Before the internet told you what pedalboard to buy, before algorithm-approved tone stacks homogenized every rig from Nashville to Tokyo, there was a small yellow box with two knobs that changed the sound of rock guitar forever. The MXR Distortion+ — Model M104 in Dunlop’s current catalog — is one of the most deceptively simple and historically consequential distortion pedals ever built. Two knobs. One op-amp. Two germanium diodes. And a sound so specific, so unapologetically itself, that no amount of modern engineering has fully replaced what this circuit does to a guitar signal.
Randy Rhoads slammed his Marshall through one. The Distortion+ is partly responsible for the explosive, violin-toned leads on Blizzard of Ozz and Diary of a Madman — records that rewired a generation’s understanding of what heavy guitar could be. That connection was so definitive that when Dunlop created the Randy Rhoads signature RR104, they didn’t invent a new circuit. They reverse-engineered Randy’s actual pedal, the one his family kept at the Musonia School of Music in Los Angeles, and spec’d a production model to match it note for note. The M104 didn’t just inspire the RR104. The M104 was Randy’s pedal, and it still is.
But the M104’s story extends beyond one legendary player. This is a circuit that defined an era — a germanium-and-741-driven snapshot of late-1970s distortion design that proved you didn’t need a complicated circuit to make something irreplaceable. If you’ve been on the fence about the Distortion+, this guide covers every angle: circuit character, verified player history, tone recipes, and exactly where this pedal fits — and doesn’t — in a modern rig.
History: From Rochester to Rock History
MXR Innovations was founded in 1974 in Rochester, New York, by Terry Sherwood and Keith Barr. The company came of age during the pedal boom of the mid-to-late 1970s, when the effects industry was still young enough that bold engineering choices could define an entire market category. The Distortion+ arrived around 1978, landing at a moment when guitarists were pushing amplifiers hard but didn’t always have access to full-volume cranked-amp distortion on stage. The Distortion+ offered a different solution: not a standalone distortion source, but a signal slammer — a pedal designed to hit the front end of a tube amp with a hot, pre-clipped signal that would push the amp’s own tubes into saturation.
This is a crucial distinction that defines the M104’s character even today. The Distortion+ was never intended to be the entire distortion in your chain. It was designed to work with an amp — specifically to excite the input stage, accelerate tube saturation, and add its own germanium-diode character on top. Randy Rhoads understood this intuitively. His approach — cranking the Distortion+ into a high-gain Marshall — became one of the defining tonal blueprints of early 1980s heavy rock.
MXR had a turbulent business history. The company filed for bankruptcy in 1984, and the brand and assets were acquired by Jim Dunlop Manufacturing in 1987. Under Dunlop, the Distortion+ was reissued as the M104 and has remained in continuous production ever since. The Dunlop version updated the packaging and added modern power supply compatibility, but the fundamental circuit — 741 op-amp, germanium clipping diodes — stayed true to what made the original great. The M104 is currently priced at $99.99 MSRP, making it one of the most affordable entry points to a genuinely storied piece of pedal history.
Specifications
The Circuit: Why Two Knobs Are Enough
The MXR Distortion+ circuit is a lesson in doing more with less. At its core is a 741 operational amplifier — one of the most fundamental op-amps ever designed, developed by Fairchild Semiconductor in the 1960s and widely used through the 1970s. In the M104, the 741 runs as a non-inverting gain stage with the input signal amplified to a gain determined by the Distortion knob. The key to the circuit’s character is what happens after that amplification: the signal hits two 1N270 germanium diodes wired hard to ground, clipping both the positive and negative cycles symmetrically.
Germanium diodes have a significantly lower forward voltage threshold than silicon diodes — roughly 0.3 to 0.45 volts versus 0.7 volts for silicon. This lower threshold means the clipping kicks in earlier and is softer in transition, adding a degree of compression and a slightly rounded, warm character to the clipped waveform. It’s not the extreme soft-clip effect of diodes in a negative feedback loop (like the Tube Screamer family), but it’s noticeably less harsh than silicon diodes at the same voltage threshold. The result is a distortion that has grit and aggression without the glassy brittleness that characterizes some early digital or silicon-only designs.
The 741 op-amp itself contributes significantly to the sound. The 741 has a leisurely slew rate of just 0.5 V/µs — meaning it physically cannot reproduce a full-amplitude signal at high frequencies without distorting. This slew-rate limiting acts as a natural high-frequency rolloff, softening the top end of the gain stage in a way that’s baked into the physics of the component. Different 741 flavors — the LM741CN used in early production, the UA741CP in later variants, and others including JRC741 equivalents — produce subtly different character, which explains why vintage M104 hunters often obsess over chip markings. The fundamental behavior is the same, but there’s variance in the feel.
