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Can anyone suggest an LFO...?
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Coriolis



Joined: Apr 11, 2005
Posts: 616
Location: Stilling, Denmark

PostPosted: Sat Jun 06, 2009 12:47 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi Tim - those waveshapers look great!
I'm wondering about the LFO itself though: Do the coarse and fine tune pots work by limiting the current going into IC1b?

If so, would it be possible to add voltage control to the lfo core by inserting an OTA acting (as a vc-resistor) before the fine tune pot?

Just thinking aloud here...

C

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Coriolis



Joined: Apr 11, 2005
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Location: Stilling, Denmark

PostPosted: Sat Jun 06, 2009 12:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

Gaahh! Double post...

C

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fonik



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PostPosted: Sat Jun 06, 2009 3:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

hello coriolis - long time no see!?
Very Happy

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Tim Servo



Joined: Jul 16, 2006
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PostPosted: Sat Jun 06, 2009 5:10 pm    Post subject: Can anyone suggest an LFO...? Reply with quote  Mark this post and the followings unread

Coriolis wrote:
Hi Tim - those waveshapers look great!
I'm wondering about the LFO itself though: Do the coarse and fine tune pots work by limiting the current going into IC1b?

If so, would it be possible to add voltage control to the lfo core by inserting an OTA acting (as a vc-resistor) before the fine tune pot?

Just thinking aloud here...

C


Hey Coriolis,

Actually, that would work very nicely. If you look at things like the VCO-1 or the LM VCO, that's exactly how they do it. I have a couple of new VCOs in the works that also use that principle. Of course that's a secret, so don't tell anyone... oh wait. Anyway, it works nicely and I would probably just ditch the coarse and fine tune pots as they are, and put those controls in the standard place in the CV summer for the OTA. Of course, at that point you've got a basic, generic triangle core VCO, but those usually work well as a VCLFO (just use a bigger cap). The main thing to watch for is the voltage limits to the input of the OTA. Again, check the VCO-1 schematic or check Thomas' 3080 book for some examples.

And glad you like the waveshapers. I totally discovered those by accident (was trying to make a tri to sine shaper) but I'm very pleased with them.

Tim (loves those happy accidents) Servo
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Coriolis



Joined: Apr 11, 2005
Posts: 616
Location: Stilling, Denmark

PostPosted: Sun Jun 07, 2009 4:22 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
hello coriolis - long time no see!?

Hi fonik - yeah, I've been busy playing guitar and...my modular! Laughing
But of course what happens when you actually play a modular is, you find out you need more modules!
So now I'm finally building my Buchla 281 clones, some LPG's - and I really need to stop worrying and start building my Klee as well...


Quote:
If you look at things like the VCO-1 or the LM VCO

Precisely the ones I was thinking about. I would just build the LM VCO core and tack on your waveshapers, but figured it would be better to use your design, what with the extra +/-2.5 V on the signal and all.
I haven't been able to find a formula I can understand, to help me change the voltage swing of a schmitt-trigger from 10V p-p to 15V p-p...

But thanks for the help - now to the breadboard!

C

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nicolas3141



Joined: May 25, 2007
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PostPosted: Sun Jun 07, 2009 5:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

If it is an op-amp based schmitt trigger, the trigger voltages are determined by the voltage divider around the + input, relative to the output swing of the opamp. There is generally one resistor from out to +in and a resistor from +in to ground or the signal source. For example if the 1st resistor is 5k and the second resistor is 10k and the opamp output swing is +/-12V then the trigger levels will be +/-8V ... =12Vx10k/(10k+5k)

If the opamp is referenced to some voltage other than ground or its output swing is not symmetrical then the trigger voltages will be assymmetrical too. Sometimes the second resistor is replaced with back to back diodes or zeners - in which case the trigger voltages are the same as the diode voltage drop.

Cheers,
Nicolas
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Coriolis



Joined: Apr 11, 2005
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PostPosted: Mon Jun 08, 2009 12:20 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks a lot Nicholas - that helps.
Although the first thing I had in mind, was to mod the swing of the TH VCO-1, which has a discrete schmitt-trigger. I even think I understand which resistors to fiddle with, just not the ratio...

C

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nicolas3141



Joined: May 25, 2007
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PostPosted: Mon Jun 08, 2009 1:44 am    Post subject: Reply with quote  Mark this post and the followings unread

Ah well then ... a very asymmetrical situation. Because it doesn't have a separate output driver stage you might struggle to get sufficient output swing for what you want. R20 is the positive feedback, but R13 is also involved on the positive output swings and R15 on the negative swings so to increase the trigger levels (and hence the triangle levels) you probably need to reduce all of them and probably quite a lot because you are getting close to the limits for a single stage. Then adjust R22 to bring the voltage levels back into symmetry.

