electro-music.com   Dedicated to experimental electro-acoustic
and electronic music
 
    Front Page  |  Articles  |  Radio
 |  Media  |  Forum  |  Wiki  |  Links  |  Store
Forum with support of Syndicator RSS
 FAQFAQ   CalendarCalendar   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   LinksLinks
 RegisterRegister   ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in  Chat RoomChat Room 
 Forum index » DIY Hardware and Software
Passive Vactrol VCA - Am I right?
Post new topic   Reply to topic Moderators: jksuperstar, Scott Stites, Uncle Krunkus
Page 1 of 1 [6 Posts]
View unread posts
View new posts in the last week
Mark the topic unread :: View previous topic :: View next topic
Author Message
Snaper



Joined: Feb 28, 2014
Posts: 65
Location: Hungary

PostPosted: Tue Mar 04, 2014 12:48 am    Post subject: Passive Vactrol VCA - Am I right? Reply with quote  Mark this post and the followings unread

Hy Guys,

Im new in here, just found this site when I was searching for passive schematics for my Eurorack.
I would like to build a simple passive vactrol based VCA, found some schematics here.

Could the attached schematic work?

Basicaly audio in and out, with 0,1uF cap, and there is a 100k pot after the CV in for att. the incoming signal.

Should I put a diode after the input to avoid negative voltage?


vactrol_sketch_schem.png
 Description:
 Filesize:  35.37 KB
 Viewed:  25 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

vactrol_sketch_schem.png


Back to top
View user's profile Send private message
Snaper



Joined: Feb 28, 2014
Posts: 65
Location: Hungary

PostPosted: Tue Mar 04, 2014 3:13 am    Post subject: Reply with quote  Mark this post and the followings unread

Nobody?
Back to top
View user's profile Send private message
JovianPyx



Joined: Nov 20, 2007
Posts: 1249
Location: West Red Spot, Jupiter
Audio files: 161

PostPosted: Tue Mar 04, 2014 7:56 am    Post subject: Reply with quote  Mark this post and the followings unread

I think it would "sort of" work. In fact, this circuit will act quite a bit like a lowpass filter (single pole). In terms of VCA action, it will reduce the output when the Vactrol is darker, but it will never completely cut off the sound. Also - IIRC, a Vactrol has a LDR as the control component. The schematic shows a bipolar phototransistor. If it really is a transistor, you will also likely get severe clipping distortion.

I understand why some people like passive circuits. They are usually quite simple and need no power supply. However, IMO they all perform poorly. The investment in a power supply and use of active circuits will perform orders of magnitude better than passive systems.

_________________
FPGA, dsPIC and Fatman Synth Stuff

Time flies like a banana.
Fruit flies when you're having fun.
BTW, Do these genes make my ass look fat?
corruptio optimi pessima
Back to top
View user's profile Send private message Visit poster's website
PHOBoS



Joined: Jan 14, 2010
Posts: 1811
Location: Moon Base
Audio files: 351

PostPosted: Tue Mar 04, 2014 8:35 am    Post subject: Reply with quote  Mark this post and the followings unread

apart from the 'Amplifier' section I think the CV input wouldn't really work either. They way you have it drawn now the pot can short the CV and
whatever the souce of it (CV) is might not be happy with that. Also to control the LED you're much better of with either a current source or PWM (with PWM
you would most likely need a lowpas filter if you use a phototransistor).

_________________
"My perf, it's full of holes!"
http://phobos.000space.com/ http://www.acidtrash.com/ Stickney Synthyards
Back to top
View user's profile Send private message Visit poster's website AIM Address Yahoo Messenger MSN Messenger
JovianPyx



Joined: Nov 20, 2007
Posts: 1249
Location: West Red Spot, Jupiter
Audio files: 161

PostPosted: Tue Mar 04, 2014 8:47 am    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
... I think the CV input wouldn't really work either. They way you have it drawn now the pot can short the CV and whatever the souce of it (CV) is might not be happy with that.


Heh - I didn't notice that, but yeah.

Another way to light the LED besides a (true) current source would be to simply put the variable resistor in series with the CV. Also another fixed resistor in series to force a maximum possible current amount.

However, all that said, I still think it will perform poorly. A way to make a small improvement is to replace the 0.1 uF capacitor with a resistor. I'd suggest trying 1K for a start. The size of that resistor will control how close to cutting off the signal you can get with smaller being closer. Still, it will never cut off completely, but possibly to an acceptable amount.

_________________
FPGA, dsPIC and Fatman Synth Stuff

Time flies like a banana.
Fruit flies when you're having fun.
BTW, Do these genes make my ass look fat?
corruptio optimi pessima
Back to top
View user's profile Send private message Visit poster's website
diablojoy



Joined: Sep 07, 2008
Posts: 795
Location: melbourne australia
Audio files: 11

PostPosted: Thu Mar 06, 2014 6:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

look up scott stites mutant vactrol filter , check out the cv drive circuitry
used for the vactrol its a very good method and works really well for what you want to acheive.

_________________
In an infinite universe one might very well
ask where the hell am I
oh yeah thats right the land of OZ
as good an answer as any
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic Moderators: jksuperstar, Scott Stites, Uncle Krunkus
Page 1 of 1 [6 Posts]
View unread posts
View new posts in the last week
Mark the topic unread :: View previous topic :: View next topic
 Forum index » DIY Hardware and Software
Jump to:  

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum
e-m mkii

Please support our site. If you click through and buy from
our affiliate partners, we earn a small commission.


Forum with support of Syndicator RSS
Powered by phpBB © 2001, 2005 phpBB Group
Copyright © 2003 through 2009 by electro-music.com - Conditions Of Use