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 Forum index » DIY Hardware and Software » Thomas Henry designs
The VCO Maximus
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halfshavedyaks



Joined: Jul 12, 2018
Posts: 65
Location: UK

PostPosted: Sun Aug 26, 2018 2:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

I replaced R19 with 56k, even though my chip says CEM3340 not AS3340

it improved it considerably the error is now less than 10 cents - still not usable but much better.

I think i need to try with a different power supply - which I don't have.
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halfshavedyaks



Joined: Jul 12, 2018
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Location: UK

PostPosted: Sun Aug 26, 2018 2:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

can the maximus be adapted to sync to a positive trigger? would changing the direction of D1 achieve that or is a positive voltage at pin 6 of the 3340 a bad idea?
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halfshavedyaks



Joined: Jul 12, 2018
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PostPosted: Sun Sep 09, 2018 5:43 pm    Post subject: Reply with quote  Mark this post and the followings unread

I looked up the CEM3340 datasheet and it is quite clear that the maximus can't do conventional hard sync.

The datasheet does give an implementation for that but the maximus doesn't use it.
The sync on the maximus doesn't reset but rather changes the direction of the core.

no wonder it seemed wrong it's a different thing entirely.
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halfshavedyaks



Joined: Jul 12, 2018
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PostPosted: Wed Sep 12, 2018 1:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

Proper hard sync that resets the oscillator to zero rather than just reversing the direction is essential for me so I have implemented the conventional hard sync as outlined in the CEM3340 datasheet.

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

It works, but it only responds to a negative pulse (as expected)

It is also finicky about the level of the pulse.

has anyone else done this?

I don't actually understand this circuit - since I don't know what's going on inside the 3340.

does anyone know how to make it respond to a positive rather than negative pulse? (other than inverting the pulse first with an attenuverter)

EDIT I used a BC557 - just what I had on hand I'm not sure how appropriate it is.
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halfshavedyaks



Joined: Jul 12, 2018
Posts: 65
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PostPosted: Wed Sep 12, 2018 1:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

I have tried some more to improve the PWM pitch shift but with no luck.

I did however notice that flexing the board by pressing on the edges creates a pitch shift. Is that normal or could it be a clue to a problem?
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halfshavedyaks



Joined: Jul 12, 2018
Posts: 65
Location: UK

PostPosted: Wed Sep 12, 2018 1:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

this is a useful page - it also has schematics that are much clearer showing the hard sync circuit.

https://electricdruid.net/cem3340-vco-voltage-controlled-oscillator-designs/
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elmegil



Joined: Mar 20, 2012
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PostPosted: Wed Sep 12, 2018 3:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

might be worth putting a comparator between your pulse input and this? Up to you whether you invert the incoming pulse at the same time...
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halfshavedyaks



Joined: Jul 12, 2018
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Location: UK

PostPosted: Thu Sep 13, 2018 10:02 am    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
might be worth putting a comparator between your pulse input and this? Up to you whether you invert the incoming pulse at the same time...


I can see the benefit in that but I'm not really confident about how to do it. I can sort of imagine it with an opamp inverting comparator - but I'm a little beyond my expertise to actually design a suitable circuit.

also i really don't have space to add an opamp, unless I make a whole daughterboard for the mod.

would anyone be willing to draw a suitable circuit?

I had hoped there would be an easy way to change the standard circuit to respond to a positive rather than negative pulse.
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elmegil



Joined: Mar 20, 2012
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PostPosted: Thu Sep 13, 2018 11:26 am    Post subject: Reply with quote  Mark this post and the followings unread

normally to change it you'd invert it. You can do that part with a transistor inverter ... which could also then drive a stronger signal, so that *might* work.

See the response with the NPN and PNP transistors and 5V supply here:

https://electronics.stackexchange.com/questions/30238/how-to-invert-a-digital-signal

Instead of 5V use 12V.
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halfshavedyaks



Joined: Jul 12, 2018
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Location: UK

PostPosted: Fri Sep 14, 2018 10:10 am    Post subject: Reply with quote  Mark this post and the followings unread

I've done it with an opamp. I used a TL071 - maybe I will change to a LM311 later although it isn't really necessary.

It works fine, but there's a slight catch - the oscillator isn't reset to 0 or to max negative but to the output level of the comparator, which is as close to the negative rail as the TL071 can get.

Since the tri wave only does +/- 5v this means there's a big negative spike on every reset - so you have to trim the comparator output to the level you want it reset to, which I guess is -5v

using the comparator however at least means that you can control that level rather than being at the mercy of the sync signal input level.
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Jaba



Joined: Feb 27, 2009
Posts: 48
Location: Genova, Italy

PostPosted: Wed Mar 27, 2019 8:02 am    Post subject: mods ? Reply with quote  Mark this post and the followings unread

Hi
I am planning some modifications and add-ons to my VCO Maximus, such as different sync modes (real hard sync too) and TZFM, mostly based on J.Haible's circuit.
Has anyone else tried it ?
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ixtern



Joined: Jun 25, 2018
Posts: 104
Location: Poland

PostPosted: Fri Mar 29, 2019 2:54 pm    Post subject: Re: mods ? Reply with quote  Mark this post and the followings unread

Jaba wrote:
Hi
I am planning some modifications and add-ons to my VCO Maximus, such as different sync modes (real hard sync too) and TZFM, mostly based on J.Haible's circuit.
Has anyone else tried it ?

No, but I've also done some modifications on my fonitronik's VCO Maximus. Will publish it here soon.
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Jakob1



Joined: Mar 21, 2019
Posts: 2
Location: Sweden

PostPosted: Thu May 30, 2019 4:53 am    Post subject: Reply with quote  Mark this post and the followings unread

Hey fonik.

