Following the departure of his partner, Matthews was introduced to inventor and electric engineer Robert Myer through IBM colleagues. Together they designed a circuit to create a distortion-free sustain. A simple line booster used by Myers in testing to preamplify the guitar's signal was also manufactured from 1969 as the Linear Power Booster (LPB-1), and has continued production in present day.
During the mid-1970s, Electro Harmonix had established itself as a pioneer and leading manufacturer of guitar effects pedals. Electro-Harmonix was the first company to introduce, manufacture, and market affordable state-of-the art "stomp-boxes" for guitarist and bassists
Electro-Harmonix stopped making pedals in the mid-1980s, and in the early 1990s started selling vacuum tubes re-branded with its name for guitar amplifiers, which it had also been making since the 1970s. However, due to demand and the high prices guitarists were paying for old 1970s pedals on the vintage market, it reissued the more popular old pedals in the mid-1990s, including the Big Muff Pi and Small Clone. In 2002 it started designing new pedals to add to its range. Company policy was that all reissued effects remained as close as possible to the original, vintage designs; however, casings, knobs and especially the old-fashioned mini-jack power plug were not up to later standards. In 2006 the smaller and more standardized "micro" and "nano" effect lines using surface-mount circuit components were introduced. Circuit board manufacture was outsourced, and the pedals assembled in New York.
Electro Harmonix produces pedals with many different types of sound manipulation suitable for guitar, bass, vocal, keyboard, and other instruments. It also sells rebranded vacuum tubes carrying the Electro Harmonix brand name.