Cheers,
Nicolas
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Coriolis



Joined: Apr 11, 2005
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PostPosted: Mon Jun 08, 2009 2:28 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks again Nicholas. Very Happy
Will try your suggestions if I decide to use the VCO-1 as basis...

C

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creekree



Joined: Mar 30, 2006
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PostPosted: Sat Jun 27, 2009 6:50 am    Post subject: Reply with quote  Mark this post and the followings unread

i perfboarded nicolas' simple LFO last night - got it up and running within an hour or so and i have to say that i quite like it! (i made it entirely out of used components that i desoldered from my pile of electronic junk)
the skew function is a nice touch...
when i find the time i'll tinker with different LEDs and/or caps.

thanks nicolas!
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lostinpurdy



Joined: Nov 16, 2007
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Location: Gig Harbor

PostPosted: Wed Nov 04, 2009 1:40 pm    Post subject:   Reply with quote  Mark this post and the followings unread

The 8K LFO - can this be made switchable to VCO or Attack/Release mode? I assume that I should be able to get it into VCO range by replacing the resistor that sets the initial frequency, right?
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Tim Servo



Joined: Jul 16, 2006
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PostPosted: Wed Nov 04, 2009 2:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

lostinpurdy wrote:
The 8K LFO - can this be made switchable to VCO or Attack/Release mode? I assume that I should be able to get it into VCO range by replacing the resistor that sets the initial frequency, right?


Hey Lost,

Well, you can change the timing cap (C1) to change the range of the 8K, but it won't be a Voltage Controlled Oscillator - just an oscillator with manual control over the frequency. Still, those can be useful for drones and stuff like that. The waveshapers I show definitely sound neat in the audio range. Make the cap smaller (try something around 330pF) to make the frequency higher.

As far as an Attack/Release mode, I guess you could do that by removing the feedback from the integrator (IC1b) to the comparator (IC1a). Take out R6, and put a gate signal through a resistor (try 100k) to the + input of IC1a. The output will be inverted, so you'll want to invert the gate first to get a 'normal' envelope out. Additionally, you could use a couple of diodes to have separate Attack and Decay rate controls. Check out some other AD envelope generator designs to get a better idea of how this works.

Tim (Voltage Controlled Nerd) Servo
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lostinpurdy



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PostPosted: Wed Nov 04, 2009 7:53 pm    Post subject:   Reply with quote  Mark this post and the followings unread

thanks - (I think) it should be CV controllable if I add a CV stage followed by a simple inverter and a switch to bypass the frequency pot? I really don't care much for correct key-to-key CV tracking since I intend to control that from a baby 10 type sequencer. I'm aiming to build a small modular synth with the simplest possible circuits available. It would implement three modulators which I call "VCHLFO/AR's" or "UVCM/O's" (pick one), which can be switched to VCO, LFO, AR modes. It's kind of a space saving measure to avoid having to build extra AR's and LFO's, kind of multi-function modulators, which Could also make a nice module for modulars.. at least that's what my wicked mind just cooked up Very Happy
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Ojd



Joined: Feb 22, 2008
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PostPosted: Mon May 10, 2010 5:26 am    Post subject: Reply with quote  Mark this post and the followings unread

I've made your LFO, nicolas. To get +/-6V had to put 3 LEDs in series though. Also made capacitor switch - 4,4 uF (two of 2,2uF back to back), and 10nF. From 10 second to 1000hz, not bad at all! By the way, i've changed 100k rate pot to 500k pot. I think Audio taper would give me better lower range.

You know, i didn't think i would start DIYing modular again (first time i had less experience took much complicated circuits, and failed). But with your modules its sooo easy. Especially i liked the tweaking part, it really puts a part of you into what you're doing. Waiting for more modules!
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nicolas3141



Joined: May 25, 2007
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PostPosted: Mon May 10, 2010 1:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

Brilliant. I always encourage everyone to build simple circuits and on stripboard, because I think the fun is in understanding how they work and then adapting and extending them.

