The Official LXH2 Marshall type preamp
The "Marshall" type preamp description,
Last updated 1/20/97 
          All references to ICs refer to op amp sections.  Part labels
        mentioned here are not shown on the current version of the
        schematic.

        Click on here for the schematic.

        
          U1 and its associated resistors form the initial gain stage.
        U1 has a gain of 8 dB. U2 and its associated
        resistors form a second gain stage with a gain of 28 dB.
        
          Dual potentiometer VR1 forms the gain control.
        The purpose of this arrangement is to provide a wide range of
        gain while reducing noise. The 3000 pF capacitor through which
        the first half of VR1 is fed attenuates frequencies below 530
        Hz for pre-equalization. It should also be noted here that the
        purpose of this pre-equalization is to provide for equal
        energy between strings to create the proper voicing.
        
          The "presence" circuit consisting of U3, U4, VR2, and their
        associated components, form a side chain. The components
        between U3 and U4 form a shelf filter, whose main effect is as
        a high-pass filter that attenuates frequencies below 500 Hz. U3
        is a buffer, and U4 is an amplifier stage with a gain of 20 dB.
        VR2 controls the amount of equalized signal, consisting mainly
        of frequencies above 500 Hz, which is then added back to the
        main signal path. As the presence control is increased, the
        effective turnover frequency is lowered and ultimate gain is
        thus increased.  The result simulates the effect of a change
        in high frequency negative feedback in the power amp stage.
          The presence signal and the main signal are fed to the input
        of U5 through 10K mixing resistors. U5 serves as an inverting
        mixer.
        
          U6 is gain stage with a gain of 34 dB. When the signal
        reaches a predetermined level, the maximum output from U6 will
        reach the supply rails and therefore provide an abrupt clipping
        of the waveform which is an intentional part of the overall
        distortion characteristic. The 10k resistor acts as the
        effective plate resistance of an EL34 output tube, and the
        1N914 silicon diodes duplicate the effective maximum number of
        electrons that the plate can absorb, thus producing the
        characteristic "tube type" distortion. The .1 uF capacitor
        attenuates frequencies below 160 Hz, but only when the diodes
        are conducting. This distortion-dependent low frequency roll
        off is characteristic of a Marshall style output stage.
        
          VR2, VR3, VR4, and their associated components form the tone
        control circuit, which is similar to that used in the amplifier
        being simulated. VR2 is the treble control, VR3 is the bass
        control, and VR4 is the midrange control.
        
        U8 provides a unity gain buffer, which feeds the
        output level control.  Following the output level control is
        another buffer.
        
          The following filter circuitry is used to simulate the low
        frequency response of a typical 4-12" sealed guitar cabinet driven
        by an amp with a low damping factor, such as a tube amp during
        distortion.
        
          U9 and its associated components form a 95 Hz Sallen-Key
        high-pass filter. Its damping factor is user adjustable, within
        specified parameters, centered around .5. This effectively
        simulates the low frequency speaker/cabinet resonance. This
        filter, used in conjunction with the 90 Hz peaking filter having
        a Q of .9 that is formed by U10, U11, and U12 with their
        associated components, provides a simulation of the low end
        "chunk" associated with a 4-12 closed back cabinet.
        However, the amount of bass boost associated with this circuit
        is user adjustable to suit the individual guitarist's taste.  
        


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