gaussmarkov: diy fx

To produce the graph like the one at the beginning of this tutorial, one adds a little programming. You need to create a parameter and enter another SPICE command to vary the value of the parameter. This SPICE command does not have a dialog box in LTSpice like the AC command.

Change the values of the resistors in the voltage divider representing the tone pot. Change the value of R4 from 50K to {100K*(1-wiper)} and change the value of R5 to {100K*wiper}. These expressions must be inside brace brackets. The parameter name in this example is “wiper” and this will vary from 0.1 to 0.9 so that the tone knob will be set at 10%, 20%, …, and 90% of its throw from CCW to CW.

The SPICE command that will vary “wiper” is called STEP, or DOT-STEP to acknowledge the period placed in front of the command. To enter the command, choose the SPICE Directive entry on the Edit menu and type

.step param wiper .1 .9 .1

into the space provided and click OK. Don’t forget to click the SPICE command onto your schematic which should look something like this:

Now when you run the simulation (Simulate>Run) you will get a graph like the one at the beginning of this tutorial with 9 green lines, one for each value of wiper. To get the HPF and LPF graphs as well, click on their labels or nets in the schematic window and they will also appear.

Now all this work does not replace the Duncan TSC. The TSC is a much slicker way to play with tone stacks. This tutorial is just using the TSC as a simple example to show how to do an AC analysis with LTSpice. The same procedure works for a whole stompbox circuit, as for the DOD Overdrive 250, and that is the sort of application that shows the value of this tool to the DIY stompbox builder/designer.