Controlling VCA's from 25m distance...

[Rogy]

Hi all,

I’d like to control a set of VCA’s from about 25m distance.

What’s the idea? I’d like to offer musicians a possibility to alter three stereo elements of their in-ear mixes from a small remote attached to their music stands.
Instead of sending three stereo balanced signals to their remote, which would cost way too much cable pairs, I’d make a box which takes three stereo inputs, have them pass sets of VCA’s that can be controlled from the remotes and then sum these three mixes to one final mix, that enters their in-ear wireless systems.

I know great care should be taken to drive the control ports of the VCA’s with a noise free and very low impedance signal.

Ideas that come to mind (not necessarily inspired by technical knowledge…):

a) Send a way too high DC voltage (eg +48V) to the remote, put it on the top of three potmeters and send the wipers back to the VCA’s; then rescale the voltage using a resistive voltage divider, then further on thru a control port buffer.

b) Instead of the above, use 3 three-pin voltage regulators in the masterstation to drive the voltage divider and use the remote pots to set the voltage (connected between Adjust-terminal and GND via long cables)

Is any of the above possible? Would a voltage divider do for rescaling the control voltage, or is a more esoteric approach needed?

In the end the VCA’s should be adjustable from 0 to -12dB attenuation, with the center point of the pots being -6dB. So the talents have a mix range from +/- 6dB.

Thanks,

Rogy

 

[chr1s]

With low impedance you could use for example a (about) 5 mA constant current source. Giving the musicians a 500 Ohm pot would give you a voltage between 0 and 2.5 V.

Of course the source should have little noise, but you would only need a cable with one wire + ground.


Chris

 

[abbey road d enfer]

[quote author="Rogy"]Hi all,

I’d like to control a set of VCA’s from about 25m distance.

What’s the idea? I’d like to offer musicians a possibility to alter three stereo elements of their in-ear mixes from a small remote attached to their music stands.
Instead of sending three stereo balanced signals to their remote, which would cost way too much cable pairs, I’d make a box which takes three stereo inputs, have them pass sets of VCA’s that can be controlled from the remotes and then sum these three mixes to one final mix, that enters their in-ear wireless systems.

I know great care should be taken to drive the control ports of the VCA’s with a noise free and very low impedance signal.

Ideas that come to mind (not necessarily inspired by technical knowledge…):

a) Send a way too high DC voltage (eg +48V) to the remote, put it on the top of three potmeters and send the wipers back to the VCA’s; then rescale the voltage using a resistive voltage divider, then further on thru a control port buffer.

b) Instead of the above, use 3 three-pin voltage regulators in the masterstation to drive the voltage divider and use the remote pots to set the voltage (connected between Adjust-terminal and GND via long cables)

Is any of the above possible? Would a voltage divider do for rescaling the control voltage, or is a more esoteric approach needed?

In the end the VCA’s should be adjustable from 0 to -12dB attenuation, with the center point of the pots being -6dB. So the talents have a mix range from +/- 6dB.

Thanks,

Rogy[/quote]
Solution a is what I used in the Universal Buffer Series; I used the internal +15v rail, send it to a 10k pot via a 4.7k resistor so I end up with ca. 0-10v DC control, then scale it down to 600mV for 100dB attenuation. You need a voltage follower to provide low impedance to the control port. You will need to scale the voltage divider ratio for 12dB range. Use a low noise opamp for the voltage follower (5532 ok). You may find the need to add a Zobel (47R in series with 0.1uF) across the opamp's output - stability/noise issues. I had no problems with 200metre distance. You need one cap across the lower leg of the voltage divider. I used 100k and 6.2k and 47uF across the 6.2k.
I cannot recommend solution b, because three-pin regulators are way too noisy for the control port.

 

[jdbakker]

We discussed a similar design idea here:

http://www.groupdiy.com/index.php?topic=25108

HTH,

JDB.