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 Forum index » DIY Hardware and Software » YuSynth
VCO to LFO mod
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agustindead



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PostPosted: Mon May 23, 2016 3:50 pm    Post subject:  VCO to LFO mod Reply with quote  Mark this post and the followings unread

Hi all, maybe someone can lend me a hand with this... I'm going to build a Yusynth VCO and I'd like it to double as an LFO. I'm guessing the part I'll need to change is C8, the 1% cap, is that correct? I plan on putting a switch on the front panel to change between two values. I suppose changing/bypassing R8 and trimming T1 won't get me to a frequency low enough, right?

And, again, assuming C8 is the component to change: which value should I use? Or how can I calculate the approximate frequency range related to its capacity? I really have no idea about the math involved in this, sorry Embarassed

I'm asking because I'm a bit afraid of messing around with the caps on my already finished VCOs and I'd like to order a 1% cap for the new one. Or is 1% unnecesary for LFO duties?

Thanks!

Edit: well, I just checked and there's no way reducing R8 will make the VCO go slow enough. It gets into "fast LFO" territory, but soon freaks out and stops oscillating.

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Last edited by agustindead on Mon May 23, 2016 9:36 pm; edited 1 time in total
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Grumble



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PostPosted: Mon May 23, 2016 9:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

maybe it's a good idea to add a schematic to your post, because if someone who is willing to look into this doesn't have to search for this.
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agustindead



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PostPosted: Mon May 23, 2016 9:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

True, I completely overlooked that, thanks. Edited first post with a link to Yves' schematic.
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Grumble



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PostPosted: Mon May 23, 2016 11:36 pm    Post subject: Reply with quote  Mark this post and the followings unread

Since you already plan to add a switch to choose between VCO and LFO, the easiest way is to just add a capacitor in parallel with R8 with the switch.
Keep this extra capacitor off board so it is easy to change the value.
Importand is to have a capacitor with a low current leakage and low dependance of temperature (much more important as accurate value since there are enough trimpots to compensate for the lack of accuracy).


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agustindead



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PostPosted: Tue May 24, 2016 11:57 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Grumble for the help!

Grumble wrote:
Since you already plan to add a switch to choose between VCO and LFO, the easiest way is to just add a capacitor in parallel with R8 with the switch.

I'm assuming you mean C8 here.

Since just putting a cap in parallel is a pretty simple mod, I just tried on my already built VCOs. It definitely lowers the frequency, but as I go lower and lower, weirdness ensue. I found 470nF gets the VCO into a nice LFO range, but:
- sine and triangle outputs start working more like ramp up, and with something like a 3.3v offset.
- saw and pulse wave seem to have a normal shape (though the PW control is completely off center), but also with a big offset, this time around -3.3v

I don't have a scope to check the exact waveforms and voltages so I'm judging with my ears and voltmeter.

I didn't try re-trimming the oscillator, but it seems like there is no trimming that will work with both LFO and VCO frequency ranges......

Maybe the circuit is just not appropiate for what I want to do?
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elmegil



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PostPosted: Wed May 25, 2016 5:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

Possibly not.

I'd suggest maybe breadboarding a Thomas Henry VCO1 and seeing if that works any better in this mode. I was going to make the same suggestion of a bigger cap in parallel to the timing cap.

http://www.birthofasynth.com/Thomas_Henry/Pages/VCO-1.html
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agustindead



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PostPosted: Wed May 25, 2016 10:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
Possibly not.

I'd suggest maybe breadboarding a Thomas Henry VCO1 and seeing if that works any better in this mode. I was going to make the same suggestion of a bigger cap in parallel to the timing cap.

http://www.birthofasynth.com/Thomas_Henry/Pages/VCO-1.html


Well, in fact my first plan was using VCO-1. I decided not to because I'd really want a saw out (I looove saw LFOs! love ) and the layout size is a bit big for my Eurorack gabinet. I guess I can sort both things redesigning and adding a saw shaper, but I'd rather have something more simple. In fact, tuning or scaling are not that necessary since I'll be using to make FX mainly.

THanks!
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elmegil



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PostPosted: Wed May 25, 2016 11:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

Fits fine on Euro:

https://github.com/TomWhitwell/TH-VCO1
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gdavis



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PostPosted: Thu May 26, 2016 11:49 am    Post subject: Reply with quote  Mark this post and the followings unread

agustindead wrote:
elmegil wrote:
Possibly not.

