Electro-Harmonix Black Finger Operating Instructions

Black Finger
All-tube Optical Compressor
— OPERATING INSTRUCTIONS —
Congratulations on your purchase of the Black Finger! The Black Finger is a warm sounding
compressor pedal useful for both guitar and bass, as well as for home recording. This
pedal features an all-tube signal path using two premium 12AX7EH vacuum tubes. Carefully
reading these instructions will ensure that you will get the most out of your Black Finger.
Included with the Black Finger is a unique 12VAC/1000mA adapter. Use only the adapter
supplied. Using any other adapter can cause harm to you, the Black Finger and the adapter.
WARNING: There are extremely high voltages in the Black Finger
even when it is unplugged – DO NOT OPEN THE BLACK FINGER.
— THEORY OF OPERATION —
Optical compressors work by treating an incoming signal with an optically controlled
attenuator (a type of volume control) comprised of a fixed resistor and a shunt opto-
coupled photocell. Because this attenuator is "passive"— as opposed to the more common
"active" AGC (Automatic Gain Control) compressor circuits — it doesn't add an electronic
signature to the signal, thus preserving the tone and character of the original sound. Some
gain is needed to adjust the output level so that it matches the original level, and this is
accomplished with an all-tube fixed gain pre-amp circuit. In general, compressors are used
to sustain melody lines, minimize changes in signal level, or add a "forward" percussive
effect to instruments or drums.