The design was modeled after simplified impulse response curves for a Marshall 1960A cabinet driven from a solid state amplifier and impulse responses for the G12M Greenback speaker driven from a high output impedance tube amplifier.
On the schematic below the output of the tube amp driven section is the junction of C6 and R6 - C16 is the input coupling capacitor for the Marshall 1960A cabinet response simulator.
The first graph shows the frequency response of the Quad box driven from a typical low impedance flat transistor power amplifier.
The second graph shows the frequency response of the tube amplifier driven raw celestion G12M speaker (red line) and the combined response characteristic for a tube amp driven Marshall 1960A cabinet loaded with Celestion G12M Greenbacks (grey line).
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