5. Inputs & Outputs
For flexibility, most acoustic amps have several inputs:
Active & Passive Instrument Inputs
Guitars with passive piezo or magnetic pickups require a high impedance input for optimal performance, but modern electro-acoustic guitars usually have active piezo pickups which ideally need a low impedance input. Most acoustic amps have either an active/passive switch, or separate inputs - some also feature a pad switch to ensure distortion-free sound from high output active systems.
Many acoustic amps feature a mic input, either via a combination mic/line input, or a dedicated microphone channel. This feature can be very convenient for singers playing small venues who then dont need to take along a full PA system, and can also be used to amplify acoustic instruments without electronic outputs. Some instruments are best captured using a high quality condenser microphone, and this requires a mic input with +48V phantom power. For maximum flexibility, each input or channel should have its own EQ and FX controls.
As acoustic amps are generally designed with a flat response in mind, they are also ideally suited for amplifying full-range devices such as CD players and drum machines. Many amps therefore have an auxiliary line level input, increasing their usefulness for players gigging with backing tracks.
Most acoustic amps are equipped with a variety of output options. A line output can be connected to many PA mixers, but most engineers would connect this straight to a DI box anyway, so many amps have incorporated one into their design. (A DId signal is balanced which avoids noise being picked up in cabling.) For maximum flexibility, the DI output should have a pre/post switch so that the signal can be taken from either before or after the amps EQ and master volume most PA engineers prefer the former option, so that the guitarist can adjust their on-stage volume independently of the front-of-house mix. Some amps also have a ground lift on the output as a precaution against mains hum.