Rise & Ramp Mod to Easy Vibe

[axeman010]

Hi

has anybody modded an Easy Vibe to add the Rise & Ramp function that a VB2 has ?

From what I can make out the Rise time and ramp emulate a Leslie coming up to speed
when engaged rather than just a constant speed.

Any ideas gratefully received. 

Axeman.

[R.G.]

It's not a mod to that specific circuit, but rather a mod for most LFOs. Will that do?

Go read GEO - in this case the article on building LERA, the Leslie Effect Rotor Adapter.
http://geofex.com/Article_Folders/lera/lera.htm

[axeman010]

Many thanks R.G.

Will dig through the archives as well to look at others experiences.

Thanks again !

Axeman.

[R.G.]

Along those lines, it is modestly simple to make an LFO like most of the triangle-square types used in effects into a version that is both wider range and can be made to follow the speed control to a loose degree - that is, you turn the speed control or switch speeds with a footswitch and it ramps up/down to that speed.

Is that desirable enough to write up a buildit article about?

[Jay Doyle]

Is that desirable enough to write up a buildit article about?

YES!!! But if I were you I would expect it to be in a boutique effect a couple of days after you offer it.

But I imagine you are used to that by now... 

[joelap]

Yes!  I'm very interested in a ramp switch and would be incredibly grateful if someone could figure one out.  I'll give you my first born... and second... hell take 'em all.   

[R.G.]

I did the sim work on it this afternoon during a break.

The classical integrator/Schmitt trigger LFO is what is in almost all modulated effects. The integrator is the classical inverting integrator, with the + input biased to half the total power supply (either Vbias, or ground for bipolar supplies) and its output ramps at a rate of dv/dt = Vin/(Rin*c). The output is fed to an inverting Schmitt trigger that has only two output states, as high as it can go and as low as it can go. The input makes the trigger switch at some voltage Vt above the + input's bias point, and positive feedback resistors cause the output to have hysteresis- it switches at +Vt and -Vt around the bias point. The output is then fed back to the input resistor of the integrator.

Since the Schmitt output is a fixed amount, either full high or full low, if the integrator + input is biased in the middle of those input voltage, the integrator will then ramp up until its output hits Vt, makes the schmitt trigger flip, and then start ramping down because the schmitt inverted its input. It ramps down to -Vt, then flips again. You control speed by changing Rin, or (rarely) by changing the fraction of the Schmitt's output voltage that gets fed to the integrator.

This circuit can be converted to a voltage controlled oscillator with a couple of minor tweaks. The big one is in the integrator. The + and - inputs are both fed through a resistor from a control voltage Vin, and each has a resistor to the most negative supply - ground for single supply use. The + input resistors are equal, and the - input resistors are in a 2:1 ratio. There is also a transistor used as an open/close switch between the - input's ground resistor and actual ground.

The input voltage sets the + input at half the input voltage, whatever that is. The - input is fed through its input resistor if the transistor switch is open, or through 1/3 of the input voltage through an equivalent impedance of 2/3 of its input resistor if the switch is closed to ground. This sets it up as a

differential integrator

. That is, it integrates the difference between the two inputs. And since the difference voltage is always proportional to the input voltage Vin, it always ramps at a rate determined by Vin.

The Schmitt trigger comes in by sensing the ramp output voltage of the integrator, and again flipping at +Vt and -Vt around some bias point. However, it does not feed the input to the integrator directly. It turns the transistor switch on and off. If you juggle the resistor values right, you get the same ramp up and down and square wave out of the Schmitt, but now the speed is controlled by an input voltage over a very wide range, not by some resistor value.

So far, all I've done is complicate things with extra resistors, transistor and speed pot setups. But here's where it gets good.

How about if I make the Vin be the output of an opamp. This opamp is just a follower, but with a high input impedance. Now I can make my speed pot be the input to the follower, but because it's a high impedance, I can separate the follower from the speed control voltage by a high value resistor and a capacitor to ground. Now when I change the speed pot, the voltage on the capacitor at the input to the follower cannot respond instantly, but can only follow the speed pot voltage, lagging behind at a rate determined by the resistor between the speed pot and the cap feeding the follower.

And now we get to switching. If you switch the input voltage between two levels, the voltage at the follower output ramps up and down at a rate determined by one resistor and one cap. If you want to have different ramp up/down rates, you use two ramping resistors, with a diode in series with each one to separate their effects, and you get different ramp up/down rates.

Easy, yes?

No, I probably lost you describing circuits in words. I'll draw it up. Here's the critical parts.
- the opamps for this thing must have inputs that can go to the most negative supply, ground in the case of the single supply version.
- the opamps must be able to swing their outputs reasonably near ground to control the switch transistors

Fortunately, the garden variety LM324 quad and the LM2904 dual will do this nicely, as will others.

I'll gen up a schematic.

[axeman010]

Awesome work R.G !!!!!!!!!!!

Thanks for putting the time in to this one - I am sure there will be  many people who will appreciate it 

Axeman.

[d95err]

Way to go RG!

This is one of those things I've always though it should be fairly simple to do. However, it has always been slightly too complex for my poor electronics theory...

Dang, now I need to start another modulation project...

[R.G.]

OK, the first schemo is up at GEO.

A couple of notes. The first box is the theoretical voltage controlled LFO. It requires that the opamps be able to work with their inputs near the negative rail, ground in the single supply case. The LM324 quad, LM2904 dual, and LM358 dual all do this, as do many others. I would say just use an LM324, but I hate quads for being hard to lay out. All of the rail-to-rail input/output opamps should work as well. More ordinary opamps like the TL072 are likely not to oscillate in this circuit at all.

The second box is the original easyvibe LFO design, shown for reference in a similar layout.

The third box is the voltage controlled LFO adapted for the easyvibe. I have not prototyped this yet, but there are a number of sim runs that are quite believable. Still, it has to be regarded as experimental until some brave soul hacks it into his easyvibe. Note the changes:

- the inputs of the Schmitt trigger are swapped, inverting input for non-inverting input. This is because the transistor used to switch the integrator inverts it again.
- the integrating capacitor is 5x bigger. I think I could have dinked with the input resistors to fix that, but using a bigger cap is simpler.
- the opamp control voltage follower has been replaced by an NPN emitter follower. This is because the ordinary opamps like the LM324 which will let their inputs go to ground do not do a great job of pulling their outputs up to nearly the power supply. The 324 only gets to about 7V on a 9V supply. A fancier rail-to-rail output opamp would work there, but I got about 90% of that goodness by using an ordinary NPN. It seems to work if the gain of the NPN is over 200. I didn't really want to give up the high input impedance, but it seems to be OK.
- the speed ranges of this setup are similar to the stock easyvibe LFO. The VCO version will run much, much faster under the right conditions, as it happily accepts input voltages of twice the available power supply. But I limited it to the stock 9V for simplicity's sake.

So - who's gonna breadboard?