One of the keys to good audio is proper gain structure, and by now you’re familiar with the basics such as connecting microphones to mic inputs and line-level sources to line inputs. That’s easy enough, but uncertainty creeps in when interfacing equipment with different operating levels. Let’s unravel some of the confusion.
‡‡        Multiple Standards
We commonly see two different operating levels for line level audio: “+4 dBu,” and “-10 dBV” (please notice the upper case “V”). The majority of pro audio gear operates at +4, but most consumer and semi-pro audio gear operates at -10. The voltage level for gear operating at -10 dBV is 0.316 volts; the voltage level for gear operating at +4 is 1.228 volts.
If you look quickly, you’d think that the difference in dB between -10 and +4 is 14 dB, but that’s not true — it’s more like 12 dB (11.78 if you want to be nit-picky). That’s because -10 and +4 use different references: “dbV” versus “dBu.” 0 dbV is defined as 1 volt of AC regardless of impedance load. 0 dBu is defined as 0.775 volts of AC into an unterminated load (the “u” is for unloaded). This is different from 0 dBv (small “v”), which is defined as 0.775 volts of AC terminated into a 600 Ohm load. The reference to 600 Ohms is left over from the days when audio gear typically had 600 Ohm inputs. Modern audio gear has much higher input impedances, so dBv is kinda outdated (as is its cousin dBm). If you encounter a piece of gear where the operating level is spec’d as -10 dB small-v, double check the specs to make sure it’s not a typo. It’s okay if your eyes glazed over reading that last paragraph — you won’t need to worry about the math.
An operating level of +4 generally results in lower noise, increased headroom, and the ability for a device to drive signal over longer distances of cable. It’s important to remember, however, that there are other parameters in play, most notably whether a signal is balanced or unbalanced. A balanced signal will be less susceptible to interference across a longer cable run. Generally, +4 devices utilize balanced I/O, while -10 devices may have balanced or unbalanced I/O.
The type of connector used on a device is not an absolute indicator of operating level. Gear employing RCA connectors usually operates at -10, and gear using XLR connectors typically operates at +4. Gear using ¼” TS or ¼” TRS connectors could go either way, but keep in mind that there are still some power amps out in the working world that use TS connectors for their speaker outputs.
‡‡        Achieving Zen in the Real World
When all of your gear operates at -10, or all of it operates at +4, you can take operating level for granted. Hey — as long as the console shows that your mix bus level is somewhere around “0,” you should be able to get a decent signal-to-noise ratio, avoid distortion, and probably have some headroom. Likewise, there’s little or no need to think about the fact that your home entertainment gear all plays nicely together at -10. But when you interface +4 gear with -10 gear, all hell breaks loose.
A common example is the analog insert. Most analog mixers (and a surprising number of digital mixers) have an analog insert I/O on each channel. The most common insert type is that pesky ¼” TRS insert where tip = send and ring = return (or vice-versa). A TRS insert is an unbalanced connection where the tip is positive for the send, the ring is positive for return, and the ground conductors for both are tied to the sleeve (that’s how four fits into three). Just about every TRS insert you’ll ever encounter operates at -10 dBV, even if the console inputs and master outputs operate at +4 dBu. (It’s worth mentioning that the inserts on high-end analog consoles are probably balanced +4 if there are separate jacks for the insert send and insert).
Let’s look at the dbx 166xs compressor/limiter/gate (see Fig. 1). Rear-panel switches change the operating level between +4 and -10. If you plan to use the dbx 166xs with a TRS insert on (for example) a lead vocal or a bass DI, the operating level should be set to -10. This ensures proper gain structure through the signal path, and the meters on the 166xs will agree with the input channel meters. It also ensures that the threshold controls will allow you to accurately dial in settings for the comp and gate.
What happens if you set the operating level switch to +4? No small animals will be harmed, but the dbx unit will not behave the way it’s supposed to behave because you are feeding it a signal that’s lower than it expects to receive. The compressor threshold will need to be set very low in order to get any compression, and the gate won’t open until you set the threshold very low. There will also be an increase in the noise floor because the 166xs is receiving a signal far below its operating level, and you may have to increase the channel trim control to make up lost gain.
‡‡        You Can Sleep While I Drive
The need for matching operating level between a mixer and a drive processor is more critical.
The dbx DriveRack PA2 (shown in Fig. 2) has XLR inputs that switch between +4 and -10. If you set the DriveRack PA2 to -10 and patch it between the output of your mixer operating at +4 dBu and the input to a power amplifier, there will be problems. The mixer will likely overload the input of the PA2, resulting in clipping because the mixer is operating at a higher output level than the PA2 is expecting to receive. Setting the compression threshold on the PA2 to “0” will result in tons of compression because even at low outputs, the mixer is pushing too much signal into the PA2.
The problem may be compounded by what happens between the drive processor and the power amps. If the amp inputs are at +4, the drive processor may not drive them hard enough to achieve maximum power output. You’ve lost gain in the signal path. So why provide the option? Because there’s a ton of semi-pro gear out there operating at -10 and they need love too.
‡‡        Shifty Drivers
You may encounter a situation where you’re stuck with a source that operates only at -10 and a mixer or power amp that operates only at +4 (such as if you’re connecting the audio output of a laptop to the line inputs on a +4 mixer). The solution is a “line driver” or line-level “shifter” such as the J+4 from Radial Engineering (see Fig. 3).
The J+4 is designed to “make up” the gain difference between a consumer-level, -10 output (from a CD player, computer etc.) and a +4 input. A bonus is that the device outputs a balanced signal, enabling you to use short cables from the source to the J+4, and use long balanced cables from the J+4 out to a mixer without worry of adding noise.
The ART’s CLEANBox Pro (see Fig. 4) converts signals in both directions. One section of the CLEANBox Pro changes two channels of -10 unbalanced input into two channels of +4 balanced output, while the other section converts two channels of +4 balanced audio into two channels of -10 unbalanced audio. At a street price of just $79, it’s definitely a useful thing to have in your audio toolbox.
Interfacing consumer or semi-pro gear with pro gear may be a bit of a challenge — but it’s not impossible. Knowing the difference is half the battle.
Steve “Woody” La Cerra is the tour manager and FOH engineer for Blue Öyster Cult.
