Author |
Message |
Zoomby
Joined: Aug 28, 2006 Posts: 18 Location: Germany
|
Posted: Tue Sep 19, 2006 12:37 pm Post subject:
OTAs rare? Alternatives? |
 |
|
Hi,
Although I'm still a newbie and never used OTAs I noticed that these ICs seems to be rare and increasing in price steadily. The only affordable one I saw is the LM13700.
Are there any alternatives to OTAs? I mean, if an OTA is used as a voltage controlled resistor for example (and I think it's a useful and often needed function) why isn't there simply a voltage controller resisor IC? (or does it exist?)
..just some newbie thoughts. Can you enlighten me?
Bye
Chris |
|
Back to top
|
|
 |
Freq Band

Joined: Sep 05, 2006 Posts: 57 Location: Los Angeles
|
Posted: Tue Sep 19, 2006 9:04 pm Post subject:
|
 |
|
Do a good solid search in the net. There are many papers written about it's uniqueness.
Newer designed circuits , yes, will use the 13700/13600.
Chips for older circuits can still be found (CA3080, etc..)....try ebay.
Quote: | why isn't there simply a voltage controller resisor IC? (or does it exist?) |
Yes there are, and are often tailored to different kinds of signals.
Most simple of all is the Vactrol ....light from LED shinning onto a LDR (light dependant resistor). Many classic synth circuits use them.
=FB= |
|
Back to top
|
|
 |
StephenGiles
Joined: Apr 17, 2006 Posts: 507 Location: England
|
Posted: Wed Sep 20, 2006 12:02 pm Post subject:
|
 |
|
Not forgetting, of course, the CA3094 - now sadly out of production, and I need some! |
|
Back to top
|
|
 |
mosc
Site Admin

Joined: Jan 31, 2003 Posts: 18240 Location: Durham, NC
Audio files: 224
G2 patch files: 60
|
Posted: Wed Sep 20, 2006 6:57 pm Post subject:
|
 |
|
You can use a multiplier IC, like the AD633 as a voltage controlled amplifier. These are much easier to use than the OTA in many applications. _________________ --Howard
my music and other stuff |
|
Back to top
|
|
 |
saul2600

Joined: Jul 13, 2006 Posts: 14 Location: variable
|
Posted: Wed Sep 20, 2006 8:47 pm Post subject:
|
 |
|
Don't forget the SSM2164, either. It's four VCA's in one - with current inputs and outputs. And SSM2018 - one high quality VCA in a chip. Both still in production, and should be for quite some time, as far as anyone knows. |
|
Back to top
|
|
 |
zipzap
Joined: Nov 22, 2005 Posts: 559 Location: germany
Audio files: 24
|
Posted: Thu Sep 21, 2006 6:46 am Post subject:
|
 |
|
A Fet can be used as a vc resistor. Although i´m still not sure how exactly. Is there a simple method for designing?
Like, if you want 1k to 100k in resistance, controlled from 0 to 10v, use these values:
As faar as i know the vactrol is the only thing that acts like a real resistor in any aplication. _________________ http://www.myspace.com/lorolocoacousticpop
http://www.myspace.com/petrolvendor
music and transcribed jazz basslines |
|
Back to top
|
|
 |
Uncle Krunkus
Moderator

Joined: Jul 11, 2005 Posts: 4761 Location: Sydney, Australia
Audio files: 52
G2 patch files: 1
|
Posted: Thu Sep 21, 2006 6:56 am Post subject:
|
 |
|
Even a vactrol is not a common thing. I mean there's nowhere in Australia I know of that I could even mail order them from. The only easy way for me to play with vactrols would be to roll my own. That's not very convenient.
I've got a big stock of 13700s which cost a mint. I just have to rely on the idea that any circuit I might build which uses an OTA will take an LM13700 as a substitute. _________________ What makes a space ours, is what we put there, and what we do there. |
|
Back to top
|
|
 |
zipzap
Joined: Nov 22, 2005 Posts: 559 Location: germany
Audio files: 24
|
|
Back to top
|
|
 |
Uncle Krunkus
Moderator

Joined: Jul 11, 2005 Posts: 4761 Location: Sydney, Australia
Audio files: 52
G2 patch files: 1
|
Posted: Thu Sep 21, 2006 7:35 am Post subject:
|
 |
|
Oh I'm not saying I don't like the idea of making my own vactrols. I love that sort of thing. But I'm glad that I can.
But if I find a circuit for a great VCO and in the parts list there's a complex function generator IC which is no longer made and the only answer is to "make your own" I'm probably moving on to the next circuit idea, you know what I mean? _________________ What makes a space ours, is what we put there, and what we do there. |
|
Back to top
|
|
 |
State Machine
Janitor


Joined: Apr 17, 2006 Posts: 2810 Location: New York
Audio files: 24
|
Posted: Thu Sep 21, 2006 12:00 pm Post subject:
|
 |
|
Quote: | Don't forget the SSM2164, either. It's four VCA's in one |
Wow, YES, another State Machine .... Hi State! We will take over the world !!!
Kidding aside, I agree, the SSM2164 is a fantastic IC! The response time of this IC is incredible so it can be used in high rate modulation experiments like audio granulation! I will have some real nice QUAD VCA double sided printed circuit boards available based on this IC in a month or so. If anyone will be interested, I will supply the details and price. Right now I am laying out a prototype with only a single channel active for testing purposes.
Some details:
It will support (0-10) and (0-5) CV standards, have 1 to 20 dB boost, support multiple inputs, MOD DEPTH controls, MOD LEV indicators, 120 dB dynamic range, INVERT and OFFSET functions, etc ... This will be a LOG response VCA unit
Bill |
|
Back to top
|
|
 |
StephenGiles
Joined: Apr 17, 2006 Posts: 507 Location: England
|
|
Back to top
|
|
 |
toppobrillo

