dBV
Signal Level
Power into
Gain Between Two Points
dB → Ratio
dBV (re 1 V)
-20.00
dBu (re 0.775 V)
-17.78
Peak / Peak-peak
141 mV
283 mVpp
Power into 8 Ω
1.25 mW
1.0 dBm
Stage gain
100.00×
Gain in dB
40.00 dB
20.0 dB voltage
10×
20.0 dB power
100×
Engineering Notes
How it works
The decibel expresses ratios on a logarithmic scale, matching how we hear and making cascaded gains simple to add. For voltage the factor is 20 (because power ∝ V²), for power it is 10:
dB = 20·log₁₀(V₂/V₁) dB = 10·log₁₀(P₂/P₁)
Absolute levels need a reference voltage:
- dBV: referenced to 1 V RMS (consumer gear, −10 dBV line level).
- dBu: referenced to 0.775 V, the voltage that dissipates 1 mW in 600 Ω (pro audio, +4 dBu).
- dBm: power referenced to 1 mW.
dBu = dBV + 2.21 Vpeak = √2·Vrms Vpp = 2√2·Vrms
A typical 12AX7 stage gives ~35 dB of gain; three of them cascaded give about 105 dB, which is why high-gain guitar amps need careful layout to stay quiet.