Joined: May 04, 2007 Posts: 1734 Location: NM USA
Audio files: 54
Posted: Thu Feb 07, 2008 11:45 am Post subject:
The Threeler Subject description: My new, third order VCF
Here's you a look at my latest project. It's a third-order VCF with four switchable modes and three outputs. The idea for this came from the MS-20 filter, which Rene and Scott were using to get some crazy sounds by overdriving with the resonance turned way up.
I began by wiring up a "Filter Foundry" (shown schematically below), which had three independent filter sections and a high-gain clipping amplifier. These could be patched up arbitrarily to produce various kinds of filters. I played with this for several weeks until I found a set of four configurations that worked well for me.
The final unit has a big switch (actually made from DG409 muxes) to select from four configurations. This is also shown below. The configurations are: (1) 3 LP sections, (2) 2 LP and 1 HP sections, (3) 1 LP and 2 HP sections, and (4) 3 HP sections.
The mp3 clip was done using mode 1. It has some bass, so please listen to it through real speakers. The signal path used is a straightforward VCO-VCA-VCF one.
Posted: Thu Feb 07, 2008 3:41 pm Post subject:
wow
this looks really great. i love the filter foundry idea.
i am going to have to see if i can handle a circuit of this complexity...maybe i just need to get a few more breadboards! i dont quite understand the threeler diagram though.i get that each square is a stage of the filter and i think all the little dots are poles of the switch you mentioned??
am i correct in understanding that an expo converter can be connected to any point R instead of the pot as a means of adding voltage control?
or i might be way off on that. i take it i could use any expo converter?
Joined: Dec 23, 2005 Posts: 4127 Location: Mount Hope, KS USA
Audio files: 97
Posted: Fri Feb 08, 2008 2:50 pm Post subject:
Yes, a quite sensational sound indeed - it seems to have a bit more substance than the filter I'm using (partially, I'm sure due in part to the extra pole) - particularly the lowpass, which I'm assuming is 18 dB (in Mode 1)? What a novel configuration (which I'm still grokking). In any event, it sounds great! I hear that definite "squawnk".
As a test to see if I understand it correctly, I'll see if I can answer Loss (please correct me if I'm wrong Ian):
The filter blocks are configurable as either high pass or low pass - if you inject the input signal (R) through the resistor, that produces the low pass response, and if you inject the signal (C) into the capacitor, that produces the high pass response. The switch therefore controls the response of each stage:
But, that's not all - Ian - I notice the resonance signal is applied to opposite parts of the first section (LP or HP, depending on mode), so is that playing a part in the response as well?
The expo converter connects to the "a" input of each block. The control b is the initial resonance control, and connects to the resonance gain cell.
Ian - on the Schmitz Late MS-20 filter, when the high pass isn't used, I have the leg of the cap connected to ground. I notice here, you have it connected to the output of the A1b - that accomplishes the same thing, doesn't it? I mean, with that 1M to ground on A1b's positive input, that would leave the potential at ground, right? And, when a signal is applied, you get the signal instead. Very cool indeed...
wow
so does that mean that using a rapidly switching switch (could a 4016 be set to close and open in response to a clock (square wave creating low and high?) could cause the signal to quickly jump between high pass and low pass?
Joined: May 04, 2007 Posts: 1734 Location: NM USA
Audio files: 54
Posted: Fri Feb 08, 2008 5:25 pm Post subject:
Scott Stites wrote:
Yes, a quite sensational sound indeed - it seems to have a bit more substance than the filter I'm using (partially, I'm sure due in part to the extra pole) - particularly the lowpass, which I'm assuming is 18 dB (in Mode 1)?
Glad you like it. I'm having great fun learning to use it. Yes, it's 18dB/Oct -- something you don't see very often. I decided on third order because I wanted to use dual OTAs and because it seemed like it would be a good idea to include VC resonance. Also I wanted to look at the coupled hi-Q oscillations in a higher order system.
I'm finding that third order sounds pretty good to my ear, for some reason. I know that the audiophile folks like third order for their x-overs, second order being too slow and fourth having phase problems.
