How to Mix Sound, Session 6: Operating a Mixer

Our examples come from Allen and Heath. If you haven’t done so before, go to Allen and Heath’s website and pull down a PDF of the manual for one of their current mixers. I’ll use them as the basis for all the terminology, as they use industry-standard signal flows and function names. Choose a board without a monitor embedded in it, just for simplicity’s sake.

Channel Gain

Signals come in the back of the board and appear on a channel. At the top of the channel, you have a Channel Gain adjustment (also known as a ‘pot’ for potentiometer) that sets the incoming signal level. You adjust the Channel Gain and watch the signal indicators for that channel. You want the green area to be on, and the yellow to be winking only occasionally. Never the red.

Technically:

The adjustment for the Channel Gain is actually an attenuator – after the signal comes in, it is run through a very high-gain, low-distortion, low-noise amplifier. This adjustment is downstream of that, so it is technically a pad. You’re actually knocking down the output of that amplifier to match it to your needs in the channel.

Next, as you adjust the Channel gain, watch the signal level indicator, which for most Allen and Heath boards is down by the fader. You set for flashing zero indication with no blinking of the ‘Peak’ light. This gives you the best signal level through the channel and its processing without distortion from peaks or noise from having to use too much gain.

If whatever’s connected requires phantom power from the board, you push the red ‘Phantom’ switch to send power down the cable to it. It’s called ‘phantom power’ because the power comes from the mixer and not from the device itself. NEVER send phantom power to a line-level device; if what’s connected is not well-engineered, you could burn it up. If you have a dynamic mike connected, keep it off, also. Sending power to a dynamic mike will cause the microphone element to overheat and possibly damage it. Some dynamic mikes have a power blocking mechanism to prevent this very thing from occurring.

If what you have connected is a line-level device, you push the PAD button to knock down the signal level coming in.

Phase is next. Say you have a person, standing at the podium. They’re wearing a body pack in addition to being heard through the podium mic. You are getting sound from both sources, but instead of sounding clean, it sounds kind of muffled or like it’s from the inside of a barrel. That’s because you have sound coming from two mikes that’s out of phase. Pushing this button reverses phase on one mike, so now the sound adds instead of cancels. This is always the first button you hit when you have two mikes in proximity and the sound coming out is ‘just not right’.

As you set up the equalization for the channel, it’s best to listen to that single channel on headphones; but that will be covered in detail later.

Set Your Routing

Now you set up where you’re sending your channel’s output. You can send it directly to the main Left and Right outputs, or you can route it through a Group.

Grouping

Grouping allows you to set up a submix of any number of sources, then just have the submix available on one fader. One example of a submix is a drum kit: Once you have your drummer’s style and sound tuned in to your liking by adjusting the levels and EQ from each mike, you can send the drums to one of your Groups. This allows you to make the entire kit either more or less prominent with just one fader.

Inserts

Inserts are a way to have processing available on just one channel of the board at a time. The insert is handy when you want to have effects like compression applied to just one source, for instance when you don’t want to compress background vocals as much as foreground vocals; since the more compression you add, the louder the sound, and the more ‘air’ that you take out of it.

There is a trick to using an inserted effect: there is no way to control the level coming back from the unit, to allow for headroom downstream of the insert, particularly in your EQ section. So you have to watch both the levels going to the inserted effect and the levels coming back from it. If you get distortion or low gain for no reason, it’s usually due to the inserted effect unit. When using a compressor as an inserted device, you listen to this channel only (Solo the channel) as you set up your compressor. Once it’s set, you control the output of the channel in the normal way – with the fader. You typically do NOT adjust the input gain once you have your compressor set up, as doing so now greatly affects the compressor’s operation.

Sends

Sends are the ‘mixer within a mixer.’ Using the sends, you can set up completely independent submixes that don’t change with the fader levels. These are really useful for ‘mix-minus’ applications, such as on-air ear monitors – you send all the feeds to the anchor, except their own mike. In this ear-monitor situation, you work with the anchor to send them actuality audio, or the raw mike feed from the person they’re interviewing remotely.

The most common use for sends in live work is for stage monitors. You’ll set up one set of sends for the drummer, who needs to hear only the lead’s mike and instrument, another for the bass player, who needs to hear his own instrument, the lead, the lead’s instrument, and the drums… You get the idea: Sends are anything you want them to be.

