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Single Op Amp SAW VCO
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beep



Joined: May 05, 2013
Posts: 105
Location: Germany

PostPosted: Tue Jun 16, 2015 4:08 pm    Post subject: Reply with quote  Mark this post and the followings unread

MAN! *-* thank you very much, you're my hero. it really works.

I'm going crazy. gonna build a battery powered cardboard arp 2600
THANK YOU SO MUCH!

https://www.youtube.com/watch?v=_omtCarsNs4

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Shock_Hazard



Joined: Jul 17, 2015
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Location: Gwynedd

PostPosted: Tue Jul 21, 2015 11:03 am    Post subject:   Reply with quote  Mark this post and the followings unread

This is a fantastic little circuit Smile However I've been playing with the exponentiator all night, but I can't get any kind of scaling;

Every pot adjusts the frequency, but when supplied with a 1V/oct CV signal I can't seem to adjust the scaling; that is the ratio between key 1 and key 2 is allways the same, maybe at a lower or higher frequency but allways the same. e.g. key 1 might be at 20Hz and key 2 at 50Hz, but if I increase the offset voltage or adjust the trimmer I end up simply shifting that ratio to say 200Hz and 500Hz, when what I really want is to stretch and shrink ratio or gap between the frequencies untill it fits a standard scale. I tried an attenuation pot on the CV input but it didn't seem to make any difference... Confused
The circuit definately results in an exponential voltage to frequency relationship... but I can't seem to get it in tune! Any help?
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JL



Joined: Jun 25, 2012
Posts: 20
Location: New England

PostPosted: Tue Jul 21, 2015 11:40 am    Post subject: Reply with quote  Mark this post and the followings unread

I've had the same issues getting the version Kassu posted to track with an approximate 1V/oct response. Which I suppose isn't surprising considering how simple the circuit is - there's a reason more sophisticated expo converters are the norm. I don't care about proper 1V/oct tracking for what I'm building with these, but the more "natural" response of turning the frequency knob sure is nice. The temperature sensitivity was an issue though, so I replaced R5 with a 1k NTC resistor and R6 with a 2k trimmer. With some adjustment of R6 the frequency drift is much less dramatic. Just don't breathe on the board or touch the transistor... Very Happy
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beep



Joined: May 05, 2013
Posts: 105
Location: Germany

PostPosted: Thu Mar 03, 2016 2:37 pm    Post subject: Re: Exponential CV input Reply with quote  Mark this post and the followings unread

I took Kassu's circuit and replaced the op amp with a 40106.
All other inputs tied to ground and some PSU filtering @ 12V wall wart.

For buffering I used a lm324.
I should try a Jfet like Mr. Schmitz recommended.
What easy to get type should I use?

All part values remained the same, but I got rid of four resistors and
a cap.

I have only cheapo single turn trimmers at the moment,
but after some careful tweaking I got a somewhat nice tracking
about 4-5 octaves. Way better than anything I got when trying around
with the op amp version.

The interesting thing: the lowest and the highest notes played amazingly
well in tune, just in the middle range the detuning increased a bit
but all in all the tracking is musically more or less in tune.

I used the microbrutes internal oscillator as reference.
For such a low parts count single supply WOW!

I think, it's possible to improve the tracking a bit when using
multi turn trimmers and a better buffering (jfet).

I also would like to power the 40106 from 5v instead of 12v.

What do you think?

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JL



Joined: Jun 25, 2012
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PostPosted: Thu Mar 03, 2016 3:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

You can use something like a 2N5457 JFET as a buffer, or a single-supply dual opamp like an LM358 wired as a voltage follower.

A few months ago I revisited this circuit and was determined to find a better single-supply expo converter that was more stable than the single-transistor version Kassu posted a while back. Eventually I came across this and this. It is an expo circuit designed as an upgrade to the Gakken SX-150. What interested me most was that it runs on a single supply and uses only three transistors. The circuit in the first link seems to work well with the VCO in this thread, and I imagine it would also work well with the similar 40106 oscillator. Temperature stability is much better with matched transistors and a 10K NTC in series with each CV input resistor. I'd suggest replacing R6 with a trimmer to adjust scaling.


