preamp investigations

What Is This?

This page contains data from a spice simulation of an instrument amplifier's preamp stage (Fender 5F4 type).

The Preamp Model

A previous analysis suggested that sampling a linear ramp response from a valve preamp is a viable way to recreate its particular harmonic distortion characteristics.

For this work, a preamp stage from an existing instrument amplifier has been simulated using the spice package (Here's the spice netlist). This preamp operates the valve at different voltages, producing more pronounced tone forming. In addition, its filtering effect is reduced wrt. the previous model, resulting in simulation data more suitable for DSP emulation purposes.

Simulation Data

To get an idea of the model's filtering and amplitude response characteristics, it was run on an input signal of 2 ms of 0 V, then a ramp from -5 V to 5 V over another 2 ms, and finally the input voltage was ramped over the same difference within 4 ms.

plot of input voltage
plot of output voltage

From the plots, we can deduce that a relatively simple input to output voltage mapping will perform well in capturing the preamp's tone coloring properties in a DSP emulation, because memory effects seem to have little to no influence on its response.

It can also be seen that the original design intention was to provide a fairly linear amplitude response in an input voltage window of about -1.2 to +1.2 V., February 18 2004.