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 Forum index » DIY Hardware and Software » Thomas Henry designs
An Analog Synthesizer for the 21st Century
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chieffrancis



Joined: Feb 07, 2013
Posts: 88
Location: california, united states

PostPosted: Fri May 24, 2013 9:01 am    Post subject: Reply with quote  Mark this post and the followings unread

i turned in a non-functioning project so i'm awaiting my grade. depending on my grade, i'll either go back to finishing it or, if i pass, i'll probably put it off for a few months so that i can focus on producing music Very Happy

thanks for everyone's help. really valuable information going around here
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State Machine
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Janitor


Joined: Apr 17, 2006
Posts: 2757
Location: New York
Audio files: 24

PostPosted: Fri May 24, 2013 10:51 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Francis

OK, I was wondering if you completed most of what you needed to. I am sorry your project was not finished in time and do hope that during this adventure you learned many things. Good luck for a passing grade and hope that your professor sees the hard work put into your project thus far.

We are here if you decide to continue your project I know I still have to clarify some questions you had about the IKC interconnection. Wink have fun making music !!!

Bill
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chieffrancis



Joined: Feb 07, 2013
Posts: 88
Location: california, united states

PostPosted: Thu Mar 20, 2014 5:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

hey guys, i noticed this thread hasn't seen any love in a while.

just wanted to say thanks again to everyone about the help. i learned a lot about synthesizers and about signal flow in audio applications.

right now, i'm working with a small start-up company working with digital signage and video file transcoding and stuff. hopefully i'll get to pitch my intended projects soon of a loudspeaker (crossover-transducer network) and eventually, a modular synthesizer!

right now, my synth is on the backburner because i think i either messed up on the power supply side, or somewhere soldering the oscillator module because i'm getting weird negative voltages on some of the nodes that shouldn't be having negative voltages.

since i'm in the middle of moving also, i'll have to put my projects on the sidelines for now.

i'm not moving away from the synth world though, i've recently delved deeper into audio production (having way loads of fun with subtractive synthesizers like massive and a little tinkering in fm synthesis like FM8). soft synth for now until i get back to developing the hard synth again.

i'm also interested in simplifying the analog for the 21st century designs with microcontrollers, but that would also be on the backburner. if anyone has any ideas on this development or any thoughts on the other topics, definitely shoot me an email!

email: i.chieffrancis@gmail.com
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fonik



Joined: Jun 07, 2006
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PostPosted: Tue Oct 20, 2015 4:42 am    Post subject: Reply with quote  Mark this post and the followings unread

i am almost ready to publish PCBs and panels for a DIY eurorack module from the VCF/VCA.
while testing the prototype i had hard times to get the filter into self oscillation.
the schematic shows the resonance potentiometer wired as retrostat. however, this was not sufficient for my build. i had to wire the pot as voltage divider to GND. this way the potentiometer would cut the feedback path completely, forcing the filter to oscillate.

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cheers,
matthias
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Last edited by fonik on Thu Sep 15, 2016 3:40 am; edited 1 time in total
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elmegil



Joined: Mar 20, 2012
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PostPosted: Tue Oct 20, 2015 5:50 am    Post subject: Reply with quote  Mark this post and the followings unread

That is great information, fonik!

I have never been able to get this filter to oscillate -- I built on protoboard, and I even went through a bunch of gyrations working with dr sketch & etch in PM trying to make mods that would do it, and got nowhere.

I will have to see if I can make this mod and be able to get there....

Thanks!
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elmegil



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PostPosted: Tue Oct 20, 2015 6:00 am    Post subject: Reply with quote  Mark this post and the followings unread

So like this?


analog-synth-alternate-vcf-resonance.jpg
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 Filesize:  37.34 KB
 Viewed:  2460 Time(s)

analog-synth-alternate-vcf-resonance.jpg


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fonik



Joined: Jun 07, 2006
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PostPosted: Tue Oct 20, 2015 9:46 am    Post subject: Reply with quote  Mark this post and the followings unread

exactly.

just be shure it is wired backwards, so that CW connects the wiper to GND, something i did wrong in the past several times. Embarassed

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cheers,
matthias
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fonik



Joined: Jun 07, 2006
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PostPosted: Thu Oct 22, 2015 2:42 am    Post subject: Reply with quote  Mark this post and the followings unread

here is a raw video of the prototype


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



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PostPosted: Thu Oct 22, 2015 5:50 am    Post subject: Reply with quote  Mark this post and the followings unread

Are you going to have enough pieces to build a full "Analog Synth for the 21st Century" voice? Smile
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fonik



Joined: Jun 07, 2006
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PostPosted: Thu Oct 22, 2015 11:05 am    Post subject: Reply with quote  Mark this post and the followings unread

no Sad
buuuut, as you can see in the vid, i have enough other TH pieces...

and more to come. i already started working on a tuned digital noise module.