The Output knob controls the final level post-clipping, allowing anything from a significant signal drop for subtle amp-front color to a massive clean-boosted level that will push any tube amp hard. Set low Distortion and high Output, and the M104 works almost like a clean boost with germanium character. Set high Distortion and moderate Output, and it’s a fully saturated hard-rock distortion stage. The range across just two controls is wider than it first appears.
The M104 uses an LED-indicating bypass rather than true bypass. This means there is a buffer in the signal path even when the pedal is off, which in practical terms affects impedance interaction with other pedals — particularly fuzz pedals, which are sensitive to input impedance. If you’re running a vintage-style fuzz before the M104 in your chain, the fuzz should come first, before any buffered pedal, to preserve its impedance interaction with the guitar’s volume and pickup. This is standard buffer placement wisdom, not a flaw unique to the M104.
Famous Players: The Honest Assessment
The MXR M104 Distortion+ has one confirmed player connection so primary-source solid that Dunlop built an entire signature product line around it. Everything else in the M104 player legend requires honest qualification.
Randy Rhoads
Randy Rhoads is the defining Distortion+ artist, and the evidence is as primary as it gets. In producing the MXR Randy Rhoads Distortion+ (RR104), Dunlop’s design team received access to Randy’s actual pedalboard — displayed by the Rhoads family at the Musonia School of Music in Los Angeles, the family’s music school, for the first time in over four decades. Engineers painstakingly spec’d Randy’s vintage Distortion+ unit directly, with the involvement of his sister Kathy Rhoads. The resulting RR104 was tuned to reproduce the specific overdriven tones heard on Randy’s most celebrated recordings. The Rhoads family quote accompanying the release — “We are very proud of and honored by this special signature pedal. It is a true tribute to Randy’s musical legacy” — speaks to the authenticity of the sourcing.
Dunlop’s description of Randy’s technique is precise and instructive: he “harnessed its raw energy to slam his high-gain amplifier with a torrent of harmonics to create the sonic signature that would etch an indelible mark on music history.” This is the M104 in its natural element — not as a standalone distortion box, but as an amp exciter. Randy was using a high-gain Marshall, and the Distortion+ was hitting the front end of that amp to push it further into saturation, adding its own germanium character to the amp’s own harmonic content. The result — those violin-like, super-saturated lead tones on tracks like “Crazy Train,” “Flying High Again,” and “Over the Mountain” — is one of the most recognizable guitar sounds in rock history. Randy Rhoads’ gear history is inseparable from his Distortion+ connection, and the M104 is equally inseparable from Randy Rhoads.
Zakk Wylde — The Honest Picture
Zakk Wylde is often mentioned in the same breath as the Distortion+ given his role as Ozzy Osbourne’s guitarist and his deep association with the MXR brand. However, Zakk’s documented pedal of choice is his own MXR ZW44 Berzerker Overdrive — a signature pedal designed in collaboration with Dunlop that reflects his approach of using overdrive as a clean boost to push his already-distorted Marshall heads further into saturation. The 2016 Premier Guitar Rig Rundown with Zakk, filmed during the Generation Axe tour, focuses on his Wylde Audio signature gear, and no primary source — no PG rig rundown, no interview, no manufacturer documentation — specifically confirms Zakk as an M104 Distortion+ user. Zakk Wylde’s rig is a masterclass in Marshall-and-humbucker tone, but the M104 is not a documented part of it. The Rhoads-Wylde connection in the Ozzy camp is real; the Distortion+ specifically belongs to Randy’s documented history.
The Broader Legacy
Beyond Randy Rhoads, the MXR Distortion+ was widely used during the late 1970s and early 1980s by a generation of hard rock and blues-rock guitarists who were pushing amplifiers hard and needed front-end drive. Bob Welch (Fleetwood Mac, Paris) and blues-rock players of the Bloomfield era are cited in M104 histories, and the pedal appears across Equipboard’s documented user list in community-sourced configurations. The M104 is one of those pedals that was ubiquitous enough in the late ’70s that many players used it without anyone taking detailed notes — its presence in the recording chain of countless records is confirmed by the era, even where individual players’ usage isn’t documented with primary-source precision. For the purposes of this guide, Randy Rhoads is the anchor artist whose connection is primary-source verified and historically definitive.
