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 Forum index » DIY Hardware and Software » Thomas Henry designs
[Received help] Trying to make a the 555 VCO
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Thomas73951



Joined: Jan 21, 2020
Posts: 4
Location: UK

PostPosted: Tue Jan 21, 2020 10:07 am    Post subject:  [Received help] Trying to make a the 555 VCO
Subject description: Having issues, tried to remove sections
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http://electro-music.com/wiki/pmwiki.php?n=Schematics.VCO555ByThomasHenry
I am trying to make this with little success. I know I can easily remove things like EXP FM as that is just an input into a summing op amp, but I'm not sure about other parts. I feel like I may have removed a crucial part of the design.

The linear to exponential part seems to be working, I hooked up a simple current to voltage converter using a standard op amp and that was exponential. However, I haven't had any output waves. I thought I had ramp working but the signal I'm picking up on my oscilliscope doesn't vary its frequency when I change the 1V/oct. I have built it on breadboard. I had all the decoupling capacitors in but I removed the electrolytic ones as I was concerned about them as they were warm.
The aim is to get something working, it doesn't have to track perfectly or make the most beautiful waves at the moment, I just need basic functionality. So I am removing parts, as there is rather a lot for me to troubleshoot all at once.
So far I have removed:
- EXP FM
- HF trim
- Hard sync
- Entire pulse section


Can I remove more, or have I removed something that will stop it working entirely. I would like to removes some of the waveforms and just have one if possible to isolate the issues.
Or are there any other 1v/oct VCO designs I should look at using instead, that can be made with easy to get parts like the design I'm trying to make.
Also I understand how the linear to exponential part works but I'm not entirely sure how the rest of it works, so explanations are appreciated.

Thanks for any help,
Thomas Very Happy

Last edited by Thomas73951 on Sat Jan 25, 2020 2:25 am; edited 1 time in total
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Blue Hell
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PostPosted: Tue Jan 21, 2020 2:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

welcome Thomas73951

The elecrolytics getting warm is worrysome ... maybe you have exchanged + and gnd or - and gnd or + and - in some places?

The parts you have not built should not cause any trouble I'd think.

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Jan
also .. could someone please turn down the thermostat a bit.
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Thomas73951



Joined: Jan 21, 2020
Posts: 4
Location: UK

PostPosted: Tue Jan 21, 2020 3:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks I only started my DIY synth journey a few months ago. I removed the capacitors when I noticed it as I'd rather not have them blow up. I'll have a look at how noisy the rails are and if it's bad I'll try adding the capacitors again.
Are there any more sections I could remove. Could I have just one waveform for now?
And are there any test points I can use to see what parts are and aren't working?
I've heard the output of the LM13700 (pin 5 if I remember right) can be used and shifts from negative saturation when there is control voltage in, which I need to try.
Also how well does this circuit work on +-9v instead of +-15v (which I'm currently using)? Because running it on 9v batteries would be nice.

Thomas Very Happy
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Blue Hell
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PostPosted: Tue Jan 21, 2020 5:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

What output wave form do you want?

As .. when you do not need sine and triangle things could be simplified to a basic saw core thingie.

Like the VCO core here : https://sowa.synth.net/modular/quadvco.html : although it has a bit of a strange expo converter.

or this one :: http://musicfromouterspace.com/analogsynth_new/VCO20090724REV0/VCO20090724REV0.html

What will you be using it for?

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Jan
also .. could someone please turn down the thermostat a bit.
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Thomas73951



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PostPosted: Wed Jan 22, 2020 3:35 am    Post subject: Reply with quote  Mark this post and the followings unread

I don't particularly mind what output wave form I get at the moment as long as there is one. I'm at a bit of a roadblock at the moment because I can't test some of the other parts of the synth that I've made as I don't have a wave to test it with, like a Look Mum No Computer low pass filter.

The aim of this project is to make a somewhat simple analogue synth that uses control voltage. I have a 12 key (1 octave) layout of buttons, and octave up and down buttons for the control voltage. This is tuned and put through an envelope generator (just attack and release). It's then converted to -18mv/oct for the linear to exponential converter and then the VCO. After that is a low pass filter, pre amp (if it needs it), power amp and then one speaker. I think that's all in the right order.

I'm going to test the Thomas Henry 555 VCO without the triangle and sine parts, tomorrow. To see if that now works. I'll try swapping the ICs out too to see if any of them are broken.

If that fails, I'll either make the MFOS VCO. However, I don't have any OP275's, SSM2210's or a PN4391 so I would have to replace them with TL082's, 2N3904's and probably a J111 respectively.
http://electro-music.com/forum/topic-35244.html
The only alternative listed here that hasn't been discontinued at RS components is the J111.

Or I will make the VCO core but I will make a different linear to exponential converter and just feed that into the second half of the design. Also I would have to replace the BF245 with something else as.

Thanks for your advice so far Very Happy
Thomas
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Blue Hell
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PostPosted: Wed Jan 22, 2020 5:12 am    Post subject: Reply with quote  Mark this post and the followings unread

So basically .. for now .. just for testing stuff .. then .. why not just use that 555 to make a simple square wave oscillator tunable with a potentiometer?

( something like :: https://www.electronics-tutorials.ws/waveforms/555_oscillator.html )

Assuming you have a scope and a digital multimeter you could check envelopes, VCA's and filters with that.

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Jan
also .. could someone please turn down the thermostat a bit.
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Thomas73951



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PostPosted: Thu Jan 23, 2020 2:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

I removed the triangle and sine parts of the Thomas Henry 555 VCO and for some reason it now works Very Happy . Not sure why but I'm not complaining. After quite a bit of tweaking the inputs from the keys and the octave up and down is all working along with the envelope generator. The filter might be working but I need to test it, and I think I need a VCA but I found a simple one that just uses one normal op amp, a LM13700 and some resistors.

The ramp wave I got from the VCO looks surprisingly 'ideal', if that's the best way of describing it. I'm going to try the other wave forms from the VCO and the filter tomorrow. I feel I may be near completion, but then again there is always something else that could be added to make it a better synth.

Thanks for your help Very Happy
Thomas
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Blue Hell
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PostPosted: Thu Jan 23, 2020 5:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

Cool

And yes . . there is always this one little extra thingie to make it nicer Laughing

Have fun!

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Jan
also .. could someone please turn down the thermostat a bit.
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