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cheers,
matthias
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elmegil



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PostPosted: Thu Oct 22, 2015 1:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

I was just looking at stripboarding that recently, you should throw the S&H in for good measure, it's basically 1 chip, a good cap, and a FET Smile
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mrand



Joined: Mar 30, 2014
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Location: Yukon and London Ontario

PostPosted: Fri Jul 29, 2016 12:39 am    Post subject: error in sample hold? Reply with quote  Mark this post and the followings unread

Does anyone know if the transistor gate in the sample hold circuit is supposed to go to -15v, rather than +15 as marked in the schematic? Can't get the circuit working as is. Could be that my JFET is broken too...
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elmegil



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PostPosted: Fri Jul 29, 2016 6:22 am    Post subject: Reply with quote  Mark this post and the followings unread

I haven't built the circuit, but yes, that should be correct.

Normal state with no clock input on the clock comparator drives the output of the op amp to -15V (well, not rail-to-rail, so probably more like -13V). This will hold the FET cut off. Then when a clock pulse comes in, depending on the time constant of the cap and the resistor to ground, the clock is converted into a brief pulse, which opens up the FET and lets the input signal charge C2 to whatever its new level might be. Since the pulse is brief, the FET is then cut off again, holding the new value on C2.

You don't describe the behavior of the circuit to know what "not working" looks like....

is it always passing a signal? Is it never sampling a signal?

Look at the signal on the gate of the FET and see if it's doing what it should when the clocks come in.

Also, FETs more than others seem to be subject to not always matching the pin out on the standard data sheet -- you may want to put the FET on a transistor tester and verify that it's really oriented the way you think it should be.
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mrand



Joined: Mar 30, 2014
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Location: Yukon and London Ontario

PostPosted: Thu Aug 04, 2016 2:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

Back from a camping trip and tried your recommendation to check for discrepancy between FET datasheet and actual part. Indeed this wasthe case and now my S&H works as expected. Thanks elmegil!
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elmegil



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PostPosted: Thu Aug 04, 2016 2:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

You're quite welcome

Glad you got it going! Smile
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mrand



Joined: Mar 30, 2014
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Location: Yukon and London Ontario

PostPosted: Wed Aug 17, 2016 9:25 pm    Post subject: 2164 svf Reply with quote  Mark this post and the followings unread

Working through some 21st century designs, and hit a block with my breadboarded 2164 vfc. Things are working well only for CV input ranging from about 5.6v-6.1v. At 6.1v it sounds all the way open, which seems correct. Below 5.6v, however, is "snaps" off with a bit of a click. Also perhaps noteworthy is that CVinput voltages over 6.4v introduce some kind of spurious HF oscillation.

My cap situation is not the best. I'm using 1000pf caps rather than the specified 100pf. I've tried smaller values (500pF and 220pF), but the limited range of filtering is most audible when using 1000pF. (I don't have 100pF except in ceramic). Also, I'm using 500pF rather than the 560pf specified near the VCA inputs. Changing values for any of these caps does not seem to affect the "snap off" point, though, so I kind of ruled them out as being the source of my problem. That said, I don't really understand how the circuit works to begin with.

Any guidance or suggestions would be welcome!

Thanks,
Colin
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elmegil



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PostPosted: Wed Aug 17, 2016 10:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

560pf -> 500pF is only a 10% difference.

1000pF vs 100pF is an order of magnitude. I would imagine that would be a major problem in the functioning of the circuit, especially since those are the integrator caps that are key to the filter's function.