Quote:
The filter blocks are configurable as either high pass or low pass - if you inject the input signal (R) through the resistor, that produces the low pass response, and if you inject the signal (C) into the capacitor, that produces the high pass response. The switch therefore controls the response of each stage:
Mode 1 LP->LP->LP
Mode 2 LP->HP->LP
Mode 3 HP->LP->HP
Mode 4 HP->HP->HP
Exactly.
Quote:
But, that's not all - Ian - I notice the resonance signal is applied to opposite parts of the first section (LP or HP, depending on mode), so is that playing a part in the response as well?
Well, I just did that the same as the MS-20 and other Sallen-Key-like structures. I didn't look at every single possible combination of connections. It's convenient, though, because otherwise extra summing amps would be needed.
Quote:
The expo converter connects to the "a" input of each block. The control b is the initial resonance control, and connects to the resonance gain cell.
Correct. There is an expo source that drives the three filter stages, and also a linear CV circuit for the resonance. I indicated this with just a pot, but the final circuit does have voltage control.
Quote:
Ian - on the Schmitz Late MS-20 filter, when the high pass isn't used, I have the leg of the cap connected to ground. I notice here, you have it connected to the output of the A1b - that accomplishes the same thing, doesn't it? I mean, with that 1M to ground on A1b's positive input, that would leave the potential at ground, right? And, when a signal is applied, you get the signal instead. Very cool indeed...
I was wondering if anyone would pick that up.
It was a big breakthrough for me, seeing that little trick. It saves a bunch of switching! Actually 1 M turns out to be too big, because of pickup on the lead to the panel. So I actually use 5.6 k.
And of course, for the first stage this trick is not needed because both inputs are always connected. So it just has a buffer after the input pot, and the buffer is switched to the input side of the filter stage.
At some point, I need to draw up a complete schematic because of the final circuit being somewhat modified from the original filter foundry stages.
Joined: May 04, 2007 Posts: 1734 Location: NM USA
Audio files: 54
Posted: Fri Feb 08, 2008 6:17 pm Post subject:
RF wrote:
Some really nice sounds on the mp3, frijitz.
Thanks; I really appreciate that, since making the demos is pretty difficult for me. I have some more done for mode 2, which I will try to edit and post tonight or tomorrow.
Joined: May 04, 2007 Posts: 1734 Location: NM USA
Audio files: 54
Posted: Fri Feb 08, 2008 6:30 pm Post subject:
loss1234 wrote:
so does that mean that using a rapidly switching switch (could a 4016 be set to close and open in response to a clock (square wave creating low and high?) could cause the signal to quickly jump between high pass and low pass?
Right. The mode is selected by two digital signals going (in parallel) to the mux control lines. I just have those wired to a 2P4T panel switch, but you could use two 1P toggle switches, or digital control lines. The filter doesn't have the same center frequency for the different modes, though, so I'm not sure how useful this would be.
Joined: May 04, 2007 Posts: 1734 Location: NM USA
Audio files: 54
Posted: Fri Feb 08, 2008 6:45 pm Post subject:
andrewF wrote:
Do you have any particular expo converter cct in mind?
oh- Does D3 indicate much? I think it has other duties than blinking in this cct.
Any of your usual OTA expos should work OK. I use a 2SA798 pair, with the converter transistor driving all three OTAs. The MS-20 uses a pair of LEDs for its limiter. I have had good luck with the zener pair shown; but the LED can be switched in for asymmetric limiting.
i always love your samples and this is no exception.
one thing i was wondering about which i dont see you commenting on too much here, what, if any, are the chaos properties of this circuit?
i know you had had a lot of luck getting interesting behavior out of the original ms20 circuit, so i was wondering if that carried over into this schematic as well, or if this is purely being worked out as just a great sounding filter without non-linear behavior.
Joined: Dec 23, 2005 Posts: 4127 Location: Mount Hope, KS USA
Audio files: 97
Posted: Sat Feb 09, 2008 9:14 am Post subject:
Wow - I can't believe the formant quality of the first section of that sample. I've never pulled anything like that out of the Late MS-20. I've gotten formants before, but generally of the type one would associate with cartoon characters.