Joined: Dec 10, 2005 Posts: 766 Location: oakland, ca
G2 patch files: 1
|
Posted: Thu Sep 21, 2006 4:39 pm Post subject:
|
 |
|
hey i was looking for that but at 'adadepot' you gotta pay.
also, check out the PWM phaser schematics at rene schmitz site. |
|
Back to top
|
|
 |
sneakthief

Joined: Jul 24, 2006 Posts: 569 Location: Berlin
|
Posted: Fri Sep 22, 2006 8:39 am Post subject:
|
 |
|
State Machine wrote: | Quote: | Don't forget the SSM2164, either. It's four VCA's in one |
Wow, YES, another State Machine .... Hi State! We will take over the world !!!
Kidding aside, I agree, the SSM2164 is a fantastic IC! The response time of this IC is incredible so it can be used in high rate modulation experiments like audio granulation! I will have some real nice QUAD VCA double sided printed circuit boards available based on this IC in a month or so. If anyone will be interested, I will supply the details and price. Right now I am laying out a prototype with only a single channel active for testing purposes.
Some details:
It will support (0-10) and (0-5) CV standards, have 1 to 20 dB boost, support multiple inputs, MOD DEPTH controls, MOD LEV indicators, 120 dB dynamic range, INVERT and OFFSET functions, etc ... This will be a LOG response VCA unit
Bill |
oh count me in! shoot me a PM or email when your PCB is done.
i was just in the midst of planning a dj-style crossfader based on the schematic below. of course i would be using all four channels . i was thinking of using a dual-slider-pot with one pot wired to stereo channel 1 (vca1+2) and the other pot wired in reverse to channel 2 (vca3+4)
...the 50k pot at the top changes the crossfader curve
(the schematic is somebody's slight modification to the 2164 datasheed - it was grabbed from this thread - http://www.skratchlounge.com/forums.html?showtopic=1942&st=0 )
cheers,
michel _________________ Sneak-Thief - raw electrofunk |
|
Back to top
|
|
 |
State Machine
Janitor


Joined: Apr 17, 2006 Posts: 2810 Location: New York
Audio files: 24
|
Posted: Fri Sep 22, 2006 10:30 am Post subject:
|
 |
|
Quote: | i was just in the midst of planning a dj-style crossfader based on the schematic below. of course i would be using all four channels . i was thinking of using a dual-slider-pot with one pot wired to stereo channel 1 (vca1+2) and the other pot wired in reverse to channel 2 (vca3+4) |
OK sure, I will keep a list of everyone who would be interested in a PCB.
Taking a look at your schematic, you might want to get more usable range from the slider by using 3.3 volts instead of 5 V since at 3.3V, the VCA will be attenuating at it's 100 dB point from unity (0V). The response transfer characteristic is -33 millivolts/dB. If you leave it biased at 5V, you will lose lots of range. You may find that even 100 dB may be too much attenuation and may want to go a slight bit less than 3.3V.
My VCA design accepts a 0-10 V or 0-5 V input and scales it to 3.3 to 0 V. (Remember 0V is unity on this chip) |
|
Back to top
|
|
 |
sneakthief

Joined: Jul 24, 2006 Posts: 569 Location: Berlin
|
Posted: Fri Sep 22, 2006 11:02 am Post subject:
|
 |
|
State Machine wrote: |
Taking a look at your schematic, you might want to get more usable range from the slider by using 3.3 volts instead of 5 V since at 3.3V, the VCA will be attenuating at it's 100 dB point from unity (0V). The response transfer characteristic is -33 millivolts/dB. If you leave it biased at 5V, you will lose lots of range. You may find that even 100 dB may be too much attenuation and may want to go a slight bit less than 3.3V.
My VCA design accepts a 0-10 V or 0-5 V input and scales it to 3.3 to 0 V. (Remember 0V is unity on this chip) |
The person who made schematic this intended it for a special "scratch fader" used by turntablists - the whole point of using 5v to both the fader and bias is to change the curve of the fader. In this case, using 5v makes sense here.
Check out Rane's slightly-biased history of the crossfader:
http://www.rane.com/note146.html
Rosie - the first known dj mixer from 1965:
 _________________ Sneak-Thief - raw electrofunk |
|
Back to top
|
|
 |
zipzap
Joined: Nov 22, 2005 Posts: 559 Location: germany
Audio files: 24
|
Posted: Fri Sep 22, 2006 12:29 pm Post subject:
|
 |
|
hey that´s a nice link!
But how does that pot in your shematic change the curve?
33mv/db, is that measured directly at the controll in?
I never found any infos how the chip really responds in the datasheet. _________________ http://www.myspace.com/lorolocoacousticpop
http://www.myspace.com/petrolvendor
music and transcribed jazz basslines |
|
Back to top
|
|
 |
State Machine
Janitor


Joined: Apr 17, 2006 Posts: 2810 Location: New York
Audio files: 24
|
Posted: Fri Sep 22, 2006 6:32 pm Post subject:
|
 |
|
Quote: |
The person who made schematic this intended it for a special "scratch fader" used by turntablists - the whole point of using 5v to both the fader and bias is to change the curve of the fader. In this case, using 5v makes sense here. |
Now that I took a look at the graph for the scratch response in the link you sent, now it's makes sense to me why the circuit uses the configuration it does.
Cool link BTW !!  |
|
Back to top
|
|
 |
|