I finished this module way back, but now i am switching formats for my modular and is rewiring all the panel mount controls however i cant find your schematic for the maximus? Its not in the build doc or on this forum. Am i just blind or have you removed it for some reason?
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fonik



Joined: Jun 07, 2006
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PostPosted: Thu May 30, 2019 5:46 am    Post subject: Reply with quote  Mark this post and the followings unread

Arrrgh. I uploaded a new version of the documentation but missed to add the 12V schematics. My fault. Will upload it later today and let you know.
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cheers,
matthias
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fonik



Joined: Jun 07, 2006
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PostPosted: Thu May 30, 2019 6:35 am    Post subject: Reply with quote  Mark this post and the followings unread

fixed
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cheers,
matthias
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tom303



Joined: Jun 11, 2019
Posts: 2
Location: austria

PostPosted: Tue Jun 11, 2019 3:50 am    Post subject: Reply with quote  Mark this post and the followings unread

Hello everyone,

I've got my AS3340 recently and did a little breadboard test circuit yesterday. Having read this thread in advance, particularly regarding the pwm/pitch issue, I connected pin 3 and the resistors from pin1 and 2 directly to a regulated -5V source expecting that this would prevent the issue entirely.

However the pwm to pitch influence was very noticable. (haven't measured it, but I'd say at least 30cts). Unusable.

After tinkering around for a while, I tried something more close to the datasheet implementation, hooking Pin 3 to -12V via a resistor (I think around 600Ohms), so Pin 3 measured -5V. The issue was gone!

Altering the pulse width, I can see the voltage at Pin3 floating around a bit - plus minus 100mV. The pitch however stays pretty constant. Again, not measured, but it's so small, it's perfectly usable in a synthesizer application.

Has anyone else experienced this? After this it seems to me like the potential at Pin 3 has to be able to float a bit. Pinning it to a regulated constant -5V does not work for me. Any comments?

best regards.
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fonik



Joined: Jun 07, 2006
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PostPosted: Tue Jun 11, 2019 3:56 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes. It is crucial to have pin3 as close to -5V as possible.
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cheers,
matthias
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tom303



Joined: Jun 11, 2019
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PostPosted: Tue Jun 11, 2019 4:04 am    Post subject: Reply with quote  Mark this post and the followings unread

But apparently not too close, since a direct connection to regulated -5V does not work, at least here with an AS3340
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fonik



Joined: Jun 07, 2006
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PostPosted: Tue Jun 11, 2019 4:06 am    Post subject: Reply with quote  Mark this post and the followings unread

I did not test this. Maybe a current is needed? I used shunted 11V and a trimmer.
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ixtern



Joined: Jun 25, 2018
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PostPosted: Wed Jun 12, 2019 10:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

I can confirm that in Maximus with AS3340 pin3 connected to the stable -5V, during PWM change frequency deviation is about 6Hz when VCO is tuned to 1000Hz.
Changed R58 (the one going from pin 4 to 5) to the 10 Mohms (as in Kassutronics 3340 VCO) but the problem still persists.

Not tried to change -5V to the Ree resistor for Vee (pin 3) yet but will try it soon. Seems that for -12V Ree should have value of 575 Ohms - from the AS3340 datasheet's formula: Ree = (Vee - 7.4)/0.008.

Even if problem vanishes with Ree resistor, it seems that the best solution is to make separate PWM circuit getting input from saw output.
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fonik



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PostPosted: Wed Jun 12, 2019 11:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

ixtern wrote:
Even if problem vanishes with Ree resistor, it seems that the best solution is to make separate PWM circuit getting input from saw output.

I totally agree on that, however, this was not the design goal, I think.

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ixtern



Joined: Jun 25, 2018
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PostPosted: Thu Jun 13, 2019 1:38 am    Post subject: Reply with quote  Mark this post and the followings unread

fonik wrote:
ixtern wrote:
Even if problem vanishes with Ree resistor, it seems that the best solution is to make separate PWM circuit getting input from saw output.

I totally agree on that, however, this was not the design goal, I think.

Sure, but how you explain the birth of AS3340-HYB? Smile

For me, linearity and stability of VCO is the priority (more than plenty of features) as I am planning to have many of them in one voice (how many? don't know yet) to get rich sound so detuning surprises are the most unwanted feature.
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fonik



Joined: Jun 07, 2006
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PostPosted: Tue Jul 09, 2019 1:07 am    Post subject: Reply with quote  Mark this post and the followings unread

Oh, I did not consider any other designs. I was talking about Thomas' design goal here to make a 3340 VCO with as much funktions and as few parts as possible. And with having it running on 15V, as all Thomas' designs do originally, we did not experience any issues at all.
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cheers,
matthias
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ixtern



Joined: Jun 25, 2018
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PostPosted: Tue Jul 09, 2019 6:26 am    Post subject: Reply with quote  Mark this post and the followings unread

fonik wrote:
Oh, I did not consider any other designs. I was talking about Thomas' design goal here to make a 3340 VCO with as much funktions and as few parts as possible. And with having it running on 15V, as all Thomas' designs do originally, we did not experience any issues at all.

+-15V supply is not so common these days as +-12V. Before starting to building my synthesizer I was thinking what standard to implement: 15 or 12V and choose 12V as most currently avaiable modules are for 12V. Maybe I should go with 15V and install individual voltage regulators for every module... What a perfect supply bus separation Smile Neverless I am going to test Maximus with CEM3340 (G) also as I have ordered two of them.
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