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



Joined: May 05, 2013
Posts: 105
Location: Germany

PostPosted: Fri Mar 11, 2016 3:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

After messing around with Kassu's Expo converter I got stuck cause
it's extremely temperature-dependent. Not only the tuning changes
with ambient temperature, but also the scaling.

In the meantime some other guy got the expo converter of Rene Smitz'
design working fine on single supply.

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

I'll give it a try.

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jean bender



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PostPosted: Tue Apr 19, 2016 3:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi everybody
I'm just going into opamps, and that's pretty cool even if i don't get everything. As i come from lunettas, i'm more empirical than mathematical..

Well, the schematic used here is great, it works for me, but i absolutely can't use a 10nf or 100nf cap. It's way too high to be in audio range. I can make it working with a cap btw 2,2uF and 10uF.

Where shoul i be wrong ?

I've also tried to build the osc you can get in the LM324 datasheet , and it's the same. The datasheet talk about a 0;05uf, but nothing happens.

I'm working with a single supply, 9v, and the only difference is that i'm using a 50k lin pot instead of 100K ( also : which law assume this kind of change ? as the pot melt V* and GND, if it's bigger, it can offer more possibilities ?)

thanks per advance for any help.. i'll be happy to learn some more and fo further !

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Kassu



Joined: Apr 17, 2015
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PostPosted: Thu Apr 21, 2016 5:07 am    Post subject: Reply with quote  Mark this post and the followings unread

What version of the circuit are you building? If you need a bigger capacitor, it probably means that you are using a larger current also, defined by the transistor and the resistors going into it's base. Maybe you have a different value resistor there?

Otherwise, haven't been around here during the last year, but I see people have tried my version of the circuit, super nice! Indeed, the single transistor exponential converter is very poor in terms of stability and tracking, the idea is just to get a somewhat exponential response. For example if you would use a ribbon potentiometer to play the VCO it would be good enough scaling as you would play by ear anyway Smile.

What I remember from trying before: buffering the CV before the expo transistor with an opamp helps a lot for the tracking, but not for the temperature stability. As noted, both the frequency and the tracking/scaling change with temperature.

The PNP buffer + NPN expo converter circuit beep posted is probably much better. Have you had a chance to try it? I have used a similar circuit in Yusynth's moog VCF, and there the scaling works really surprisingly well. The first (PNP) transistor is not only a buffer, but it also somewhat compensates temperature drifts (it drifts in opposite direction, but it is not true compensation)
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jean bender



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PostPosted: Sun Apr 24, 2016 6:36 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi ! thanks for your answer !
I haven't checked the schematic you're talking about. I've also tried to build the osc you can find in the lm324 datasheet.
Actually, i get it working.

I'm using a 9v power supply, not regulated (so around 12v). I've discovered i get it better working if instead 9V i connect the frequency pot to 1/2 v+.. Don't know why. Maybe because i'm using 50k pots ? i don't think so, it's just a voltage divider...

Also, the transistor doesn't work as it should. I'm using BC547B ones, and sometimes it's working, sometimes not, so i have to bypass it.
Could someone explain me how the transistor works ? What should it do, or not ?
Well, that's a good starting point to me for discovering op amps and so on...
best
J.

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elmegil



Joined: Mar 20, 2012
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PostPosted: Wed Jan 11, 2017 3:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

Another necropost Smile

I'm pretty frustrated; I've tried to build this and cannot get it to work.

I've built on stripboard and on breadboard.

I'm using one side of a TL072, planning to buffer with the other side. Disconnecting the buffer makes no change. I am NOT using the transistor buffer from the original circuit.

I've made it as simple as possible with just the pot for frequency control.

I am referencing the pot's CCW pin and the transistor's collector to -V.

I've been using +/-12V for the most part, but also tried +/-15. I have not tried 9V.

One thing I'm not 100% clear about is the description of "If you want to use a dual supply, simply exchange the lower voltage divider with a connection to ground". Does this mean remove *all* of the 10K resistors and add a ground on the + terminal of the op amp? Or does it mean leave the 10K feedback resistor from 1 to 3 there, and another 10K from + terminal to ground?

