Most mid- to high-end bass amplifiers include both active and passive inputs. For the uninitiated, the choice of input can cause confusion, even though a noticeable drop or increase in volume output is immediately apparent when the bass is plugged in. The primary difference between the inputs is known as "attenuation," which regulates the volume depending on the type of circuitry built into the bass guitar.
Bass Guitar Circuitry
Bass guitars are equipped with either active or passive pickups and electronic circuitry. Active basses require a source of power -- typically in the form of an onboard battery -- to drive the specially made pickups and built-in preamp. Preamps boost the volume of the bass, providing a louder signal than passive instruments. Passive basses do not require battery power, are not equipped with preamps and are consequently lower in output volume. The choice of active or passive basses is largely a matter of sound preference.
Bass Amp Inputs
Since active basses are louder than passive instruments, they may cause the amplifier to produce a distorted sound. For this reason, the active input uses an internal attenuator to reduce the sensitivity of the input by about 10 decibels to eliminate distortion. Passive basses produce normal sound levels with no boost or distortion issues, and therefore don't require any reduction in attenuation.
Which Input to Choose
Generally, choosing the active amplifier input for active basses will provide optimal results, just as choosing the passive input for passive basses will produce the clean sound desired. There are no strict rules, however, and either bass may be safely plugged into either output to achieve subjective tone. With lower-output active basses, they can actually benefit from the "hotter" passive input. Higher-output passive basses can benefit from the active input's lower output. Sound output variances in both bass types will vary according to the manufacturer, pickups and electronics used.
Older or inexpensive bass amplifiers that include only one input are meant for passive basses. However, active basses can be used by lowering the instrument's volume control to compensate for the volume difference and limit distortion. Amplifiers with hybrid single inputs with a "pad" switch can be used for either bass type. Pad switches lower the attenuation by about 10 decibels when used with active basses and return to normal attenuation for passive basses when switched to normal mode (0 decibels).