Filters

Discrete transistor filters considered here will be active types. Passive types may get makeup gain from a following transistor amplifier. The last stage of a Big Muff or Superfuzz is a good example of this type of makeup gain amplifier.

Vox Wah

Unquestionably, the king of all discrete transistor filters found in guitar effects would be the Vox Wah. This is a bandpass type filter that sweeps by varying the amount of feedback via a foot controlled potentiometer. In addition to being a discrete transistor design, it also features an inductor in the filter circuit which is a big no-no in modern electrical engineering. Many wah pedals use inductorless circuits, but these circuits never seem to quite have the original Vox wah sound made famous on many 1960s recordings.

In the above Vox wah circuit, there are two transistors at work. The first transistor is a common emitter that gives gain and inverts the input signal. The network of passive components form a bandpass filter, and we take our output from the collector here. Before considering the second transistor, the circuit is very simple.

The key to how we are going to sweep this filter is that the inductor isn't referenced to ground, but to the output of the second transistor. The second transistor is an emitter follower that serves to buffer our output so that we can inject it back into the circuit as feedback. The feedback is positive with respect to the output voltage at the first collector, and negative with respect to the input base. Because our inductor sits between the feedback signal and the collector output, its effect on the bandpass equation shifts depending on how much feedback we provide via the foot controlled potentiometer.

The wah is a pretty neat circuit, isn't it?

BBD Anti-Aliasing and Reconstruction Filters

Boss BBD delay effects have some nice discrete transistor filters in them. BBDs are clocked at relatively low frequencies that approach or dip into the audio band. To avoid distortion known as "aliasing," an "anti-aliasing" filter is usually placed before the BBD input to limit the high end of the signal. Q2 forms a Sallen and Key low pass filter to before this function. After exiting the BBD, audio signals are mixed with the clock signal and other cumulative effects from the long series of sample and hold circuits inside the BBD chip. Some additional Sallen and Key low pass filtering from Q3 and Q4 serve as "reconstruction" filters to smooth out the high frequency jitter imparted by the BBD chip.

I will add more examples as I find them.