More than just the right types of connectors and/or adapters are required to interconnect various pieces of audio gear: input and output voltage levels must be properly matched. As a result, it’s important to understand what voltage level each piece of equipment needs in order to function properly. Getting it wrong can result in things like an increased noise floor, signal clipping, and even electrical damage to console preamps.
Let’s do a quick study of the typical voltages found in standard audio connections and then go over how to convert from one signal level to another. But first, we need to get the pesky decibel stuff out of the way.
In the audio world, 0 dBu is the standard reference level to compare everything else to. This was defined by Bell Laboratories back in the 1930s for the original telephone system. Bell Labs used the reference of 0 dBm to represent 1 milliwatt, and the voltage level needed to provide 1 milliwatt of power across a 600-ohm load just happens to work out to 0.775 volts.
But since we’re not driving 600-ohm loads with modern sound systems, we just use the “unloaded” voltage reference and call it 0 dBu (with the “u” meaning “unloaded”). The math can be checked pretty easily since volts squared divided by resistance (or impedance in a reactive circuit) is equal to watts. So (.775 volts x .775 volts) / 600 ohms = .001 watts (which is 1 milliwatt).
Again, in the audio world, +4 dBu is typically used as a console output reference for driving power amplifiers and other equipment, plus decibels work really cool in that they’re just added together. So, a +4 dBu signal is 4 decibels hotter than 0 dBu, and +4 dBu works out to 1.23 volts rms, which means that a console is outputting 1.23 volts rms when the meter is hitting +4 dBu, or 0.775 volts rms when the meter is actually showing 0 dBu. Also note that the dBV standard is often confused with the dBu standard. It uses 0 dBV = 1 volt as a reference.
Some of the other dB levels found in a typical sound system, along with the approximate voltages that go with them, include:
• Mic level XLR: – 50 dBu, ~ 1/500 volt (0.002 volts)
• Consumer RCA: – 10 dBV, ~ 1/3 volt (0.36 volts)
• Line level XLR: + 4 dBu, ~ 1.23 volts
• Speaker level: + 30 to +60 dBu, ~ 25 to 60 volts
By the way, if you want to run your own numbers, there’s a nifty online dB-to-volts calculator at sengpielaudio.com.
Now let’s define connectors and the voltages normally associated with them. First up, the ever-popular XLR connector, found on every console and professional microphone (with the possible exception of a few AES/EBU digital mics). Note that there are male and female versions of the XLR, with the pins pointing in the direction of signal flow.
In Figure 1, the one on the left with the pins going in is the “input” while the one on the right with the pins pointing out is the “output” (at least in audio). The lighting/DMX world is backwards of this, but don’t blame me – we’re talking audio here.
What’s not so obvious, perhaps, is that depending on where an XLR is located in a console, it’s either a line level output with +4 dBu level (1.23 volts) or a mic level input expecting something around -50 dBu level (0.002 volts).
Take a look at the back panel of a Behringer X32 rack mixer (Figure 2) for the plethora of connectors, and while you’re at it, note the other decidedly non-analog signals such as the AES/EBU output and the AES50 ports. (We’ll save digital connectivity for another article.)
In addition, many devices have XLR outputs that can be switched between line level (1.23 volts) and mic level (.002 volts). We often see this as a switch on the back of wireless microphone receivers listed as mic/line (Figure 3).
The most dangerous levels of all (except for AC power, of course) are courtesy of the 1/4-inch jacks on most amps for guitars and bass (Figure 4). As noted earlier, speaker level voltage can easily be in the 25-volt to 50-volt range, which works out to +30 to +60 dBu. And there are quite a few amps that include both speaker level and preamp/line level outputs on the same panel, using the same 1/4-inch jacks. That’s an accident waiting to happen.
What does all of this mean? To interconnect properly and safely, we have to convert the style of connector and we have to match the level.
