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vactrol driven vc lfo resetable with symetry control
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zipzap



Joined: Nov 22, 2005
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PostPosted: Tue Mar 07, 2006 1:29 pm    Post subject: vactrol driven vc lfo resetable with symetry control
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The reason i´m asking is because i want a vclfo that is simple to build - like a square vco. Of course it should have at least tri output.
Or maybe does anyone have a simple vclfo i could use? What i love about the asm1 ´lfo is that it´s so simple. Can a circuit like that be cv controlled with a fet or transistor instead of the pot? i tried for hours but couldn´t manage.
cheers zip

Last edited by zipzap on Wed Apr 05, 2006 4:32 am; edited 2 times in total
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Macaba



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PostPosted: Tue Mar 07, 2006 1:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

I tell you what, i'll try simulating a few setups. See if I can make the ASM-1 LFO voltage controllable.
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Macaba



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PostPosted: Tue Mar 07, 2006 2:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

EXPERIMENT with this. Its probably totally impratical for synths but maybe it can be improved. The 100k resistors round the FET is for linearization, so perhaps you could leave these out and it'll give a different response?


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bugbrand



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PostPosted: Tue Mar 07, 2006 4:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

I think Macaba's circuit is certainly headed in right directions - 'fraid I can't add much because I've never fully figured how to implement FETs.. But, thinking about it & depending on how linear you wanted things, how about vactrols (probably your own DIY ones for economy).

But, there's also a simple VCLFO in the Elektor Formant:
Page 5 of this pdf from CAG Cloned Analogue Gear --- this particular pdf is in german so I wasn't able to figure some things out for setting up - I made the circuit and it correctly generates the waveforms, but the CV is barely working and very strange and the LED indicator didn't seem to be functioning. Haven't had time to properly debug it all yet. ANyone else had experiences with any of the Elektor designs? There's full Elektor documentaion on the CAG site

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Macaba



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PostPosted: Tue Mar 07, 2006 5:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

I added an inverting amplifer on the CV input so it behaves the correct way round. (Higher voltage = higher frequency)
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toppobrillo



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PostPosted: Tue Mar 07, 2006 8:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

heres some links

there is this one:
http://www.musicfromouterspace.com/analogsynth/lfo.html,
a modification to rays earlier VCO

there's this one: http://m.bareille.free.fr/asm1/vclfo.gif at papareil site, based on the formant one

then there is one based on a moog design using an OTA http://members.tripod.com/urekarm/synth/srcvclfo.pdf
i have built this, modified it for a sawcore [was a triangle core] and it works very well, has a really good useful range..

josh
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zipzap



Joined: Nov 22, 2005
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PostPosted: Wed Mar 08, 2006 3:36 am    Post subject: Reply with quote  Mark this post and the followings unread

thanks for the links, the formant looks interesting! Yesterday night i breadborded the core of this http://www.musicfromouterspace.com/analogsynth/coolnewlfo.html
the waveshape pot is buffered by its own opamp. Now the waveshape is almost independant from the speedpot (btw, anyone know why the speedpot should be connected to ground on one lug? if its just a variable resistor seems to work as well).
Anyway,
maybe the 4th opamp (three have been used so far) can be used to somhow insert a controll voltage like in the formant module. i´ll check this out.
What i also did try was to replace almost every resistor (there aren´t that many) by a fet or transistor. Maybe i have the wrong parts lying around, don´t know, it never really worked.
If i replace the speedpot with a fet i can voltage controll the speed, but only on one side of the triangle. i mean it only affects the charging of the cab. I never really understod how to implement a fet or transistor as a resistor as well...
Maybe otas respond different, worth a try. Vactrol is cool also! i just don´t have any ldr lying around right now, but it should work nicely.
Another thing i discovered (for myself, this propably isn´t new) yesterday is that if you aply a short pulse of pos or neg voltage to the cab the lfo is reset to either its highest or lowest point. is that something like a sync circuit known from many vcos? I have to find a way how to correctly implement this....
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bugbrand



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PostPosted: Wed Mar 08, 2006 4:28 am    Post subject: Reply with quote  Mark this post and the followings unread

zipzap wrote:
Another thing i discovered (for myself, this propably isn´t new) yesterday is that if you aply a short pulse of pos or neg voltage to the cab the lfo is reset to either its highest or lowest point. is that something like a sync circuit known from many vcos? I have to find a way how to correctly implement this....