I'd suggest maybe breadboarding a Thomas Henry VCO1 and seeing if that works any better in this mode. I was going to make the same suggestion of a bigger cap in parallel to the timing cap.

http://www.birthofasynth.com/Thomas_Henry/Pages/VCO-1.html


Well, in fact my first plan was using VCO-1. I decided not to because I'd really want a saw out (I looove saw LFOs! love ) and the layout size is a bit big for my Eurorack gabinet. I guess I can sort both things redesigning and adding a saw shaper, but I'd rather have something more simple. In fact, tuning or scaling are not that necessary since I'll be using to make FX mainly.

THanks!

In that case, if VCO-1 still doesn't work for you I'd suggest looking at the Yusynth VC-LFO. Might give you an idea how to fix the trimming problem on the VCO. Might also be able to modify it to give the frequency ranges you want instead of using the VCO, but I think the layout is a bit long for euro.

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agustindead



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PostPosted: Fri May 27, 2016 9:56 am    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
Fits fine on Euro:

https://github.com/TomWhitwell/TH-VCO1

Hey thanks, didn't know about that layout. I had etched another layout with the idea of testing some mods (saw shaper, LFO frequency) but then realized it didn't fit into my cabinet Confused so I didn't make any further progress.
It still puts me down the lack of a saw output... I don't know how people can live without saw LFOs... Laughing

gdavis wrote:

In that case, if VCO-1 still doesn't work for you I'd suggest looking at the Yusynth VC-LFO. Might give you an idea how to fix the trimming problem on the VCO. Might also be able to modify it to give the frequency ranges you want instead of using the VCO, but I think the layout is a bit long for euro.

Sadly, the VCO and the VCLFO seem pretty different (the LFO is based on the 7555). I just checked the VCLFO schematic though and it seems like a good candidate anyway, since tuning stability doesn't really bother me (which the redesign will probably lack) and it has a saw output, FM and a PW control, which are the main things I'm looking for. A PWM input seems easily doable as well. But yes, as you pointed out, the layout won't work as it is since it's too big... Sad I guess I'll have to finally learn how to use Eagle haha.

Basically what I'm looking for is an LFO design with:
-saw out
-FM input
-PW and PWM input
-audio rate capabilities
And that's why I thought modding a VCO would be easier than finding such an LFO...
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gdavis



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PostPosted: Fri May 27, 2016 2:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

agustindead wrote:
elmegil wrote:
Fits fine on Euro:

https://github.com/TomWhitwell/TH-VCO1

Hey thanks, didn't know about that layout. I had etched another layout with the idea of testing some mods (saw shaper, LFO frequency) but then realized it didn't fit into my cabinet Confused so I didn't make any further progress.
It still puts me down the lack of a saw output... I don't know how people can live without saw LFOs... Laughing

gdavis wrote:

In that case, if VCO-1 still doesn't work for you I'd suggest looking at the Yusynth VC-LFO. Might give you an idea how to fix the trimming problem on the VCO. Might also be able to modify it to give the frequency ranges you want instead of using the VCO, but I think the layout is a bit long for euro.

Sadly, the VCO and the VCLFO seem pretty different (the LFO is based on the 7555). I just checked the VCLFO schematic though and it seems like a good candidate anyway, since tuning stability doesn't really bother me (which the redesign will probably lack) and it has a saw output, FM and a PW control, which are the main things I'm looking for. A PWM input seems easily doable as well. But yes, as you pointed out, the layout won't work as it is since it's too big... Sad I guess I'll have to finally learn how to use Eagle haha.

Basically what I'm looking for is an LFO design with:
-saw out
-FM input
-PW and PWM input
-audio rate capabilities
And that's why I thought modding a VCO would be easier than finding such an LFO...


They're really not that different. The expo converter is simplified in the LFO and the integrator and comparator are done with the 7555 instead of discreetly. But the functional elements are very similar.

The problem you're having with adding a cap in parallel in the VCO sounds like a saw bias issue, which is handled the same way in both, the LFO just has two trimmers that are switched based on the selected cap. You could probably do the same thing with the VCO. Gonna be hard to trim and verify without a scope though. IMO a scope is a critical instrument for DIY.

Learning Eagle isn't a bad idea, then you can always design exactly what you want Wink

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wackelpeter



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PostPosted: Sat May 28, 2016 12:05 am    Post subject: Reply with quote  Mark this post and the followings unread

What about the Serge DUSG or VCS?

Don't know if Ken has some PCB's for Euro but they'll run on 12 or 15V without modifications and at least the schematics are available and you can design your own PCB...

