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 Forum index » DIY Hardware and Software
Help! vc resistor
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SuperKoopa



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PostPosted: Thu Jul 20, 2006 6:13 am    Post subject: Help! vc resistor Reply with quote  Mark this post and the followings unread

I want to built a VC delay (visit:http://www.tonepad.com/project.asp?id=27)
simply with a VC resistor connected in parallel with the time pot's pins.
I can do it with a LED/LDR cauple or with a Jfet.
the problem is that each method accepts a Voltage from 0 to +V and i want to drive one of this with a -V/+V range.
How i can do it?
please help me!
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zipzap



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PostPosted: Thu Jul 20, 2006 1:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

Easy way could be like this:
Say your vactrol wants 0-10v. You can use a simple mixer to add an offset of 5v. So with no cv applied your vc resistor get´s 5v. If you put in 5v cv the circuit will get 10v out 0f it. If the cv gets to -5v the vactrol or fet will see 0v.
In case you need other ranges you have to adjust scaling and gain of the amps.


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SuperKoopa



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PostPosted: Thu Jul 20, 2006 2:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

yeah I've thought to this too but i wish something automatic Very Happy

Your schematic is very useful for adsr modulation which in the most of cases starts always with the same voltage (Sound lab starts with -V) .
but with LFO-type modulation the signal amplitude will change with the amplitude pot.

thanks for interesting!! but I don't want any manual regulation for this!
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zipzap



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PostPosted: Fri Jul 21, 2006 1:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Sorry, i don´t get it. Manual regulation for what? Automatic? Sounds interesting
Or do you meen that the circuit should stop going further once the cv reaches the extremes?
Quote:
with LFO-type modulation the signal amplitude will change with the amplitude pot.

What signal?
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SuperKoopa



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PostPosted: Fri Jul 21, 2006 1:44 am    Post subject: Reply with quote  Mark this post and the followings unread

Sorry I have some problems with english!!

let's take the sound lab as example:

The ar gen output starts from -9 V and in the attack time goes to the "output level" (it could be max +9V) rigth?

Adding +9V to this output will start from 0V and theoretically the max level will go to 18V (with a Voltage divider we can reduce this to an half)

The lfo amplitude spans from 0V to +/-9V so the minimum level will change with the lfo output pot
So if lfo output is +/-2V I must add +2V if is +/-3V i must add +3V.......

when i say manual i mean that i must add another pot to regulate this adding to the desired level.

I want that the circuit realizes automatically wath is the minimum level and shift this to 0V Confused
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zipzap



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PostPosted: Fri Jul 21, 2006 7:41 am    Post subject: Reply with quote  Mark this post and the followings unread

I know what you meen now. I have had problems like that. I talked about it in the waveshaper post. Basically i don´t think the circuit can know what will happen. The only thing that can be done is limit the summ of all cv-levels. If you are driving a vactrol a parallel resistor will determine the minimum resistance. So if the summed cv goes too low from a certain point on it will have no effect. In my case i used a diode going toward a voltage divider to limit the rising voltage. as soon as that goes above a certain level nothing will happen anymore. so than you have to use your ears to find the center of the world where the lfo can swing freely.
It´s one of those inconveniences with analog circuitry.
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Scott Stites
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PostPosted: Fri Jul 21, 2006 9:27 am    Post subject: Reply with quote  Mark this post and the followings unread

Actually, IMO, I think ZipZap's solution will work just fine. Just get rid of P5 - it's not needed. Make the second op amp stage fixed unity gain.

Now, consider what the pot you want to parallel with the Vactrol in the delay circuit is doing - varying the delay right? What do you think P4 is going to do? Right - you don't need the original delay pot anymore - replace it with a fixed resistor.

More importantly, with anything that you want to control with unipolar or bipolar CV's, you want to be able to set an intial value and modulate around that. In other words, you don't want to set the delay at max delay, then modulate that with a bipolar signal - it'll move up for the positive half of the CV, but the other half won't do anything - you're chopping it off - it's already at max delay, so it's not going to go anywhere.

So, P4 not only sets the delay, but it sets the inital point of where the modulation starts when you want to modulate the delay. If you're modulation is positive unipolar, then setting the delay at max is just fine - you're only moving up. If you want delay induced vibrato using a bipolar CV, set the initial delay to some mid point and modulate positive and negative from there. That's where the other input pots would come in handy - P4 sets the delay and, at the same time, the initial point of modulation, and the the CV input pot determines how much modulation you want.

Well, something to think about, anyway.

Take care,
Scott
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SuperKoopa



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PostPosted: Fri Jul 21, 2006 3:13 pm    Post subject: Reply with quote  Mark this post and the followings unread

I have no choice! Laughing

The schematics proposed by zipzap require opamps and stripboard free pieces) and there's no more space inside SL box.

do you think that this could work?


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dnny



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PostPosted: Sat Jul 22, 2006 2:52 am    Post subject: Reply with quote  Mark this post and the followings unread

SuperKoopa wrote:

The schematics proposed by zipzap require opamps and stripboard free pieces) and there's no more space inside SL box.


you could "space wire" those op amps.

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Scott Stites
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PostPosted: Sun Jul 23, 2006 12:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Hey SuperKoopa,

Check out Grant Richters treatment in his #3 Wogglebug design here:

http://www.musicsynthesizer.com/WoggleBug/WOGGLSCH.GIF

This is prety much the circuit I use to control Vactrols linearly. R9, R10, R20, R24 and Q1. R20 pretty well determines how low in resistance you want your Vactrol to go (this will determine minimum delay in your application). You can make it smaller, perhaps stick with the 1K of your design. Don't go much lower, or you may end up shortening the life of the Vactrol LED. Wire up a pot like R9 to feed the CV to R24 and Bob's yer Uncle.

Cheers,
Scott
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SuperKoopa



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PostPosted: Thu Jul 27, 2006 2:54 am    Post subject: Reply with quote  Mark this post and the followings unread

hey just curiosity but what does it do?


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zipzap



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PostPosted: Fri Jul 28, 2006 5:53 am    Post subject: Reply with quote  Mark this post and the followings unread

it controlls the brightness of the leds.
With cv or the pot.
In a way that somehow gives a led and a vactrol a more linear usable feel, rather than driving it directly. Don´t ask me why.

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Blue Hell
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PostPosted: Fri Jul 28, 2006 6:35 am    Post subject: Reply with quote  Mark this post and the followings unread

zipzap wrote:
Don´t ask me why.


It is because the LED's current is controlled now and not it's voltage. The light output relates in a more or less linear way with the current, not with the voltage (which is more or less fixed on a value depending on hte LED's color - higher for blue, lower for red, approx. 2.2 V for green I think).

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zipzap



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PostPosted: Fri Jul 28, 2006 8:54 am    Post subject: Reply with quote  Mark this post and the followings unread

ok, but it´s two transistors here. something like a log. current source?
With one transistor the brightness doesn´t seem to follow the cv as well as with the two. Or is that just my imagination?

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SuperKoopa



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PostPosted: Fri Jul 28, 2006 10:54 am    Post subject: Reply with quote  Mark this post and the followings unread

Sad

uh... mmm...

Sad

so this don't solve my problem
sincerely my hope was that this circuit could shift the CV level!
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zipzap



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PostPosted: Sat Jul 29, 2006 4:33 am    Post subject: Reply with quote  Mark this post and the followings unread

it can. see the resistors 5 and 6. they are both 47k. you can put another one to the base of the transistor and add whatever offset you want. Or place a 1meg trimmer from the base to +12v and give it some offset.
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