moog ladder filter - nonlinear resonance

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Dimitree
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moog ladder filter - nonlinear resonance

Post by Dimitree »

hi everyone,
I was reading an article about the Moog Source on retrosynth.com,
and this part made me curious:

" the resonance on the Minimoog filter is nonlinear. "Nonlinear" means the resonance on a Minimoog filter does not stay constant across the full frequency cutoff range - it disappears as the filter cutoff falls to low frequencies. So when a "spoinky" bass is dialed up on a Minimoog, the filter resonance disappears as the EQ sweeps the filter toward low cutoff - the net effect is the bass sound is full sounding.

Moog subsequently viewed this nonlinear resonance as an engineering "fault" and corrected it in new designs - including the Source. So the resonance on the Source filter stays constant across its full cutoff frequency range and the engineers were happy. But this was also why later Moog instruments also didn't sound like a Minimoog and the customers were not happy. Moog went through a lot of head scratching to find out why and never realized that their "correction" was the source of the difference."

I guess that the same happens on the same Moog synths of that period, like the Moog Prodigy/Rogue/etc..

so what's that fix that makes the resonance linear?
this is the schematic of the Source filter

source vcf
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Post by guest »

i had a similar problem with the last filter i designed, and it was because the amplifier stage after the ladder filter had too low of an input impedance. the ladder filter changes its output impedance based on how much current is flowing through it (ie what frequency it is at). so at low frequencies, where not much current is flowing, the transistors have a high output impedance. if the input impedance of the next stage isnt really high, you get a voltage divider effect at low frequencies, knocking down the gain, which in turn knocks down the feedback, and in turn knocks down the resonance.

on the source, they use JFET buffers on the amplifier stage, which have a really high input impedance.
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Broadwave
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Re: moog ladder filter - nonlinear resonance

Post by Broadwave »

Dimitree wrote:so what's that fix that makes the resonance linear?
this is the schematic of the Source filter
The only Moog VCF I've ever built was the Minimoog one (resonance roll off at low freq). It's taken a while but I now appreciate the subtleness of it :)

I can only guess that the 3080 OTA in the Source VCF feedback loop is what "corrects" the linear response.
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Post by Dimitree »

guest wrote:i had a similar problem with the last filter i designed, and it was because the amplifier stage after the ladder filter had too low of an input impedance. the ladder filter changes its output impedance based on how much current is flowing through it (ie what frequency it is at). so at low frequencies, where not much current is flowing, the transistors have a high output impedance. if the input impedance of the next stage isnt really high, you get a voltage divider effect at low frequencies, knocking down the gain, which in turn knocks down the feedback, and in turn knocks down the resonance.

on the source, they use JFET buffers on the amplifier stage, which have a really high input impedance.
thank you,
so comparing it with:

the Rogue:
rogue schematic

the Prodigy:
prodigy schematic

the Rogue looks like the Source since LF353 opamp buffers are JFET too..
what about the Prodigy, that uses NPN but those inside a 3046 IC?
did they corrected the impedance in other ways or it is essentially like the minimoog, with the "nonlinear resonance" response?
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Post by guest »

yes and no.

both the rogue and prodigy have 1M resistors to ground at the input of the amplifier stage, which is pretty high, but not as high as a JFET input. the minimoog, on the other hand, used 47k resistors, which is really low. so you can dial in the effect by changing that resistor (as long as the transistors/opamps have a higher impedance than the resistor you select).
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Dimitree
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Post by Dimitree »

ok, that's pretty easy if I understand correctly,
so if one wants to implement that design issue on the minimoog, you simply change the input resistors from 1M to 47k on the Prodigy and the Rogue, while you need to add that resistors to ground on the JFET buffer inputs of the Source.

I guess you also need to change the values of the 2 caps on the buffer inputs to match the new resistors value?
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Post by guest »

yes, the capacitors need to change as well. and if this schematic is correct for the minimoog:

http://www.fantasyjackpalance.com/fjp/s ... -schem.gif

then there is also a roll-off due to much smaller caps (proportionally to the resistors) being used. .22 and 47k give 16Hz, whereas .1 and 1M gives 1.6Hz. and although those both seem well below audible frequencies, those are the -3dB points, and the roll-off starts much earlier (higher frequencies). in a feedback circuit, a small change in the gain has a much larger effect, as it regenerates on it.
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Dimitree
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Post by Dimitree »

clear :yay:

and why on the prodigy/rogue/source there's an opamp in the regeneration path while there is no need on the minimoog?
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Post by guest »

just different ways of accomplishing the same thing. it might have more to do with the relative cost/availability of opamps at the time of design of the various synths.
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Post by drip.feed »

Stirling information, wigglers. :tu:
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Post by The Real MC »

Dimitree wrote:and why on the prodigy/rogue/source there's an opamp in the regeneration path while there is no need on the minimoog?
Four reasons:

1) there was a conscious decision to replicate the 904A lowpass VCF module from the modular, which was discrete.

2) opamps were in their infancy and were not considered good enough fidelity to be used in audio. Early opamps were also prone to latchup and didn't have robust short circuit protection.

3) back when the minimoog was in the design stage, opamps were only available in single unit packages. There were no dual or quad packages like 4558s or TL084s. Minimoog PC boards were already pretty dense.

4) business conditions at the time - a recession caused by market saturation of modulars due to quick-buck yankee slickers failing to profit from copying Carlos' SOB - - mandated quick transition from concept to market. Not a lot of effort on re-design, there wasn't time. The oscillators were pretty much the only new design, much of the minimoog was copied from modular circuits which at the time used zero opamps.
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Post by cloudberry »

Does anyone know what the transfer characteristic of resonance and cutoff in the minimoog VCF is? It seems like it would be a good thing to design into other filters. One could relate resonance level to pitch cv, but it would only be a guess. I fear the mathematics here is a little beyond me...
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