Buchla. The filter consists of a peculiar rearrangement of elements in a classic Twin-T configuration; to our knowledge, it has not been previously addressed in the literature. As an example, we explore its specific application in the Model 295.

Figures (8)

• Figure 1: Active bandpass filter used in the 200 Hz to 3.2 kHz filter blocks of Figure \ref{['fig:b295']}.
• Figure 2: Passive variant of the bandpass filter in Figure \ref{['fig:passive-twint']}.
• Figure 3: Simplified schematic of the Buchla Model 295 10 Channel Comb Filter. Capital K in resistance markings represents k$\Omega$ throughout the figures. The open circles represent connections from the 33 k$\Omega$ and 39 k$\Omega$ summing resistors to the virtual ground of the opamp in the lower left corner. The right-facing arrows between the 220 $\Omega$ and 50 k$\Omega$ resistances represent individual channel outputs. The analysis in this paper assumes the opamps are ideal, so some components meant to address nonideal opamp effects have been omitted for clarity, namely resistors between the positive input terminals of opamps and ground and DC blocking capacitors in series with the 33 k$\Omega$ resistors at the initial input and connecting the bottom two opamps.
• Figure 4: Lowpass Sallen-Key filter for the below-100 Hz channel.
• Figure 5: Highpass Sallen-Key filter for the above-7 kHz channel.