[sdiy] Buchla 194 Fixed Bandpass Filter analysis

[Aaron Lanterman]

To get some practice analyzing circuits with transistors, I started  
looking at the Buchla 194 Fixed Bandpass Filter, as redrawn by Marjan  
Urekar:

http://members.tripod.com/urekarm/synth/buchla_bpf.pdf

The filters are all Sallen-Keys.

Numbering them from top down, the 1st filter is a 2nd order highpass,  
the next two are 2nd order lowpass followed by 2nd order highpass, and  
the 4th one is a 2nd order low-pass.

There are three emitter followers in the circuit; one as a buffer  
right at the beginning, and another two as part of lowpass filters in  
the 2nd and 3rd filter (from the top). The 1st and 4th filters, and  
the second filter section of the 2nd and 3rd, have two transistors  
forming a buffer in the "Sziklai" configuration, according to my  
trusty Horowitz and Hill.

The circuit runs off a single supply, so the transistors are biased at  
around 6 volts via 68K and 100K resistors. That's a little less than  
7.5, which would be halfway between the rails. A biasing network  
appears at the initial emitter follower, and also in the highpass  
filters (since they appear after a cap which would block the DC bias).

Questions:

1) The highpass S-K has a "feedback resistor" and a "resistor to  
ground." When I'm considering the "resistor to ground," should I think  
of the 68K and 100K biasing setup as both being resistors in parallel  
to "AC ground?" If so, my "to ground" resistor is actually 68K || 100K  
= 40K. (It's kind of embarassing to ask this, being an ECE prof, but  
it's been a long time since I actually dug into a transistor circuit -  
probably when I took the sophomore electronics class, uh, 17 years ago).

With this figured out, I can compute the cutoffs and Qs of the various  
filters.

2) All of the bases of the voltage buffers, except for the first one,  
have 10K resistors going into the bases. Why are those 10K resistors  
there?

3) There's a 2K2 and 100K voltage divider right at the beginning that  
attenuates the signal by a factor of 50, presumably to get the signal  
into the "linear small signal analysis" range. But... I don't see  
anywhere where the gain is brought back up at the end! Is there  
something I'm missing?

I thank the list, as always, for its collective wisdom!

I'm thinking it would be fun to create a modern version of this that  
just uses op amps and a split supply.

- Aaron