Tone Recipes: Seven Essential M104 Settings
1. The Randy Rhoads Marshall Slam
Set Distortion to 2–3 o’clock and Output to maximum, running into a high-gain Marshall (JCM800 or equivalent) with the amp’s own preamp gain at moderate-high. This is the approach Randy documented: the Distortion+ is not the primary distortion source but the front-end exciter, pushing the Marshall’s input stage hard and adding germanium compression on top of the amp’s own saturation. Neck or bridge humbucker (Gibson Les Paul, Flying V) works best. The result is a super-saturated, harmonically dense lead tone with violin-like sustain and fast attack transients.
2. Classic Clean Boost
Distortion at 7–8 o’clock (minimum), Output at 3–4 o’clock. At minimum Distortion the circuit is barely clipping — mostly adding gain and the 741’s mild frequency character. The germanium diodes barely touch the signal. This setting works as a clean boost with very slight germanium warmth, useful for pushing a clean amp into natural breakup or adding presence and level to a lead without obvious distortion coloring. Single-coils (Stratocaster, Telecaster) benefit particularly from this application.
3. Low-Gain Blues Crunch
Distortion at 9–10 o’clock, Output at unity (typically 12 o’clock or slightly above). Set this way, the M104 delivers a bluesy, slightly compressed crunch that responds well to picking dynamics. Picking hard produces grit; picking light cleans up. Les Paul into a Fender or Vox amp produces that classic early-British-blues character — not quite overdrive, not quite full distortion, but the zone between them where most expressive playing happens.
4. Classic Early ’80s Hard Rock
Distortion at 12–2 o’clock, Output at unity. This is where the M104 does its most iconic work as a standalone distortion into a clean or moderately pushed amp. Full harmonics, good sustain, and the characteristic germanium compression that makes chords feel thick and riffs feel heavy without becoming a wall of noise. Humbuckers into a Marshall or Hi-Watt-style amp. This is the setting range that defined dozens of hard rock records.
5. Lead Presence Boost Into Driven Amp
Distortion at 9 o’clock, Output at 3 o’clock. Running the M104 with low gain but high output into an already-driven amp — the same approach Zakk’s Berzerker is designed for, applied here to the M104. The pedal’s germanium character adds compression and harmonic density to the amp’s existing drive without adding much additional gain. Solos cut through the mix without getting fizzy or undefined. The 741’s slew-rate limiting provides natural high-frequency softening that keeps lead tones musical at high volumes.
6. Maximum Saturation (Maximum Distortion)
Distortion at maximum, Output at unity. The M104 at full Distortion is a fully saturated, tight distortion with a compressed character. Compared to modern high-gain pedals, it sounds vintage — less scooped, with more upper midrange presence and less extreme low-end extension. Single notes sustain well; chords can get muddy (the two-knob design has no tone control to compensate). This setting is more useful for single-note work, riffs, and leads than for open chord work.
7. Always-On Mild Drive (Under-Unity)
Distortion at 8 o’clock, Output at 9 o’clock (below unity). Running the M104 as a permanent mild colorant — always in the signal chain, always on — at below-unity volume creates subtle germanium character without obvious gain. The result is a slightly warmer, more compressed baseline tone from the guitar into a clean amp. Not every player runs an always-on drive, but for those who do, the M104’s germanium character is a distinctive choice: more compressed and rounder than a silicon-clipping drive at the same gain level.
The M104’s two-knob interface is direct and honest. Distortion controls gain; Output controls volume. There’s no tone stack, no mid scoop, no filtering — just the circuit and the player. Here are seven settings that illustrate the full range of what the M104 delivers.
Signal Chain Placement
As a hard-clipping gain pedal, the M104 sits naturally in the standard signal chain after any wah or pitch effects and before modulation, delay, and reverb. The most important placement consideration is the M104’s LED bypass (buffered when off): if you’re running a vintage-style germanium fuzz — a Fuzz Face-type circuit particularly sensitive to input impedance — that fuzz must go before the M104 in the chain, not after. Running a germanium fuzz into the M104’s buffer when the M104 is bypassed will load the fuzz circuit and degrade its tone. In practice: fuzz → M104 → everything else.
