VCA's Explained

In Uncategorizedby tfwm

Some of the most common questions from those researching mid- to large-format live sound consoles for their houses of worship involve VCA groups. What are they? How are they used? What are the benefits?

A VCA, or Voltage Controlled Amplifier, is an electronic device in the form of an integrated circuit, or chip, that sits in the audio circuit of each input channel in a mixing console. The audio signal through each input channel passes via the VCA, which controls the level of that signal based on a DC voltage applied to it from an external controller such as a fader.

Unlike a conventional console, the channel fader in a VCA console does not pass audio, instead controlling the VCA in the input channel circuit. This offers several advantages, one of which is that it drastically cuts down on the wear and tear normally associated with standard analog faders.

A conventional fader is essentially a resistive circuit, like a rotary level control stretched out into a straight line, or a lighting rheostat. Constant use of a conventional fader, which passes the actual channel signal, leads to a deterioration of the fader’s resistive surface, requiring it to be carefully maintained and eventually, more than likely, replaced. In a VCA console, noise filtering may be applied to a VCA fader circuit to reduce the audible effects of fader wear.

VCAs are by no means new. They are also used in outboard signal processing devices such as compressors, limiters, and noise gates, where circuitry acts on VCAs to produce the desired effect. They are not new to consoles either, having originally been fitted into high-end recording studio consoles, where mix automation systems recorded and played back the voltage changes at each input channel VCA, using a computer synchronized to timecode on the multitrack tape.

Except for elaborate Broadway-style shows, live sound consoles generally do not benefit from timecode-based automation. But VCA control is most definitely beneficial. Since each input channel VCA responds to an external DC voltage, the signal level can be controlled not only by its respective channel fader but by other faders, also. That has allowed the development of VCA group faders, dedicated faders in the center section to which input channels may be assigned, in much the same way as conventional audio subgroups.

Compared to recording consoles, VCA technology has only relatively recently been applied to live sound consoles, especially large desks suitable for major touring applications, where the ability to create VCA subgroups, plus the associated muting functionality, is extraordinarily useful. As the technology has matured, it has trickled down to become more commonplace in live consoles that are within the budget of many houses of worship.

So how do VCA groups differ from audio subgroups?

Assigning an input channel to an audio subgroup sends the signal through a buss to the group channel with its associated gain stage, subgroup fader, and output connector, plus, on some consoles, effects insert connectors (see Audio Group diagram- print version). Since no audio passes through a VCA fader, a VCA group fader simply acts as a control device for the channels assigned to it (see VCA Group diagram- print version).

An audio subgroup is absolutely essential in certain circumstances. For example, groups of inputs can be grouped, submixed, and sent to a recording device in situations where there are insufficient tracks available to record each channel from the respective direct outputs. In distributed speaker systems, the audio groups can be mixed into the matrix output system to feed separate loudspeaker clusters or locations.

Most importantly, where circumstances do not allow individual signal processing channels to be used on each input channel, outboard devices can be inserted into groups of inputs. Thus a single compressor can be applied to multiple choir vocal microphones, or to an entire drumkit.

But audio subgroups also have disadvantages when compared to VCA groups. Whereas it would require two audio subgroup channels to control input channels with stereo panning (one subgroup panned to the left, the other to the right), or three for LCR panning, a single VCA group fader can control such inputs.

That’s because, in a VCA console, the VCA is located in the channel circuit before the panpot. Panning, routing, and pre-fade auxiliary sends are therefore unaffected by the VCA.

Controlling stereo or LCR groups using VCA group faders has the additional benefit of freeing-up audio subgroup faders. There are a number of FOH mixing consoles available that allow the audio group faders to be used to control auxiliary send levels, and create stage monitor mixes with fader control and insert capabilities. The more audio group faders available, the more monitor sends that can be set up on a front of house console doing double-duty as a monitor desk.