Or .... leave the feedback there but ground the + terminal??

I have been assuming the two 10Ks would stay and the "voltage divider" was simply the two that run from +9V to ground in the original schematic.


What happens is that 1) I don't get any oscillation and 2) the transistor gets REALLY hot. With a 15V supply I measured a 12V drop across the 100K resistor, that's 12mA going to the base of the transistor!

I have tested the transistor in a transistor tester (it was fine), and tried another (same results). I connected the op amp as a voltage follower and it too is behaving correctly.



So I think my question really boils down to: with a bipolar supply/op amp, do *any* of the 10K resistors stick around?
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Grumble



Joined: Nov 23, 2015
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PostPosted: Thu Jan 12, 2017 12:47 am    Post subject: Reply with quote  Mark this post and the followings unread

umschmitt wrote:
Hey. Is it me, or shouldn't the CV transistor have emitter and collector swapped ? Or does this resistance thing work also with collector at a lower voltage than emitter ? Just asking, I'm pretty ignorant.
(ok I should just try it myself)

I still don't understand how this transistor could be working with its collector to GND and being an NPN transistor.
Could somebody explain this?
Or... is it just used as a diode?
Still PUZZLED Confused Shocked Sad
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elmegil



Joined: Mar 20, 2012
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PostPosted: Thu Jan 12, 2017 1:07 am    Post subject: Reply with quote  Mark this post and the followings unread

I agree about that too Grumble, but in simulation what I found was it does appear to work both ways, but much faster frequency range when you're connected the way you'd expect to work.

Of course if I can get it to work at ALL in the real world, that would be a bonus Smile I've tried it with the transistor both ways though, no improvement either way.
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brock



Joined: May 26, 2011
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PostPosted: Fri Jan 13, 2017 6:59 pm    Post subject: Reply with quote  Mark this post and the followings unread

Kassu pointed out the problem you are having in his post, last one on the first page:
Quote:
1. Dual supply opamp: It's actually not so easy to use a dual supply opamp, and probably its not worth the trouble. To use dual supply, the transistor and the CV input should be referenced to the negative supply in stead of ground (the oscillation will be between +V/2 and -V/2, so you have to discharge to something more negative).

If you don't change all the biasing around the CV transistor and discharge the cap below -V/2 you are going to stay within the hysteresis range of the comparator so no astable action.
I think you need to turn the discharge transistor around for bipolar operation too, otherwise it will be reverse biased a lot of the time, probably in breakdown range.
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elmegil



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PostPosted: Fri Jan 13, 2017 10:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yes, I'm doing the negative supply to the input pot and the transistor.

Quote:
I am referencing the pot's CCW pin and the transistor's collector to -V.


And I've tried with the transistor both ways around. I'll try again later when I'm less frustrated.
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gasboss775



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PostPosted: Fri Jan 27, 2017 5:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
Yes, I'm doing the negative supply to the input pot and the transistor.

Quote:
I am referencing the pot's CCW pin and the transistor's collector to -V.


And I've tried with the transistor both ways around. I'll try again later when I'm less frustrated.


Have you tried again elmegil?

When Iam in a situation like this I'd say 90% of the time its down to a stupid over site in construction.

Rebuild everything from scratch and yes probably sticking to a single +Ve supply. If you want an AC output you could stick a capacitor on the output of your buffer.
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elmegil



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PostPosted: Fri Jan 27, 2017 5:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

I ended up going with a completely different design for the saw VCO I needed, using a 555 and 2 op amps.
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gasboss775



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PostPosted: Sat Jan 28, 2017 2:21 am    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
I ended up going with a completely different design for the saw VCO I needed, using a 555 and 2 op amps.


Could you post the schematic or a link to it, please? I have a bit of a fascination with anything interesting based around 555's.
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elmegil



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PostPosted: Sat Jan 28, 2017 10:20 am    Post subject: Reply with quote  Mark this post and the followings unread

Sure, here's my schematic. The weird P4/P5 stuff is the way I set things up to show a connection from the main PCB to a panel PCB with controls. Honestly, I did that originally a long while back, and I cannot remember what exactly I was thinking Very Happy, but it works out in the PCB layout.

