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 Forum index » DIY Hardware and Software » Thomas Henry designs
VCA for the 21st Century
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Thomas Henry



Joined: Mar 25, 2007
Posts: 298
Location: Southern Minnesota
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PostPosted: Tue Mar 18, 2008 9:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

Scott Stites wrote:

One reason would be that if your control voltage, say an ADSR EG, for whatever reason, has a positive offset on it, the only way you could close the VCA while the CV is connected would be to mix a bit of negative voltage with it.


Exactly! And that happens all the time in various other patches.

Which is one more reason to commend the forward-thinking Bernie Hutchins for having figured all this out some thirty years ago. Ever since 1980, I've followed his input/output/impedance/voltage standards and have never rued the day.

Thomas Henry
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fonik



Joined: Jun 07, 2006
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PostPosted: Wed Mar 19, 2008 12:16 am    Post subject: Reply with quote  Mark this post and the followings unread

Coriolis wrote:
fonik: did you have success with this layout?

C

i have not finished it yet. most of the components are placed, but all the DIY stuff is in packing cases these days...

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Coriolis



Joined: Apr 11, 2005
Posts: 616
Location: Stilling, Denmark

PostPosted: Mon Mar 24, 2008 2:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

Oh yes, you are moving, right? Sorry to bug you. Embarassed
Anybody else?
Phil? Did you get to finish this?

C
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philpeery



Joined: Nov 08, 2006
Posts: 125
Location: new jersey, usa

PostPosted: Tue Mar 25, 2008 8:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

I've got a few boards made, but have not even had the time to drill them out yet. Hopefully soon! I am also sending a couple to my pal loss1234 most likely by friday. He's been pretty hot with DIY lately, maybe he will beat us all to the punch and get one of these vca's built! One of us will be sure to post our results.

Hang in there loss! The pcb's are coming!

Phil
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Coriolis



Joined: Apr 11, 2005
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Location: Stilling, Denmark

PostPosted: Wed Mar 26, 2008 12:51 am    Post subject: Reply with quote  Mark this post and the followings unread

Aww heck - I should stop bothering other people and take part in the testing of this layout myself! Laughing

Think I will do one of these in the very near future, then we can compare results, yes?

C
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Sound



Joined: Jun 06, 2006
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PostPosted: Sat Apr 05, 2008 9:41 am    Post subject: Reply with quote  Mark this post and the followings unread

Hello,

Looking at the schematics of the XXI Century VCA, I see that the line output; in addition of the attenuator, have a 15K resistor (impedance?) instead a 1K resistor of the synth output.

Since that I have an attenuator on my Line mixer input it means that I can connect the synth output to the line input of the mixer...

But theoretically... It is better the 15K output? Why? Because the impedances?

It's Ok 15K or may will be better put the value resistor at the same impedance of the Input line mixer?

Since I have an attenuator on my line mixer input, Do I need the output attenuator In my VCA module?

In my modular Synthesizer project I would like to have 8 voices directly connected to my line mixer. I understand the mixer like a heart of my system, 8 voices from the modular synthesizer more two chanels from two microphones- flute-voices and two chanels more for other synths. All perfectly integrated like a more big modular concept.

It's very important to me the clean of the sound and its quality. It is because I'm interested in this kind of details.

So I would like to have 8 XXI Century VCA like a final step of my modular Synthesizer connected permanently to the mixer.



Any answer to my questions? Any advice? Please Very Happy a little of light over this matter.
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Coriolis



Joined: Apr 11, 2005
Posts: 616
Location: Stilling, Denmark

PostPosted: Sat Apr 05, 2008 10:51 am    Post subject: Reply with quote  Mark this post and the followings unread

If you didn't have the pot and the 15k resistor, you would have 10V PP signals going straight into your line mixer - much too hot a signal level for that. Then you would find it difficult to adjust the volume of the synth with the attenuator on your mixer - just turning it a tiny, tiny bit, would result in too much level for the mixer input.
So to make it easier to adjust levels, the 15k resistor pads it down, and the pot makes that pad adjustable.


Hope that helps (and hope it isn't wrong)

C
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Sound



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PostPosted: Sun Apr 06, 2008 6:45 am    Post subject: Reply with quote  Mark this post and the followings unread

Coriolis wrote:
If you didn't have the pot and the 15k resistor, you would have 10V PP signals going straight into your line mixer - much too hot a signal level for that. Then you would find it difficult to adjust the volume of the synth with the attenuator on your mixer - just turning it a tiny, tiny bit, would result in too much level for the mixer input.
So to make it easier to adjust levels, the 15k resistor pads it down, and the pot makes that pad adjustable.