VCA groups can also be overlapping, with inputs assigned to more than one group (see diagram 3- print version). This allows sophisticated control of instruments and voices. For example, a VCA group could be created for all of the choir microphones, with a second VCA group controlling only those mics used by the soloists, allowing the overall vocal level to be controlled with one fader and the balance of the solo voices against that group with another.

In the same way, instruments can be grouped. The drums and percussion might be on one VCA group fader while the entire rhythm section — drums, percussion, bass, and piano — is on a separate fader.

For further sophistication, if different groups of instruments and voices must be made more or less prominent in different pieces of music, they can be grouped accordingly and controlled on separate VCA faders. To offer a simple example, a solo vocal mic, piano, bass, and drums may be in one VCA group, with a different solo vocal mic, guitar, bass, and drums on another VCA group fader.

Audio and VCA groups are not exclusive, either. Where processing is to be inserted, an audio group is essential. But that exact same group of inputs can also be assigned to a VCA fader to provide overall level control.

VCA grouping also allows the balance between input sources and effects returns to be maintained more easily. A reverb, for example, inserted via an audio group and applied to several input sources, will maintain its return level even though that audio group fader is pulled down. But a VCA group can also include the channel into which the reverb is returned, maintaining the balance between effected input sources and effect return.

If a single effects device is shared by two groups, VCA control allows the post-fade aux sends for the individual groups of inputs to also be attenuated. When the first group fader level is reduced, the balance between source and effect for that particular group is again maintained, while the second group’s effect return level is unaffected.

The benefits of VCA group faders also apply to muting. VCA group faders have associated mute switches, thereby acting as an additional layer of mute groups. How they are implemented may differ; on some consoles it is a master on/off switch, while on others the VCA group mute switch turns any assigned input channel mute on/off.

Using ‘soft’ switching for VCA group and mute assignments also opens up the possibility for console manufacturers to incorporate those settings into ‘snapshot’ memory automation using MIDI, onboard memory, or RS232 to an outboard controller.

In practical use, and leading on from the discussion on console gain structure in the previous issue of this publication, each input channel VCA fader should be set at the ‘0’ position. That represents 0V, in other words, no change to the audio level through the VCA. Console manufacturers design VCA consoles such that 0dB unity gain is passed through at 0V. That allows multiple input channels at 0V to be summed to a VCA group without producing a gain change.

The 0dB/0V position is usually marked clearly on the channel fader and VCA group fader faceplate. On some consoles there are even LED indicators that show that the fader is at or close to 0.

Lowering the VCA group fader by 10dB lowers each input level while retaining the relative balance. An input set at 0dB now becomes -10dB, an input at -10dB goes to -20dB, and a channel set at -20dB now produces -30db.

The flip side of this is that where a single channel is assigned to multiple VCA groups the output level is the sum of the VCA group levels. With the input channel set at -5dB, group 1 at -10dB, group 2 at +5dB, and group 3 at 0dB, the resulting output is -10dB (see diagram 4- print version). One ‘gotcha’ for those new to the concept: Be aware that if any one VCA group fader to which an input channel is assigned is all the way down at infinity, then that input channel will not be audible.

As mentioned previously, pre-fade aux sends are unaffected by changes at the channel VCA. Monitor feeds from a FOH desk generated on aux sends will not therefore alter level in response to input channel VCA level changes.

A single VCA fader could be set up to control all of the choir and instrument inputs, allowing the operator to reduce the level or mute all of the performers while a single pulpit microphone is open. Such a fader, controlling all or a majority of the inputs, is referred to as a ‘grand master.’ If the monitor feeds are set to pre-fader, then there is the possibility of noise — a cough from one of the choir members, for example — being heard through the onstage speakers. Setting the monitor sends to post-fade will prevent this from happening, as it will not only eliminate input channel feeds to the main speaker system, but also to the monitor system when the grand master VCA fader is pulled down. This is especially useful if radio microphones are in use, so as to avoid unwanted ‘backstage’ or handling noise to enter the monitor system.