Electric Mistress Filter Matrix mode - can someone explain how this works?

[kvandekrol]

Now that I've got the Electric Mistress working (thanks Thomeeque!), I've been playing around with it and it really is a cool piece of equipment. I am especially fascinated by the Filter Matrix mode, which disables the Rate control and sort of "freezes" the flange in place (for lack of a better way of describing it) based on the position of the Range control. It's intended to provide a unique coloration to your tone which is an effect in itself. (I found a demo of it - 4:10 in this video: http://www.youtube.com/watch?v=5pwxGE5I7rA)

But what I noticed was that by moving the Range knob up and down while playing, you can achieve a sort of manual flange. The gears started turning with the possibilities... An external expression pedal to control this pot would be very cool, but I opted for an easier experiment: I put a photocell in between lugs 1 and 2 of the Range pot that can be switched in and out, so it flanges in reaction to light. I like it a lot.

I have two questions. The first is: can someone explain to me how this mode works? What exactly is going on in the circuit to make this happen?

The second is: Could it be adapted for any other time-based effects - specifically a phaser? I don't know all the technical differences between a flanger and a phaser so this may be ridiculous, but to my ears a flanger sounds more metallic while a phaser is more lush and organic, and I'm wondering what a "frozen" phaser would sound like - especially one where you could control the position of the phase.

Schematic:
http://www.generalguitargadgets.com/diagrams/dmistsc.gif

[Taylor]

A flanger is a very short delay line, with its time parameter being swept longer and shorter with an LFO. The delayed signal is then mixed with your clean signal, and they cancel each other out at certain frequencies.

So the filter matrix just stops the oscillation, and uses the range knob to set a static delay amount. You then get cancellations that just stay at the same frequency (unless you manually sweep the range pot around). Because the peaks/notches are at a linear frequency ratio, whereas we hear pitch on a logarithmic scale, the sound is "metallic" because in real life metallic objects resonate with inharmonic overtone relationships. The resonance ("color") makes the delay line ring at certain frequencies.

Google Karplus-Strong.

A static phaser is not a very fun sound. You'd be unlikely to know it was on at all - it just sounds like a particular EQ setting on a graphic EQ.

[Mark Hammer]

An LFO is simply a varying voltage.  As you are well aware, that change can be made very slow.  The filter matrix mode is just very very VERY slow, such that the notches stay in one place.

You will note that when the switch shown in the GGG schematic is in the filter-matrix position, SW2 connects IC4b to R31 rather than the output of the LFO via R29.  What's at R31?  Well, that goes to the junction of R23/R24, which is just a divided-down version of the 15vdc supply; i.e., a stable DC voltage.

SW2 also connects R41 to ground.  And when it does that, it makes IC4b a non-inverting gain stage.  When you adjust the Range pot you effectively amplify the miniscule DC voltage arriving at pin 5, and that (amplified) DC voltage drives the clock.

Actually, a number of flangers provide the same sort of thing, except that allow you to pan between a static DC voltage and an LFO, rather than force you to choose one or the other as in the DM.

Whenever you see a flanger (e.g., the BF-2) with Rate, Manual, Depth controls, the circuit will essentially have a variable DC voltage, and an LFO.  The depth control is really a panpot that turns the one up as it turns the other down.  So you can adjust the starting point of the sweep with the manual control, but the more LFO you want to apply, the less impact the manual control will have.

Some phasers are like that too.  I have a Boss RPH-10 which includes a manual control in addition to depth and rate.  Works exactly the same way.

[kvandekrol]

Thank you! Both of those explanations were very helpful.

A static phaser is not a very fun sound. You'd be unlikely to know it was on at all - it just sounds like a particular EQ setting on a graphic EQ.

The reason I'm interested is not so much for the static sound, as much as the possibility of outsourcing control of the phase to an expression pedal or a photocell. The range of the expression pedal would be somewhat equivalent to the full sweep of the phaser, but it can be controlled like a wah.

Are the common phasers like the Phase 90 possible to set up this way with some circuit surgery, or are there factors inherent to the design that would make it impossible (or ill-advised)?

[Mark Hammer]

Oh they're quite amenable to it.  In fact, if you tinker with the bias trimpot on a Phase 90 during setup, the thought of turning it into a foot-control may just cross your mind.