The M104 responds strongly to the Output knob as a gain-staging tool. A low-Distortion, high-Output M104 into the front of a tube amp is a different tonal proposition from a high-Distortion, low-Output M104 used as a distortion box before a clean amp’s power section. Experimenting with these two extremes — and the many configurations between them — is how the two-knob interface reveals its depth. The pedal’s simplicity is not a limitation; it’s a different philosophy of control.
Comparisons: Context in the Distortion Landscape
M104 vs. Pro Co RAT 2
The Pro Co RAT 2 is the M104’s closest philosophical sibling in the hard-clipping distortion category — op-amp based, designed to slam an amp front end — but with a significantly different execution. The RAT uses an LM308 op-amp (slower slew rate than the 741, which contributes to its dark, woolly character), silicon clipping diodes, and a three-knob layout that adds a Filter control. The Filter is the RAT’s greatest advantage over the M104: it allows high-end rolloff adjustment that the M104 fundamentally lacks. The RAT is more flexible. The M104 is more specific — its character is fixed by the circuit, not adjustable by the player. Players who love the M104’s compactness and the way it forces a commitment to a single voice often find the RAT’s Filter option unnecessary; players who want tonal adaptability across different rigs will find the RAT’s extra knob invaluable.
M104 vs. Boss DS-1
The Boss DS-1 represents a different distortion philosophy: silicon diodes in an asymmetrical clipping configuration, with a Tone knob for EQ shaping and a Level control for output. The DS-1 has more of a mid-forward character when the Tone is set flat, a brighter and more cutting tone at high settings, and considerably more high-end aggressiveness than the M104’s germanium warmth. The DS-1 is cheaper and more widely available. The M104 is warmer, more compressed, and more historically rooted in the amp-slamming approach. These are different pedals for different approaches, not competitors in the same tonal space. Many players have owned both for different applications.
M104 vs. MXR M75 Super Badass Distortion
The MXR M75 Super Badass Distortion is what happens when you keep the MXR hard-clipping distortion approach but add a 3-band active EQ (±15 dB per band), true bypass, and significantly expanded tonal range. The M75 is the modern, flexible MXR distortion; the M104 is the original, character-specific one. If you need a single distortion pedal that can cover multiple rigs and genres, the M75 wins on versatility. If you know exactly what you want — the germanium-741 character that Randy Rhoads used — the M104 is the answer. They’re the same brand but represent opposite design philosophies: maximum control (M75) versus maximum commitment (M104).
M104 vs. MXR M78 Custom Badass ’78 Distortion
The MXR M78 Custom Badass ’78 Distortion is the most direct evolutionary descendant of the M104: it’s explicitly a “souped-up” version of the original Distortion+ circuit with a Tone knob added. The M78 provides the M104’s fundamental character with the one addition most players wish the M104 had — the ability to roll off or brighten the top end. If you love the M104 sound but have found yourself wishing for a tone control, the M78 is the answer. If you believe that the M104’s fixed character is part of its virtue — that forcing a single tonal commitment is part of its discipline — the M104 remains the more honest choice.
Buying Guide: New vs. Vintage
The current Dunlop M104 at $99.99 MSRP (street ~$78–100) is the most straightforward purchase: identical circuit in a modern, reliable package with full warranty. Street pricing often dips below MSRP at major retailers, making the M104 one of the best value-for-character propositions in the distortion market. Used M104s in excellent condition typically trade for $50–75, representing minimal savings but the possibility of a unit with a specific 741 variant the buyer prefers.
Vintage MXR Distortion+ units — original pre-bankruptcy 1974–1984 production — are a more complicated proposition. The circuit is fundamentally the same, but component drift over decades of use (particularly in the germanium diodes and electrolytic capacitors) can produce subtle tonal differences. Some collectors specifically seek out vintage units for their specific 741 chip markings (LM741CN for the “best” vintage tone is a commonly held belief in the vintage pedal community). Vintage units in good original condition trade for $100–250+ depending on date and condition, representing a significant premium over the current production M104 with no guaranteed sonic superiority. The Dunlop reissue is faithful; vintage hunting here is more collector territory than a practical tone upgrade.
The Randy Rhoads RR104 signature occupies its own category: it’s spec’d to Randy’s actual vintage unit and features a custom finish inspired by his Flying V guitar. For players who specifically want the Randy Rhoads story in a pedal — and the production-line quality control that comes with a modern Dunlop build — the RR104 is the premium option. Its pricing reflects the limited-edition positioning.
