Lost in the LFO forest

[CrocoDuck]

Hi cool people!

So, I am designing an envelope filter. Basically, it is a state variable filter (http://www.daycounter.com/Filters/StateVariableFilters/State-Variable-Filter-Design-Equations.phtml) that I'd like to control with:

Potentiometers

or

The signal from an envelope detector

or

The signal from an LFO.

The state variable filter works pretty well. I am now trying to design the LFO. I've tried a Wien bridge configuration with a(n half of a) TL082CP as well as a tuned oscillator with an active filter based on the Antoniou circuit... unsuccessful. Moreover, I am trying to do something with a 555 and a 40106. If you want I can post some schematic of my LFOs wannabe, but my problems are:

1) 555 and 4016: they make oscillate also the supply and (virtual) ground voltages (I am using a split supply obtained from a single supply with a buffered virtual ground), putting noise in the signal path. Moreover, it is hard to figure out how to obtain a sine wave from them.

2) The other ones are not stable at low frequencies (0.01 - 20 Hz): the active one cannot oscillate at low frequencies, the Wien has a huge ammount of distortion.

How you would design a LFO you can tune with just a potentiometer? I like to be able to deeply understand the circuits I use, to design accurately my own stompboxes rather then copy some schematic I don't understand. What resources are available online to understand oscillators?

[Seljer]

Getting a wide range while keeping it sinusoidal and capable of very flow frequencies is very tricky.

The majority of LFO based pedals used the much more simple comparator based relaxation oscillator (kind of whats inside a 555 timer) or with opamp based integrators. There no need for it to be sinusoidal for the effect to be musical , triangles sound just fine . The design is very simple and if the shape is not acceptable it's easy to deform it afterwards to suit your purpose (filtering, clipping diodes, etc...).


In a couple of designs where the LFO requires a reference voltage for the opamps, I've noticed that the designs often use a separate voltage divider for the LFO from the audio portion of the circuit. In any case the solution to any noise and ticking is better seperation and more filtering. And using a lower power device for LFO is possible (eg: use the CMOS 555 chip instead of regular one and low current opamps).

[CrocoDuck]

Getting a wide range while keeping it sinusoidal and capable of very flow frequencies is very tricky.

The majority of LFO based pedals used the much more simple comparator based relaxation oscillator (kind of whats inside a 555 timer) or with opamp based integrators. There no need for it to be sinusoidal for the effect to be musical , triangles sound just fine . The design is very simple and if the shape is not acceptable it's easy to deform it afterwards to suit your purpose (filtering, clipping diodes, etc...).


In a couple of designs where the LFO requires a reference voltage for the opamps, I've noticed that the designs often use a separate voltage divider for the LFO from the audio portion of the circuit. In any case the solution to any noise and ticking is better seperation and more filtering. And using a lower power device for LFO is possible (eg: use the CMOS 555 chip instead of regular one and low current opamps).

Cool! I will dig into and reply what I end up with. Thanks!

[duck_arse]

crocoduck, I am currently messing with "an" lfo with envelope modulation via ldr. parts of it might apply to your work, PM me and I'll send you what I have if you are interested.

[Blitz Krieg]

Check out digi2t's schematic of the Foxx Guitar Synth.  the Mutroxx.  it is an envelope controlled state variable filter with an lfo.

[CrocoDuck]

Hi! Got some nice improvement! I have designed a 555 astable stage like this one https://en.wikipedia.org/wiki/555_timer_IC#Astable supplied by an independent filtered voltage regulator and virtual ground circuit. Now I can have rectangular waves from 0.6 Hz up to 2000 Hz or so, with a duty cycle very close to 50% in all the LFO band. No noise is noticed in the state variable filter circuit, even if I have a regular NE555, thumbs up! I am planning to put a similar supply stage on the state variable filter circuit too, this should be quite aggressive against noise.

Now I am going to transform this rectangular waves into triangular ones. If you have hints I am listening! I was going to adapt something from this pages:

http://www.schematicsforfree.com/archive/file/Oscillators%20and%20Generators/Triangle-Sawtooth-Square%20Wave%20Generators/Triangle-Sawtooth%20Generators.pdf (there is a part with a 555 circuit)

http://www.allaboutcircuits.com/vol_6/chpt_6/8.html

http://www.all-electric.com/schematic/eticircuits/555-triangle-with-independent-slopes.htm (this looks pretty sexy to me)

Thanks to all! Help really appreciated!

[Seljer]

Your 555 is already generating a triangular waveform on the timing capacitor. But hooking it up to something will draw the current away from the capacitor and stop the circuit from behaving as it is so you need to add a buffer with high input impedance before using the triangle LFO in your circuit.