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 Forum index » DIY Hardware and Software » Thomas Henry designs
VCO 1 questions
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Cfish



Joined: Feb 24, 2016
Posts: 451
Location: Indiana

PostPosted: Wed Feb 24, 2016 9:56 am    Post subject: VCO 1 questions Reply with quote  Mark this post and the followings unread

I am new here. I have built the VCO1 and am having a little trouble figuring some things out. I will give you a little background to help understand my situation. Most of my electronics experience is in the guitar stomp box field. I have Ben experimenting with Sound generating circuits for years, but often get lost quick when things get in depth. I built my VCO 1 using TL071 opamps spread out on a 5 x 7 perfboard to give easy access with my scope to try to better understand the circuit. I am currently using a random pair of 2n3906s in the Q5 and Q6 positions. The VCO sounds awesome. Thanks Thomas Henry!

This is my first attempt at a forum of any kind

My problem is that it takes 1.8 volts to achieve one octave of tone shift. No matter where I adjust the volt per octave trim.
Is that because I do not have a matched pair at Q5 and Q6? Or do I have another problem?

Last edited by Cfish on Wed Mar 02, 2016 7:25 am; edited 2 times in total
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wackelpeter



Joined: May 05, 2013
Posts: 427
Location: germany
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PostPosted: Wed Feb 24, 2016 10:57 am    Post subject: Reply with quote  Mark this post and the followings unread

Have tried to trimm the V/oct Response already?
can it be that your V/oct trim pot is turned fully in one direction?
when you turn it fully in the opposite direction does it Show the same behaviour?
two unmatched trannies should be no Problem to get it working roughly... have used unmatched from the same batch too... on my other Thomas Henry VCO's 4x555 and 1x4046 VCO...

the resistor in the feedback path of the Input summing op amp IC2a is 2K?
the tl071 gets +15V on pin 7 ( i usually do that mistake and try to power it on pin8 Very Happy )

hmmm on my latest 4046 VCO i had to play a bit with R8, R4 and R1 (those are the resistors numbers for the VCO1 schematic) by increasing or reducing the resistance a few ohms to get it accurate tracking... but it was roughly tracking from start...

maybe a good idea would be to measure the current flowing from the collector of Q6 to the 3080's pin5... is there a Change? maybe how much with each Volt applied to the V/oct Input...

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Cfish



Joined: Feb 24, 2016
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Location: Indiana

PostPosted: Wed Feb 24, 2016 11:13 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes I have tried tuning the volt per octave response. I get very little change in response no matter where the trimmer is set. Used a 10 turn trimmer. My next step was to pull it and test it. But I was not sure if the transistor matching would have an affect that drastic on this?

What affect should I expect from a random transistor set. Am going to build the MFOS circuit for matching transistors. But there are a few to do items on my list before I get there
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Cfish



Joined: Feb 24, 2016
Posts: 451
Location: Indiana

PostPosted: Wed Feb 24, 2016 11:29 am    Post subject: Reply with quote  Mark this post and the followings unread

What type of voltage response should I expect at pin5 of the OTA with a 1 volt shift in control voltage?
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wackelpeter



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

you should measure the current flowing between those both Points...
I really have no idea how much it should be exactly with each voltage step, but somewhere in the very low mA regions...
Maybe someone with more insight (i'm a newbie myself) can tell you how much that should be in average...

But the whole circuitry that leads to pin 5 of the CA3080 works as a voltage to current converter/current source to control the CA3080....

As far as i remember there where some values in the original WASP VCF schematics... also the datasheet of the ca3080 might give some values...
i could measure that myself but i had to pull off my whole "cabinet" and that'S bad because i have a patch running i'm planning to work on a bit more...

but 2 unmatched trannies should not give such an behaviour... well they wouldn't track accurately maybe... but at least no nearly normal response... hard to imagine...

