Wiardgroup

> i see terms describing, would it be called "filter topology" ?, 
>and i`m often not sure what that contributes to the sound of a 
>particular filter as opposed to thing`s related more to the 
>device`s general design and component selection - like 
>"sallen-key" or "butterworth" or "moog" transistor ladder (i`m 
>excluding here things like ems diode filter clones or edp wasp 
>tortured cmos clones) - ?

The two major variables in filter design are the topology and the 
transconductor type. A transconductor for synthesizers in simple 
terms is just a voltage controlled resistor.

The Moog transistor ladder, EMS diode filter and the Boogie filter 
are all 4 pole Butterworth lowpass filters. Each LP filter "pole" or 
stage reduces the output by 6 dB per octave. What that techno 
speak means is each time the frequency doubles, the amplitude 
is reduced by half. Putting 4 stages in series makes a 24 dB per 
octave filter, or each time the frequency doubles, the output is 
reduce to 1/16.

The Moog ladder uses transistors as the transconductor, the 
EMS uses diodes and the Boogie filter uses Vactrols as the 
transconductor. In theory, there should be no difference in the 
"sound" of all three. But the ear is such a fantastic discriminator, 
that it can detect the mathematical defects in each design, so 
they sound slightly different.

The Boogie (from Boog "Buchla + Moog") uses Vactrols as the 
transconductor. This should be the closest to mathematically 
perfect, because the photo resistors are true conductors, not 
semi-conductors used in a current starved mode to behave as 
AC resistors. Also the Vactrols optically isolate the control 
signals from the audio path, eliminating any control voltage 
bleedthru or thumping.

Another advantage is that resistors are not active noise sources 
like transistors or diodes, so there is much less "hiss" to deal 
with. There is always some hiss because electricity does not 
flow in a perfectly smooth stream but rather in "clumps" of 
various size groups of electrons. When this "clumpy" current 
passes through a resistor, it produces a clumpy voltage 
because of Ohms Law (E = I * R). That is called thermal noise, 
because the molecular vibrations cause the chaotic electron 
behavior which we perceive as clumpyness. It is actually a 
chaotic system of coupled oscillators where each molecule is an 
oscillator, but that is getting too far out.

Semi-conductors have an added noise source which is the 
"diode drop" a kind of quantum waterfall, where more 
clumpyness is added as the electron groups try to decide if they 
are going to go over the waterfall or not. This is where the noise 
comes from in a reverse bias transistor. You just hook the 
transistor up in the worst possible way to bring out the quantum 
waterfall noise.

So the Boogie is designed to have less noise and distortion than 
the Moog or EMS implementations of a Butterworth 4 stage filter. 
This may disappoint people who LIKE noise and distortion. My 
philosopy is to start with the least possible distortion, then use 
either over driving or special circuitry to add it back in when you 
want it.