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 Forum index » DIY Hardware and Software » Thomas Henry designs
The VCO-555
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YashN



Joined: Jun 27, 2011
Posts: 95
Location: Canada

PostPosted: Sun Feb 18, 2018 4:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
So let's talk our way through control voltages.

wiper of the coarse pot should swing from -15V to +15V


Thanks elmegil. For me, it's -12V | 0 | +12V.

elmegil wrote:

Set the exp FM pot all the way CCW and the fine pot as close to center as you reasonably can (you could measure it at the wiper and make sure it was 0V).

Set the linear FM pot all the way CCW as well.


I had removed those pots for troubleshooting, so I had to re-solder them. Almost OK as in my little soldering session, I wired the pot the other way round, so I had to adjust for that in the following measurements (fully CW instead of CCW for the FM pots).

elmegil wrote:
IC3a pin 1 should output something close to -.02 x the voltage at the coarse pot's wiper -- for example if the wiper is set at 10V, the output of the op amp should be -.2V


Setting Coarse to 10V and Fine close to 0V using the DVM, I get:
I3P1 = -7.41V

elmegil wrote:
The output of the wiper of the 100R V/Oct trimmer (connects to base of Q3) should be a little bit less, somewhere above .16V when the coarse pot is at 10V.


For the V/Oct Trimmer Pins, I get:
P1 = -4.79V
P2 = -4.81V
P3 = -7.41V

elmegil wrote:
When you change the coarse pot, these should all move in the direction you expect.


Haven't checked this yet, as it seems to me the outputs seem quite a way off despite adjusting for the +/- 12V PSU instead of the +/-15V, what do you think?
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elmegil



Joined: Mar 20, 2012
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Audio files: 16

PostPosted: Sun Feb 18, 2018 4:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yeah, that's wack.

10V in is 10V in regardless of the supply, those values depend on the resistors and op amp behavior, not on the supply rails.

DOUBLE CHECK. R29 is 100K. R11 is the 2K tempco.

Sounds to me more like R11 is ~75K or R29 is 2.7K


edit 1: 7.5V where you expect .2V WOULD give you ridiculously high frequency though Very Happy

edit 2: verify that IC3 pin 7 is actually somewhere near 0V when the Linear FM is all the way CCW (at 0V itself)
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YashN



Joined: Jun 27, 2011
Posts: 95
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PostPosted: Sun Feb 18, 2018 6:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
DOUBLE CHECK. R29 is 100K. R11 is the 2K tempco.


You'll laugh: R29 is indeed correct. I had no tempco! I even wonder how it at least worked once without anything there to bridge the two opamp pins... (an Expo rather than linear Pot at Coarse and Fine at inital build might have helped, I am not sure).

elmegil wrote:
edit 1: 7.5V where you expect .2V WOULD give you ridiculously high frequency though Very Happy


I installed what I had at hand - the phenomenon of the missing resistor value, couldn't even get two 1Ks to fit in there, found only a single one - a 2.7K resistor.

elmegil wrote:
edit 2: verify that IC3 pin 7 is actually somewhere near 0V when the Linear FM is all the way CCW (at 0V itself)


A more sound-looking -.291V!

Tested the Pulse, Tri, Ramp and Sine outputs with my little test speaker. All good! Varying Coarse does what it should. So now I can either wire the other inputs, pots and the ramp output again or start working on a little circuit to control it!

Thanks for hanging there with me and helping me out with this, elmegil, I appreciate it! Very Happy

Last edited by YashN on Mon Feb 19, 2018 7:15 am; edited 1 time in total
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elmegil



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PostPosted: Sun Feb 18, 2018 8:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

YashN wrote:
elmegil wrote:
DOUBLE CHECK. R29 is 100K. R11 is the 2K tempco.


You'll laugh: R29 is indeed correct. I had no tempco! I even wonder how it at least worked once without anything there to bridge the two opamp pins...


The op amp would have acted as a comparator, sticking mostly at one rail or the other, with no feedback resistor.

YashN wrote:

A more sound-looking -.291V!

Tested the Pulse, Tri, Ramp and Sine outouts with my little test speaker. All good! Varying Coarse does what it should. So now I can either wire the other inputs, pots and the ramp output again or start working on a little circuit to control it!

