My take on the Seek Wah

[Morocotopo]

Here´s my take on the ZVex Seek Wah.

I started researching this some time ago, no Seek Wah schems in the net, so I searched for similar stuff, and found the Vanishing Point, the Dig Dug and, ehh, some other one. I settled for the Vanishing Point 2.1 for the sequencer part, since it had some desirable features. That took some experimenting, since some of the values are left to experimentation in the original article. Then, since that schem doesn´t include the audio part, I had to find a suitable one. I tried the Colorsound inductorless wah and the morley opamp wah, and chose the Morley, ´cause it seemed a bit more consistent then the Colorsound one, that seemed more dependent on the particular transistor´s Hfe and a bit dirtier. Then came the opto selection. Tried VTL5C4´s, 9´s, 8´s homemade ones, but finally settled on the Silonex NSL32, that one gave the best results. The real key of this type of circuits is fine tuning the filter for the LDR range, so that it gives a good frequency sweep between the max and min LDR resistance. I adjusted some of the audio part values to get the best response.

Then I had to decide on the control pots. I tried every value and taper combo I could, with and without parallell resistors, none really satisfied in terms of the "controllability" of the frequency range. Supposedly (I´ve never tried one) the Seek Wah has an uneven response in the pots turn-to-change ratio, it only responds in the last part of the turn... What this really needed were antilog pots, but those are hard to get, more so in 9mm. size (I believe they are not even made). So I settled on A50K ones, but, but, but...
wired backwards, so turning the pots clockwise lowers the filter freq. Counterintuitive? Yes. Awkward? Yes. Gives good range of control? Yes!

[YouAre]

You may want to experiment with the voltage divider setup of the sequencer pots. They're set up to give a pretty large range of voltage output (from full 5-7v all the way to ground possibly), when in reality, you only need a fraction of that. See what voltage input to the vactrol is most useful/usable, and see if you can adjust the output of the vanishing point to be within that range. It may involve adding 2 resistors to every pot, but that's better than half the range of the pot being useless.

Hope that helps.

[Morocotopo]

For some reason the forum doesn´t let me write too much text on a single post, so...

Part 2

Then Came the PCB design stage. I made three PCB´s one for the audio, another for the sequencer and another one for the pots and LED´s. It was quite a challenge to fit 8 9mm pots on a reasonable size box, it all fit quite tight. The jacks went in superclose to one of the boards, and I had to file down the back cover lip were the jacks are to be able to close it! But it all fit in the end.

So, pics:





And a little video:

[rousejeremy]

Is it just me or is it cycling kind out of time for the first two steps?

[Morocotopo]

Part 3

Mods: I added a switch to get, besides 4, 8 or 6 steps, 5 and 7 steps. Long live additive time signatures! Also, I had to add a cap to the control pots output to avoid a pop at every change of step, noticeable mostly at high speeds.

Conclusion:
The thing works well. The random mode doesn´t really work as a random step selection mode, it tends to stay on one or two steps mostly, but is another usable sound, more like a chaos mod, since between steps it sort of closes the filter completely, so you get a kind of pulsing sound. See the video. Finetuning the filter is crucial. One cap value change to the next one up or down totally destroys the sound! Works best after a OD.
Also, when the step next to the last one selected is at min resistance (zero), sometimes it flashes briefly or keeps going, I believe the pots need a bit of resistance in series so as to never have zero resistance.
Another thing I noticed is that sometimes the steps don´t last the exact same time! It´s subtle, but it´s there.

Well, hope you find it entertaining. I can post the schem later if anyone wants it.

[gregfrusciante]

Good job Morocotopo!

Could you share the schem please? Because like you said, it's not easy to find Seek Wah schems in the net 

Thank you!