New OTA Phaser Design - “Causality 4”

[frequencycentral]

Important Update:

I've been experimenting to get a wider LFO sweep. I played around with the LFO on the breadboard and found that if I replace R5 with a 47k resistor (was previously 220K) I get a broader LFO sweep, which is very pleasing.

For anyone who has already built this and wants to apply the 'wider LFO sweep mod' without removing R5 from the board, just parallel it with a 68k resistor, which will give you 51k.

The mod also has the effect of slowing down the LFO, so the fastest speed is slower, but so is the slowest speed.

It also has the effect of making the LFO sweep slightly asymetrical. Subbing a 68K for R5 gives a deeper sweep ,though not as deep as a 47K, but the symetry is better. If you want to do this without removing R5, parallel it with a 100K

[effdub]

Awesome. As soon as all this holiday stuff is over, I'm going to try out your LFO on my phaser circuit.

[frequencycentral]

Awesome. As soon as all this holiday stuff is over, I'm going to try out your LFO on my phaser circuit.

One thing I'd love to see is a thread dedicated to one and two opamp LFOs. I may have to do it myself. I spent some time with that LFO on the breadboard yesterday, changing values etc. I also played around with the second LFO, making it into an amplifier rather than the unity gain buffer, ie sticking a resistor between -ve input and output, and another resistor from -ve input to vref. This can widen the sweep even more. I can feel an article on LFOs coming up..........

[effdub]

I can feel an article on LFOs coming up..........

I think that's a capital idea. 

[earthtonesaudio]

I can feel an article on LFOs coming up..........

I think that's a capital idea. 

Me too.   

[frequencycentral]

After tweaking about with the value of R5 a little more, trying values from 22k to 220k, I've come to the conclusion that R5 really ought to be a control in it's own right, ie a 220k lin pot in series with a 22k resistor. This new control should be called 'Depth', and the original Depth control should be relabelled 'Range', as strictly it governs where in the spectrum the phasing happens.

I haven't got room to implement another pot in my build, so I may go with a switch to selcet between a couple of different values. Or maybe I'll sell my prototype and make another..............

[effdub]

Or maybe I'll sell my prototype and make another..............

Now you're thinking.

[effdub]

You know, this whole time I thought the name was "Casualty 4". 

You know, like a series. Murder One and then Casualty 4. haha

[frequencycentral]

frequencycentral in association with doc_drop are proud to present “Causality 4” MkII:

What's new:

- Width pot added, this defines how broad an LFO sweep the OTA's see
- Shape pot added, waveform now continuously variable from upward sawtooth through triangle to downward sawtooth
- The original Depth pot has been re-named as Range, and defines where in the audio spectrum the phasing happens
- Regen pot is now 10k log for a more useful range
- Speed pot is now 470k rev log for a more useful range
- R7 reduced from 10k to 4k7 for faster maximum speed
- R12 reduced from 100k to 47k to correct the gain structure
- R5 increased from 220k to 330k
- I've switched the two OTA stages around in the schematic to match up with the perf/PCB layouts



Chris (doc_drop) has been good enough to breadboard the original circuit and implement some mods I had in mind, he's done some great work fine tuning and tweaking them to perfection, massive respect to Chris for taking this on! He's even provided some soundclips of his breadboarded circuit in action, I believe he's perf'ing it up right now. I'm pretty much stoked with how this turned out, I need to order some parts before I can get my MkII together. How cool is the internet, that two guys living over 5000 miles apart can work on something like this together???

I'm just putting the finishing touches to the MkII perf and PCB layouts, actually pretty close to the original layouts, and I'd even say that anyone with a MkI should be able to mod it to MkII spec without too much fuss. If you're not sure, just ask and I'll point you in the right direction.

The original concept for Causality was to create an 'easy build' good sounding phaser, which I think the MkI still is. MkII takes it a step further, more options and sonic variation, more knobs, more offboard wiring. The really cool thing is that the new PCB layout will be able to be used to build either a MkI or a MkII, you could even build a MkI on it and later mod it to MkII spec without to much fuss.

To round off, here some very entertaining soundclips kindly provided by doc_drop, I'll be adding my own soundclips too when I get the chance. Take it away doc!

Clean clip: http://dl.dropbox.com/u/967492/DROPed-Cause-01-31-10.mp3

Dirty clip: http://dl.dropbox.com/u/967492/DROPed-Cause-Thru-Fuzz-01-31-10.mp3

[frequencycentral]

Here's the artwork for the 5 knob MkII PCB, it includes pads for the addition of an extra stages daughter board - more on this some other time! The 3 knob MkI version can also be built using this layout.

[frequencycentral]

[CountryBoyAtHeart]

Multicolor LED?

[frequencycentral]

Multicolor LED?

Never used them. Do they vary in colour according to voltage? That would be cool, as one went up through the spectrum the other would go down through the spectrum. Is this correct?

[earthtonesaudio]

Multicolor LED?

Never used them. Do they vary in colour according to voltage? That would be cool, as one went up through the spectrum the other would go down through the spectrum. Is this correct?

No, they're not that cool.   
Basically you have one package (lens) that contains several separate LEDs of different colors.  They can be independent, or have one connection in common, or sometimes arranged back-to-back like clippers (two leads).
There are also multicolor LEDs that have a built-in circuit that cycles through the colors smoothly... which is really cool to look at but unfortunately these sometimes have an audible whine when used near sensitive audio.
You can get some extra use out of them by the fact that your eye will mix colors together if they're strobed fast enough.  For instance there are bidirectional red/green LEDs that will look yellow if you give them AC voltage, because red+green looks yellow to your eye.  That's something I want to experiment with someday.

[frequencycentral]

Ah ok, thanks Alex. I have a salvaged bi-colour from a PC, but that has a common cathode, so it wouldn't be suitable for this application. I'm gonna sell my MkI and build a MkII, so I'll check into the other multi-colour types you mentioned, though I still think two seperate LEDs would look cool too.