Ah-ha --- interesting - that's something I'd tried to locate - will try it out. Further details/ideas would be damn useful.!.

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zipzap



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PostPosted: Wed Mar 08, 2006 5:14 am    Post subject: Reply with quote  Mark this post and the followings unread

It is interesting. the bad thing is that i just fried my last 4584. So no more short pulses right now... (the lfo is stoped for the duration of the pulse - also useful if you aply variable gate length. But for now i just want short reset)
I think what we need is a small circuit that creates a positive or negative pulse from a signal aplied to a reset input.
I guess something like a slope detector and some kind of swich that can deliver the neg or pos pulse. i don´t know how thats done. cmos only gives pos voltage and i haven´t realy figuered descrete switching with transistors (althoug its basic stuff i guess).
What i also tried was the concept of vc from the formant vclfo.
this is how i did this. it works, but only for the rising ramp side. the othe half of the triangle is unchanged... thats usaless. if i swithe the summing and the inverting opamp the effect is reversed. also useless.


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zipzap



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PostPosted: Wed Mar 08, 2006 3:08 pm    Post subject: Reply with quote  Mark this post and the followings unread

just swithed the speed pot with an lm13700. just like in the shematic posted josh. it works, but again it only affects the rise time. so a triangle wave gets faster by turning into a falling ramp...
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v-un-v
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PostPosted: Wed Mar 08, 2006 5:39 pm    Post subject: Reply with quote  Mark this post and the followings unread

Macaba wrote:
I tell you what, i'll try simulating a few setups. See if I can make the ASM-1 LFO voltage controllable.


"Simulating"?

Mac, are you using any circuit simulation software? If so which one?

I've just got a PC- loading it with stuff the Mac can't do. It's turning out to be a LOT of fun Smile
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toppobrillo



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PostPosted: Wed Mar 08, 2006 10:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

im not sure exactly how you implemented the OTA in place of the pot in that drawing, but the circuit you saw in the link works very well for your purpose..

yes you are right about sync. it is an ouside reset of the integrator. in the closed-loop configuration, the comparator handles that.

oh yeah, you can operate CMOS gates to give negative swing by powering vdd from 0v and vss from -v.


josh
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Macaba



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PostPosted: Thu Mar 09, 2006 9:16 am    Post subject: Reply with quote  Mark this post and the followings unread

v-un-v wrote:
Macaba wrote:
I tell you what, i'll try simulating a few setups. See if I can make the ASM-1 LFO voltage controllable.


"Simulating"?

Mac, are you using any circuit simulation software? If so which one?

I've just got a PC- loading it with stuff the Mac can't do. It's turning out to be a LOT of fun Smile


I'm using Proteus Professional 6 Demo:

http://www.labcenter.co.uk/download/prodemo_download.htm#professional

Works very nice, the only limitation to the demo is that you can't save or load circuits. If I need to do that on large designs, I just use Proteus at college. If you wanted to use this software to produce schematics and didn't care about simulating, then you could use Proteus Lite, which allows saving and loading, but doesn't have simulation features.

http://www.proteuslite.com/

ISIS is the circuit schematic drawer and simulator, and ARES is the circuit board designer.

This isn't the only simulation software by any means, but its my preferred one, and it has VERY cool features on the professional version where you export the net list from ISIS to ARES, and when you drop components into the circuit board design, it SHOWS you where the tracks have to be connected to. A standard feature in these packages, but I found it cool.
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Macaba



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PostPosted: Thu Mar 09, 2006 9:48 am    Post subject: Reply with quote  Mark this post and the followings unread

Here is a nice design I found on the datasheet for the LM358.

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

Parts of numbers are missing. I'm guessing this is due to the software I use to 'clean' jpegs up to get rid of all that useless data in jpegs.
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Macaba



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PostPosted: Thu Mar 09, 2006 11:01 am    Post subject: Reply with quote  Mark this post and the followings unread

I'm sorry for the triple post, but I want to make sure no-one misses anything. Here is a link to free simulation software:

http://ktechlab.org/
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v-un-v
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PostPosted: Fri Mar 10, 2006 5:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

Macaba wrote:
I'm using Proteus Professional 6 Demo:



Cheers Mac- that's wicked!

