I recently rewired my studio and patched a compressor and an effects unit into my mixer’s master inserts. The mixer’s manual says “a single three‑pole TRS jack carries the unbalanced insert signal. Tip = send, Ring = return, Sleeve = common ground. The L‑R mix inserts are pre‑fader and operate at ‑2dBu. These should work fine with line‑level signal processing equipment such as compressors, outboard EQ, delay units and so on.” Both processors were set to work with +4dB signals but the levels from the processors seemed low, so I switched them to their ‑10dB positions and the levels seem healthier. Have I done the right thing?
SOS Forum Post
SOS Technical Editor Hugh Robjohns replies: In all matters concerning signal levels the dB suffix is important, as without it the numbers are confusing and misleading! The two common studio and semi‑pro operating levels are +4dBu and ‑10dBV, equivalent to signal voltages of 1.228V RMS and 0.316V RMS. Voltages aren’t as easy to compare as decibel values, so if we convert ‑10dBV into the same format as +4dBu we get ‑7.8dBu. Rounding that to +8dBu, then, there’s a roughly 12dB level difference between the two standards.
The common studio and semi‑pro operating levels are +4dBu and ‑10dBV... There’s roughly 12dB of level difference between the two.
Relating that knowledge to the console’s master insert points, which are designed to operate at a nominal ‑2dBu, we can see that they are running 6dB below the +4dBu studio level, and 6dB above the nominal ‑10dBV (‑8dBu) semi‑pro standard level. In other words, if your compressor is plugged into the master inserts (with no gain reduction being applied) and set to its +4dBu mode, its input and output levels will be 6dB lower than normal. If switched to the ‑10dBV mode they will be 6dB higher than normal.
Both options should be perfectly workable, as most modern devices have sufficient headroom and a low enough noise floor to cope with running 6dB high or low. The obvious worst cases would be a slightly more prominent noise floor in the +4dBu mode, and a slightly greater risk of clipping in the ‑10dBV mode — but both risks can be managed with careful setting of signal levels. In the specific case of the compressor, when dialling in gain reduction you are likely to find yourself setting the threshold to lower (smaller) values for a given amount of compression in the +4dBu mode (because the average signal level is 6dB lower) but significantly higher (louder) values in the ‑10dBV mode, as the input signal level is 6dB higher.
By the way, the reason the console manufacturer has chosen such an odd nominal level for the insert point is simply because that’s the optimum level from a headroom point of view at that point in the signal path of the mixer. As the manufacturer states, although non‑standard, it doesn’t usually cause any significant practical problems — but it may get confusing if the inserted device has meters, since they won’t align with those of the console!