The Boss HM-3 Hyper Metal is a distortion pedal in Boss's metal line, sitting alongside the better known HM-2 Heavy Metal. Boss describes it as dishing up "smooth, high-gain distortion with a tight, controlled character," with wide-ranging control over low and high frequencies12. Builders who have reproduced the circuit describe it as the younger brother of the HM-23, and one verified build on Guitar FX Layouts confirmed exactly that: "Its verified! Hm3 the younger bro to hm2 !!"3
What it does
The HM-3 is a high-gain distortion pedal. Boss's own description is of smooth, high-gain distortion with a tight, controlled character and wide-ranging control over low and high frequencies1. The circuit analysis at Guitar FX Layouts quotes the effect as able to "put out a richand truly powerful distortion sound"3. One builder who finished a version of the circuit put it plainly for players after extreme gain: "If thrash/deathmetal is your thing, BUILD IT! =)"3
The pedal's tone shaping comes from its two Color Mix controls. Guitar FX Layouts describes them as "Two specially designed Color Mix controls provide for wide ranging tone control"3. On the related HM-2, TONEHOME describes the equivalent control as "the ability to boost or cut mids using the H (high) Colour Mix"4, and MusicRadar notes the HM-3's front panel controls are the same as the HM-2's, with a similar EQ profile5.
HM-3 versus HM-2
The two pedals look like close relatives, and in some ways they are, but the resemblance stops at the panel5.
- MusicRadar: "Although the controls on the front panel are the same as those on the HM-2, and the EQ profile sounds similar, that's wh[ere the similarity ends]"5
- Builders on Guitar FX Layouts call the HM-3 "the younger bro to hm2"3
- The HM-2 launched in 1983 and was later replaced by the MT-2 Metal Zone, which offers a similar midrange control46
The HM-3 itself is the odd one of the trio, a high-gain pedal with the HM-2's layout but its own circuit517.
The circuit, for builders
The HM-3 has drawn attention from the pedal-building community, and the technical analysis at Guitar FX Layouts covers its parts in detail3.
- The circuit uses gyrators, and the transistors in them (Q2 to Q4) are not readily available today. The published layout uses a CBE pinout so a modern NPN such as a 2N3904 or 2N5088 can be used3
- The buffer JFET (Q1) is a 2SK184, still obtainable from some sellers, with same-pinout substitutes including the 2SK117, 2SK170, 2SK30 and BF244A3
- The opamp is an M5218L dual opamp made by Mitsubishi; a DIP8 version exists but may be hard to source, and any common dual opamp works, such as the JRC4558, TL072, NE5532 or LM3583
Builders report the circuit can oscillate at high gain. Mortua's build oscillated at around three quarters of the Distortion setting and was cured by replacing the 47p capacitor with 1nF and the 100p with 220pF; a second build with those changes plus a TL072 and 2N3904 was reported working perfectly. Another builder's version, using 1N4148 diodes, 2N3904, 2SK30 and TL072, came out as "a... Noisy Fuzzy Beast!" with high pitched squeals at higher gain settings3
The HM-3 today
The original pedal is long out of Boss's catalog, but the circuit has a current life in the PX-1 Plugout FX. Boss's PX-1 product pages carry the same description of the HM-3's smooth, high-gain distortion with wide-ranging low and high control17.
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Questions
How is the Boss HM-3 different from the HM-2?
MusicRadar notes that although the controls on the front panel are the same as those on the HM-2, and the EQ profile sounds similar, that is where the similarity ends. Builders on Guitar FX Layouts describe the HM-3 as the younger brother of the HM-2. The two pedals share a look and a control layout, but the circuit behind them is not the same.
Does the Boss HM-3 use gyrators?
Yes. The independent technical analysis at Guitar FX Layouts identifies gyrators in the HM-3 circuit, and notes that the transistors used in the gyrators are not readily available today. The published layout uses a CBE pinout so a modern NPN transistor such as a 2N3904 or 2N5088 can be used in those positions.
What replaces the 2SK184 JFET in an HM-3 build?
The 2SK184 JFET in the HM-3 works as a buffer, and Guitar FX Layouts notes substitutes with the same pinout include the 2SK117, 2SK170, 2SK30 and BF244A. For the M5218L dual opamp made by Mitsubishi, any common dual opamp can be used, such as the JRC4558, TL072, NE5532 or LM358.
Why does an HM-3 build squeal or oscillate at high gain?
Builder Mortua reported oscillation at around three quarters of the Distortion setting on the Guitar FX Layouts build, and cured it by replacing the 47p capacitor with 1nF and the 100p with 220pF. Another builder described the pedal as a noisy fuzzy beast with high pitched squeals at higher gain settings, so noise at high gain is a known trait of the circuit.
What are the Color Mix controls on the HM-3?
The HM-3 carries two specially designed Color Mix controls, which Guitar FX Layouts describes as providing wide ranging tone control. Boss describes the pedal as offering wide-ranging control over low and high frequencies, so the Color Mix knobs are where a player shapes the low and high end of the distortion.
Sources (7)
- BOSS - PX-1 | Plugout FX, BOSS (www.boss.info)
- BOSS PX-1 Plugout FX: HM-2, HM-3, and MZ-2 Model Passes, BOSS (articles.boss.info), 2026-09-28
- Guitar FX Layouts: Boss HM-3 Hyper Metal, Guitar FX Layouts (tagboardeffects.blogspot.com), 2021-08-04
- TONEHOME, TONEHOME
- The FX files: BOSS MT-2 Metal Zone, MusicRadar, 2019-11-12
- Boss MT-2 Metal Zone: how a divisive pedal conquered the world, Guitar World, 2021-09-17
- BOSS - PX-1 | Plugout FX, BOSS (boss.info)