Also nice to see another British product! Very Happy
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toppobrillo



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PostPosted: Sat Mar 18, 2006 12:20 am    Post subject: Reply with quote  Mark this post and the followings unread

heyya if yer still thinking about this simple triangle vclfo business ive got something even easier for you to make... i sure hope osamu don't mind..

josh


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Macaba



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PostPosted: Sat Mar 18, 2006 4:45 am    Post subject: Reply with quote  Mark this post and the followings unread

Niceee. I've completed the board design for a VCLFO with Tri/Square/Sine outs, so gotta order the parts sometime. Interesting design based on LM13700. Smile
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zipzap



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PostPosted: Sat Mar 18, 2006 11:10 am    Post subject: Reply with quote  Mark this post and the followings unread

With a little help from my friends...... LFO´s running
Nice stuff indeed. yup, i have been thinking about that lfo buisnes,
In the last days my little experiment-board got messier and messier, i blew up 5 ics (literarly blew up), burned some pots and npns, killed my psu.
Most of the time i didn´t even know what was wrong... must be my lack of education. Even when i go for a walk i got that burning smell up my nose...I´m learning.
Anyway, i´ve been working with the vco design macaba posted from the lm358 datasheet (after killin my only lm13700), i think it works realy well. I leaft out r 7,8,9. Don´t know what they´re for, works without.
Here are some circuits that somehow happened.
1
Depending on swith 1 the triangle wave can be "shortened" to either falling or rising ramp (with then double speed)
I use some resistors around the pots to get good usable scaling. I first wanted a single pot, for symetry controll from rise to fall. Of course its possible, but i wasn´t happy with the scaling. a single log pot is much better controll. Problem is that the scaling can change a little with different speeds. So now p1,2 are one stereo log pot, the swich gives the direction. (can´t have falling and rising ramp at the same time anyhow).
2.
The reset input works similar, i used a cmos 4584 to form a short pulse from what comes in, sw2 decides for up or down, pot 2/4 set the amount of reset. This can produce stairlike effects for example. that i like. more than i was hoping for. Again u need the right resistor values around the pots for good scaling.
There propably is a way to do this with single pots, save a few bucks, but i haven´t found it.
Two more things: the basic vco design has a wide range from about 0,5v to 15v input, but again the scaling is not the way that feels realy good. Am i right that some type of expo-converter is needed?
At very slow speed i can hear the voltage trembling a bit, only notice it if a vco is controlled. Is there an easy way to get rid of that? i already got 100nf cabs to ground in most places.


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zipzap



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PostPosted: Wed Apr 05, 2006 4:31 am    Post subject: Reply with quote  Mark this post and the followings unread

Well, i´d like to bring this up again. I was working on this thing once in a while over the last weeks, trying different things, and now my vclfo project has turned into a vactrol thing. Those other threads concerning vactrols were a lot of inspiration. this is the first one i put together, i was instantly hooked up!
Anyway, this lfo is working nicely, it can be tuned over a huge range with a single cab (1uf goes from approx. 0,02hz to audio range)
What really suprised me is how nice a vactrol reacts with that log-converter circuit!
The resetoption now works without cmos, the double swith sets the direction, P1 the amount, tr1 is the timing resistor, must be trimmed for good response.
Next thing i´ll do is make a board layout, but first i wanted to show this shematic to you guys. Its already working, but maybe its missing some obvious stuff that i don´t know about?
The values on the shematic, well they represent what could be found on my workdesk. I guess it´s all not very critical.
It´s got some small bugs, maybe someone knows how to get rid of them:
1 not really a bug, but 2 LEDs draw max 40ma. Can i use lc leds + some series resistor instead? do they respond different with the ldr?
2 with large r1 reset can be triggered by just touching the input. how small can r1 be? i don´t want to shorten the thing connected to the input...
3 At very slow speeds the symmetrie pot P2 doesn´t give a clear cut anymore when sat to saw. Of course its impossible for the outputvoltage to jump, so there must be some glide. I would like to minimise that glide. Might have to do with the sice of P2. Maybe 2 or 4 meg are better than 1m.
Also i´m not sure about the real effect of tr2. smaller values make this lfo really slow, but i´m not sure if it also affects the waveform. I never calculate anything, so i never really know whats going on...
Also the reset part of the circuit is just experimental, mabe this is not the "professional" solution.
Any suggestions are gladly welcome! I´ll soon post the layout for this.
Greetings zip


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