ICEinLAVA's Domain

In my June article, “Gain Structure Madness,” I hit a nerve with a few readers’ philosophy of drive levels exiting the mixing console and entering the power amplifiers. Those who commented on my idea of keeping drive levels near the zero dBu mark rebutted using the rationale of Signal-to-Noise Ratio (SNR) optimization, and also the idea that every piece of gear in the drive line should reach clipping at the same point.

For me, this puts us into two camps. There is my camp of old codgers that lived and died by the VU meters, with everything defined as line level, never pegging the needle to the end stop (about +4dBu). Then there is the “run it hot” crowd that believes it ain’t clipped until every last red LED on the LED ladder has been lit. Both practices are valid, but each has its downside operationally. Obviously, the run it hot crowd has to back off the amp gain controls by about 10dB or more. The ol’ codgers can leave the gains at full tilt, and never have to worry about someone getting the “crank it up” impulse at the amp rack.

Running the Numbers

To partially justify my bias towards keeping my levels cool while throwing a few decibels of SNR away, I look at the highest “line” level I need to achieve to get every drop of power from my power amplifiers. Right now, I have been using a rotating group of subwoofer amps set up at 32dB of gain (40V/V), the QSC PL-6.0 II and the Lab Gruppen fp6400. Both are capable of delivering about 2,200 watts into a 4-ohm load, which happens to be a pair of 2- by 18-inch subwoofers. Taking the power (2,200w) multiplied by the nominal load (4 ohms) and performing a square-root function leaves us with 94 volts RMS output drive to the speakers. Dividing the output by the 40V/V value gives us the 2.35 volts peak input signal, or about +9.7dBu.

At this point, the “run it hot” crowd will dial in a -10dB setting at the subwoofer amp so that they can put +19.7dBu at the amps’ XLR jack to get +9.7dBu at the gain control wiper contact. Us cool campers are running the amps wide open, and still bending the rules by having to run into the “yellow” to get +9.7dBu. Granted, most us are not pushing the limits in most gigs, and tend to leave the +9.7 and +19.7 dBu levels as our signal peaks, and average most likely six to 12dB lower as average levels.

Our run it hot crowd spares no rest to mention that limitations on most mixing consoles create noisy signal sources that need the extra 10 to 15dB signal excursion. While my impulse is to strike out at your choice of crappy mics, crappy console and poor gain structure tactics, I do share the compassion that many times things do not roll our way and noise can be a constant companion. I usually am happy if my noise floor leaving the console is about -60dBu. That typically means I can easily hear the hiss in the mid and high frequency drivers at one meter away with no program input, but it is pretty much undetectable when quiet program material (sound check CD, speeches, etc.) is being run through the system.

Clip the Amps First

Call me forever poisoned by Hartley Peavey’s Distortion Detection Technique (DDT) feature, but having amplifiers with soft input signal limiters has been a godsend. Such limiting circuits on most power amplifiers today means that driver-killing clipping can be replaced by softer distortion that is audibly detectable, and serves as polite warning you have hit the system limits. Added to that is the knowledge that nothing upstream in the signal path has reached clipping beforehand. This is the first reason why I do not hang with the run it hot crowd, but pay a modest SNR penalty to do so.

With the ever-increasing assurance that our power amplifiers are modern and include signal limiters, it is prudent to actually set the clip levels of the loudspeaker processor, equalizer, or crossover higher than the amplifier limit levels. The run it hot crowd has to sweat the details of how many dBs higher, but the codgers have no worry, with +22dBu minus +9.7dBu as “headroom.” I confess to rarely paying attention to drive signal path levels, and just glance at the power amp rack for proper level indications. My rule is that amps soft-clip (limit), and EQs, comps and crossovers hard-clip; and hard clipping is a driver-destroying practice.

Not Fade Away

Part of getting the mixing console drive levels correct is the backward impact imposed on subgroups and input channel levels. When kicking off a gig, I start with the master faders (a.k.a. the gas pedal) at the -10dB mark and push up or pull back as required with the lead vocal and its sub-group riding the 0dB marks. Small rooms with little stage wash will cause me to back off many dB on the gas, and the outdoor gigs are likely to get me goosing the pedal a few dB as the show mix settles in.

Subgroup fader position philosophy can be all over the map, depending what the console operator thinks is most important. I have tended to fall into the “leave it at the 0dB marks during the show” camp with rare exceptions. Lately, I have begrudgingly begun working the subgroups more, mostly to drop instruments in the quest of keeping the vocals on top of the mix with varying stage wash and performance dynamics. But with the subgroups near zero, I still have to keep a watchful eye on metering the subgroups to avoid running out of headroom.

The subgroup headroom issue then trickles down to the channels. I used to love working the channels hot, like in the +3 to +10dBu range, and throw away the levels in the channel fader or gas pedal. However, I have mended my ways towards cooler peak values in the 0 to +3dBu range so that my compressors and insert effects are not running on limited dynamic headroom. By running cooler at the channels, not only do I avoid summing amp clipping at the subgroups (in non-VCA desks), but it forces me to step on the gas pedal (faders) a bit more. I find these practices avoid the problem of “running out of fader” in any signal path on the console.

The above paragraphs are not meant to be just educational, but to stir thought on the mix creation process. Please feel welcome to share your insights, tips and criticism on these fader and headroom tactics. Let us also discuss what not to do, and anecdotal stories are welcome as letters to the editor and in the FOHonline forums.