Nicholas' LM324 LFO circuit

wax+wire · Joined: Sep 28, 2014 Posts: 17 Location: Australia

so i've built Nicholas's LFO circuit ( http://electro-music.com/forum/topic-32372.html )

works great running off a single supply 9V power source converted into dual supply.

The output however oscillates between +3/-3V where what I want is it to oscillate purely in the 0-6V +.

I'm using it as an LFO for Tom's Q&D Filter (http://www.jiggawoo.eclipse.co.uk/guitarhq/Circuitsnippets/simplevcf.gif)

thoughts?
Thanks

elmegil · Posted: Thu Oct 09, 2014 9:22 am Post subject:

Not clear what you mean "converted to a dual supply".

You need to add a +3V offset, you could run the output into an additional op amp buffering stage to accomplish that..... but only if you have a supply that's capable of going up to 6V. Without understanding your supply that's possibly a sticky problem.

wax+wire · Joined: Sep 28, 2014 Posts: 17 Location: Australia

sorry. I'm running this connected to a casio SK5

So I've changed the power from single supply 9V+ to bipolar power supply 4.5 +/- with two resistors and a capacitor ... (bipolar power is something new to me) -- the circuit is designed for bipolar power

The output triangle/square wave swings from -3V to +3V

The Q&D Filter runs on 9v+ and expects for its CV input 0-9V

at the moment I'm already using 4 stages of the LM324 Quad OPAmp - so I'd be keen if possible to modify this circuit possibly to make it swing from 0-6V rather than have to get another opamp involved

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Cynosure · Posted: Sat Oct 11, 2014 7:22 am Post subject:

Change it back to running on 0/+9V and use a virtual ground. Then you output will be 1.5V to 7.5V.

http://tangentsoft.net/elec/vgrounds.html

JovianPyx · Posted: Sat Oct 11, 2014 7:41 am Post subject:

wax+wire · Joined: Sep 28, 2014 Posts: 17 Location: Australia

i don't know exactly why, but i tapped the triangle out before 1K resistor, stuck and depth resistor in series, and *woot* it was working.

I'm not sure if the wave is being truncated when it drops below 0V, but it is sounding good. the skew knob works and the rate works - though only in the middle of the range.

I'll play around later with the 100K resistor like nicholas says, and see if i can extend my rate - i want it to be able to go slower.

I wonder how this would work inside a commodore 64 as an LFO for the paddles? (at the moment i use a 555 circuit, but i really don't like it - the triangle is too weak in voltage, and the square too harsh)

nicolas3141 · Posted: Thu Nov 06, 2014 1:00 pm Post subject:

The voltage swing is set by the voltage drop of the LEDs, so if you try different colours such as red you can get smaller swings or try zeners (back to back rather than in parallel) if you want higher voltage swings. Using anti-parallel LEDs like this you are also not restricted to symmetrical swings, you could try red/blue for an asymmetrical LFO.

To make it go slower you can increase the size of the cap C1 as big as you like, but remember that unfortunately you can't use anything polarised like an electrolytic in there so you can't easily buy caps much bigger. You could also try increasing the size of the skew pot P2. But probably the easiest change would be to reduce the size of R6 down from 100K to something like 22k.

nicolas3141 · Posted: Thu Nov 06, 2014 1:04 pm Post subject:

Also, if you want it to go really slow you probably should look at upgrading to a better spec opamp. Doesn't need to be anything expensive, but the LM324 is not ideal for a really slow LFO because of fairly high bias current and noise.