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zipzap
Joined: Nov 22, 2005 Posts: 559 Location: germany
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Posted: Thu Feb 22, 2007 4:58 am Post subject:
diode vca? yes! Subject description: it´s running. now for the tweaking! |
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Hi People!
Visiting again, been a while...
I´m wondering how those so called diode limiters work...
Like this one:
http://www.uaudio.com/webzine/2006/august/index2.html
I have seen the diode ladder used in filters, i´m not quite sure how it´s working though. Apparently the resistance of a diode changes whith the current thats going through it, or something.
Well, the only information i could find on the diode ladder vca is that it exists, like in some of those classic compressors.
But no circuits or further descriptions...
Ok, when i take this filter as an example:
http://guitarfool.com/modules/images/Diode_mod_sch.jpg
What would happen if i kick out all the cabs in the ladde? A vca i guess. And if i kick out all the diodes and replace them with a resistor? Still a vca.
So that can´t be the Diode-vca.
Anyone know something about this stuff? _________________ http://www.myspace.com/lorolocoacousticpop
http://www.myspace.com/petrolvendor
music and transcribed jazz basslines Last edited by zipzap on Thu Apr 12, 2007 3:13 am; edited 1 time in total |
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blue hell
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Posted: Thu Feb 22, 2007 12:51 pm Post subject:
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Look at a graph giving the current that flows through a component having two terminals vs the voltage that is over it as a result of that.
When you do so for a resistor you'll se that the graph is a straight line, a fixed relation between current and voltage. Make the current twice as large and the voltage will be twice as large as well.
When you do the same for a diode you will not see a straight line but an exponential curve instead. Because the line is not straight it means that the resistance varies with the current that flows through it. The resistance at any point on the curve can be determined by looking at the angle the curve at that point makes with the horizontal axis, or it's coefficient of direction (don't know the proper english word).
Now this effect is usable only in a small signal range, as it is essentially non linear. Large linear changes in the current would not result in linear changes in the voltage over the diode. This makes it a bit troublesome to use the principle, as it will always distort a bit or it will have to use very small signal levels resulting into relativly high noise levels.
For the ladder filter the distortion might actually be the thing that makes it's characteristic sound. _________________ Jan
also .. could someone please turn down the thermostat a bit.
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zipzap
Joined: Nov 22, 2005 Posts: 559 Location: germany
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vtl5c3
Joined: Sep 08, 2006 Posts: 425 Location: PDX
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Posted: Fri Mar 02, 2007 9:19 am Post subject:
Re: diode vca? |
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There's the Korg MS50 VCA:
http://machines.hyperreal.org/manufacturers/Korg/MS-synths/schematics/ms50filt.gif
Never heard one in action, but if it's like the VCF, it's got poor CV rejection (clicky clicky click!).
Romeo
zipzap wrote: | Hi People!
Visiting again, been a while...
I´m wondering how those so called diode limiters work...
Like this one:
http://www.uaudio.com/webzine/2006/august/index2.html
I have seen the diode ladder used in filters, i´m not quite sure how it´s working though. Apparently the resistance of a diode changes whith the current thats going through it, or something.
Well, the only information i could find on the diode ladder vca is that it exists, like in some of those classic compressors.
But no circuits or further descriptions...
Ok, when i take this filter as an example:
http://guitarfool.com/modules/images/Diode_mod_sch.jpg
What would happen if i kick out all the cabs in the ladde? A vca i guess. And if i kick out all the diodes and replace them with a resistor? Still a vca.
So that can´t be the Diode-vca.
Anyone know something about this stuff? |
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blue hell
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Joined: Apr 03, 2004 Posts: 24137 Location: The Netherlands, Enschede
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Posted: Fri Mar 02, 2007 3:40 pm Post subject:
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zipzap wrote: | might look something like this, right? |
I don't really have ideas about how to make a practical circuit, I'd expect something different, more like :
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--+---- gnd
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But rhe Korg thingy might be a much beter idea ! _________________ Jan
also .. could someone please turn down the thermostat a bit.
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zipzap
Joined: Nov 22, 2005 Posts: 559 Location: germany
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Posted: Wed Mar 07, 2007 1:35 pm Post subject:
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I think our ideas might both work.
One thing about the korg vca:
there is the diode-ring formed inside the ca3019. where is pin 5 going to? open?
i´ll definetly check this thing out and post some samples... soon _________________ http://www.myspace.com/lorolocoacousticpop
http://www.myspace.com/petrolvendor
music and transcribed jazz basslines |
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vtl5c3
Joined: Sep 08, 2006 Posts: 425 Location: PDX
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Posted: Wed Mar 07, 2007 1:42 pm Post subject:
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Pin 5 connects to Pin 2, which connects to a 100K resistor....
zipzap wrote: | I think our ideas might both work.
