RC low-pass filter
Tone stacks and tone controls from amps and pedals, from their schematics, with the stage that drives each one and what it feeds. Turn the knobs and watch the curve. Pin a setting to compare, change any part, and listen through a speaker. Pickups and guitar knobs are on the Guitars page.
A resistor into a capacitor to ground. It’s flat below the corner and falls 6 dB per octave above it, 3 dB down at 1 / (2πRC).
Swipe sideways for the whole drawing. Tap a part to change it. The source drives the end marked in, and the load hangs on the end marked out.
At this setting
0 dB is the source’s voltage with nothing hooked to it, so the curve shows what the stack cuts. A tube source changes the curve only through its output resistance, not its gain.
Pin this curve keeps what the graph shows, up to three pins in their own colors. Pins stay when you switch stacks, so you can lay a Marshall treble sweep over a Vox one. In a sweep, the lightest line is the knob at 0 and the darkest is 10.
Listen
You hear a picked string through a guitar on full, with 15 feet of cable. Play without it leaves the stack out.
Source and load
Driven by
Driving
The knobs
- Frequency
- A 100 kΩ reverse-audio pot used as a variable resistor, in series with 1.2 kΩ. Its resistance drops as you turn it up, so the corner rises from about 160 Hz to about 13 kHz. It sits near 1.4 kHz at noon, and the sweep is spread evenly on the log scale.
Parts
- R min
- 1.2 kΩ, Sets the highest corner, with the pot at full.
- R
- 100 kΩ, The filter’s resistance. It drops as you turn the knob up.
- C
- 10 nF, The filter’s capacitor. The corner is at 1 / (2πRC).
Source and load
Driven by: ideal. Driving: none. Generic values, not from a product. Try a resistive source or a load to see how the corner and the level shift with what’s around the filter.
Warnings. The summary above the graph turns red if the stack can’t be solved as set, usually from a part at zero or a shorted pot track.
How this tool works. We solve each stack with the circuit bench’s engine: the source stage’s operating point first, then every part at 161 frequencies from 20 Hz to 20 kHz. A stack that’s also in a bench circuit is built from that circuit file, so the two always match.
The curves are small-signal. They don’t show clipping or grid current in the next stage. For the stage before or after a stack, try the gain stage tool.
Spot an error, or want a circuit added? Email info@swellamps.com. A schematic or a clear photo of one helps. Anything you send may be used to improve these models and our desktop app.
© 2026 Swell Amplification LLC. All rights reserved. The simulator and tools, their circuit models, drawings, presets and text are the property of Swell Amplification LLC and may not be copied, reproduced, redistributed or used to build another product without written permission. The original circuits are historical designs documented in public sources, and no claim is made to them. Everything here is provided as is, without warranty of any kind, for study and personal use. Results are simulations of typical parts, not measurements of any real unit. Don’t rely on them for the safety of any build or repair. Tube amplifiers carry lethal voltages.
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From Swell Amplification, swellamps.com.