Let’s do a little math. (I know, you hate math.) But do it anyway. Let’s suppose a 32-channel digital mixing console has a retail price of $15,000. And let’s say that this mixing desk has 16 outputs. That yields a total of 48 I/O channels, each of which requires a certain amount of audio circuitry. Granted, most of the audio processing happens in the digital domain, so we’re not exactly in need of 48 hardware parametric EQs, but we certainly do need 48 amplifiers plus A/D and D/A conversion. That comes to around $312 per channel.
Not a bad budget, but when you throw in “frills” such as a power supply, metalwork, faders, switches, LEDs, display screens, plus profit margin and product development, the actual amount of cash that goes toward the analog circuitry is way less. Being the audio nuts that we are, it’d be nice if each channel path in a console had a budget of $1,000 — but such is not the case, and so real-world compromises must be made. The state of the modern mixing console art is indeed quite high (even on the lower shelves), but there’s no substitute for the audio quality delivered by a premium input channel. If we can’t afford that premium for every channel, we certainly can afford it for some channels, or at least the money (“star”) channel. That’s where all those beautiful outboard “channels” spotlighted in this month’s Buyer’s Guide come into play. (See page 47).
Inside the Money Channel
Outboard channel strips give us the ability to have one or two high-end, super-high quality audio paths for a few critical channels. Typically a channel strip contains a microphone preamp, EQ, compressor, possibly a limiter and expander, and maybe an A/D converter. As channel strips have dedicated power supplies, they typically have considerably more headroom than console preamps, and let’s face it: spending $1,000 or more on a single channel has got to provide better audio than the input channel found in most consoles. When you don’t tour with a mixing console, carrying a rack-mount channel strip for the lead vocal helps provide consistent sound from venue to venue, with less variation resting on le console du jour. Your singer would like it.
Creature Features
What are some of the valuable features for a channel strip? Starting with the mic preamp, phantom power is a must, even if you are using a dynamic microphone at the moment. La Cerra’s Corollary to Murphy’s Law states “You will need phantom power as soon as you don’t have it.” Confirm that the phantom supply actually delivers 48 volts. I have occasionally run into preamps that supply 24 volt phantom — which is sufficient to run most (but not all) condenser microphones. Next on the list for the mic preamp section would be a high-pass filter, preferably one that’s adjustable so you can sweep the filter cutoff frequency. Less critical — but sometimes equally helpful — is a low-pass filter, which can help tame the audibility of spill from the stage. I’d consider a pad, polarity reverse and an input level meter a must. Some channel strips have meters that can be switched to different points in the signal path, which is also quite useful.
A parametric EQ is much more versatile when dialing in your sound than simple high and low-shelf EQ controls. If the EQ is not a true parametric and omits the bandwidth control, look at least for mid-sweep controls so you can home in on problem areas. An in/out EQ bypass switch will allow you to hear whether you’re doing more harm than good.
For the compressor section, threshold, ratio and output controls are a must. I’d like to also see attack and release controls, knee, bypass, gain reduction metering and perhaps a high-pass filter for the sidechain. A separate limiter can help ensure that the output of the channel doesn’t exceed a desired level.
‘Round back, we of course need a microphone input, but a line input is also a good idea, and an insert point opens up a world of possibilities for advanced processing or splitting the signal for recording purposes. We need at least one analog line output; two would be even better (read on). If the channel strip has onboard A/D or a card slot for future expansion, look for AES and S/PDIF digital outs. An optical output can facilitate interfacing the unit with a wide variety of gear, and word clock I/O can be pretty important (also see below).
When Will You Return, Jack?
Usually, the output of a channel strip is connected to the line input of an analog console, but you can bypass some of the console’s input path by using the channel insert return. Mixing desks that provide separate jacks for insert send and insert return make this easy. Simply use the insert return jack as a line input. If the console has a single TRS insert with tip=send, ring=return (or vice versa), you’ll need to make a cable that connects the channel strip’s output to the return on the TRS. In that case keep the cable short, because TRS send/return inserts are, by nature, unbalanced.
Routing one channel strip to separate consoles for monitors and front of house can be a bit tricky. We’d typically patch the output of a microphone to the input of the split, but since the output from a channel strip is line level, you’ll need to verify that the split can handle the signal level without overload. Passive splits should be able to accommodate line level, but if there are microphone transformers in the signal path, it’s entirely possible to overload them. Some channels may offer the ability to output at mic level, but that sort of defeats the whole purpose of avoiding the console’s mic preamp in the first place. If the channel has two line-level outputs, feed one to the monitor desk and the other to the FOH desk. Be aware that the line inputs on some desks go through the mic preamp anyway, just at lower gain levels. You may need to carry your own passive or transformer-isolated splitter, and if so — don’t cheap out.
Worms, Anyone?
Some channel strips have onboard A/D conversion or an expansion card slot for adding a digital output, allowing you to route audio digitally into the console. If you use the console’s digital input, you’ll probably have to assign the input to a channel fader, because AES and S/PDIF digital inputs often default to some sort of stereo return. This opens up quite the can of worms if you want to digitally feed two consoles, because exchange of digital audio requires that all devices share the same word clock. Only one device can be the word clock master, leaving you with two options: send word clock from the channel to the mixers or vice versa. It’s probably easier to implement sending word clock from one of the mixers to the channel, so you’ll need a channel strip that has the ability to sync to an external clock signal via BNC clock input, or AES or S/PDIF digital input. Not all do.
Stay tuned to T&P. Next month, we’ll discuss the word clock in more detail.
Steve “Woody” La Cerra is the tour manager and front of house engineer for Blue Öyster Cult.
