An audio player has a −10 dBV output. The next device expects +4 dBu. How much gain separates those two levels?
Subtracting the labels gives 14. The answer is about 11.8 dB, because dBu and dBV start counting from different voltages. Once you see where their zero points sit, the calculation becomes much easier.
The zero is a reference, not silence
A reading of 0 dBV means 1 volt RMS. A reading of 0 dBu means approximately 0.775 volt RMS. RMS is the voltage measure used here; keep it separate from a peak-voltage reading when comparing specifications.
Think of two rulers whose zero marks are offset. The same point receives two different numbers. Likewise, a 1 V RMS audio signal is 0 dBV and about +2.21 dBu. Changing the label does not change the signal.
Analog Devices’ reference calculator identifies these voltage references. Both units describe voltage level. Neither tells you power in watts without further information about the load.
First put both levels into volts
The conversion is V = Vref × 10(L/20). L is the level in decibels; Vref is 1 V for dBV or 0.775 V for dBu. The answer is volts RMS. This article uses the rounded 0.775 V reference throughout.
| Label | Calculation | Voltage RMS |
|---|---|---|
| −10 dBV | 1 × 10(−10/20) | 0.3162 V |
| +4 dBu | 0.775 × 10(4/20) | 1.2283 V |
Now the comparison is ordinary division: 1.2283 ÷ 0.3162 ≈ 3.88. The target voltage is about 3.88 times the source voltage. Express that ratio as gain:
Gain = 20 log10(Vtarget / Vsource) ≈ 11.8 dB. The volt units cancel inside the ratio. The logarithm is base 10.
There is a useful shortcut for later: dBu ≈ dBV + 2.21. So −10 dBV is about −7.79 dBu. From −7.79 dBu to +4 dBu is 11.79 dB, giving the same rounded answer. The Rane BB22 guide, English page 5 discusses this reference trap and rounds the required gain to about 12 dB.
Why the bigger number may describe a limit
The calculation compares two stated levels. Before applying that much gain, find out what those levels mean on the actual products. A nominal operating level describes one thing; the maximum input before overload describes another.
Suppose a hypothetical input has a +4 dBu nominal level and a +18 dBu maximum, measured on the same basis. The 14 dB difference is the space between those ratings. It is not an instruction to add another 14 dB to every signal.
A few reference points help when reading a specification:
| Level | Voltage RMS |
|---|---|
| 0 dBu | 0.775 V |
| 0 dBV | 1.000 V |
| +4 dBu | 1.228 V |
| +18 dBu | 6.156 V |
A digital meter marked dBFS introduces another reference. You need the device’s analog calibration before translating that reading into volts. Biamp’s gain-structure guidance explains why the input and output settings matter.
For the connection in front of you, write down the source’s nominal and maximum output, then the receiving input’s nominal level and overload limit. Convert comparable ratings to one scale. Check wiring and loading separately. That gives you a reason for the gain setting, which is the useful part of building a gain structure.