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.
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
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.