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OTAs rare? Alternatives?
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Zoomby



Joined: Aug 28, 2006
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PostPosted: Tue Sep 19, 2006 12:37 pm    Post subject: OTAs rare? Alternatives? Reply with quote  Mark this post and the followings unread

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
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Freq Band



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PostPosted: Tue Sep 19, 2006 9:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

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=
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StephenGiles



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PostPosted: Wed Sep 20, 2006 12:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

Not forgetting, of course, the CA3094 - now sadly out of production, and I need some!
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mosc
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PostPosted: Wed Sep 20, 2006 6:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

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.
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saul2600



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PostPosted: Wed Sep 20, 2006 8:47 pm    Post subject: Reply with quote  Mark this post and the followings unread

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.
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zipzap



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PostPosted: Thu Sep 21, 2006 6:46 am    Post subject: Reply with quote  Mark this post and the followings unread

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.

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Uncle Krunkus
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PostPosted: Thu Sep 21, 2006 6:56 am    Post subject: Reply with quote  Mark this post and the followings unread

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.

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zipzap



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PostPosted: Thu Sep 21, 2006 7:19 am    Post subject: Reply with quote  Mark this post and the followings unread

you´re right, i never had a finished vactrol, i always roll my own. But i like it, little other type of handcraft than soldering once in a while.
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Uncle Krunkus
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PostPosted: Thu Sep 21, 2006 7:35 am    Post subject: Reply with quote  Mark this post and the followings unread

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?

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State Machine
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PostPosted: Thu Sep 21, 2006 12:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

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 !!! Very Happy

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
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StephenGiles



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PostPosted: Thu Sep 21, 2006 1:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

http://www.tasteundtechnik.de/bilder/adafinalph1.jpg

Look at this - a phaser with PWM from a VC 4047 into resistors switched by 4016 gates!
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toppobrillo



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PostPosted: Thu Sep 21, 2006 4:39 pm    Post subject: Reply with quote  Mark this post and the followings unread

hey i was looking for that but at 'adadepot' you gotta pay.

also, check out the PWM phaser schematics at rene schmitz site.
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sneakthief



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PostPosted: Fri Sep 22, 2006 8:39 am    Post subject: Reply with quote  Mark this post and the followings unread

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 !!! Very Happy

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) Wink

Posted Image, might have been reduced in size. Click Image to view fullscreen.

...the 50k pot at the top changes the crossfader curve Cool

(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

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State Machine
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PostPosted: Fri Sep 22, 2006 10:30 am    Post subject: Reply with quote  Mark this post and the followings unread

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)
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sneakthief



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PostPosted: Fri Sep 22, 2006 11:02 am    Post subject: Reply with quote  Mark this post and the followings unread

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:

Posted Image, might have been reduced in size. Click Image to view fullscreen.

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zipzap



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PostPosted: Fri Sep 22, 2006 12:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

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.

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State Machine
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PostPosted: Fri Sep 22, 2006 6:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

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 !! Cool
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