In fact, I don't think I've ever gotten anything like any of those sounds out of it. I'm impressed as hell!
My hat's off to you, Ian! That's a fabulous filter.
One thing I wanted to comment on from your first post in this thread - I'm surprised that anyone would say that the original MS20 filter is thin sounding. Absolute news to me. Back when I had my MS20, I found it could sound thin, but more often it was pushing my speakers to the limit. Maybe it's people who are working with just one of the MS20's filter sections, i.e. 2 pole response.
Joined: May 04, 2007 Posts: 1734 Location: NM USA
Audio files: 54
Posted: Sat Feb 09, 2008 12:42 pm Post subject:
loss1234 wrote:
one thing i was wondering about which i dont see you commenting on too much here, what, if any, are the chaos properties of this circuit?
i know you had had a lot of luck getting interesting behavior out of the original ms20 circuit, so i was wondering if that carried over into this schematic as well, or if this is purely being worked out as just a great sounding filter without non-linear behavior.
Good question. One of the criteria in picking the four modes to use in the final circuit was that they should have lots of good action above resonance. It did seem to be a little less prominent in general than for the MS-20. Another configuration I liked was more similar to the MS-20, in than it had a second order feedback loop, with the third filter section as just an extra section in series with the output. (Mae West: "When forced to choose the lesser of two evils, I always pick the one I haven't tried before")
In the demos you usually hear two distinct pitches. These are the main VCO pitch and the pitch of the filter oscillation or some phase-locked combination tone from the nonlinear interaction of the two. For example, in the third section of the demo the steady high pitch is the filter oscillation turning on and off. In the fourth section, the VCO is the bass line and the higher ring-modulator like sound is phase locked combination tones from the nonlinear interactions above resonance.
There are also regions of chaos, but as with the MS-20 they occur over a fairly small range of parameter space. I kept away from those in the demo, although since large amounts of dynamic FM and QM were used, the chaotic regions were probably transited dynamically.
This is all new for me, these nonlinear signals. I noticed in one of the old ARP patents, Perlman explained in great detail how he designed his filter to avoid these effects:
"Because of the potential instability problems imposed by positive feedback, it is necessary to limit the audio input signal, the feedback signal or both. Typically the limiting is accomplished by utilizing diode clippers. Such clippers are characterized by abrupt transitions into conduction, and thus do not allow smooth control of resonance. Because of this, higher order harmonics are frequently created which distort the audio signal and may also cause the resonant filter to lock onto one or more of these harmonics uncontrollably."
Joined: May 04, 2007 Posts: 1734 Location: NM USA
Audio files: 54
Posted: Fri Feb 15, 2008 3:27 pm Post subject:
mode 3 demo and schematic
Here is a demo of Threeler's mode 3 (1 LP stage, 2 HP). For this clip I used my Dynamix and 8-phase VCO to make a wave scanner, with a varying mix of four waveforms from three VCO's. Two of the VCO's drive 5Pulser shapers.
The VCO pitches and waveforms are slowly varied from my Cyclic Chaos generator. The scanner drives the filter through a VCA, which is controlled by an AD+AR generator. The AD and AR envelopes also modulate the filter FM and Q.
Also attached is the schematic of the current version of the filter. I'm not planning on changing the filter any further, but I may try adding a switch to add a special configuration for multiscroll chaos. I found some literature on how to do this that looks like it might fit in pretty well.
Ian
threemx3a.mp3
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Last edited by frijitz on Sun Feb 17, 2008 11:49 am; edited 1 time in total
Posted: Sun Feb 17, 2008 12:23 pm Post subject:
Re: schematics removed
frijitz wrote:
because of a threat of them being misused.
erm ... ... _________________ Jan
also .. could someone please turn down the thermostat a bit.
9 3 4 .. erm .. not 13 then? .. hmm, ah eight! .. yeah yeah as in 8647 .. 47 is an 88 .. pwew .. numbles!
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