Guitar Patch Cord

Posted on Feb 5, 2014

The PhoneAxe is an original design by the author of a passive filter network built into a 1/4` phone plug/coaxial cable, and is designed to allow electric-guitar matching to the magnetic-phonograph input on common hi-fi amplifiers. Most guitar amplifiers have `high input impedance` typically greater than 200 kohms. The reason for this is that the

Guitar Patch Cord
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typical electric guitar pickup is a inductive magnetic transducer with frequency-dependent impedance. At high frequency, the mutually induced pickup voltage is dropped across the pickup-coil self-inductance (back EMF) if the pickup load impedance (i. e. amplifier input) is too low. Therefore, a high impedance amplifier input is required to prevent high frequency "loading" effects which are audible as a high-frequency attenuated "dull" sound. Most hi-fi audio amplifiers have two types of signal inputs: the line-level auxiliary inputs (for tuner, tape, CD etc. ) typically require 200 mV or more signal-level for efficient output power, have a nominal flat frequency response from 20 Hz to 20 kHz and an input impedance <= 30 kohms. This input by itself is not well matched to the guitar pickup output because of : The typical active solution for guitar/line-in matching is a flat- response dual op-amp circuit which provides impedance buffering and sufficient gain (typically 10 dB) for driving the line-level input. This is generically the type of input stage used in most solid-state guitar amplifiers. A simplified schematic diagram is shown below: The second type of hi-fi amplifier input is the classic magnetic-phono input. A simpler solution for guitar to hi-fi amplifier interfacing can be realized using this phono input. The phono input is characterized by : Since there is abundant gain at the phono-input, a simple compensating...

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