Bridged-T mid notch
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.
Two caps in series, a resistor from their junction to ground, and a resistor bridging both caps: a mid notch with no inductor and no loss outside the notch.
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
- Depth
- The notch sits at 1 / (2πC√(R1 R2)). Turning the pot up raises R1, which moves the notch down in frequency and makes it shallower. Turning it down deepens it and moves it up.
Parts
- C1
- 10 nF, First series cap.
- C2
- 10 nF, Second series cap.
- R2
- 220 kΩ, Bridging resistor across both caps. It carries the lows around them.
- R1
- 10 kΩ, Resistance from the caps’ junction to ground. It goes up as you turn the knob up.
- R1 min
- 470 Ω, The smallest resistance to ground, with the pot at zero.
Source and load
Driven by: ideal. Driving: none. Generic values, not from a product. With equal caps the notch gets deeper as R2 / R1 grows. Try a load: the notch only reaches full depth into a high-impedance load.
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.
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From Swell Amplification, swellamps.com.