LXH2 "Marshall" Simulating Preamp Schematic

LXH2 "Marshall" simulating preamp circuit Last updated 02-04-06

Latest updates:

I have made a few updates and revisions to this circuit.

I have provided for a seperate output level control for the "non cabinet" output. The cabinet simulator input level control is provoded to prevent clipping of any of the subsequent filter circuitry. This control can be a trimpot used only for initial setup and calibration of this circuit. Even though the "non cabinet" output includes the "chunk" and "damping" circuitry, it is not considered to be part of the cabinet simulator circuit. This is because I have found that when using the preamp output into a guitar amp, the cabinet simulator was largely redundant, as it simulates the frequency response of a guitar speaker in a cabinet. However, in most cases, I found that the "chunk" and "damping" sections are a welcome addition to the preamp output, providing a more chunky bottom end.

Recently I have been informed of several devices currently on the market that claim to do something similar to the amp and speaker simulators described here. I have not heard them and will not pass judgement on them.

I have also added a switch in the diode circuit to facilitate a degree of asymmetry in the waveform. This is a matter of what what suits your ear. I originally designed this circuit to get as close to the real thing as possible without getting into digital signal processing. However, you may want to experiment with using different diodes in different combinations to suit your taste.

Includes both preamp and speaker/cabinet simulator sections.

The cabinet simulator frequency response sonically duplicates the response of this graph.

The control called "Chunk" is also referred here as "Cabinet" or "low frequency boost"

!!!!! IMPORTANT WARNING !!!!! Do NOT connect this circuit to the speaker output jack on your amp. Serious damage can occur to both your amp and this circuit and can cause a fire hazard. This is a line level device. Use only with preamps, effects processors, and other line level equiptment. Line level refers to the output level of most mixers, preamps, and effects processors.

FILTER DATA TABLE:


 Notes:

 1. Connect all [peaks] MFB filters together in paralell where shown.
 2. Connect all [dips] MFB filters together in paralell where shown.
 3. Keep all leads as short as possible to prevent oscillations.
 4. All power supply connections are not shown in this drawing.

MFB FILTERS FOR PEAKS

  In all MFB filters below, C represents C1 and C2.
   
   MFB FILTER  320 HZ    800 HZ     1.2 KHZ    1.8 KHZ     2.5 KHZ
   
       R1       270k      270k       180k       120k        330k
       R2       4.3k      3.0k       2.2k       1.2k        5.1k
       R3       110k      150k        82k        68k        180k
       R4        12k       12k        12k        12k         11k
       C       .022uF    .01uF      .01uF      .01uF       .0022uF

   MFB FILTER  5 KHZ     8 KHZ      11 KHZ

       R1       247k      390k       68k
       R2       5.4k      4.4k       1.15k
       R3       202k      270k       570k
       R4       18k       16k        15k
       C       .001uF     560pF      560pF

MFB FILTERS FOR DIPS    

   MFB FILTER  600 HZ    1.45 KHZ    2 KHZ     7 KHZ       9 KHZ

       R1       220k      120k        330k      220k         56k
       R2        6.8k     1.2k        1.5k        9k        8.2k
       R3       120k      100k        180k      202k         39k
       R4        11k       10k         22k       10k        5.6k
       C       .01uF     .01uF       .0047uF    560pF      .001uF

TECHNICAL NOTES:

1. All op-amps are TLO84.

2. The "chunk" and "damping" circuitry in the lower portion of this schematic provides an adjustable resonant boost around 100 Hz for use with guitar speakers powered by high damping factor solid state amps. The total low frequency response is more like the resonant response of a sealed 4-12" guitar cabinet powered by a low damping factor amp such as a distorting tube amp.

3. For best results, all potentiometers should be audio taper except the 250K treble control which should be linear. For best results, the 10k "damping" control should be audio taper, using the "low" and wiper leads (left two leads if you view the front of the pot with the leads pointing down). This makes the resistance increase more slowly as you increase it, for best taper in damping. Counterclockwise rotation decreases damping (increases bass resonance). Otherwise, make this potentiometer linear.

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