Hope that helps (and hope it isn't wrong)

C


Hi Cariolis, it is easy. But is that the only difference within line or synth level? I think (but i don't understand exactly) that it have relation with the impedance. If I understood the emitter, the line, and the receiver must to have the same impedance?... maybe it is the reason of the 15K resistor?


http://en.wikipedia.org/wiki/Characteristic_impedance

http://en.wikipedia.org/wiki/Transmission_line
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fonik



Joined: Jun 07, 2006
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PostPosted: Mon May 19, 2008 2:29 am    Post subject: Reply with quote  Mark this post and the followings unread

just wanted to let you know (expecially you, coriolis) that i finished the dual VCA last night and it works like charm - as expected Very Happy
i built it from my layout using my documentation so everything seems to be okay.
i will post some pics as soon as the front panel arrived and i mounted the whole module.

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Coriolis



Joined: Apr 11, 2005
Posts: 616
Location: Stilling, Denmark

PostPosted: Mon May 19, 2008 5:01 am    Post subject: Reply with quote  Mark this post and the followings unread

That's good news (as expected Cool ). Puts it on top of my to-do list.

Thanks!

C
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numbernone



Joined: Aug 16, 2006
Posts: 477
Location: new york city

PostPosted: Sun Aug 10, 2008 7:35 am    Post subject: Reply with quote  Mark this post and the followings unread

Murphy's law in action!

This VCA was fine on the breadboard, but now that I have finally gotten my boards made, and populated the first quadrant of it with the LAST of my applicable parts...NADA!!! No signal is passed, no nothing, what a drag!!

Anybody have a nice build of this running that could give me some voltages etc. to look for?? I have checked the layout vs. the schematic 10x+, it is spot on. Very confusing and maddening, I spent far too much of a beautiful Saturday inside trying to figure this one out.

Should have some more parts by the end of the week to re-breadboard it to test, but if any kind soul can lend a hand I would be most appreciative.

EDITED!! Man am I stupid, murphy's law indeed, sometimes we miss the most obvious things, and I mean THE MOST OBVIOUS!!!

Hats off to Thomas for this great simple VCA.


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Coriolis



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PostPosted: Sun Aug 10, 2008 10:37 am    Post subject: Reply with quote  Mark this post and the followings unread

Aww, c'mon - tell us what you did! Very Happy
Actually I recently finished one of these (fonik's dual vca layout), and it worked right off the bat! That's rare for me...

C

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numbernone



Joined: Aug 16, 2006
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Location: new york city

PostPosted: Mon Aug 11, 2008 5:52 am    Post subject: Reply with quote  Mark this post and the followings unread

Lets just say it was a missed connection...

A rather IMPORTANT one...

Very stoked with this one, this is going to provide me with not only regular old VCAs, but big banks of voltage controlled mixers, and eventually quad panning joystick modules. I imagine I will think of some others as well.
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LetterBeacon



Joined: Mar 18, 2008
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PostPosted: Mon Jun 22, 2009 1:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'm planning on building this dual VCA using Fonik's layout and I want to add the line out jack as well. If I drill holes in the PCB at the points marked in red below, would they be good places for me solder in one leg of R13 (the other leg leading up to the attenuator)?


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macumbista



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PostPosted: Mon Nov 09, 2009 5:18 am    Post subject: Reply with quote  Mark this post and the followings unread

numbernone wrote:
Very stoked with this one, this is going to provide me with not only regular old VCAs, but big banks of voltage controlled mixers, and eventually quad panning joystick modules.


Just found this as well, looks very interesting! TH's designs haven't failed me yet. My dream is a 4x4 joystick controlled matrix mixer for CV. Doing the math, that means 16 VCAs!!!! Which leads to my question:

How big was your board, numbernone? I'm using the 100x160 Bungard boards for etching, but I can only fit two of Fonik's layouts on one (and in a very inefficient, space-wasting way). Turns into a lot of boards. I have enough space on the height and width, as I lay my boards parallel to the faceplates which are 4U high. But I don't have the depth in the case for 4 PCBs (+ extra for DC mixers and room for joysticks...)!

I'm still pretty bad with Eagle/KiCad, but perhaps this case would be a candidate for SMD-ization? I'm imagining a 100x160 (or a bit smaller actually) PCB with 8 linear VCAs (i.e. 4 x 2164 ICs) on it. Then I'd only need two PCBs and my matrix would be complete!

Would anyone be interested in helping me with this project? I might be willing to invest in some production if so... reply here or by PM.

best!
Derek

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julian



Joined: Jan 11, 2008
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PostPosted: Wed Dec 23, 2009 9:50 am    Post subject: Reply with quote  Mark this post and the followings unread

...just recieved the book through the mail : )

Am looking at the schematics, and have a quick query, partly, i guess, in relation to Matthias' layout...

There is provision for Input attenuation (R36) but also Gain (Initial) using (R34).

I havent built the module, but wouldnt attenuating the input, and applying a -ve voltage (using R34) have similar effects?

Im tempted to sacrifice the input attenuation in place of CV attenuation, and still keep the panel components to five.

