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.
Playing Nicely
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.