And I say a couple op amps, actually, they don't have a lot to do with the clock function. First one's a buffer, then a DC blocking cap, then inversion and scaling (coming off the 555 is actually a ramp, not a saw). The last stage there, the LM311, is to provide a variable pulse width clock off of the saw.


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gasboss775



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PostPosted: Sun Jan 29, 2017 2:36 pm    Post subject: 555 oscillator Reply with quote  Mark this post and the followings unread

So its a square wave LFO with variable pulsewidth. Is it part of something bigger?

BTW Thanks for posting the schematic.

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Grumble wrote:
umschmitt wrote:
Hey. Is it me, or shouldn't the CV transistor have emitter and collector swapped ? Or does this resistance thing work also with collector at a lower voltage than emitter ? Just asking, I'm pretty ignorant.
(ok I should just try it myself)

I still don't understand how this transistor could be working with its collector to GND and being an NPN transistor.
Could somebody explain this?
Or... is it just used as a diode?
Still PUZZLED Confused Shocked Sad


Because all NPN have two similarly composed N regions with a P region in the middle they can be used the other way round. However the two N regions are not equal in size and in the back to front configuration the transistor operates with poorer performance ( less gain )

Last edited by gasboss775 on Sun Jan 29, 2017 2:47 pm; edited 2 times in total
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elmegil



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PostPosted: Sun Jan 29, 2017 2:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yeah, it's a clock for a gate/trigger sequencer
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Tchidu



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PostPosted: Wed Mar 20, 2019 3:38 am    Post subject: Reply with quote  Mark this post and the followings unread

I built the first schematic from this topic and it works great with a lm358!
Only thing is that when I connect the 'narrow' pulse to a vca it gets almost mute... when connected direct to speaker it's same volume as the saw one.
Also, is it possible to get a square and triangle wave from this with some improvement?
(I'm a beginner and don't understand maths very much... all of the things I do is testing and hearing!)

btw, great simple vco!!!
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gabbagabi



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PostPosted: Wed Mar 20, 2019 8:44 am    Post subject: Reply with quote  Mark this post and the followings unread

usually there is almost always a way,
but the quality of the triangle depends on the quality of the saw - of course

would be good to know: what kind of power supply u are using?
should the square have variable pulse with?
can u make a scope-shoot of the saw output?
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JovianPyx



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PostPosted: Wed Mar 20, 2019 9:51 am    Post subject: Reply with quote  Mark this post and the followings unread

When it comes to music, for me (that is, in my opinion) quality is a high or highest priority. Too often low parts count and simplicity detracts from quality of sound. Saw with a curved ramp has a different harmonic content than a true straight line ramp and near instantaneous reset.

So make sure to consider what you want to use it for and what you hope it will sound like before choosing simplicity and low parts count. After all, electronic parts are not very expensive in the first place and doing more complex circuits are a learning experience.

Again - just my opinion.

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Tchidu



Joined: Mar 20, 2019
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PostPosted: Wed Mar 20, 2019 12:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

g.gabba wrote:
usually there is almost always a way,
but the quality of the triangle depends on the quality of the saw - of course

would be good to know: what kind of power supply u are using?
should the square have variable pulse with?
can u make a scope-shoot of the saw output?


The quality is not my main interest at the moment... just want the oscillator sending saw, triangle and square waves... if its possible of course

I made the video showing all scopes.
My power supply is 9V 1A (lots of people already told me to go into dual suply, and I will, it's just for this project I'm doing all from 9V).

It should be good having variable pulse widht!!!


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gabbagabi



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PostPosted: Thu Mar 21, 2019 7:25 am    Post subject: Reply with quote  Mark this post and the followings unread

ok,
it looks like the half of the saw is missing,
so if you would try it to convert it in a triangle it would look somehow like this

may the transistor buffer does something strange?
for that, can you make a scope-pic:
when you are removing the connection from the 100n integration-cap to the transistor-base, and the probe is then measuring on the integration-cap


and of course welcome to the forum


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