and continued making Hiwatt amplifiers. There were some minor innovations to the circuitry produced during this time, including the "OL" (overload) model which had an extra gain stage added. Some were labelled "OL", while many were not. Some had an additional gain control added to the front panel labeled "overdrive". Technical Classic Hiwatt circuits Input stage variations: The early input circuit version (Input 1) used a traditional resistive mixer to combine the two input channels. Half of V2 went unused. The later input circuit (Input 2), used both halves of V2 to do the input channel mixing. The OL and LEAD input circuits (Input 3) went back to the resistive mixer, and used the second half of V2 as an additional gain stage. These can be easily recognized by the "flying" components on the V2 socket. Phase Inverter (PI) Variations: The earlier circuit (PI 1) used a cathode-follower directly connected to the PI circuit to both set the DC level and buffer the signal. The middle and later units (PI 2) capacitively coupled the last preamp stage to the PI. The former cathode follower section now has its input connected to a DC voltage divider, and is used strictly as a low impedance voltage reference. Combination I: The earliest DRs with 4-inputs used Input 1 and PI 1. Combination II: At some point, the circuit was changed to use Input 1 and PI 2 (this version is represented by the Audio Bros and Hiwatt.com schematics). Some 4-inputs may have been made this way. Combination III: Finally, the circuit was changed to use Input 2 and PI 2. This change was definitely in all the 2-input heads, and is represented by the widely circulated freehand-drawn schematics in Pittman and elsewhere. The OL Model (early 1980s): Biacrown's "high gain" model moved back to the Input 1 circuit (still keeping PI 2), but used the "extra" half triode as an additional gain stage. The LEAD Model (early 1980s): This was basically the same as the OL model, but with an extra level control after the extra gain stage, which was marked "Overdrive" on the front panel. DR103 amplifier See also The Who's influence on musical equipment References External links Hiwatt.org Hiwatt history and technical information TheWho.net page documenting Pete Townshend's transition from Sound City amps to Hiwatt Guitar amplifier manufacturers Manufacturing companies based in London Audio equipment manufacturers of the United Kingdom