Thanks for hanging there with me and helping me out with this, elmegil, I appreciate it! Very Happy


Awesome Smile

You're quite welcome. Enjoy your oscillator.
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Project_2501



Joined: Jan 31, 2018
Posts: 49
Location: Los Angeles

PostPosted: Wed Mar 07, 2018 4:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

lysergist wrote:
Hi !
I don't know if PCBs are still available, and i have all the components, so i decided to try a stripboard version. I know i should etch, maybe soon.

So... that's my first stripboard EVER. Really hard to make it compact and to save some place. But it fits perfecly in my board (9x15cm) That's maybe one of the worst layout ever hehe, too many jumpers, holes etc... I know it could be improved. That's my first stripboard... we learn from errors.

And it works ! Triangle, Sine and Pulse are perfectly stable, even if the sine sems to be a little bit weak. I have an some issue with the Ramp, wich is not a ramp, i have a weird waveform, but it sounds cool.




Thank you for this Layout!

On the layout, what are those things called for the inputs. Looks like a philips screw. Just curious. I'm new, haven't seen that before.

I'm about to attack this project or start collecting parts for some kind VCO.
Has anyone built this layout besides the creator? Anyone pointers/updated layouts? Greatly appreciated.
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YashN



Joined: Jun 27, 2011
Posts: 95
Location: Canada

PostPosted: Sat Mar 10, 2018 12:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
You're quite welcome. Enjoy your oscillator.


Thanks @elmegil, having a lot of fun listening to it and twiddling knobs. Tested a few internal patching options too - that was quite fun! e.g. : feedback into the 1V/Oct and also taking its own PWM into the SYNC while listening to the RAMP I think, and moving the PWM knob.

I am thinking of a couple of adjustment or mods that could be helpful, namely getting the SINE output to the level of the other outputs, and secondly, enabling the switching of a larger capacitor in parallel to the main timing cap C4 for an LFO mode.

I have a few more thoughts about the PCB layout (mine is a Fonitronik - not sure if I was on the 2nd or 3rd run), so I would like to hear feedback on each of those:

- Adding another bigger bypass cap to the 555

- Putting some of the op amp feedback resistors nearer the op amps

- There are some unused pins in the LM13700 and I believe they are left floating: one Darlington Buffer and the two diode biases if I'm not mistaken. What would be the best practice here with regards to tying them all to some ref point (or not)?

- A slow sweep of the PWM sounds so wonderful, I am tempted to try and add an onboard little LFO! But then again, this should really be a separate module in a...er... modular Very Happy

If there any additional tweaks or mods or useful functionality I could add, let me know!

Great VCO! I love how it sounds really clean in such a wide range.
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wackelpeter



Joined: May 05, 2013
Posts: 430
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PostPosted: Sat Mar 10, 2018 2:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

YashN wrote:
[I am thinking of a couple of adjustment or mods that could be helpful, namely getting the SINE output to the level of the other outputs, and secondly, enabling the switching of a larger capacitor in parallel to the main timing cap C4 for an LFO mode.

.


As far as i can tell from my 4 builds they all go into the LFO region with the regular Timing cap already...

There was on a few sites earlier a few notes regarding the sine output.... don't know if it was already implemented in your build docs or BOM but it was very simple to do.

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YashN



Joined: Jun 27, 2011
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PostPosted: Sun Mar 11, 2018 7:12 am    Post subject: Reply with quote  Mark this post and the followings unread

wackelpeter wrote:
from my 4 builds they all go into the LFO region with the regular Timing cap already...


That's not what it's about though because I didn't say my VCO build doesn't go to LFO region. See fonik's own post on Page 2 on this thread.
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linwop



Joined: Mar 24, 2018
Posts: 7
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PostPosted: Sat Mar 24, 2018 2:46 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi all

I'm just getting started with DIY synths and this is the first module I'm trying to build. Just populated the PCB from fonic.
The waveforms sound good so far and I'm getting a frequency response from sub until above audible.

However, I'm having a problem tuning the module.
When I apply CV (0V-5V) to the V/Oct I get an exponentially rising frequency response.
Can't figure out what could be the problem. Do I have a wrong IC?

Would really appreciate if someone could hint me in the right direction.
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fonik



Joined: Jun 07, 2006
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PostPosted: Sat Mar 24, 2018 3:00 am    Post subject: Reply with quote  Mark this post and the followings unread

Isn't exponential what we want?
for each 1V increase the frequency doubles.

