Guitar Patch Cord

Copyright M. Gallant 6/6/92


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 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:

Typical commonly used op-amps for this purpose are the MC4558 , LM833 etc. The circuit requires several components and a power supply.


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 equalization network can be inserted between the guitar and phono-input to realize effectively an overall flat response. Since the RIAA playback equalization is really a multiple low pass filter network, we insert a high pass network with sufficient nominal attenuation at some reference frequency (e.g. 50 mV ---> 3 mV or -24 dB attenuation at 1 kHz). The schematic diagram below shows typical component values. The boxed section at the left represents the equivalent circuit of the guitar pickup with Vp the EMF induced in the guitar coil, Lp the coil inductance (4 Henries being a typical value for a Gibson Les Paul hum-bucking pickup) and Rp the coil series resistance, typically 7 kohms. These values will vary with pickup design. Indeed, the exact details of how the pickup coil is wound, the geometry of the magnet/coil layout, the adhesive used to fix the coil in place all play a role in shaping the sound characteristics of the electric-guitar pickup. The equivalent circuit for the pickup presented here is very simplified:

The guitar cable shunt capacitance Cj is 200 pF for standard musical-instrument coaxial cable of 6 foot length. The PhoneAxe filter network described here consists of just two components: a series capacitor C and a shunt resistor R . Typical values for these components which produce good results in practise (for a Gibson Les Paul hum-bucker pickup) C = 330 pF and R = 100 kohms.

The Java design applet below shows the filter response at the phono-input (represented by a 47 kohm load, the typical input impedance of magnetic-phono inputs). The net amplifier response (not shown here) is obtained by convolving the filter response with the RIAA playback response, and the users BASS and TREBLE response settings. The user can modify the R and C values of the PhoneAxe network as well as the cable shunt capacitance Cj, the series resistance of the pickup Rp, the input resistance of the amplifier Ramp and the inductance of the pickup coil Lp. The changes are updated in the graph. Note the characteristic 20 dB/decade low frequency rolloff. The frequency of the LC peaking feature is also displayed:

The filter components (R and C) can be mounted directly inside a standard 1/4" phone plug for the guitar as part of a simple PhoneAxe patch cord. Use the smallest available components, and carefully solder and pack the resistor and capacitor inside the plug using hear-shrink or spaghetti. For best shielding, use a good quality metal phone-plug. The total cost including a 6 foot cable with a phono connector at one end (to plug into standard stereo inputs) is about $5.00

Filter Modifications:

In general, a more complex passive filtering network can be employed to "shape" the sound, especially the high-frequency resonance. The figure below shows a generic T impedance bridge to tailor the filtering response:

Each complex impedance Zi may consist of networks of inductors, capacitors and resistors. The high gain of the phono-input allows trading off gain to manipulate the frequency-response shaping.

Internal Amplifier Modifications:

The simple passive filter network described here can be easily included in the amplifier unit itself by simple rewiring of the phono input. A basic DPDT switching arrangement can be used to select guitar or phono input.

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