It’s time for me to come out of the closet: I’m a dyed-in-the-wool unity gain structure freak.  There, I said it and I sure do feel better. Catharsis is a good thing. 

Now, for those of you who aren’t sure what the heck I’m talking about, I’ll explain, and maybe get a few of you over to the dark side with me.

First some clarification. Gain in sound parlance is the act of adding to (positive), subtracting from (negative), or leaving unchanged (unity) the amplitude of an audio signal. 

The first gain stage in the typical signal path is the channel preamp, followed by the channel fader, bus master, and main master. This is then followed by all the gear that’s patched between the console and the amps. 

The method by which you set the many gain stages in the signal path after the preamp is known as gain structure.

There are two prevailing methods of establishing the gain structure of a sound system - unity and optimal. (There are also sound systems that don’t follow any established gain structure whatsoever.) 

Unity structure means that the gain for every piece of gear in the signal chain is set at a 1/1 ratio.  That is, the level of the signal going out is the same as that coming in. 

Optimal gain structure means setting the clipping point of every piece of gear in the signal chain at the same level. The outcome of this is that any one particular piece may have positive, unity or negative gain, because the amount of headroom in individual components varies by manufacturer and product tier.

The only obvious practical result in a gain structure comparison is that optimal gain structure yields an overall lower apparent noise floor in a sound system. But what’s not so readily apparent is that optimal gain structure reduces the available headroom, which is a big no-no.

Headroom is an oft-misunderstood concept. Back in the day, headroom meant the difference between the level at which a circuit would clip and the current peak level of the signal coming from that circuit.  If a circuit clipped at +22 dBu, and the signal was currently peaking at +10 dBu, then there was 12 dB of headroom. 

Today, however, headroom is often used to describe the difference between peak and RMS (root mean square) values, especially in digital signals where an RMS value at –10 dBFS is said to have 10 dB of peak headroom.

Thanks to one Peter Dix, I had my head unscrewed and adjusted regarding my interpretation of headroom and my gain structure agnosticism.  Peter is somewhat of a dark wizard of the audio arts, and has been building custom amplifiers and loudspeakers at his shop near Vancouver B.C. for years. (Just be careful striking up a conversation with Peter - it may take awhile to get your ear back.)

Anyway, the point Peter emphasized is that there should always be headroom left unused in a circuit, ideally never less than 6 dB. The closer a circuit runs to its maximum, the greater the non-linearities (distortion), so by staying well under the maximum, the quality of the sound through that circuit will be sweeter.

This ties in with the term “earlash” that was coined years ago by U.K. designer Ben Duncan (http://benduncanresearch.com/). Earlash is the quality in a sound system that causes one to wince.  It is not a function of loudness, but of distortion, whether it comes from an electrical origin in a gain stage, mechanical (i.e., throat distortion in a horn), or spectral imbalance. 

My first priority when setting up a system is to avoid earlash, and I’ve found that a unity gain structure helps do that.

Note that there is one place in a sound system that requires some special gain structure attention.  Looking at power amplifier specs, you’ll see that the input level sensitivity for full output is usually right around the “old 0VU standby” of +4 dBu (1.23 volts). This means that while your console and subsequent gear may have 18 dB or so of headroom available above +4 dBu, the amplifiers don’t. 

What the heck?

Getting a usable gain level out of an amp, in order to power multiple loudspeakers, requires the amp to run at higher average levels with much less available headroom. Thankfully, that’s exactly what they’re designed to do, while the typical line level gain stage is not.

All professional amps I’m familiar with have passive input attenuators and integrated clip limiters. The clip limiters prevent the amp from clipping when an audio peak exceeds the input sensitivity (thus protecting your loudspeakers), and the input attenuators can be used to pad the signal into the amp.

While there are instances when the amp input needs to be padded, I often come across systems where the signal from the console has been over-attenuated. When the knobs on the amp are turned down, the signal from the console has to be hotter. And this means:

Hotter signal = less system headroom. Less headroom = poorer sound quality. 

Therefore, it’s better to set amp inputs wide open and control the system volume at the console.

Next time, I’ll be talking about unity mixing.

Check out another fine article by James Cadwallader here on PSW: No Slave To Gear - Maximizing What You Get Out Of What You Have