Small stone manual control and cv input?

[dirk]

Hello people,
I'm an electronic newbe and need some help.
I have bought a new small stone and it sounds very good.
I searched the forum for mods, but I could only find the level drop mod. Witch will be done in a few day's.

But I want to switch off the lfo and give it a manual control.
And I would like a cv control input, so that the phase shift can be controled with external (audio, adsr and lfo) signals.

Thanks in advance.

[dirk]

I've done the volumedrop mod and the univibe mod. They work fine.

But I need help with the LFO. How does it work, how to make a manual control on it (like on the Bad Stone) and how to make a cv input.

Please.

[sfr]

I've been meaning to do something similar to a small clone for a while now, I just haven't had the time to get around to it.   If I get to have that afternoon mucking around with the scope and LFOs and hacking at my modulation pedals, I'll try and post more. 

I'm not the most skilled person - I'm think if I sat down and beat my head against the wall long enough, I could give you a step by step, but what I can think of off the top of my head -

If you head over to Tonepad, there's a schematic for the Small Stone - their version uses a CA3080 for the OTA, which may be different than the current production units from EH, and there will be a different layout than what's in your box, but the general schematic should be the same. 

The nice bit about the Tonepad schematic is that the sections are separated well, making it fairly easy to see what's going on - the "top" of the schematic shows the four phaser stages, the lower left shows the input section and bias network and the lower right shows the LFO.  In fact, there's two points labelled "LFO" showing where the LFO signal exits and then enters to control the OTA.  Breaking this connection will kill the LFO's control of the phase shift stages; to control it with a different LFO, you'd simply want to hook it up there.  (Actually, you'd want to break it just before the 100 ohm resistor, which is located just prior to the LFO point on the schematic) To control it with a pedal, or a knob for "stuck" sounds, you'd simply want to insert a non-moving DC voltage at that point - a variable voltage divider may be good enough, although many pedals I've seen with this control have used a voltage divider into an op-amp with variable gain to adjust the voltage coming out.  A manual knob, to adjust the sweep up and down (like on the ADA flange) just adds a DC offset to the LFO.  (Another interesting trick from the ADA book is to use another knob to blend the control signal between the DC offset and LFO, allowing a variety of sounds from "stuck" to full sweep, with sweeps being shifted high/low between.)

Probably a good idea to figure out what sort of voltage range your LFO swings across to figure out what sort of voltages you should be feeding into that LFO point.  A scope would tell you this easily, or if your LFO swings slow enough, you can probably get a decent reading with a meter.  There's probably a way to determine what sort of voltages the LFO puts out by looking at the schematic, but without racking my brain for a bit I'm not the guy to figure that out.   You could just look at the schematic for whatever the OTA is and figure out what sort of levels it accepts at the control point - depending on what sort of voltages and currents your working with, you may want to change the 100 ohm resistor to another value.  (If I remember correctly, the control points on those OTA's is a current, not voltage controlled thing?  I haven't messed with them much)

Well, that should get you started.  Hopefully someone else will chime in and fill in some of the blanks I've left and correct any mistakes I've made, but I hope I've pointed you in the right direction to figure the rest of it out.   

Also, there's an article, I think "the technology of phasers" or something similar, over at Geofex, that has a lot of cool information as well. 

[dirk]

Thanks sfr, that's just what I needed.
I will attempt it after the weekend.
Will let you know how it went.

[haima]

oops, posted in wrong thread....!