Signal Flows

Rather than use block diagrams (which can be hard to understand if you’re not used to the symbology), just continue to refer to your Allen and Heath manual, as it gives you a  typical board, with a typical layout. And a block diagram which makes sense when used as a reference to your particular mixer.

Input Stage

The signal (whether mike level or line level) enters the back of the board at the connector. The individual channel inputs typically feature a 48-Volt Phantom Power supply which can be switched in or out. It is then amplified through a special high-gain, low-noise amplifier. This amplifier is self-protected against clipping and distortion, and is a fixed-gain amplifier.

It is the output of this fixed-gain amplifier that feeds your Input Gain pot. Your Input Gain pot is really only a variable pad to the output of the input amplifier. You’re just adjusting the output from that amp to feed the rest of the channel electronics.

EQ Section and Aux Sends

The EQ section applies frequency-based attenuation to the signal. Consider this: The signal enters as a ‘clean’ signal, then you ‘dirty it up’ by adding equalization. Adding EQ will never ‘clean up’ a signal; instead you’re changing it with this filter section.

A good example of this might be a guest artist’s electric guitar that sounds pretty good in the lower registers but has a screechiness to it when he plays the high notes. So you fix the sound by rolling off the highs, beginning with the area where it really starts to sound bad. (We’ll cover specific EQ work in an upcoming session.) This isn’t really ‘cleaning up’ the sound, you’re just making the unpleasant parts much less audible. Meanwhile, the filter effect from the EQ section has its own distortion, and at high levels of EQ, it can be subtly uncomfortable to listen to.

The next section beyond the EQ is the Aux Sends. Here, you can actually create a completely independent mix by adjusting the amount of sound you send to any particular Aux channel. The advantage of the Aux Sends is that your output mix never changes, no matter where the fader is set. Recording studios sometimes use the Aux Sends for a backup recording, where every track is recorded at its optimum level; but the master mix goes through the rest of the board. This assures that none of the performance in the studio is lost, and enables the recovery of the mix, or even a later remix of a particular performance. The Aux Send level for any channel is affected by the Input Gain pot, however. So if you change this master gain on any channel, it will affect all of your Aux Send levels from that channel.

Groups and Mains Out

By setting the switches next to the fader (on the board pictured) you can elect to send a channel’s output to either a Group or to the Mains Out. Grouping works well for when you’re miking a drum kit, and you can just bring the whole kit up and down with one fader. Sometimes background vocals can be handled through one Group; decisions like this are really up to the person on the board. Groups typically send directly to the Mains outs, which are of course controlled by the faders.

One innovative use of Grouping I’ve seen was when one of the mains faders had noise in them somewhere. We  just set the mains at a quiet spot, and ran everything using the Group faders.

Okay, time for homework. It’s going to seem familiar.

Homework:

(Same as last session. Yup.)

Find yourself a copy of some ‘classic’ music and listen to how the sound crew put together the three-dimensional stage of sound. Find something that matches your tastes and give it a good, deep listen with your eyes closed. Some examples would be:

  • Classical and symphonic music from most any era
  • Jazz music from any era up to the 80’s
  • Rock/Pop from any era up to the 80’s
  • Vocals from any era up to the 80’s

So now do something different: find a well-produced piece from, oh, let’s say the Disco era. (Yes, go ahead and laugh.) But – hear me out – listen closely to any Bee Gees album and you find several things: Heavy rhythm, heavy bass, clean sounding drum kit, lots of guitar and keyboard in the mix, super-clean vocals. Every voice and instrument is distinct.

But notice something: It’s INTELLIGIBLE!

Now pick up most any later-produced “rock” or “grunge” piece and try to sort out the vocals. Try to sort out the individual instruments. Try to understand the lyrics with only one listen-through.

Harder than you thought? You’re hearing the result of an indifferent mix, the lack of any art in sound mixing. The result of “well we just recorded it, make us an air cut with the remainder of the 2 hours we paid you for”. Now go back to the Bee Gees and listen again. You can hear the space. Compare with the “grunge” again. Sound like it was recorded in a parking garage?

Now you’re getting it.

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