~900 is a compact, a 4-knob fuzz that delivers impressive versatility and range with its interactive control set. The "about 900" is the successor to Fairfield's Four Eyes and features a mu-amp input and cascading J-FET gain stages that when cranked create rich, saturated, spitty, nasty fuzz.
The INPUT section is a 'mu-amp' design. The INPUT knob controls the level going into this first gain stage - you can go from clean with lots of headroom, to completely slammed and distorted - and everything in between.
The FUZZ knob controls the level going into the second JFET gain stage. This section is very dependent on the level of the INPUT.
The BIAS knob adjusts the bias of the second gain stage. This gives control over the saturation symmetry, and can also allow gating when turned up high enough. The BIAS knob is especially interactive with the FUZZ knob.
The VOLUME controls the master volume output. This pedal has a lot of gain to offer
and can get quite loud.