Direct (DI) boxes are often a key in this quest. Not only do they convert the 1/4-inch phone plug output of a keyboard or guitar to XLR, they also drop the level about 22 dB.
So, if we take the line level output of a keyboard (maybe +4 dBu) and attenuate it by 22 dB, the result will be a mic level signal of -18 dBu, because +4 dBu minus 40 dB equals -18 dBu (see how that works?).
DIs (Figure 5) incorporate transformers that do other good things as well, such as letting us lift the chassis grounds between stage gear and consoles to eliminate ground loop hum, as well blocking 48-volt phantom power from back-feeding into gear and blowing up the outputs. This is why we really don’t want to use a 1/4-inch phone to XLR male cable to plug in stage gear. The signal level may be too hot for a mic level input on a console, plus there’s no ability to do a ground lift to stop hum.
But the real danger is when this cable is (accidentally) plugged into the loudspeaker output of a stage amp and then connected to the snake feeding a console’s XLR inputs. Recall the stage gear with both preamp and loudspeaker outputs on 1/4-inch jacks? Here’s a story that should make us think twice about utilizing adapter cables to connect a guitar or bass amp to a console instead of using a DI.
Solving The Mystery
Over the past 20 years or so, I’ve taught audio seminars at hundreds of churches, and occasionally, I come across a console with a fader or two taped over with the universal sign of a big “X” to indicate a dead channel. I ask the tech folks if they know what happened, and the universal answer is usually something like “lightning must have hit the building and burned out the channel (s).” However, the fact is that if lightning actually did hit the facility’s electrical service, it would burn out more than just a channel or two on a console.
I’d never received a better explanation until a visit to a church with a console with three faders in a row marked as dead. I asked what had happened, and they indeed knew the circumstances: there was a long-standing habit of using a 1/4-inch to XLR male adapter cable to connect the preamp output of a guitar amp to the main PA system. Because it was only sending +4 dBu line level to the console, and the preamp was OK with +4 dBu (which any good preamp should be), it worked fine for years.
But one Sunday, a newbie accidentally plugged this same 1/4-inch to XLR male cable into the speaker level output on the guitar amp. This sent about 50 volts down the XLR cable into a console preamp that was expecting maybe 0.01 volts. One big guitar chord, and wham, the preamp fried.
When this happened, no one knew why, so the XLR cable was simply swapped to the next channel. Wham, another channel was immediately fried. The cable was then transferred to a third channel with the same result before someone got the idea that maybe something from the stage was destroying the preamps.
Sure enough, it was discovered that the cable was plugged into the wrong output of the guitar amp. The result was three useless channels, and even inexpensive consoles can be expensive to fix.
The lesson is to always use a DI to interconnect stage amps with consoles, not just adaptor cables. Generally, speaker level output plugged into a DI won’t sound great, but at least it won’t kill the preamp (and therefore then entire channel) on the console.
Finally, let’s look at changing +4 dBu to -10 dBV signal, and visa versa. We can get into trouble taking the +4 dBu (1.23 volt) output of a console to try to drive a recorder with RCA inputs looking for -10 dBV (1/3 of a volt). Or going the other way, we might want a component with RCA -10 dBV outputs to drive something looking for line level.
Nothing horrible will happen if the signal levels aren’t matched perfectly, but there will be increased noise level, and perhaps some signal clipping. As a result, the best solution is to employ a line level shifter to convert +4 to -10 and -10 to +4. An added bonus is transformer isolation that prevents ground loop hum between various pieces of gear.
The Ebtech line level shifter shown in Figure 6 will not only step the signal level up or down needed, it also adapts the cable type from XLR to 1/4-inch phone. It will also convert balanced signal to unbalanced, and unbalanced signal to balanced.
Understanding the various signal levels each component requires is key to successfully getting everything to “play nice” together within a system. Finally, I regularly carry a gig box with all sorts of DIs, line level shifters, and adapter cables to correctly deal with stray stage amps and processors, and I suggest you do the same.