Boss MT-2 Metal Zone tone stack
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.
The first of the MT-2’s two EQ stages, a non-inverting op-amp with the Low and High pots between its inputs. A gyrator branch hangs on the Low wiper and an RC branch on the High wiper.
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
- Low
- A 100 kΩ pot between the op-amp’s inputs, with its wiper to a gyrator branch tuned near 105 Hz. Toward the minus input it boosts. Toward the plus input it cuts. It’s marked G and modeled as linear, and its direction is assumed.
- High
- The same, but with a 10 nF and 2.2 kΩ branch instead of a tuned one. That makes a shelf above about 7 kHz.
Parts
- R014
- 22 kΩ, Input resistor of the bass and treble stage.
- IC1a
- Bass and treble stage, non-inverting, unity gain with both controls centered.
- R015
- 22 kΩ, Feedback resistor.
- C010
- 10 pF, Across the feedback resistor.
- Low
- 100 kΩ, Low, 100 kΩ (marked G, modeled as linear, flat at noon). Its wiper feeds a branch tuned near 105 Hz. Toward the minus input it boosts, toward the plus input it cuts. The direction is assumed.
- High
- 100 kΩ, High, 100 kΩ (marked G, modeled as linear, flat at noon). It works the same way with a 10 nF and 2.2 kΩ branch, a shelf above about 7 kHz. The direction is assumed.
- C008
- 220 nF, Series cap of the low branch.
- R012
- 2.2 kΩ, Sets the low branch’s resistance at resonance.
- IC1b
- Follower of the low-branch gyrator. With 2.2 kΩ, 47 nF and 100 kΩ it acts as about 10 H, resonating with 220 nF near 105 Hz.
- C009
- 47 nF, Gyrator cap.
- R013
- 100 kΩ, Gyrator resistor, from half supply.
- C044
- 10 nF, High branch cap.
- R061
- 2.2 kΩ, High branch resistor.
Source and load
Driven by: op-amp buffer. Driving: resistance, 47 kΩ. Source: the inverting buffer before it. Load: the middle stage’s 47 kΩ input resistor. An op-amp drives this stage, so the load barely matters. The Middle and Freq controls are in the next stage, the other version of this stack.
Values and wiring are the same as in the op-amp distortion with resonant emphasis and a swept middle control circuit on the bench, so the two always match.
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.
Product and company names are trademarks of their owners. They are used here only to identify the circuits these models are based on; this site is not affiliated with or endorsed by any of them.
From Swell Amplification, swellamps.com.