Pseudo-Random LFO

[SonusFluxa]

Here's my take on an interesting LFO. I drew this up the other night in anticipation of someone creating a layout for the Psycho LFO. This is a synthesis of ideas from the Psycho and the Pseudo-Random LFO from GeoFX.

This is also my first vero layout! I'm very excited about this, and really hope to get some feedback.

[edo1123]

Is the output a kind of voltage? I mean do I have to connect a led cathode there?

[Beaker]

No you don't. LED goes between your footswitch and power socket like this:

http://tagboardeffects.blogspot.co.uk/2012/02/offboard-wiring.html

There is a small minority of layouts on here, that include an LED connection (and resistor) on the board, but not this one.

[edo1123]

So, if I want to use it as a LFO to modulate a delay how am I supposed to use it?

[Neil mcNasty]

EDO1123, you are Correct!
Beaker is confusing your question with something else...

An LFO puts out a voltage swing (in this case) trough lug 2 of the Depth pot (normally around 5-6v at top of the peaks).
When hooked up to a LED/LDR (optocoupler), you then change the "voltage swing" to a "ground swing" via the LDR in the other end. ...or sometimes you'll see it go via a JFet transistor that flips it to a ground signal, instead of using a optocoupler.

The JFet version is often used in Phasers and Flangers, and these are the cases where they ask for matched JFet transistors, so that it gets an even response across several LFO points.
The optocoupler approach is the most common one, and is great for making Tremolos, AutoWahs, and specially when you are replacing a pot with an LDR in order to drive it with an LFO!

[edo1123]

Well. If I connect a led positive lug to the output and I face it a ldr I would get my modulated delay (I'll connect the ldr lug to the time pot), am I wrong?

[Neil mcNasty]

Sounds correct to me, but I am not familiar with how it is done in a delay pedal...
I guess you'll have to fine tune it quite a bit by trying different LEDs and LDRs, maybe even hook a resistor in parallel with the LDR to lower the total value, if the effect is too dramatic...

[SonusFluxa]

I think the LDR approach is a good idea. Also try connecting the output of the LFO directly to the delay time pot (from what I've seen, usually lugs 2/3).

As far as a resistor in parallel for the LDR, I haven't tried this yet, but I think that between 47k and 100k is probably a safe bet, since anything over that would give a lot of noise with a PT2399. Unless noise is what you're after! If you try this, definitely report back with what you find. I've got some more experimenting to do now also.

[luca.metal]

Can this be used to drive a VCO? I'm trying to build something similar to the EHX Random Tone Generator.
Thanks in advance.

[zombie_rock123]

Completely untested but you could try hooking the PRLFO up to the input of this - http://www.tradeofic.com/uploadfile/ic-circuit/200962343045612.gif and see what happens.

[SonusFluxa]

I'm totally going to try that circuit out! I was going to say something similar about sending it to a CV in on whatever oscillator you like.

[edo1123]

Guys, I have a question. Can I use this as a "sample and hold" for a filter in order to achieve the sample and hold function of the madbean shark fin? How far could this be from the sample and hold mad with the transistor? I mean, from the graphic I've seen on geofx it seems to do the same, but I'm not sure that it is the "same".

[nocentelli]

It would give an approximation of the sample+hold effect, but doesn't quite replicate the FSH-1. I breadboarded a modified version of the pseudo-random LFO above, and hooked it up to a simple resonant wah circuit (colorsound inductorless "twin tee") and filmed the result: Judge for yourself -  

https://m.youtube.com/watch?v=IdChTFGOfSw


If you were planning to just drop it in to replace the S+H part of the Sharkfin, I think it would be more hassle than it's worth - I can't see much benefit. If you're thinking of building a similar sounding pedal but with a different part set, there is plenty of scope to design around this.

[Addy Bart]

The filter with this sounds very good. I'd love to see how you connected it to do that, nocentelli. It's still unclear to me if the output of the random LFO can directly modulate a PT2399 or if an LED/LDR combo is needed.

[nocentelli]

In my video, it's an LED/LDR with the LED driven directly by the LFO, and the LDR in place of the wah pot. I'm sure the LFO would work directly on the PT2399 pin 6, with the output injected between the minimum delay resistor and the original delay pot.

[edo1123]

I was thinking to combine the peseudo random LFO with the wasp vcf. It has a cv input, but I'm not sure I can directly feed the LFO in there. This is the layout from Alex: http://guitar-fx-layouts.42897.x6.nabble.com/jurgen-haible-edp-wasp-vcf-filter-clone-td31274.html
The video you made is close to a sample and hold, in my opinion, so it would be cool to see a vero of that circuit.

[nocentelli]

edo1123 wrote

I was thinking to combine the peseudo random LFO with the wasp vcf. It has a cv input, but I'm not sure I can directly feed the LFO in there. This is the layout from Alex: http://guitar-fx-layouts.42897.x6.nabble.com/jurgen-haible-edp-wasp-vcf-filter-clone-td31274.html
The video you made is close to a sample and hold, in my opinion, so it would be cool to see a vero of that circuit.

The Wasp CAN be fed the CV output from the pseudo random LFO directly: The control pins of the 13700 (1+16) have a upper limit for current, but since there is 14k7 of series resistance you will be fine.

The Colorsound wah circuit I used in the video already has a layout here at tagboardfx:

http://tagboardeffects.blogspot.co.uk/2012/03/colorsound-wah.html?m=0

The LFO part is essentially the same as the layout posted at the top of this thread: The CV output just needs to go to an LED to ground, lined up as an optocoupler/vactrol with the LDR legs replacing the wah pot's connections. If you want a bit more resonance, you can replace the 33k with a bigger resistor, or run the 33k to a 100k pot before it goes to ground so you can dial in more resonance.

[Addy Bart]

Thanks for sharing your methods nocentelli. I like the sound of that wah.

I hooked up the LFO to my Multiplex (by connecting the output to lug 3 of the delay pot and the ground to lug 1) and it works. It's completely over the top though! The PT2399 was freaking out with the LFO depth pot turned up past 12 o'clock. I think a lower value might be more useful in this case.

[nocentelli]

You could try coupling it with a vactrol with the LDR in parallel with the delay time pot, or in parallel with a 10k that is in series with the delay pot: LDRs have a slower response to the changes in voltage which will probably smooth it put a little (check out the pitch pirate or pitch pirate deluxe to see how Doug hooks a standard LFO to a pt2399.

[Addy Bart]

Cheers for the suggestion - I've already built the Pitch Pirate Deluxe so I'll take a look at that. Guess this is why vactrols are used for their smoothing, and more musical qualities. If anyone is interested in the unholy racket it makes without the vactrols, I put a clip here:

https://youtu.be/7spG0nrUraA

I was wondering if increasing the capacitor values or the resistors would slow down the LFO... Had a cursory glance at a thread about Ken Stone's Psycho LFO (over at electro-music) and this was suggested by some.