# If a bridged amplifier(s) has 39 db gain ......

#### Osvaldo de Banfield

##### Member
Depending on how did it be measured. If from input to differential outputs, it will be a certain gain, if to only one output it will be 3dB less.

#### rayma

##### Member
Each amp would have 33dB voltage gain. The two amplifiers in series and in opposite polarities
doubles the output voltage across the speaker, increasing the total voltage output by 6dB.

#### Mark Tillotson

##### Member
In electronics dB (as opposed to dBV, dBu, dBm) is a always refering to a notional power ratio, even when not talking about power, which means for current and voltage you square the ratio first, and sometimes you need to be careful to explain if you are talking power, voltage or current.

In RF systems with 50 ohm impedance everywhere then power/voltage/current gains are identical expressed in dB.

#### RJM1

##### Member
Voltage gain (dB) = 20×log (Audio output voltage / Audio input voltage)

#### Mark Tillotson

##### Member
Just to be absolutely clear, its 20 * log10 (Vo/Vi)
making it explicit its the base-10 logarithm. The factor of two comes from the squaring of voltage, you can also do the equivalent:

10 * log10 ((Vo/Vi)^2)

voltage-squared (and current-squared) scales as power (since impedance is held constant).