[sdiy] Fixed filter bank questions

David G Dixon dixon at mail.ubc.ca
Fri May 10 09:23:50 CEST 2019

Brian said:

"I don't know why Yves would have simply doubled every cap. Maybe it was
purely for the improvement to tolerance."

Yves didn't just double the caps.  He used two caps in parallel (where their
values add together) to make capacitors with specific values, within the
confines of the E6 series.  Hence, if he needed an 18.3n cap, he combined a
15n cap and a 3.3n cap in parallel.  Of course, there is a limit to how
precisely one can derive values this way, but he came very close to his
target capacitances doing that.  He also ignored tolerances.

What I did instead was to select a single cap value from the E6 series, and
then design the resistor networks to give the desired frequencies.  There is
a much larger selection of readily available 1% resistor values, so this is
an easier, more accurate, and much cheaper way to get specific frequencies
than Yves' method.  I can buy 100 of any 1% resistor value I need from
Digikey for about $1.70, and from that I can build a few dozen fixed filter

However, as I said a little while ago, I'm thinking now that I'm just going
to use the same resistor network (100k-51k-2.7k) for every filter, and every
value of E6 cap between 100n and 1.5n, and just take whatever frequencies I
get -- they will be almost evenly distributed since the successive E6 caps
have value ratios which are between about 1.42 and 1.5.  If this doesn't
give pleasing results, then I can always change it.  My layout for all 12
bandpass filters is on a board which is only 2.1" x 6", and it will take me
about 30 minutes to print, etch and drill the board, and about another 30
minutes to stuff and solder it so I can try this circuit several different
ways to see which version I like the best -- it will just be a matter of
swapping out the filter board.  All the other circuitry will be on a
separate board which is also 2.1" x 6".  If the "quick and dirty" one sounds
good, then I will go with that.

I'm going to add a few other little improvements and innovations to the
design.  First of all, I'm going to make a panel PCB to eliminate pot
wiring.  Also, Yves and a few others have created separated outputs for the
odd and even filters.  I'm going to do that as well, but I'm also going to
give the option of separate inputs (the odd and even inputs will be normaled
to each other).  Also, I'm thinking of putting a white noise source in the
unit, and normaling that to the inputs so that the bank is also a
stand-alone wind machine.  Finally, I'm going to lay out the circuit so that
I can retrofit voltage control to each channel later.  I will do this by
inserting a linearized 2164 VCA between each attenuator pot and input
resistor and the summing output amps (that's 14 linear VCAs requiring 7 2164
chips and associated circuitry).  I will rig it so that this VCA network can
be inserted into the circuit simply by unplugging and replugging a few pin
headers.  I'm thinking that the basic FFB will be 2U wide, and there will be
a 1U panel next to it for the voltage control option.  I'm very curious to
hear what it sounds like when the filters are driven by the octature sine
waves from my Dr. Octature filter.  Finally, I might put a crossfader on the
odd-even outputs.

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