Cheers,
Nicolas
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emdot_ambient



Joined: Nov 22, 2009
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PostPosted: Mon May 10, 2010 2:43 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for reviving this old thread...would have missed it otherwise thumb up
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nicolas3141



Joined: May 25, 2007
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PostPosted: Mon May 17, 2010 3:19 am    Post subject: Reply with quote  Mark this post and the followings unread

Here's a simple LFO I have just built (x2). Only two opamps, but with the skew control I like so much. Reduce R1 to something smaller (not less than 10k tho) if you want the LEDs to shine a bit brighter.

Cheers,
Nicolas


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2thick4uni



Joined: Feb 20, 2009
Posts: 113
Location: UK

PostPosted: Mon May 17, 2010 2:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Here's a simple LFO I have just built (x2). Only two opamps, but with the skew control I like so much. Reduce R1 to something smaller (not less than 10k tho) if you want the LEDs to shine a bit brighter.


Brilliant Nicolas, I have been following your posts with great interest. I can't believe you can get a sawtooth thru to tri LFO with such a low component count!

Is there any way this circuit could be made to work on a single rail psu, ideally +9v? I'd guess I'd need a lower voltage opamp and a virtual ground, however I'm not clever enough with circuit design yet to implement it - any suggestions or hints?
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nicolas3141



Joined: May 25, 2007
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PostPosted: Tue May 18, 2010 12:49 am    Post subject: Reply with quote  Mark this post and the followings unread

Running this little one off a single 9V battery is actually harder than the others because it needs to sink/source up to a milliamp from ground. On the other hand the first LFO I posted will run fine, just use yellow/green LEDs rather than blue. Increase R4 from 22K to 47K. Connect the 9V battery + to pin 4 of the LM324, battery - to pin 11. Connect the ground (0V) of the circuit to a voltage divider 10K to battery + and 10K to battery -. And augment the 100nF power smoothing caps with 10uF electrolytics in parallel with the little ceramics. Does that make sense?

Cheers,
Nicolas
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2thick4uni



Joined: Feb 20, 2009
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PostPosted: Tue May 18, 2010 10:38 am    Post subject: Reply with quote  Mark this post and the followings unread

nicolas3141 wrote:
Running this little one off a single 9V battery is actually harder than the others because it needs to sink/source up to a milliamp from ground. On the other hand the first LFO I posted will run fine, just use yellow/green LEDs rather than blue. Increase R4 from 22K to 47K. Connect the 9V battery + to pin 4 of the LM324, battery - to pin 11. Connect the ground (0V) of the circuit to a voltage divider 10K to battery + and 10K to battery -. And augment the 100nF power smoothing caps with 10uF electrolytics in parallel with the little ceramics. Does that make sense?

Cheers,
Nicolas


Excellent, many thanks!

I can understand all of the the changes, the 2 of 10k resistor pd puts the "ground" at 4.5V, hence the term virtual ground?.

I've just checked the LM324 datasheet and it will work on 3v to 32v so I don't even need a more expensive opamp Smile

I'll build it on stripboard at the weekend.

Cheers,
Allan
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Ojd



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PostPosted: Sun Jul 04, 2010 2:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

Is there any way to make D3 and D4 shine brighter? Each is in series with 2 more LEDs, so its quite a drop in brightness
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nicolas3141



Joined: May 25, 2007
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PostPosted: Sun Jul 04, 2010 3:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

The brightness of the LEDs is set by R4 (shown as 22k). Reduce this to 2k or 3k or 4k to get them bright for daytime use. Will run your batteries down quicker though.

Cheers,
Nicolas
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Ojd



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PostPosted: Mon Jul 05, 2010 1:19 am    Post subject: Reply with quote  Mark this post and the followings unread

No worries, i use 15 volt bipolar supply.
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Ojd



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PostPosted: Thu Aug 05, 2010 12:02 am    Post subject: Reply with quote  Mark this post and the followings unread

I use linear 500K pot for speed, with little resistor at node 1, to give it some speed even at lowest position (without this resistor if you turn pot CCW it seemed like LFO gets frozen, i mean cycle time would be so long that it was annoying). Nice toy you made, nicolas!
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oozitron



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PostPosted: Mon Feb 07, 2011 8:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

I am stumped by my LFO... the chip is getting it's +15 and -15 volts. But both LEDs stay lit, and the output is a slight amount of negative voltage.

It's just the plain LFO (no freq CV) with just the two pots. Neither pot changes the output.

Are there specific voltages I should be seeing on the various pins of the chip?

I've tried swapping chips and the 1u cap to no avail. I'm no EE genius, but this is such a simple circuit Rolling Eyes

Drew
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