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linwop



Joined: Mar 24, 2018
Posts: 7
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PostPosted: Sat Mar 24, 2018 4:51 am    Post subject: Reply with quote  Mark this post and the followings unread

Hmm, true. I was thinking that'd be a linear relationship with a slope factor of 2.
In that case I just don't get the right adjustment range with the V/Oct trimmer.
Checked the Resistors and they should be fine.
Powering from +-12 V and did the resistor adjustments you suggested in your thread.


Rplot.png
 Description:
These are the frequency responses I get with the V/Oct trimmer set to min (black) and to max (red).
The voltages is actually from 0-5 and not 1-6
 Filesize:  22.83 KB
 Viewed:  94 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

Rplot.png


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fonik



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PostPosted: Sat Mar 24, 2018 5:35 am    Post subject: Reply with quote  Mark this post and the followings unread

the resistor adjustments for 12V don't change anything in the expo converter, though. they just correct the output levels.

Don't touch the HF trimmer for the start. Does the 1V/oct trimmer do anything?
[edit: i get it from your post that the trimmer works. and the curve you posted above shows very roughly the response we expect. it is just not right to the point that it would be musically useful yet]

For each 1V change on the 1V/oct input you should see a change of about roughly 18mV at IC3A/pin1.

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linwop



Joined: Mar 24, 2018
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PostPosted: Sat Mar 24, 2018 6:10 am    Post subject: Reply with quote  Mark this post and the followings unread

fonik wrote:

[edit: i get it from your post that the trimmer works. and the curve you posted above shows very roughly the response we expect. it is just not right to the point that it would be musically useful yet]


For each 1V change on the 1V/oct input you should see a change of about roughly 18mV at IC3A/pin1.


Correct!
I did some quick measurements before I went out:
0V: 0.026 mV
1V: 0.011 mV
2V: 0 mV
3V: -0.013 mV

Looks like it's off by 5mV.

Thanks a lot for your help fonik!
I'll check it out more thoroughly tomorrow!
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wackelpeter



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PostPosted: Sat Mar 24, 2018 6:52 am    Post subject: Reply with quote  Mark this post and the followings unread

maybe i'll chime in here and hint you to this excellent Explanation on how to trim for V/oct Response:

http://www.yusynth.net/Modular/EN/VCO/index.html

Can be found near the bottom of the page, hope this helps if you were not familiar with it.

I for this purpose used a step sequencer with single step advancing button and reset to step 1 button then first dialed in the CV's 0, 1, 2, 3V etc. on the individual steps in and then started at 55Hz advancing each step manually and having a DVM with a frequnecy counter hooked up to my Output, while trimming it... on most VCO builds it took not much time and effort and was working well for me.

Cheers Bastian

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fonik



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PostPosted: Sat Mar 24, 2018 7:03 am    Post subject: Reply with quote  Mark this post and the followings unread

With an 100k input resistor and 2k in the opamps feedback the calculated the actual change before(!) the trimmer should even be 20mA.
Something seems to be wrong here. Either it is one of the resistors, or it is your voltage source. (Please, don't take this personal, but just two weeks ago someone contacted me having issues with his VCO MAXIMUS which would not track. In the his voltage source was not calibrated and way off, so 1V increase was not 1V really.)

Anyway, the problem is pretty much encircled, which is good.

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linwop



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PostPosted: Mon Mar 26, 2018 3:48 am    Post subject: Reply with quote  Mark this post and the followings unread

wackelpeter wrote:
maybe i'll chime in here and hint you to this excellent Explanation on how to trim for V/oct Response:

http://www.yusynth.net/Modular/EN/VCO/index.html

Can be found near the bottom of the page, hope this helps if you were not familiar with it.

I for this purpose used a step sequencer with single step advancing button and reset to step 1 button then first dialed in the CV's 0, 1, 2, 3V etc. on the individual steps in and then started at 55Hz advancing each step manually and having a DVM with a frequnecy counter hooked up to my Output, while trimming it... on most VCO builds it took not much time and effort and was working well for me.

Cheers Bastian


Hi Wackelpeter

Thanks a lot for that info!
Turns out there's nothing wrong with the circuit. Managed to tune it with this guide. Was using the 5V output of an Arduino previously and used a Voltage divider to get roughly 0-5V.
But used a sequencer as you recommended this time.

