Shoot The Moon - Min/max speed help

[Stomptown]

So I'm building a Shoot the Moon in an expression enclosure and I am trying to limit the minimum and maximum speed. Does anyone know how to do this? I've tried an ICAR 100K, a C100K and a C50K (special thanks to Joe for special ordering the C100K and for giving me the C50K on the house!). The ICAR and C100K are both okay but it goes excessively slow for this application and a little too fast as well (not as critical).  The C50K does clamp the slow end, but it bunches up the middle in a bad way. Are there any modifications I can make to this circuit so I can use the C100K and clamp the speed a bit???

Thanks in advance,
Jon

[rullywowr]

So I'm building a Shoot the Moon in an expression enclosure and I am trying to limit the minimum and maximum speed. Does anyone know how to do this? I've tried an ICAR 100K, a C100K and a C50K (special thanks to Joe for special ordering the C100K and for giving me the C50K on the house!). The ICAR and C100K are both okay but it goes excessively slow for this application and a little too fast as well (not as critical).  The C50K does clamp the slow end, but it bunches up the middle in a bad way. Are there any modifications I can make to this circuit so I can use the C100K and clamp the speed a bit???

Thanks in advance,
Jon

Hey Jon,

Very interested in this myself!  I think if you are happy with the sweep of the C100K, then you should just put a shunt resistor across the lugs of the pot to clamp the lower value.  Maybe find the spot where you think the lowest speed should be and measure the resistance.  Then put a resistor across lugs 1-2 (or 2-3 if it's reversed). 

-Ben

[Stomptown]

I thought about that, but I was hoping that it was possible to adjust a couple resistors on the PCB. I was looking at the schematic and R3 and R5 are connected to lugs 3 and 2 respectively. I'm a novice when it comes to theory so I'm not sure if this is a possibility, but it would be nice to avoid soldering resistors to the pot. Here is the schematic for anyone who might be able to throw in their 2 cents...

[DutchMF]

Increasing R3 would mess with IC1A (not sure in what way.....), but increasing R5 seems like a good idea to limit the max speed. Rotating the speed knob clockwise decreases resistance between lugs 2 and three and so increases speed, if I read the schematic correctly, so maybe 20k for R5 would do the trick? I say socket and experiment....

Paul

[Stomptown]

Increasing R3 would mess with IC1A (not sure in what way.....), but increasing R5 seems like a good idea to limit the max speed. Rotating the speed knob clockwise decreases resistance between lugs 2 and three and so increases speed, if I read the schematic correctly, so maybe 20k for R5 would do the trick? I say socket and experiment....

Paul

Thanks for the input Paul. I was thinking the same thing. I'll let you all know wat I find!

[midwayfair]

This is the simplified "how jon understands stuff" explanation. There's probably a better one out there.

[R5 + speed], C1, and C2 are mostly responsible for setting the minimum and maximum speeds.

Let's break it down. We have a gain loop between the non-inverting input and output, which puts out a signal with one phase (let's call that "up"). But we also have a gain loop from the inverting input THROUGH the speed pot and R5, which is then connected back to the output (let's call that "down") with a 10nF capacitor also between those pins. We're feeding non-inverted signal back through to in inverting signal, which under the right circumstances will create enough gain to freak out the chip and trigger oscillation.

This is actually the same type of oscillation that is heard as things like whine from power supplies, or squealing in high-gain effects. What we need to do is make it a really, really low frequency. The lower the frequency, the slower the speed between pulses. There are a few ways to do that.

One way is to filter out the high frequencies. Those 100uF and 100nF caps in the power section of every effect (e.g. C6 here)? They are the equivalent of C2 in our oscillator. Increasing the size of C2 will lower the frequency of the oscillation in the gain loop.

For the other way, let's go back to what I said about oscillation and squealing in high-gain effects. You might recall that a lot of effects have really small bypass capacitors for cutting treble, either between the base and collector in a transistor or (looks familiar ...) between the pins in an op amp. These capacitors prevent certain frequencies from being amplified by the device. In this case, we have C1, which is much, much larger than we're used to seeing. It turns out that making it smaller will vastly increase the maximum frequency, even put it into the audio range -- some people mod this sort of oscillator to become a ring modulator just by changing that cap (though it doesn't work particularly well with a light dependent resistor, which is not really fast enough for true ring modulation ... though an H11F1 might be ...). Increasing the size of it will also slow down the oscillation by filtering out frequencies.

What's the speed pot + R5 doing? They're setting a low-pass filter along with C2. (This is where it becomes really helpful to remember that filters of any sort are altering time constants.) As you decrease the resistance, the frequency increases and the speed of the oscillation goes up.

R5 is simply a minumum resistance.

The takeaway from this is that you can use a pot with a really great taper and change other components to get around the limitations of the pot's resistances.

[Stomptown]

Thanks Jon! I just tied modifying R5 and it did seem to limit the max speed but thats not what I'm after. I also put a resistors across lugs 2 and 3 and was able to get what I was after using a 220K. I'm going to try messing with the caps though and see if that works. Your explanation was very helpful as well!

[Stomptown]

So I experimented with C1/C2 a bit and found that I like it most with a 33n cap in C1 and a 10uF in C2.

I noticed that as the values of C1 and C2 converge (i.e. C1 gets larger and C2 gets smaller) that oscillation stops at max speed. It's actually kink of a cool effect. You can ramp up your speed until oscillation disappears completely. I'm not sure I would need this feature but it really depends on the user. IIRC, this happened with C1=39n and C2 = 4.7uF. If anyone is interested in trying it out just socket your parts and experiment a bit.

I also noticed that changing both C1 and C2 greatly impacts the sweep of the C100K. My original intent was to limit how slow the oscillation goes, but modifications C2 in particular seems to have a negative impact on the sweep of the C100K.

As I mentioned, I liked C1=33n and C2=10uF most. It limits max speed a bit and seems to improve the sweep (when C2 = 10uF) and open up the end of the pot a bit. It does limit the wacky modulation at the end of the pot slightly, but it's still wacky.

I also added a 220K resistor across lugs 2 and 3 of the C100K while using the 33n for C1 but it also had a negative impact on the sweep.

In conclusion, it is really up to the builder to experiment here and the differences are fairly subtle. I would recommend sockets for both C1 and C2 so you can find what works for you.