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 MT-2’s second EQ stage, an inverting op-amp at unity gain with a band-passed copy of the signal on its plus input. The Middle pot sets whether that copy comes from the stage’s input or output, and the dual Freq pot tunes it.
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
- Middle
- A 100 kΩ pot from the stage’s input to its output. Its wiper drives the band-pass network through a follower. Toward the output end the band is boosted, and toward the input end it’s cut. It’s marked G and modeled as linear, and its direction is assumed.
- Freq
- A dual 50 kΩ reverse-audio pot. Both halves are variable resistors in the band-pass network, one in series and one to ground. They drop together as you turn it up, which moves the band from about 240 Hz to about 5 kHz.
Parts
- C011
- 1 µF, Coupling cap to the middle stage.
- R038
- 47 kΩ, Input resistor of the middle stage.
- IC2a
- Middle stage. It inverts at unity gain for the direct signal, while its plus input adds the output of the swept band-pass network below. Depending on the Middle control the band is added or subtracted.
- R035
- 47 kΩ, Feedback resistor.
- C026
- 100 pF, Across the feedback resistor.
- R050
- 330 Ω, From the stage input to one end of the Middle pot.
- C037
- 1 µF, From the stage output to the other end of the Middle pot.
- R049
- 330 Ω, Series resistor at the output end of the Middle pot.
- Middle
- 100 kΩ, Middle, 100 kΩ (marked G, modeled as linear, flat at noon). It sets what drives the band-pass network, the stage’s input (cut) or its output (boost). The direction is assumed.
- IC2b
- Buffer driving the band-pass network.
- C036
- 22 nF, Band-pass network: series cap.
- R048
- 2.2 kΩ, Band-pass network: minimum series resistance.
- Freq (VR02a)
- 50 kΩ, Freq, one half of a dual 50 kΩ pot (reverse audio, C, as marked), as a variable resistor in series.
- Freq (VR02a)
- 50 kΩ, Freq, the other half, as a variable resistor to ground. Both halves turn together: turning Freq up lowers both resistances and moves the band from about 240 Hz up to about 5 kHz.
- R062
- 2.2 kΩ, Band-pass network: minimum resistance to ground.
- C043
- 8.2 nF, Shunt cap in the band-pass network.
- C038
- 100 nF, Couples the band-pass output to the middle stage’s plus input.
- R039
- 1 MΩ, Bias to half supply.
Source and load
Driven by: op-amp buffer. Driving: volume pot, 50 kΩ. Source: the first EQ stage’s op-amp, through the 1 µF coupling cap. Load: the 50 kΩ Level pot at full, then the output switch’s bias and follower, about 80 kΩ. The coupling caps after the stage are left out. The Low and High controls are in the stage before, 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.
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