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wax+wire
Joined: Sep 28, 2014 Posts: 17 Location: Australia
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Posted: Thu Oct 09, 2014 4:17 am Post subject:
Nicholas' LM324 LFO circuit |
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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?
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elmegil
Joined: Mar 20, 2012 Posts: 2177 Location: Chicago
Audio files: 16
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Posted: Thu Oct 09, 2014 9:22 am Post subject:
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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. |
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wax+wire
Joined: Sep 28, 2014 Posts: 17 Location: Australia
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Posted: Thu Oct 09, 2014 1:43 pm Post subject:
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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
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Cynosure
Site Admin
Joined: Dec 11, 2010 Posts: 966 Location: Toronto, Ontario - Canada
Audio files: 82
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JovianPyx
Joined: Nov 20, 2007 Posts: 1988 Location: West Red Spot, Jupiter
Audio files: 224
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Posted: Sat Oct 11, 2014 7:41 am Post subject:
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According to the datasheet, the LM324 can get to 0 volts, but the upper limit is Vcc - 1.5. So potentially, the range is from 0 to 7.5 volts. However, that's a span of 7.5 volts and the circuit shown runs -3v to +3v which is a span of 6 volts. The OP wants a span of 9 volts, but this is impossible with a PSU of 9 volts. The supply needs to be 10.5 volts to get the range the OP wants and it may require an additional opamp to get that.
And I will insert my usual warning about virtual grounds here. In a small circuit like this, a virtual ground can work. Do not add additional loads to this virtual ground or you'll swamp it.
This subject comes up again and again and the problem documented here is why modular builders use a PSU with plenty of headroom (usually +/- 12 or +/- 15). _________________ FPGA, dsPIC and Fatman Synth Stuff
Time flies like a banana. Fruit flies when you're having fun. BTW, Do these genes make my ass look fat? corruptio optimi pessima
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wax+wire
Joined: Sep 28, 2014 Posts: 17 Location: Australia
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Posted: Sat Oct 11, 2014 5:16 pm Post subject:
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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) |
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nicolas3141
Joined: May 25, 2007 Posts: 185 Location: Christchurch, New Zealand
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Posted: Thu Nov 06, 2014 1:00 pm Post subject:
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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. |
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nicolas3141
Joined: May 25, 2007 Posts: 185 Location: Christchurch, New Zealand
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Posted: Thu Nov 06, 2014 1:04 pm Post subject:
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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. |
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