Speed ramping up/down mod for Phase 90 at GEO

[R.G.]

I modified the LFO circuit for the Phase 90 to work with the voltage controlled ramping up/down circuit. It's now in "New at GEO".

This mod replaces the speed pot with three pots and a switch - how's that for adding bells and whistles, eh?. The speed pot is now two pots, a high speed and a low speed. An SPDT switch, preferably a footswitch, selects the wiper of one or the other. A ramp speed pot sets how fast the speed moves from one to the other when you press the speed change switch.

I tinkered the new LFO speeds to closely match the speed range of the original LFO.

The P90 LFO uses a one-opamp LFO, using the opamp as a Schmitt trigger and a cap for the integrator instead of an opamp integrator. This made it easy to hack the voltage controlled integrator in because the Schmitt is already the right polarity of inputs.

The addition requires an LM358 dual opamp, two 5088's, and some R's and C's. The biggest problem will be placing all those knobs and switches.

[axeman010]

HI R.G

Looks really impressive both the Easy Vibe and The Phase 90 Mods. Thanks for following  up this request for me.

Having looked at the circuits there appears to be a lot of similarity between the Phaser and The Easy Vibe. Sorry if this  a dumb question but how much difference is there between the sound of a Phaser and a Vibe unit. I have a very old (early 80's) Boss PH-1R phaser and it sounds fantastic, lovely and warm but I have not been able to get what I would describe as the characteristic vibe sound from it.  Hence the reason for wanting to build the Easy Vibe.

Eager to learn.

Regards

Axeman.

[gez]

That's almost identical to the VCO used in many Penfold projects.  I never really understood the finer points of how it works - the triangle generator I get, just not the trannie used for switching etc.  I use OTAs for this sort of thing, but then again I tend to use them for everything these days! 

Anyway, nice work!

[R.G.]

The voltage controlled differential integrator oscillator is directly from a National Semiconductor app note. All I did was adapt it. But it is flexible, and can be adapted many places.

If you are doing other things with it, it offers many, many tricks that don't show up at first glance. Having voltage control is obviously an advantage. It makes doing a simple half or double speed switching easy. The triangle can be shaped to pseudo sine easily enough since it's constant amplitude.. You can put a resistor and back-to-back zeners on the square wave output and have a lower square wave size. There are others.

[R.G.]

I never really understood the finer points of how it works - the triangle generator I get, just not the trannie used for switching etc.

The essence of the differential integrator is that set of four resistors on the front.

All opamp integrators are actually difference integrators, it's just that we nail the + input to a non-varying reference voltage. Then the input voltage on the Rin resistor is with respect to that reference. In this case, the + input is tied to a varying reference. The reference is 1/2 of the input voltage, whatever that is. That is reason #1 that the input common mode range must include ground. The inverting input gets either the input voltage through the series input resistor (when the transistor is off) or 2/3 of the input voltage from an impedance of the two minus-side input resistors in parallel (when the transistor is on).

With the ratio of the minus side input resistors at 2:1, the resulting integration is the same rate up as down, and the speed of integration is directly controlled by the input voltage.

Like with all integrator/trigger LFOs, the final peak and bottom points of the triangle wave are the trip points of the Schmitt. The classical opamp Schmitt has two modes, inverting and non-inverting, depending on where you tie the reference voltage and where you tie the input signal. For most integrator/trigger LFOs, you have to reverse the input sense on the Schmitt because the transistor switch following the Schmitt also inverts the sense of integration directon compared to the original circuit.

It is also possible to diddle with the Schmitt trip points to make them be whatever you like.

This circuit makes a good audio VCO, too. The max speed of the circuit depends on the max slew rate of the opamp you make it from and the trip points of the Schmitt. The closer the Schmitt trip points (i.e., the shorter the amount of slew needed) the higher the frequency possible.

Since it's a 0V based VCO, you can also use this thing as a VCO in a PLL. You can use the phase comparator of a CD4046 with an opamp buffer to drive the input, do the phase lock on the square wave output, and get a phase-locked triangle or shaped sine, and get that with all of the tricks of a PLL, including octaves up and harmony notes.

but I'm rambling.

So many circuits, so little time.

[gez]

Thanks for the explanation RG, much appreciated.  I haven't read through it fully yet but have downloaded it for when I have a spare moment.  Read it I will though as this one always confused me!

[pqt_bach]

So many circuits, so little time.

Indeed... any pointers as to where to start my reading?