Thanks a lot for your guidance @fonik and wackelpeter!
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fonik



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PostPosted: Mon Mar 26, 2018 3:51 am    Post subject: Reply with quote  Mark this post and the followings unread

linwop wrote:
...Was using the 5V output of an Arduino previously and used a Voltage divider to get roughly 0-5V.

that is almost exactly what the other person i was mentioning did. alas, a simple voltage divider won't be precise enough and w/o a buffer you might run into serious problems.

anyway, i am glad you gor it sorted out.

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Hashtag Octothorpe



Joined: Jun 11, 2017
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PostPosted: Wed Apr 25, 2018 1:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hey!

I've tried building this VCO literally six times using various point-to-point or pseudo-breadboard techniques, and FINALLY I got one working.

But then it quit working!!!

See, I've been building my DIY synth over the past year and a half, and I've gotten really good at my building technique, successfully building a bunch of TR-808 modules, a couple 303 diode filters, a bunch of Thomas Henry's other designs (four X-4046s)... now I'm designing my own circuits from scratch, my latest is a octo-ping-pong-delay with eight PT2399s bouncing signals around between the left and right channels of my synthesizer...

Anyway, my point is that I'm really good at building circuits without PCBs or even perf- or stripboard. But this circuit it giving my so much trouble!

My last build worked as soon as I powered it up. Beautiful triangle wave. Nice chunky pulse. Raw sawtooth. Quiet smooth sine. Everything sounded about the same amplitude. The coarse pitch control took me from infra- to ultrasonic.

So I turned it off, wired it up to my scope to trim the waveforms, turned it back on, and got nothing. Just + or - ~5V on the outputs.

Aaaarrrggghhh.

I know nobody will be able to help me on this since I'm doing things so unconventionally, I just wonder if this particular circuit pushes the LM13700 to the edge and it might break, or if the 7555 (I do have some TLC555's on order, but they're both CMOS 555s, and I thought that was good enough) is a particularly fragile chip.
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elmegil



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PostPosted: Wed Apr 25, 2018 1:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

The OTA has a pretty extreme limit on the voltage input to it, and if you exceed that it will blow up the input.

I can't remember off the top of my head what the limit is, but it's in the mV range.
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Hashtag Octothorpe



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PostPosted: Wed Apr 25, 2018 1:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

I see!

So that's why there's always resistors to ground on the inputs, usually in the few-hundred-ohm range... cool, those are there.

The only way I've blown up LM13700s is by drawing too much current out of the amp bias pins. Like on my next-to-last VCO-555 build, I switched the collector and base of transistor Q3, and that blew the OTA.

I'll move on to other projects until my TLC555s get here, then I may give it another go.

My synth, if anyone's curious...

https://www.youtube.com/watch?v=bZwo3A6RpCM
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YashN



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PostPosted: Wed Apr 25, 2018 3:47 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hashtag Octothorpe wrote:
or if the 7555 (I do have some TLC555's on order, but they're both CMOS 555s, and I thought that was good enough)


Not just 'good enough', actually a necessity in this circuit.

@elmegil is right: the OTA can be damaged.

Actually, a short while probing pins can also do it in.
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Hashtag Octothorpe



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PostPosted: Wed Apr 25, 2018 6:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

Awesome, so you're telling me that every one failed because I was using the wrong variant of 555! My first couple were built with the NE555, even I should have known that wouldn't have worked.

This is super good to know, thank you!

Actually, this VCO with the 7555 worked for one power cycle...
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elmegil



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PostPosted: Wed Apr 25, 2018 6:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

7555 is supposed to be CMOS as well. That, the TLC, or the TS 555's all ought to work; *however* I have seen a couple of SynthTech modules using CMOS 555's where one only worked with the TS type and the other only worked with the TLC. The one that wanted the TS would fry the TLC types.

That said, I don't recall this VCO being that picky, at least not the Fonik PCB version.
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hedefalk



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PostPosted: Sat May 19, 2018 2:50 am    Post subject: Reply with quote  Mark this post and the followings unread

I'm finally taking this from the breadboard to a PCB. I'm having one small issue remaining - There is a nudge around 0V on each output. This is separate from the ramp connection which is the big one on the ramp image, see pictures from oscilloscope:

https://photos.app.goo.gl/LcX986fWeo3zwp192

Any idea? Thanks!
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Hashtag Octothorpe



Joined: Jun 11, 2017
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PostPosted: Sat May 19, 2018 6:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

That's weird.

Maybe beef up your ground connections? Toss more uFs of power smoothing caps in there? Get the bypass caps closer to the power pins on the ICs?

That's all I got.
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