ChurchSoundcheck.com

What is a VCA?

by Curt Taipale

VCA stands for Voltage Controlled Amplifier. In a typical analog audio console, the audio signal actually travels through each channel fader. You could think of the fader as a rotary volume control stretched out flat. As you adjust the fader, it uses a resistive element to vary the strength of the audio signal on that channel.

With the fader pushed all the way to the top of its throw, the resistance is zero, so the full audio signal strength goes through. Pull the fader down to the bottom and it provides the maximum resistance, meaning the maximum attenuation of the audio signal, so no audio signal goes through.

Typical Analog Console

With a VCA console, the audio signal doesn’t go through the fader at all. Instead, it actually travels through a small audio amp inside the console. (One for each channel.) A DC voltage is used to adjust the gain of that VCA. So in this case the fader controls the amount of DC voltage being applied to the VCA’s control input, which in turn raises or lowers the strength of the audio signal going through the VCA.

Voltage Controlled Amplifier

Back to our analog console for a moment. Let’s say that channel five through nine have our vocalists’ mics going through them. And let’s say that we want to add some reverb to those vocals. So we dial up one of our postfade auxiliary sends on those channels that feed the reverb effects unit. The output of that effects device comes back into the console (somewhere) and we hear reverb on the vocals.

Since the reverb send is postfade, it will follow our fader movements on those vocal channels. And if we were to pull those channel faders all the way down, the direct sound would go away and the reverberant sound would go away as well.

So that’s fine. Now, we decide that we want to group those vocals on a submaster so that we can control the overall level of the vocals with one submaster fader. Everything is good, the mix is going well, we reach the end of the song and to avoid any distraction of the vocal mics between songs we pull that submaster fader all the way down. Now with the analog console, pulling the submaster down means we won’t hear the direct sound over the system. But …. those individual channel faders are still up, meaning the audio signal is still feeding the auxiliary sends for the postfade effects send.

Following me? With this relationship it is entirely likely that any or all of those vocal mics may pick up some sounds on the platform, which feeds them to the effects device, and your congregation will hear lots of highly reverberant noise or chatter.

The way to defeat that of course is to reach over and mute all of those individual vocal mic channels, or pull their faders down. But that’s an inefficient way to mix. We would rather use the submasters as they were intended.

Okay, so what if this was an analog console fitted with VCA’s on each channel? Now when you pull down the submaster fader it is simply controlling the amount of DC control voltage allowed to reach the individual VCA’s on each channel. If you pull down that submaster, it is just as though you reached over and pulled down each individual channel fader assigned to that VCA submaster. If you press the Mute button on the submaster, it is just as though you hit Mute on each individual channel.

Sweet, eh? Of course even with a standard VCA console, the faders aren’t motorized, so they don’t actually move. But it’s still a great feature to have on a console. It makes the sound operator’s job much easier.

A DCA is a Digitally Controlled Amplifier

DCA’s are found on digital audio consoles, and offer the same types of functional advantages that a VCA does for an analog console, simply done in the digital audio realm rather than analog.

Some digital audio consoles go a step further and provide motorized faders. That gives the operator a visual reference of where the faders were the last time he/she was on that layer or in that operation mode. Not that one has to have motorized faders to take advantage of the DCA feature, but they sure make life as a sound operator much easier.


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