Tuesday, June 06, 2006

Gain Structure Within Your Mixer

Today's Topic: The Basics

Today, I'll cover one last topic related to gain structure: gain structure within your mixer.

Think about the flow of signal through a mixer: it starts at the preamp, then passes through an equalizer, the pre-fader aux sends, a fader, the post-fader aux sends, a subgroup fader, a master fader, and then an output transformer. Yours may have more or less steps, but this is the basic pattern for most mixers.

Similar to the different components in your sound system, the different components within your mixer have an expected input level and a normal output level. For well-manufactured mixers, these are all aligned appropriately and the gain knob on each channel is the only "gain" you need to consider in your mixer. However, even high quality mixers will occasionally have internal gain problems, especially when all or most of the channels are being used at the same time.

Here is the basic principle I want to demonstrate:
When combining multiple input channels together within your mixer, reduce the levels at the channel strip, before they add together and are passed on to the master output level.
Consider this example from a medium-sized church: The church has a 24-channel, mid-grade mixer that it's about to outgrow. However, it cannot afford to upgrade yet, so it sticks with the smaller mixer and the band mics and direct boxes take up 23 of the 24 channels.

The operator appropriately sets each of the channels' gain levels so that the meters generally hit 0dB. He starts the faders out at around unity and works on the mix.

Everything sounds great when the band plays at a normal level. When the band plays loudly, however, he notices that the overall output meters are peaking and he hears some distortion. He pulls back the master sliders around 15dB to get the output levels to get them to a more reasonable level. All the red lights are gone. All better, right?

Not quite.

Stick with me here.

When the signal from all the channels are combined and sent to the master outputs, they add together. Every time he doubles the number of channels that are turned on, he adds 6dB to the output level. If only 1 channel is turned on, the master outputs are at 0dB. With 2 channels, they hit +6dB. With 4 channels, they charge to +12dB. 8 channels = +18dB; 16 channels = +24dB; 23 channels = +25dB to +26dB.

In reality, the operator in our example has added all his channels together onto the main output wire which physically sends the electricity through the master fader. That master fader has a preferred input level of 0dB, but it is getting +25dB instead. If the fader isn't hearty enough to handle that +25dB signal, the distortion happens here, at the "input" of the master fader, though it isn't shown until after the fader, on the output meters.

The operator error happens when he assumes he has solved the problem by lowering the master fader. While he has eliminated any red lights, the input of the master fader itself is still distorting and he's only lowered the overall level of the combined music/distortion.

Instead, he should have applied the principle I highlighted before...

When combining multiple input channels together within your mixer, reduce the levels at the channel strip, before they add together and are passed on to the master output level.

In other words, he should lower each of the individual channel sliders in order to reduce the combining effect and the distortion at the master fader.

The same thing can happen with an aux master (knob or slider) to a monitor or headphones. Routinely, I walk by our monitor board and see aux master sliders down 10dB from "unity." I know exactly what happened. The musician said, "Hey, can you just turn down my overall mix?"

The easy thing is to pull down the master. The right thing is to slightly lower each of the channels' levels in that auxiliary. Unfortunately, even our monitor operators often do the easy thing rather than the right thing. The result: because our monitor console doesn't have the best headroom, I can almost promise you that the person listening to that mix is hearing some sort of distortion.

So, what should you do? Just remember...

When combining multiple input channels together within your mixer, reduce the levels at the channel strip, before they add together and are passed on to the master output level.


More Posts on The Basics

1 comment:

David Wilcox said...

Note: If you read this post immediately when it went up, you got some bad info. I've now changed it from 3dB per doubling to 6dB per doubling, as is accurate. Sometimes its just been too long since I really studied!