LFO`

[jamiewoody]

well, LFO is where it is at!!! i am making mostly 555 based simple LFO circuits for most of my keyboards. i made one for my sk1, my pk1, my sa-2 (well, i used a 556 based APC for that, same thing in essence...).

i guess i don't have to KNOW everything, but it is nice to understand a little more of what i do each day. i am beginning to wonder if the only difference between a VCO and a VCF is just where you wire it up another  circuit, and how...comments?

[Gordonjcp]

No, VCOs and VCFs are two very different things.  They work in substantially different ways.  Granted, both use a control voltage to change the behaviour of part of the circuit, but there the similarity ends.

http://matrixsynth.blogspot.com/2008/08/gakken-sx-150-schematic.html

In the VCO block, there are two opamps IC2A and IC2B.  IC2B is set up as a comparator, and IC2A is set up as an integrator.  When C2 discharges, it triggers the comparator to recharge it via D12.  The rate it discharges at is controlled by Q2 - a higher voltage on Q2's base causes it to conduct more and thus increase the frequency.  You'd get a squarewave at the output of IC2B.

In the VCF block, Q4 and Q5 along with C9, C10 and R35 form a crude Sallen-Key filter.  As the bias on Q4 and Q5 is increased, they conduct harder effectively lowering their resistance, so the cutoff frequency increases.  Feedback from the output of IC2D is passed back to the "bandpass" input of the filter network to provide resonance.

[Circuitbenders]

At the risk of sounding simplistic, are you sure you're not getting mixed up between an LFO and a timer oscillator here Jamie? A 555 oscillator that can replace the timing crystal of something like a casio SA isn't an LFO. An LFO runs at far lower frequencies.

Or maybe i'm misunderstanding what you write sometimes 

[jamiewoody]

i am sure it is i who is getting things confused. i am basically taking stabs and hacking my way through, though  i am doing a lot using this method!

one of the bends i do using this "timer circuit" (it is a 555, a disc cap an electrolytic, a couple of resistors and a couple of potentiometers.)

on the pk-1, i hooked it up to a couple of pins on the ICs, and i hooked it up directly to the positive out on the speaker jack.

depending on how it is set, i can either use it with or without the keyboard. it definiely effects the keys in a lot of synthlike ways!

i'll try to get some vids together really soon.

until i can afford to go to school for electronics, i guess this will be how i learn. i have been doing this less than a year, actually, if i remember right, my first bending happened nov 2009.

[Ciderfeks]

This is something that I've been getting a bit confused about too. The 555 based LFO schematics that I've been following do indeed make circuits which modulate pitch or whatever, depending on where it's wired into to the "host" circuit. But also at some other points on the host circuit it becomes an audible oscillator that functions as a basic standalone noise maker  - as I thought LFO's functioned without creating an audible signal I wondered if I'd done something wrong. I must admit that I've enjoyed this dual functionality, especially when used in effects circuits, so I hadn't looked too hard into rectifying it...

I haven't tried using a 555 circuit to replace a pitch crystal - I'm guessing this would be a different circuit again to the LFO circuit? Most people seem to favour the LTC1799 approach to this, is there any particular differences in results between the two methods? I remember reading on here somewhere that the 555 method isn't so good at raising pitch.

[jamiewoody]

i didn't know a timer module can replace the crystal. it would be kinda nice i guess to have a pitch control on the sa-2 and pk-1...i wired a timer mod around the pitch points by the trimmer on the  sk1, as well as a couple of body contacts there.