Portamento for Vanishing Point?

[Taylor]

Does anyone have any thoughts on implementing some kind of slew/portamento for the Vanishing Point? Something with control over slew rate would be really useful. I'm sure there are a number of synth designs that do this, but that stuff is always designed with a bipolar power supply, and I don't know how to go about adapting it.

[frequencycentral]

This should get you there, you may have to play around with the values of the pot and cap to define the minumum and maximum slew.

[Franky]

Yep, but there is a slight problem: the slew/portamento/glide/whatever will change when the "frequency" (sequencing speed) changes.. You also want to deal with these values.

[frequencycentral]

Yep, but there is a slight problem: the slew/portamento/glide/whatever will change when the "frequency" (sequencing speed) changes.. You also want to deal with these values.

Fair point, but I have analogue sequencers with portamento in my modular synth, it's not so hard to change to slew rate manually to suit the sequencer speed. I don't know of any analogue sequencer design where the slew speed is proportionate to the  sequencer speed.

By the way, you would add the circuit above between the common connction for the pots and the optocoupler.

[MarcoMike]

excuse me... can you explain what this portamento/glide/slew does?

[frequencycentral]

excuse me... can you explain what this portamento/glide/slew does?

Instead of stepping through each sequencer step, portamento/glide/slew makes the slurs the change between each step. Like bending up/down to the next step. The portamento/glide/slew rate defines how much slur, from nil through to meaningless wobble. Clear?

[MarcoMike]

Absolutely!

[jacobyjd]

Hey Rick, I have Solderman's version of the VP on my breadboard, and I hooked up the schematic posted above using a 4558 dual op amp.

I hooked the input up to the common sequencer pot connection and the output to the LED that will serve as the light side of my optocoupler; stock values all around.

However, when I powered it up, the opto's LED stayed lit constantly, and the sequencer LED's were not lighting. I triple-checked my hookup, and it's all ship-shape.

Any ideas?

[jacobyjd]

Ah, also--I tried running it both from the 5v regulated supply I'm using for the 4017 and from the 9v battery--same results.

[frequencycentral]

Hey - I don't remember this thread! Just looking at that circuit snippet again, I think the 2.2uf cap should be referenced to a vref (ie 4.5 volts) and not to ground. 

[jacobyjd]

Haha--way to claim no recollection :-P

Ok, I tried running the opamp at 9v, then ran the cap to regulated 5v, tried flipping the cap, same results

Also tried the same thing w/ the opamp at 5v.

Any other ideas?

[frequencycentral]

Haha--way to claim no recollection :-P

Yeah I think it must have been 'that other' frequencycentral - damn him!


Hmm, just looking at the Vanishing Point schematic (v2.1) it's quite different to standard analogue sequencers for use with synths in a couple of respects:

1. It has no buffers at the clock outputs. Analogue sequencers for use with synths have a buffer between each output and the pots/LEDs - either opamp unity gain buffers or BJT buffers.
2. Analogue sequencers for use with synths implement the pots differently - lug 1 to ground, lug 2 output, lug 3 input, ie voltage dividers. Whereas Vanishing Point's pots are configured as variable resistors.

These are both perfectly understandable differences, as VP's sequencer is varying the brightness of the LED as opposed to providing a set of voltages to drive voltage controlled modules. However, the integrator schematic I posted therefore may not be suitable, as it's assuming a set of buffered voltage dividers.

Let me have a think for half an hour and get back to you. There! I've figured out why it doesn't work - that's half the job done!

[jacobyjd]

Hah--Rick, you're the man

I've tried some other ideas based on other synth port controls with no decent results. I feel like the solution is going to be stupidly simple. Either way, I'm off to bed, but I'll try any ideas you come up with on my breadboard. I'll keep looking for ideas tomorrow.

[frequencycentral]

OK, so what you want to do is to vary the brightness of the LED, the brightness being defined by the pots for each step. The way VP does it is one way. To implement that slew/portamento control you're going to have to do it another way. Here's the Baby 10 sequencer schematic, probably the simplest analogue sequencer you'll find:



See how the pots are wired? You'll need to do it that way. You'll also need those diodes after the pots, or 100K resistors in their place. Feed the CV Out into the slew circuit. Hang your LED off the output of the slew circuit, possibly with a 100K trimmer in between to give a bit of control (you can replace it with a fixed resistor when you know what value).

It also occurs to me that in a synth type sequencer the 4017 (or 4022) would be operating between ground and +ve, but the slew/portamento opamp would be operating between -ve and +ve. You could get around this by operating the 4017 (4022) between vref and +ve, and operating the slew opamp between ground and +ve. Or you cound use a bipolar supply (charge pump?). Or you might try an LM324 single supply type opamp, which will work much closer to the rails. I'd try the vref option, though you will have to make sure the clock input that the 4017 (4022) sees is appropriate, which may mean referencing the clock chip and associated transistors between vref and +ve too.

But - take it a step at a time, it might all work fine without changing all the voltage references!

By the way, the Baby 10 schematic you'll find on the 'net has a small error which I fixed before I posted it.

[jacobyjd]

Hmm--that's what I was afraid of. The problem I'll run into here is that I need to control multiple effects w/ the sequencer simultaneously. Currently, I have 4 effects w/ 5 ldrs that are going to be controlled by the light from that one LED. by using a straight up CV, it looks like I'd be tying my hands somewhat that way. 

I may just have to go without a port control if I can't find a way to slow the LED changes. No big deal--it was just a cool feature to try

[Processaurus]

http://www.diystompboxes.com/smfforum/index.php?topic=44900.0
http://www.till.com/articles/scanner/index.html

The pot plus cap is a reasonable, simple solution though, if you don't mind adjusting the amplitude after messing with either the portamento or speed.

[bram]

Hi, I'm planning on building a CV sequencer which delivers both a CV output, but which also can control 9V effect stompboxes (in the same enclosure). The trick is, I want to combine the step output (8 pots) with a pseudo-random signal (RG Keen's Psuedo Random LFO). I want to add portamento/glide to the CV mixture.

So, what am I doing in this thread? Possible grounding issues...

The CV sequencer produces a 0 to 5V output signal (as the Baby 10, but through a 2N3904 as emitter follower); the pseudo-random LFO 0 to 5V (40106 run at 5 V). If I build a summing inverting amplifier with input caps with a stereo pot with those 2 signals wired as opposites, the ingoing voltage will not exceed 5 V. The opamp would be run at 9V.

However, I need to connect the + of the opamp to Vcc/2 in order to get an inverted output and the same for the second inverter stage? And then I still need a non-inverting amplifier which delivers 0 - 9 V at the end, so I can use it in the Nyquist ALiaser (JC Maillet) and the 9V MS-20 filter (Tim Escobedo) to replace the freek and freq pot wiper respectively.

The portamento cap has to be ground to Vcc/2. Will I not get grounding issues, or do the input caps help to overcome this?

btw this is my first post; whenever I get my sequencer thingy ready, I'll definately post pics/schem (although it's a combination of known schematics).