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Old 17th May 2012, 03:14 PM   #1
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Default Vca's

Hey,

I have a question about VCA's. How do you design one if you are looking to make the volume controls last longer? I believe some mixers use this approach
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Old 17th May 2012, 04:04 PM   #2
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They don't really make the controls last any longer, they are just more immune to noise and glitches that a bad control may produce.

Good VCA design is a specialist subject and often requires tightly matched components.
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Old 17th May 2012, 04:27 PM   #3
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hi
In practice i have found that some VCA circuitry does let some of that scratchy noise through from the volume control. Do you know why?
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Old 17th May 2012, 07:11 PM   #4
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Well there is a limit to how much they can ignore. The way to ignore the noise is to filter the control signal, but that makes it less responsive. So it's a trade off.
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Old 17th May 2012, 08:56 PM   #5
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I have no experience with designing these circuits myself, although I have modified them sometimes to make a mixer work more or less normally with a damaged fader.

Besides making the filter time constant as large as possible, you should also try to keep the current throught the wiper as low as possible such that variations in contact resistance have little effect on the control voltage. If you have an RC filter right behind the potmeter and no DC load, the voltage remains constant even when the wiper temporarily doesn't make any contact at all. Besides, with carbon potmeters, any DC current through the wiper should flow into the wiper to prevent anodisation (according to an old Panasonic trimmer potmeter application note).

In normal broadcast use, the lower part of the fader typically gets damaged first because discjockeys tend to shift them very hard from -20 dB to -infinite dB. In that case it is handy to design your circuit such that it goes to -infinite dB when the wiper doesn't make contact at all. My damaged fader modification was to add a 10 Mohm resistor, an RC filter and a CMOS op-amp connected as a voltage follower between the fader and the original circuit. The 10 Mohm resistor would pull the wiper to the voltage corresponding to -infinite dB, but with only a very small DC current.
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Old 17th May 2012, 09:20 PM   #6
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http://www.thatcorp.com/datashts/THA..._Datasheet.pdf
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Old 18th May 2012, 01:12 AM   #7
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Quote:
Originally Posted by richie00boy View Post
but that makes it less responsive. So it's a trade off.
What do you mean less responsive?
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Old 18th May 2012, 01:56 AM   #8
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I'd pick a pot size that would run about a mA, and put a probably 0.1uF to ground off the wiper that would feed the control voltage into the VCA, as a default. Chemical reactions often happen when dissimilar materials are put in contact with each other for extended periods. Back at Tektronix I used to hear the phrase "molecular migration". With enough current flow, a migrated and thereby largely corrupted connection in a switch, a connector, a relay or a wiper in a pot could often be cleaned up, at least temporarily. Too much current in a pot could increase noise. 1mA seems like a good guess as to what might work good. The cap to ground will filter out any wiper noise above some hopefully low frequency, depending on circuit impedances (calculate HZ with R or Z, from the point of view of the cap). 0.1uF seems like a good starting point for that. The cap size is too big when you can sense a delay between knob rotation and VCA gain change. I'm actually building something right now that will be using the Analog Devices SSM 2018T VCA (a 2 to 5 surround sound extractor, very similar to what I think "Circle Surround" is doing) (WAS doing - their product is no longer available as a separate).
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Old 18th May 2012, 07:43 AM   #9
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What do you mean less responsive?
That the controlling of the volume is slow in relation to the moving of the fader.
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