Boss HM-2 Heavy Metal pedal schematic

The Boss HM-2 Heavy Metal pedal generates a unique distortion effect specially designed for Heavy Metal music. It has an interesting design with both asymmetric soft-clipping diodes, hard clipping diodes and a Ge diode pair in series with the signal for crossover distortion and simple noise gate. A wide variety of tone color creation is made possible with the 2 color mix controls.

Schematic diagram of Boss HM-2 Heavy Metal pedal
Fig. 1: Boss HM-2 Heavy Metal guitar pedal schematic diagram
Boss CE-2B Bass pedal board - component side
Fig. 2: Boss HM-2 PCB - component side
Boss HM-2 Heavy Metal pedal picture


  • 4 Control Knobs: Level, Color mix (Low, High) and Dist.
  • Connectors: Input, Output, AC Adaptor
  • Input Impedance: 1MΩ (FET input)
  • Recommended Load Impedance: more than 10 kOhm
  • Recommended AC Adaptor: ACA Series
  • Current Draw: 10mA (DC 9V)
  • Weight: 400g
  • Dimensions(mm): 70(W) x 55(H) x 125(D)
  • Dimensions(inches): 2-3/4(W) x 2-3/16(H) x 4-15/16(D)

ICs, transistors and diodes used in the HM-2:

  • IC1-IC3: M5218L or NJM4558S (dual operational amplifiers)
  • Q1,Q4,Q5,Q10: 2SK30A-Y (N-channel JFET transistors)
  • Q2,Q3: 2SC732TM-GR (NPN silicon transistors)
  • Q8,Q9: 2SC945-P (NPN silicon transistors)
  • Q6: 2SC2240-GR (NPN silicon transistor)
  • Q7: 2SA970-GR (PNP silicon transistor)
  • D1,D3,D5,D8-D12: 1S2473 (switching diodes)
  • D2: S5500G (400V/1A rectifier diode)
  • D6,D7: 1S188FM (Germanium diodes)
  • D13: RD5.1EB-3 (5.1V Zener diode)
  • D14: SLP135B (red LED)