not—except for reverb and other effects—alter the natural instrument sound, other than to make it louder. Amplifiers often come with a simple mixer to blend signals from a pickup and microphone. Since the early 2000s, it is increasingly common for acoustic amplifiers to provided digital effects, such as reverb and compression. Some also contain feedback-suppressing devices, such as notch filters or parametric equalizers. Roles Instrument amplifiers have a different purpose than 'Hi-Fi' (high fidelity) stereo amplifiers in radios and home stereo systems. Hi-fi home stereo amplifiers strive to accurately reproduce signals from pre-recorded music, with as little harmonic distortion as possible. In contrast, instrument amplifiers are add additional tonal coloration to the original signal or emphasize certain frequencies. For electric instruments such as electric guitar, the amplifier helps to create the instrument's tone by boosting the input signal gain and distorting the signal, and by emphasizing frequencies deemed desirable (e.g., low frequencies) and de-emphasizing frequencies deemed undesirable (e.g., very high frequencies). Size and power rating In the 1960s and 1970s, large, heavy, high output power amplifiers were preferred for instrument amplifiers, especially for large concerts, because public address systems were generally only used to amplify the vocals. Moreover, in the 1960s, PA systems typically did not use monitor speaker systems to amplify the music for the onstage musicians. Instead, the musicians were expected to have instrument amplifiers that were powerful enough to provide amplification for the stage and audience. In late 1960s and early 1970s rock concerts, bands often used large stacks of speaker cabinets powered by heavy tube amplifiers such as the Super Valve Technology (SVT) amplifier, which was often used with eight 10" speakers. However, over subsequent decades, PA systems substantially improved, and used different approaches, such as horn-loaded "bass bins" (in the 1980s) and subwoofers (1990s and 2000s) to amplify bass frequencies. As well, in the 1980s and 1990s, monitor systems substantially improved, which helped sound engineers provide onstage musicians with a better reproduction of their instruments' sound. As a result of improvements to PA and monitor systems, musicians in the 2000s no longer need huge, powerful amplifier systems. A small combo amplifier patched into the PA suffices. In the 2000s, virtually all sound reaching the audience in large venues comes from the PA system. Onstage instrument amplifiers are more likely to be at a low volume, because high volume levels onstage make it harder for the