But as you mentioned that 1,8v to get an higher octave... when you have OV at the V/oct in and maybe 100Hz then ist at 1,8V roughly 200Hz and 3,6V around 400Hz? what i mean it's tracking and needs roughly 1,8V for each step to roughly double the frequency?

edit... read somewhere that Iabc the current to the biasing Input (pin5) should not exceed 2mA... otherwise it could fry the chip... maybe that's hard to measure... when this circuit is scaled to work the OTA in the uA regions

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Cfish



Joined: Feb 24, 2016
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Location: Indiana

PostPosted: Wed Feb 24, 2016 2:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

That is very helpful. I thought that was what that section of circuit did but I wasn't sure. Had read a lot on it. But a lot of what I read seems to assume I have more prior experience with this kind of circuit.
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Cfish



Joined: Feb 24, 2016
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Location: Indiana

PostPosted: Wed Feb 24, 2016 2:13 pm    Post subject: Reply with quote  Mark this post and the followings unread

Oh, and the voltage required at higher octaves to achieve one octave goes up a little. Not quite linear. But I assume that is the transistor set. Would be going flatter as it goes up.
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Cfish



Joined: Feb 24, 2016
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Location: Indiana

PostPosted: Sat Mar 05, 2016 10:37 am    Post subject: Reply with quote  Mark this post and the followings unread

Well, I have tried everything I can with the equipment I have. So I decided to build another vco1 to compare the results. This time I used a tl074 to save some space. Is on perfboard, same as the first. This one takes .8 volts per octave, and won't seem to adjust enough to hit 1 volt per octave. I have tested every resistor value and capacitor twice in my first build and tested every value while building the second.
As with the first build I used 2n936s unmatched but from the same batch. ( I haven't built my transistor matcher yet) but have tried multiple sets. Different transistors have a drastic affect on the frequency output at 0v. But seem to have little affect on the fact that i can't tune one octave. I re centered the vpo trim and the high freq trim with each try.
I have tried switching out the 3080s just in case my (direct from China ) ICs were the problem. The problem stayed the same. Build one 1.8 volts per octave. Build 2 .8 volts per octave.

I would greatly appreciate any new thoughts on this
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Cfish



Joined: Feb 24, 2016
Posts: 451
Location: Indiana

PostPosted: Sat Mar 05, 2016 10:37 am    Post subject: Reply with quote  Mark this post and the followings unread

Well, I have tried everything I can with the equipment I have. So I decided to build another vco1 to compare the results. This time I used a tl074 to save some space. Is on perfboard, same as the first. This one takes .8 volts per octave, and won't seem to adjust enough to hit 1 volt per octave. I have tested every resistor value and capacitor twice in my first build and tested every value while building the second.
As with the first build I used 2n936s unmatched but from the same batch. ( I haven't built my transistor matcher yet) but have tried multiple sets. Different transistors have a drastic affect on the frequency output at 0v. But seem to have little affect on the fact that i can't tune one octave. I re centered the vpo trim and the high freq trim with each try.
I have tried switching out the 3080s just in case my (direct from China ) ICs were the problem. The problem stayed the same. Build one 1.8 volts per octave. Build 2 .8 volts per octave.

I would greatly appreciate any new thoughts on this
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elmegil



Joined: Mar 20, 2012
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PostPosted: Sat Mar 05, 2016 7:25 pm    Post subject: Reply with quote  Mark this post and the followings unread

First and foremost, make sure the output of the CV buffer stage is doing what it should.

1V change on the input should lead to somewhere around 18mV change on the output. The point of the scaling trimmer is to dial that 18mV in exactly, as well as adjust for minor variation in the response of the transistor whose base it's driving.

If you don't have that, then nothing about the rest of the circuit will work correctly.

My favorite discussion of the topic are these two videos by Aaron Lanterman:

https://www.youtube.com/watch?v=ElUeQKrkfqc&list=PLXnFQqfHz_iFcdOTqI-6_FpOf73LWGNnt&index=7
https://www.youtube.com/watch?v=v9oaBudlzfc&list=PLXnFQqfHz_iFcdOTqI-6_FpOf73LWGNnt&index=8



Edit: I would also suggest that you verify that the HF adjust resistor is really 2.2M, perhaps you've gotten some that are out of whack/mis-labelled. Not common, but it does happen. Quite honestly, you could probably leave that part out entirely until you verify that the rest of things are mostly working right....
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Cfish



Joined: Feb 24, 2016
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PostPosted: Sun Mar 06, 2016 7:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Elmegil
I will check out the links you sent. I had tried disconnecting the HF adjust portion of the circuit at R44 to confirm that wasn't the problem. Thank you for the IC2A output voltage reference. That will help me narrow it down.

Thanks for your time.
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Cfish



Joined: Feb 24, 2016
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PostPosted: Fri Mar 11, 2016 8:35 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks to The help of you kind people with far more experience than myself.