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Tim Servo



Joined: Jul 16, 2006
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PostPosted: Wed Dec 23, 2009 1:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

julian wrote:
...just recieved the book through the mail : )

Am looking at the schematics, and have a quick query, partly, i guess, in relation to Matthias' layout...

There is provision for Input attenuation (R36) but also Gain (Initial) using (R34).

I havent built the module, but wouldnt attenuating the input, and applying a -ve voltage (using R34) have similar effects?

Im tempted to sacrifice the input attenuation in place of CV attenuation, and still keep the panel components to five.


Hi Julian,

Actually, the two functions (R34 and R36) are very different animals. The "Initial Gain" setting is a lot like the Coarse frequency control on a VCO; it sets the starting point, and then any CV modulation will add or subtract from that point. With the Initial Gain control, you can do things like open the VCA for a drone (so that sound comes through all the time), or open the VCA a little bit (so that sound still comes through all the time, but it gets louder when the CV from an envelope is applied), or you could even bias the VCA so that the bipolar signal from an LFO gives you tremolo (normally the VCA would only respond to the positive side of the LFO signal, but you can offset the CV with the Initial Gain control so that the tremolo sounds correct).

So again, think of the Initial Gain pot like the Coarse Tune pot on a VCO: It sets the initial frequency of the VCO, and then the signal from an LFO would modulate the frequency up and down from that initial point. The Input Attenuator on the other hand, can only change the level of the audio inputs, and while this is handy (especially when mixing several inputs), it doesn't allow you to change or offset the CV going to the VCA. If anything, R36 and the CV input attenuator (R35) are much more similar, although not exactly the same. Hopefully this helps Smile

Tim (hopefully helpful) Servo
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julian



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PostPosted: Wed Dec 23, 2009 3:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi Tim,

Thanks for your reply, although it wasnt quite what i was getting at... It has caused me to think further though, and have now concluded -

What i was getting at though was that adding a -ve offset to the cv input, and attenuating the audio input would have similar (but not identical) effect.

Take, for example, a white noise input on the audio input, and the vca being controlled by a sine wave -

Adding a -ve offset would, effectlivly, make the output quieter, however, the downside (as opposed to attenuating the audio input) is that the lower portion of the sine modulation would be clipped. (this is the part i had overlooked)

The situation on an envlope would obviously almost be like a delay on the start of the envelope.

...however, if im wanting to kind of adhere to eurorack spacing, as per Matthias' design theres a compromise.

I think i will maybee use the panel space to attenuate the CV input, and use the offset (initial) control to give some approximation of audio attenuation if i need to. In more critical situations, ill have to attenuate it somewhere else.

I guess, in conclusion, when properly thinking about it, id rather have the attenuation on the CV input than the audio input, if i can only have one or the other! : )

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fonik



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PostPosted: Mon Mar 10, 2014 6:54 am    Post subject: Re: VCA for the 21st Century Reply with quote  Mark this post and the followings unread

frijitz wrote:
Tim Servo wrote:
Linear. The VCA uses the four sections of a 2164 Quad VCA chip (each section with expo control) to create two linear response VCAs. The 2164 chip is very quiet and has a VERY wide dynamic range.

OK, so it's the old Mike Irwin trick?

I always thought it would be fun to try antilogging the input CV to get a linear response, rather than dedicating a VCA stage to be a servo.

Very Happy

Ian


so anyone tried this actually? i mean a simple diode or transistor based antilog (expo) amp for the CV?

if not, i will ASAP. i need quite a few VCAs on little room and wanted to use the 2164, however it would be a shame to use 2 VCa cells for only one lin VCA in the end...

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macumbista



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PostPosted: Mon Mar 10, 2014 10:27 am    Post subject: Re: VCA for the 21st Century Reply with quote  Mark this post and the followings unread

fonik wrote:


so anyone tried this actually? i mean a simple diode or transistor based antilog (expo) amp for the CV?

if not, i will ASAP. i need quite a few VCAs on little room and wanted to use the 2164, however it would be a shame to use 2 VCa cells for only one lin VCA in the end...


Please report back on this, Matthias, I'd be quite curious to know since the 2164 is a fairly expensive part compared with a transistor.

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fonik



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PostPosted: Mon Mar 10, 2014 11:04 am    Post subject: Re: VCA for the 21st Century Reply with quote  Mark this post and the followings unread

macumbista wrote:
fonik wrote:


so anyone tried this actually? i mean a simple diode or transistor based antilog (expo) amp for the CV?

if not, i will ASAP. i need quite a few VCAs on little room and wanted to use the 2164, however it would be a shame to use 2 VCa cells for only one lin VCA in the end...


Please report back on this, Matthias, I'd be quite curious to know since the 2164 is a fairly expensive part compared with a transistor.

and the 'irwin'-method would require one more opamp as well. not expensive pricewise, but in regards to PCB real estate...

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