You'd be better off with smaller caps in parallel (to add up to 100pF) or larger caps in series (they combine like parallel resistors, two of the same value in series result in a capacitance half of the original size)
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mrand



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PostPosted: Wed Aug 17, 2016 11:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks again, Elmegil! Would you expect the circuit to function using 100pF ceramic caps? I will also test with some daisychained green pill type caps for 100pF value.
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elmegil



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PostPosted: Thu Aug 18, 2016 7:28 am    Post subject: Reply with quote  Mark this post and the followings unread

It ought to function with ceramic; if you have C0G it should be just fine, if it's old style disk, it will likely drift. The greenies ought to be just fine.
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mrand



Joined: Mar 30, 2014
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Location: Yukon and London Ontario

PostPosted: Thu Aug 18, 2016 1:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

Tested with various permutations of 100pF, (daisy chained 220pF poly caps, and ceramic disk 100nF) and still getting same results. Actually, it's worse with 100pF: it just snaps between fully open and "off".

Using much bigger values, even up to 3300pF, it sounds like a filter in the narrow range described earlier, but still snaps off at around 5.5v.

I'd re-breadboard it today, except it's a cold rainy day up here in the Yukon and my workshop is outdoors/open air! Switching the caps in was the most I could handle Sad)

Hopefully it warms up tomorrow.

C
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elmegil



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PostPosted: Thu Aug 18, 2016 2:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

So stepping back a moment:

What are you doing with the other half of the 2164? Have you breadboarded the full VCF/VCA?

If you haven't done the whole thing, what are you doing about the extra 1/4 of a TL074 used in the filter circuit?

If you used 2 TL074's have you grounded the other op amps you're not using?

I'm assuming you have the resonance hooked up per the original, not the variation Fonik and I were discussing above?

Do you have the 10pF C1 in place (that's there to tame oscillation) what about the 22pF C3 (same)?

What voltages do you get on the output of IC3a when you're swinging the input voltages?

My main thought is that you have a short or a bad resistor value in the control section (lower part of the schematic).....

I still haven't made that resonance change on mine, BTW, I could pull it out and measure the voltage swing at the 2164 for you to see what's normal, on my way to changing that.
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mrand



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PostPosted: Thu Aug 18, 2016 4:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks, elmegil, I dont want you having to go take measurements Smile
I'm using a fixed resistor of 1M in the resonance path, in place of the pot and its partner/series resistor.

I think i included those oscilation taming caps, but will double check.

Do unused VCAs in the 2164 need to be terminated somehow? For my unused VCAs (3 and 4) I added the 500k/500pf to ground as per specifiednon data sheet, but left the other pins unconnected.

I'm hoping to use VCA3 for VCresonance and VCA4 as an expo vca, which is why They remain unused.

Dang, unused opamps need to be terminated to ground? Ive left one hanging in my tl074...

Rain has abated and a warm air moved in, so I'm gonma go ground those pins!
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elmegil



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PostPosted: Thu Aug 18, 2016 8:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

mrand wrote:
Thanks, elmegil, I dont want you having to go take measurements Smile
I'm using a fixed resistor of 1M in the resonance path, in place of the pot and its partner/series resistor.


It really would be no problem, like I say I have other work I want to do on it, so it'll be on my bench anyway.


mrand wrote:

Do unused VCAs in the 2164 need to be terminated somehow? For my unused VCAs (3 and 4) I added the 500k/500pf to ground as per specifiednon data sheet, but left the other pins unconnected.


If that's what the Data Sheet calls for, then that should be sufficient.


mrand wrote:

I'm hoping to use VCA3 for VCresonance and VCA4 as an expo vca, which is why They remain unused.


That sounds like an excellent enhancement. Smile

mrand wrote:

Dang, unused opamps need to be terminated to ground? Ive left one hanging in my tl074...


The standard method is to ground the + input and tie the - input and output together, basically making it a voltage follower on a ground signal.
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mrand



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PostPosted: Thu Aug 18, 2016 8:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well none of those changes fixed it, so I started over.. but now i got nuthin! Sigh. Maybe can try again tomorrow. One thing I noticed on my second attempt was the gnd pin of the 2164... I can't recall if I saw that first time around.

I'm halfway through a KICAD layout for my proposed circuit. Once I get it working on bb, I will finish that up. Would be happy to share gerbers if anyone wants.
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elmegil



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PostPosted: Thu Aug 18, 2016 10:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

2164's don't like going without V- ....

http://www.milton.arachsys.com/nj71/forums.html?menu=2&submenu=2&subsubmenu=3

Edited: I originally was thinking it was the ground that was the problem, but no, it's the V- pin. Even so, something to be aware of when you're working with 2164's
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