Boss HM-2 Heavy Metal 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.
A non-inverting op-amp stage with two pots between its inputs. Each wiper feeds series-resonant branches to ground: a cap plus a gyrator, an op-amp follower with a resistor and cap that acts like an inductor.
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 10 kΩ pot between the op-amp’s two inputs, with its wiper to a branch tuned near 90 Hz. Toward the minus input it boosts that band. Toward the plus input it cuts it. The taper is marked G and modeled as linear, so noon is flat.
- High
- The same, but with two tuned branches, near 960 Hz and 1.3 kHz. So it boosts or cuts the upper mids, not the treble.
Parts
- R54
- 3.3 kΩ, Input resistor of the EQ stage.
- IC3a
- EQ stage, non-inverting, with unity gain when both EQ pots are centered.
- R52
- 3.3 kΩ, Feedback resistor of the EQ stage.
- C33
- 470 pF, Across the feedback resistor.
- Low
- 10 kΩ, Low, 10 kΩ (marked G, modeled as linear, flat at center). It works the same way with one tuned branch near 90 Hz.
- High
- 10 kΩ, High, 10 kΩ (marked G, modeled as linear, flat at center). It slides two tuned branches, near 960 Hz and 1.3 kHz, between the op-amp’s two inputs. Toward the minus input it boosts them, toward the plus input it cuts.
- C35
- 1.5 µF, Series cap of the tuned branch.
- R47
- 330 Ω, Sets the branch’s resistance at resonance, and so how sharp the boost or cut is.
- IC3b
- Op-amp follower of a gyrator: with its resistor and cap it acts like an inductor, so the branch resonates like a series LC. In the low branch it’s about 2.2 H with 1.5 µF, resonant near 87 Hz.
- C30
- 68 nF, Gyrator cap.
- R51
- 100 kΩ, Gyrator resistor, from the 4.5 V bias.
- C29
- 100 nF, Series cap of the tuned branch.
- R45
- 330 Ω, Sets the branch’s resistance at resonance, and so how sharp the boost or cut is.
- IC2b
- Op-amp follower of a gyrator: with its resistor and cap it acts like an inductor, so the branch resonates like a series LC. In this high branch it’s about 0.16 H with 0.1 µF, resonant near 1.3 kHz.
- C26
- 4.7 nF, Gyrator cap.
- R44
- 100 kΩ, Gyrator resistor, from the 4.5 V bias.
- C28
- 150 nF, Series cap of the tuned branch.
- R41
- 330 Ω, Sets the branch’s resistance at resonance, and so how sharp the boost or cut is.
- IC2a
- Op-amp follower of a gyrator: with its resistor and cap it acts like an inductor, so the branch resonates like a series LC. In this high branch it’s about 0.18 H with 0.15 µF, resonant near 960 Hz.
- C27
- 6.8 nF, Gyrator cap.
- R43
- 82 kΩ, Gyrator resistor, from the 4.5 V bias.
Source and load
Driven by: op-amp buffer. Driving: volume pot, 10 kΩ. The clipping stages before it are left out, so this is the EQ alone at small signals. Source: the op-amp buffer before it. Load: 10 kΩ into the 10 kΩ Level pot at full, then the output switch’s bias and the output follower, about 250 kΩ. The coupling caps are left out.
Values and wiring are the same as in the two-transistor 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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