Effects are often incorporated into amplifiers and even some types of instruments. Electric guitar amplifiers typically have built-in reverb and distortion , while acoustic guitar and keyboard amplifiers tend to only have built-in reverb. Some acoustic instrument amplifiers have reverb, chorus, compression and equalization (bass and treble) effects. Vintage guitar amps (and their 2010-era reissued models) typically have tremolo and vibrato effects, and sometimes reverb. The Fender Bandmaster Reverb amp, for example, had built-in reverb and vibrato. Built-in effects may offer the user less control than standalone pedals or rackmounted units. For example, on some lower- to mid-priced bass amplifiers , the only control on the audio compression effect is a button or switch to turn it on or off, or a single knob. In contrast, a pedal or rackmounted unit would typically provide ratio, threshold and attack knobs and sometimes "soft knee" or other options to allow the user to control the compression.