One useful application of SPICE is to see how a tone stack behaves. Looking at tone stacks is so interesting that Duncan Munro (Duncan Amplification) wrote a now famous computer program, the Tone Stack Calculator (TSC), in 1999 that is still in wide use today. You can download the Windows application from the Duncan Amplification site. This tutorial shows how to use LTSpice to make the same calculations as the TSC for the tone stack of the Big Muff Pi (BMP).
DIY stompbox builders often add the BMP tone stack onto circuits without tone controls. Using only one potentiometer, this tone stack produces a high pass filter, a low pass filter, and a mid scoop. That’s a lot of choices without requiring the common trinity of pots (bass, mid, and treble). The BMP stack uses the single potentiometer to blend a high pass filter with a low pass filter. Fully counter clockwise (CCW), almost all of the output comes from the low pass filter (shown above in red) and fully clockwise (CW) it’s almost all high pass filter (shown in blue). In between, the pot blends the two in various amounts as seen in the green lines in the graph above.
First, following the instructions below, make the following schematic for the tone stack in LTSpice (or download it here):
If you need help with placing components or wiring on the schematic, look at An LTSpice Tutorial.
The high pass filter (HPF) consists of C1 and R2 where the output is taken at their junction. The low pass filter consists of R1 and C2 where, again, the output is taken from their junction. If you would like to read more about such passive resistor-capacitor (RC) filters, check out Low Pass Filters and High Pass Filters.
To label a net like the IN, OUT, HPF, and LPF nets on this schematic, use the Label Net entry on the Edit menu. There is a button for this command between the ground and resistor symbols of the tool bar. You can also start the command with function key 4 (F4). A net label serves the same purpose as on a normal schematic, to abbreviate wiring. It is also useful for labelling graphs. If you look again at the graph above, you will see the blue label V(hpf) that indicates blue is for the voltage on the HPF net.
The 100K linear pot in the BMP tone stack is represented by the voltage divider made by R4 and R5. For starters, the pot is “set” at 12 o’clock with 50K on each side of the voltage divider. Later in this tutorial, LTSpice will vary the pot setting to produce the opening graph. If you would like to read more about voltage dividers, see the page about resistors in series.
There are only two other elements for this schematic, the AC voltage source V1 and the so-called load resistor R3. Use the standard voltage source and then edit its properties to produce the AC properties. After right-clicking on the voltage source, you must click on the button labelled Advanced and fill in the dialog window as shown here:
A key SPICE command in this file is the AC analysis command
.ac dec 50 10 30K
LTSpice helps to enter such commands through its menus. Open the Simulate menu and choose the Edit Simulation Cmd entry. Click on the AC Analysis tab and you will see this window:
I have filled in some values to get a graph that looks like the one in the TSC. After you click on OK, you will have a cursor that looks like a box and you must click somewhere on your schematic to actually insert the command into the schematic. Sometimes I forget to do this and the simulations don’t work. Fortunately when you go back to the Edit Simulation Cmd window your previous entries are still there.
Now if everything looks the schematic above, choose the Run entry of the Simulation menu. A blank, black graph will appear. Click on the OUTPUT label or net and this graph should show up:
The dashed line shows the phase shift of the filter. Remove that phase plot by clicking on the left-hand phase scale and then clicking on the “Don’t plot phase” button. Now you should see this:
That is the same graph that you will see if you open the BMP window of Duncan’s TSC. This is a mid scoop with the bottom of the scoop at 1KHz. The output of the filter is graphed in decibels. Notice that for all frequencies the output is attenuated. That is indicated by the negative decibel values. All passive tone stacks have this attenuation property and you will often see so-called recovery stages after them in stompbox circuits to amplify the signal before the output.