Pardon me, may I stand on your shoulders?

[stringy]

I'm far from the "design your own" level yet, and quite frankly I have little understanding of how the pieces of a circuit make the whole.  That said, I'm wanting to work with the Optochopto design by dano12 in this thread

http://www.diystompboxes.com/smfforum/index.php?topic=72719.0

but I want to use an LFO with more variety and less "chop" and I found this somewhere on the board



Now my question is could I simply replace everything to the left of the LED on the Opto schematic with this other LFO connected via a sp3t switch?  Would this drive the Vactrol?  Also, a couple questions regarding the LFO schematic.  I'm aware that the power and ground aren't shown on the IC's, and I know how the place them, but what about power to the FET?  Where does it go and does it need a resistor between it and Vcc?  Could I use a J201 instead of 2SK30A?  Would I have to adjust some component values?  Finally, what is the 200K linear pot for?

[JKowalski]

Now my question is could I simply replace everything to the left of the LED on the Opto schematic with this other LFO connected via a sp3t switch?  Would this drive the Vactrol? 

Yes, you could, however you might want to match the strength of the signal going into the LED. The output of dano's LFO will likely be different then the output of the one you found.

I'm aware that the power and ground aren't shown on the IC's, and I know how the place them

The LFO you found requires a dual power supply (ex. +9V, GND, -9V). The +9 volt and -9v would be your IC power connections.

If you don't want to mess around with building a dual power supply, you can also bias your ground. Basically, you use a voltage divider between your + voltage and ground to get a voltage of half the supply voltage (ex. 9V+, 4.5V+, GND). Then you replace all the ground connections shown is the schematic with the 4.5V+, and your IC connections will be +9V and GND.

but what about power to the FET?  Where does it go and does it need a resistor between it and Vcc?  Could I use a J201 instead of 2SK30A? 

The FET only has three connections, you don't need to power it from anything separate - it gets powered by the first op-amp in the circuit (trace the signal path backwards). All three FET connections are shown on the schematic, the drain, source, and gate.

The 2SK30A and the J201 have different FET properties, they do not function exactly the same. Take a look at their datasheets.

I just took a quick peek, and I am not an expert at all, but from what I saw, they are each VERY different. You might get a weird response on your sine wave output, as the FET seems to be an integral part of the sine wave shaping circuit.

Finally, what is the 200K linear pot for?

I'm not absolutely sure on this one.

[MohiZ]

Finally, what is the 200K linear pot for?

It LOOKS like it would control the amplitude of all the signals. I have a similar oscillator built. You could use that trimmer to match the amplitude of the oscillations to the circuit you plug it into.

[Mark Hammer]

You probably want to take a look at this: http://hammer.ampage.org/files/hypertriangleclock.gif

[Joe Kramer]

Hey Stringy,

I've done some work with the LFO you posted.  It was very picky about the FET and I couldn't get a satisfactory sine wave out of it running single supply unless the voltage divider was buffered by another op amp.  Even then, it was a bigger hassle and more extra parts than it was worth.

Here's my suggestion: stand on Boss's shoulders and go with the basic LFO from a Boss CE-2, which is essentially the same circuit.  Use something like a TL062 op amp.  This will give you a squarewave and a triangle wave and is a very stable, easy-to-use, low-parts circuit.  It will drive an LED on its own, but you may have to do a little resistive padding to get the tri and square waves to equal output levels.  Hope that helps.

Joe

[stringy]

Thanks guys, REALLY helpful, now to research your suggestions.

[stringy]

The LFO you found requires a dual power supply (ex. +9V, GND, -9V). The +9 volt and -9v would be your IC power connections.

How do you know this?  I mean, how is this indicated on the schematic?  I'm sorry if I sound like an inane ass, but I'm just trying to learn.

[MohiZ]

It's not indicated on the schematic, but op-amps always require either a split supply or biasing of the inputs. Without a dual supply, you see, the signal can't swing to the negative side.

[Andi]

The main clue is that there's no Vbias shown anywhere, but lots of grounds - that usually indicates a bipolar supply.

You can either derive that with a charge pump or modify the circuit to run on a single supply.