I FINALLY FOUND THE PROBLEM!!! WOO HOO!!!

A solder joint that looked good was not actually contacting ground. Now I would have found that during one of many ohm checks of the board, had there not Ben a near microscopic trace to the adjacent inverting opamp input. So basically R4 was better attached to the inverting opamp input than to ground. And the inverting input had the equivalent of a high value resistor to ground. I'm sorta surprised it worked at all.

Thanks for the help.
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alanwilder81



Joined: Sep 03, 2016
Posts: 325
Location: italy

PostPosted: Wed Dec 28, 2016 1:59 pm    Post subject: Reply with quote  Mark this post and the followings unread

apologizes if my question has been asked already,or is too dumb.But,i'm trying to figure it out how to replace the 3080 with half LM13700.

So, as far as the LM 13700 goes. is pin 1 the amplifier bias input? (corresponding to the pin 5 on the 3080)
Is pin 5 the output ? (corresponding to the pin 6 on the 3080).

The inverting and non inverting input should be obvious, so, does the replacement take some extra circuitry or wizardry of any sort, or, is it going to work that way , simply? Smile

thanks all folks for help !


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wackelpeter



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PostPosted: Wed Dec 28, 2016 3:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

That's correct... and yes it doesn't require witchcraft or magic spells to fit the LM13700 as relacement in... Wink
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alanwilder81



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PostPosted: Thu Dec 29, 2016 2:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

hey wackelpeter Smile
some more consideration over the LM13700.
i asked whether or not the LM13700 replacement would take some modification because Thomas Henry himself, in his website "Birth of a synth", mentioned it. So, my doubts arose.
Here's the copy and paste of how he worded the substitution Smile

what does he mean by "little modification" ?? Shocked Shocked Smile


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Cfish



Joined: Feb 24, 2016
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PostPosted: Thu Dec 29, 2016 4:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

Some modification is referring to the pinout.

If you were using a pre printed kit board, the modification would be more agrivating than if you are building on perf.

I have built Thomas Henry VCOs with each and can tell no notable difference.
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alanwilder81



Joined: Sep 03, 2016
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PostPosted: Thu Dec 29, 2016 4:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

hello Chris how are you doing?
it's been a while since we've last spoken. i have been very busy and little time to devote to synth DIY.
Nice profile picture by the way. Smile
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alanwilder81



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PostPosted: Thu Dec 29, 2016 4:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

and yes, i am not sure whether use a perfboard or a PCB right away.
i'll breadboard the VCO 1 first. You always sing the praise of it, so i think it's time to give it a go.
I finally bought the LM13700 from a honest italian supplier. They cost 1.80 euros each, not bad being t a dual OTA.

I know you got a bunch of workng 3080's. However, did you have any experenxe n usng LM13700's on Thomas Henry projects?

I am planning to do VCO 1, VCA 1 ,and the icing on the cake, the astonishing 555 VCO
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Cfish



Joined: Feb 24, 2016
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Location: Indiana

PostPosted: Thu Dec 29, 2016 4:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi Alanwilder81

Yes it has been a while.

We have a PhoBos reference to thank, for giving my son the idea to creat the profile picture for me.

I really like it.

I had a couple of 3080s I offered to give away in Alanp's Christmas thread that no one asked for.

if you would like a couple to compare with let me know in PM and I can mail them to you.
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Cfish



Joined: Feb 24, 2016
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PostPosted: Thu Dec 29, 2016 4:59 pm    Post subject: Reply with quote  Mark this post and the followings unread

To alanwilder81

Yes I have built the VCO1 both ways. The 555 vco uses the OTA in the same situation so it would be the same on trading the OTAs.

My first Thomas Henry VCO1 I built using TL071s spread out on a large board. It's great to experiment with.

I have never breadboarded a Thomas Henry schematic. I trust his designs enough to just sit down with the perf and start building.


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alanwilder81



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PostPosted: Thu Dec 29, 2016 5:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

sure Chris,
we all trust Thomas Henry works , plus his designs have been done and tested by gazillion people, so, there's nothing to fear about them! We can build his project pretty confidently i guess!

yes, it was a good idea to build the VCO 1 using many separate op amps, in order to access em more easily to take measurements and stuff.
Great move.
Smile
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