They're so brilliant and have many options you can use them... Filter,LFO, sort of VCO, Envelope, etc.

Okay, they're lacking a PWM and a real square output if you don't count the EndOfCircle output pulses... but i think it should be possible to get some square waves with some sort of PWM out of it with the triangle/ramp outputs fed into an comparator with variable treshhold... like in the TGTSH from Ian Fritz... then you would have independent control over rise and fall times or both and also over the comparators treshhold...

P.S. would second that what gdavis wrote... a scope can't harm when into DIY and it's not expensive at all... there are numerous floating around at the various online auction platforms... just don't try to get the defect ones... Wink

P.P.S. i also sometimes use my Thomas Henry 555 VCO'S as LFO as they cover a really wide range and go really deep into the bottom... maybe it's easier to fix them as a VCLFO and going much deeper...

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agustindead



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PostPosted: Sun May 29, 2016 1:52 pm    Post subject: Reply with quote  Mark this post and the followings unread

gdavis wrote:
They're really not that different. The expo converter is simplified in the LFO and the integrator and comparator are done with the 7555 instead of discreetly. But the functional elements are very similar.

The problem you're having with adding a cap in parallel in the VCO sounds like a saw bias issue, which is handled the same way in both, the LFO just has two trimmers that are switched based on the selected cap. You could probably do the same thing with the VCO. Gonna be hard to trim and verify without a scope though. IMO a scope is a critical instrument for DIY.

Learning Eagle isn't a bad idea, then you can always design exactly what you want Wink

My knowledge is obviously lacking to notice that, thanks! But going by what you say I'll probably try building a new VCO and modding/trimming it and see if I can make it work. I hope the problem is just the bias and the waveshapes don't need different trimming for LFO ranges, or I'll probably have a hard time switching in and out more than one trimmer for that.... Thanks again!

wackelpeter wrote:
What about the Serge DUSG or VCS?

Don't know if Ken has some PCB's for Euro but they'll run on 12 or 15V without modifications and at least the schematics are available and you can design your own PCB...

They're so brilliant and have many options you can use them... Filter,LFO, sort of VCO, Envelope, etc.

Okay, they're lacking a PWM and a real square output if you don't count the EndOfCircle output pulses... but i think it should be possible to get some square waves with some sort of PWM out of it with the triangle/ramp outputs fed into an comparator with variable treshhold... like in the TGTSH from Ian Fritz... then you would have independent control over rise and fall times or both and also over the comparators treshhold...

P.S. would second that what gdavis wrote... a scope can't harm when into DIY and it's not expensive at all... there are numerous floating around at the various online auction platforms... just don't try to get the defect ones... Wink

P.P.S. i also sometimes use my Thomas Henry 555 VCO'S as LFO as they cover a really wide range and go really deep into the bottom... maybe it's easier to fix them as a VCLFO and going much deeper...

I always liked those Serge designs, so it may be an option... I started making a raw euro PCB for the DUSG just now in fact. I don't know if it fits what I'm looking for here though. But maybe it's a good chance to go in a new direction.

Sadly, scopes are pretty expensive where I live. I trimmed everything using my audio interface and by ear, but yes, DC bias is pretty hard to get right using what I have. I'll try to get one borrowed and see if I can get the Yusynth VCO trimmed.

I'll check the 555 VCO... If they cover a wide range that might be a good option. I still can't decide if I want a switchable between LFO and audio frequencies VCO or just a REALLY wide range VCO. Thanks!
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wackelpeter



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PostPosted: Mon May 30, 2016 6:29 am    Post subject: Reply with quote  Mark this post and the followings unread

For the 555 VCO as LFO i think there are some posts around here at EMF if i remember correctly...

Also Fonik/Matthias has done it as far as i remember and maybe also included it in one of his front panel he offered...

Over here used Scopes are around 50-100 euro at this big online auction platform called ebay...
you can roughly estimate if they're functioning when the seller provides pictures of the scope with the in-built calibration generator showed...
Most people with a little knowledge do this and when someone writes in the description i don't know how to power it up and if it's working anyway, then better stay away and save the money...

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gabbagabi



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PostPosted: Thu Oct 05, 2017 11:56 am    Post subject: Reply with quote  Mark this post and the followings unread

saw core osc are always more or less sensible in terms of capacity changes, unlike tri core vcos

one way to make the mod working would be to build a extra SAW or Ramp LFO with centered wave and then use a switch to switch the input of the waveshaperstage

same would work backward for lfo to vco mod


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