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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
MFOS Dual VCA
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matrix12x



Joined: Sep 26, 2009
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Location: New York

PostPosted: Sat Oct 03, 2009 5:21 am    Post subject: MFOS Dual VCA Reply with quote  Mark this post and the followings unread

I'm new here and I just started building the MFOS Dual VCA using the Bridechamber front panel. What is the best op amp to use in this for using my VCA primarily for audio?
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Joined: May 22, 2008
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Location: Germany

PostPosted: Sun Oct 04, 2009 12:04 pm    Post subject: Re: MFOS Dual VCA Reply with quote  Mark this post and the followings unread

matrix12x wrote:
I'm new here and I just started building the MFOS Dual VCA using the Bridechamber front panel. What is the best op amp to use in this for using my VCA primarily for audio?


what do you mean by "using my VCA primarily for audio"??? isn't this almost always the case with vcas??

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matrix12x



Joined: Sep 26, 2009
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PostPosted: Sun Oct 04, 2009 3:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

I have also use my VCAs in the past for CV attenuation
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falafelbiels



Joined: Aug 13, 2009
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PostPosted: Tue Oct 13, 2009 3:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

I have used TL084 as in the schematic and it works as expected. I'd almost add another "...as expected".
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matrix12x



Joined: Sep 26, 2009
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PostPosted: Tue Oct 13, 2009 3:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks. I did not know if the TL074's lower noise figure was any help in this type of application.
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fluxmonkey



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PostPosted: Tue Oct 13, 2009 6:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

for audio levels typically found in modular synths (+/-5v), i've never heard a difference between TL074 or TL084... mic level audio might be a different story, but you'd have lots of other factors there, like powersupply and shielding, that would probably be more significant than the opamps.
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sonicwarrior



Joined: Dec 22, 2005
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PostPosted: Sat Sep 10, 2011 7:16 am    Post subject: Reply with quote  Mark this post and the followings unread

If I understand the schematic correctly the TL084 is not in the audio path and processes the CV only.

@matrix12x:

I'm using the Bridechamber front, too. Did you build it in the meantime?
I'm wondering were I have to install the Gain potentiometer as it is not mentioned in the MFOS description/schematics. Confused
Or is it the offset potentiometer without a connection to V- and instead connecting that point to ground?

Last edited by sonicwarrior on Sat Sep 10, 2011 9:23 am; edited 1 time in total
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matrix12x



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PostPosted: Sat Sep 10, 2011 7:28 am    Post subject: Reply with quote  Mark this post and the followings unread

I put mine between R17 pin 2 and J3 pin 1.
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sonicwarrior



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PostPosted: Sat Sep 10, 2011 9:19 am    Post subject: Reply with quote  Mark this post and the followings unread

That's the output level, not gain. Gain is the base amplification. E.g. if you want a constant signal without any CV input you raise the gain knob to the needed level.
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matrix12x



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PostPosted: Sat Sep 10, 2011 11:52 am    Post subject: Reply with quote  Mark this post and the followings unread

I know, I was just lazy about it.
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matrix12x



Joined: Sep 26, 2009
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PostPosted: Sat Sep 10, 2011 11:55 am    Post subject: Reply with quote  Mark this post and the followings unread

A voltage at R11 or R33 would change gain though.
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sonicwarrior



Joined: Dec 22, 2005
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PostPosted: Sat Sep 10, 2011 2:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'll try that then. Thanks for helping. Smile
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slacker



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PostPosted: Sun Sep 11, 2011 8:03 am    Post subject: Reply with quote  Mark this post and the followings unread

On the schematic here http://www.musicfromouterspace.com/mfosweb/navigate.action?MAINTAB=SYNTHDIY&PROJARG=DUALVCA/DLLVCA001.html the "Offset Adjust" pots are the gain controls. They do this "...you want a constant signal without any CV input you raise the gain knob to the needed level."
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matrix12x



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PostPosted: Sun Sep 11, 2011 12:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yeah, I just rewired mine in that manner last night.
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sonicwarrior



Joined: Dec 22, 2005
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PostPosted: Fri Jun 01, 2012 1:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

slacker wrote:
On the schematic here http://www.musicfromouterspace.com/mfosweb/navigate.action?MAINTAB=SYNTHDIY&PROJARG=DUALVCA/DLLVCA001.html the "Offset Adjust" pots are the gain controls. They do this "...you want a constant signal without any CV input you raise the gain knob to the needed level."


I have wired my Dual VCA (the old one one without the bias trimmer) with Bridechamber front panel (adds input attenuator and CV2 attenuator which I both wired the wrong way: Input jack on 1, IN B from the PCB to 2+3 instead of 3 to ground) and wired the gain pot as described above. Oddly when the pot is in the full ccw position the gain is at max and when I move it to the right aka clockwise quickly I hear nothing anymore so most of the way seems useless. Don't know if that has something to do with the wrong way I soldered the input attenuator (I guess not). The pot looks like it's connected the right way (1 to -12 V, 2 to ADJ B, 3 to +12 V).
Any idea what I could check?
I guess the obvious is to check if ADJ B gets -12 V to +12 V when I change the pot position.
Anything else?

Edit: I should note that I only build the B part so far to check for errors.
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sonicwarrior



Joined: Dec 22, 2005
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PostPosted: Thu Jun 07, 2012 3:37 am    Post subject: Reply with quote  Mark this post and the followings unread

sonicwarrior wrote:
The pot looks like it's connected the right way (1 to -12 V, 2 to ADJ B, 3 to +12 V).


I have put -12 and +12 V the other round now which apparently is in accordance with the wiring graphic from the MFOS page (which is a bit confusing as the pot is upside down). I guess the rest is just the normal behavior.

Tried to connect ground instead of +12 V but the Log mode didn't work then.
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nyd



Joined: Jul 15, 2009
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PostPosted: Tue Mar 19, 2013 6:48 am    Post subject: Reply with quote  Mark this post and the followings unread

Anyone tried wiring a pot instead of a switch to adjust the LIN/LOG response of the VCA? I'm thinking in lines of mimicing the uVCA from Intellijel that has a pot to adjust the response.

Would it work?
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sonicwarrior



Joined: Dec 22, 2005
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PostPosted: Tue Mar 19, 2013 11:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes, should work. Intellijel did also just put a pot between these two instead of a switch. The circuit developer Dr Sketch n Etch explained that @Muffwiggler in a thread about the DIY variant of the uVCA.
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