One thing about the korg vca:
there is the diode-ring formed inside the ca3019. where is pin 5 going to? open?
i´ll definetly check this thing out and post some samples... soon |
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zipzap
Joined: Nov 22, 2005 Posts: 559 Location: germany
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Posted: Thu Mar 08, 2007 3:10 am Post subject:
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that means the bottom left diode is shorted.
another thing i don´t get: R67 (100k) goes from + to a diode that goes to ground, and via R7 (360k) to the inv. in of IC10. Is that to generate a 0.6 voltage or is there something else going on? _________________ http://www.myspace.com/lorolocoacousticpop
http://www.myspace.com/petrolvendor
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zipzap
Joined: Nov 22, 2005 Posts: 559 Location: germany
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Posted: Thu Apr 12, 2007 3:12 am Post subject:
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ok, the thing about the diode being shorted cannot work.
Anyway, here are two versions of the diode vca. haven´t decidet which is better yet.
the first makes the signal pass through the whole diode ring. i found this design in Jürgen Haibles Diode vca.
The cv is symmetrical.
The other version is from a compressor schematic.
The audio signal is symmetrical, the diode ring sits inbetween the + and - signal.
The diodes should be mashed (with steak and a glass of milk)
These vcas are very simple and produce an interesting distortion. The coulouring can be quite subtile and pleasant. Problem is that the output is very very low.
That´s why i imagine that the version with balanced in out might be better to handle.
Of course it could run unbalanced too, just place the ring between signal and ground. But balanced signals have one advantage, especially with low signal level.
A transformer (don´t have any right now) or instrumention amp could be used to amplify the signal and hopefully cancle out any hum.
Now i have a problem: I have no idea how to determine the impedance on the outputs. I have some INA103 chips. Low noise, balanced inputs. But they are specialised for low impedance sources (200r). That´s where they peform best.
Or maybe a difamp with SSM2210?
Ok, if someone can help me that would be cool. I want to turn this into a working compressor/limiter/vca schematic with lots of character and good performance, rather than a dirty distortion for my modular (ok ok, got me, of course i´ll do both
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_________________ http://www.myspace.com/lorolocoacousticpop
http://www.myspace.com/petrolvendor
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zipzap
Joined: Nov 22, 2005 Posts: 559 Location: germany
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Posted: Thu Apr 12, 2007 4:54 am Post subject:
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did some measuring, 2V pp sine input gives me 40mv output. CV set to max level without clipping, only some pleasant sounding deformation of the signal. Higher level sounds very ungly.
I also included dc levels and the current through the diode ring. Diodes are BAT43.
Ok, these balanced 40mv have to be brought back to 2v. No idea what the Rout of the signal is, or how i can calculate it.
With these small signal that propably does matter for low noise amplification.
Any ideas?
![Posted Image, might have been reduced in size. Click Image to view fullscreen. Posted Image, might have been reduced in size. Click Image to view fullscreen.](http://img187.imageshack.us/img187/4759/diodevcamessungsc5.jpg) _________________ http://www.myspace.com/lorolocoacousticpop
http://www.myspace.com/petrolvendor
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blue hell
Site Admin
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Joined: Apr 03, 2004 Posts: 24137 Location: The Netherlands, Enschede
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Posted: Thu Apr 12, 2007 5:09 am Post subject:
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It varies, when no current flows through the diodes it is determinded by the C/R series impedance that is used to feed the bridge with, so approximately 50 kOhm. When current flows through the diodes the impedance will drop to the rd (dynamic resistance) specified for the diodes (which varies with the current flowing through it and the diode type), could be as low as some 10's of ohms.
re. noise I guess that an impedance mismatch would cause a few dB of extra noise. The noise floor probably will be determined here mainly by the diode noise, but this is a bit of a guess.
I'd just try to buffer with regular opamps.
Good to see you sort it out ! _________________ Jan
also .. could someone please turn down the thermostat a bit.
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zipzap
Joined: Nov 22, 2005 Posts: 559 Location: germany
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Posted: Thu Apr 12, 2007 8:52 am Post subject:
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ok, in this design the diodes are always under current.
buffering with opamps works of course. on the other hand, i would like to take advantage of the fact that the signal is balanced. So an instrumention amp should be used (the inv and noninverting ins of an standard opamp are to different for real differential amplification.
On the breadboard this design is mainly suffering from hum and noise. When i look at my workshop i can imagine it´s not the design iteself though...
Think i´ll try the ina103 later. Don´t know how much the impedance matters, but the hifi people i know are sure that it matters a LOT... especially with certain "high-end" audio opamps.
A AD797 (0,9nv) is supposed to perform worse than most standard opamps when driven by high impedance sources.
don´t know, don´t know... _________________ http://www.myspace.com/lorolocoacousticpop
http://www.myspace.com/petrolvendor
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zipzap
Joined: Nov 22, 2005 Posts: 559 Location: germany
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Posted: Sun Apr 15, 2007 8:17 am Post subject:
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Think we solved it. I use an instrumention amp with 2 OPA2132 as inputs. gain is 40. It works perfect. I´ll soon give you a schematic with the output amp and the scaling amp of the cv for conveniant range.
Here are some samples. For cv i used a compression setup.
1. bach is with feedforward, the others are feedbackward.
on the beginning of rosa the offset wasn´t adjusted correctly, it gets overdriven a little at the beginnings of the vocal phrases.
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_________________ http://www.myspace.com/lorolocoacousticpop
http://www.myspace.com/petrolvendor
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zipzap
Joined: Nov 22, 2005 Posts: 559 Location: germany
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