Living With the M104
The MXR M104 is one of the most physically compact distortion pedals on the market. Its small footprint makes it easy to accommodate on any pedalboard, including boards where space is at a premium. The LED bypass indicator is clear and reliable. Power supply compatibility with the full Dunlop power supply ecosystem (DC Brick, Iso-Brick, Mini Iso-Brick) plus the standard 9V battery makes powering it a non-issue on any standard pedalboard.
One practical consideration: the M104 has no tone control. This is the defining limitation and the defining character feature simultaneously. If you’re running it into a bright amp (some Fenders, many Hi-Watt-style circuits) with single-coil pickups, the result can be harsh at high Distortion settings. This is not a defect — it’s the circuit behaving as designed. Players who encounter this typically address it by rolling back the guitar’s tone control, adjusting the amp’s EQ, or using the M104 in lower-Distortion, higher-Output configurations where the 741’s slew-rate limiting does more of the frequency shaping work. Alternatively, placing a Boss GE-7 or similar EQ pedal after the M104 provides the tone shaping the M104 circuit doesn’t.
The buffered bypass means the M104 stays in the signal path as a buffer even when off. This is generally a positive: the buffer drives the cable run from pedalboard to amp with consistent impedance, reducing high-frequency loss. Players who are completely true-bypass purists may object, but in practice the M104’s buffer is benign in most signal chains.
The 9V battery compartment requires removing screws to access — not the most convenient battery change design, but the pedal was designed primarily for DC adapter use. In a gig context, a power supply is strongly recommended. Battery life with the M104 is respectable given the circuit’s relatively low current draw, but the installation process discourages battery-swapping as a live option.
Frequently Asked Questions
What does the + mean in MXR Distortion+?
The “+” was part of MXR’s original branding for the pedal when it launched in the late 1970s. It distinguished the Distortion+ from other distortion units of the era and became part of the pedal’s identity — to the point where calling it simply “the Distortion Plus” or “the MXR Plus” is immediately understood in guitar circles. There’s no documented origin story for the specific “+” branding choice; it was an MXR marketing decision that happened to stick.
What amp did Randy Rhoads use with the Distortion+?
Randy Rhoads ran his Distortion+ into Marshall amplifiers — primarily Marshall Major heads and JMP/JCM-era Marshalls. His approach was to use the Distortion+ as a front-end exciter: relatively moderate gain settings on the pedal pushing the amp’s own gain stage into harder saturation, rather than using the pedal as the sole distortion source. The Rhoads family’s confirmation of his pedalboard configuration for the RR104 project is the primary source for his M104 usage.
Does the MXR M104 have true bypass?
No. The M104 uses an LED-indicating hardwire bypass that includes a buffer in the signal path when the pedal is engaged and bypassed. This is the same bypass design as many classic-era MXR pedals. In practice this means the M104 acts as a signal buffer even when off, which is generally neutral to positive in most signal chains but requires attention to fuzz pedal placement (fuzzes that need direct guitar signal impedance should precede buffered pedals in the chain).
What op-amp does the M104 use?
The M104 uses a 741-type op-amp — various 741 flavors have appeared across production history including the LM741CN, UA741CP, and JRC741 equivalents. The 741’s defining characteristic for tone is its slow slew rate (0.5 V/µs), which creates natural high-frequency rolloff and contributes to the M104’s warm, non-harsh distortion character. Different 741 variants have subtly different behavior, which explains vintage unit variation, but all operate on the same fundamental circuit principle.
Is the MXR M104 good for metal?
The M104 is an excellent choice for hard rock and early heavy metal in the Randy Rhoads / early ’80s tradition — high-saturation, harmonically rich, and responsive to picking dynamics. For modern metal genres that require high gain, extreme low-end extension, and tight palm-muted chugging, the M104’s two-knob circuit (no tone control, no mid-scoop adjustment) shows its age. The M104 is most effective as a front-end exciter into a high-gain amp rather than as a standalone modern metal distortion source. Players who specifically want the Rhoads-era heavy metal approach will find it ideal; players who want djent or modern death metal will need more gain and tonal flexibility than two knobs provide.
What’s the difference between germanium and silicon diode clipping?
Germanium diodes (like the 1N270 in the M104) have a lower forward voltage threshold (0.3–0.45V) compared to silicon diodes (0.7V). This lower threshold means the clipping begins earlier and the transition from clean to clipped signal is softer — more gradual and rounded. The result is a clipping character that feels more compressed, slightly warmer, and less harsh at the clipping boundary than silicon diodes produce. Silicon diodes clip later, more abruptly, producing a brighter and more aggressive character. The M104’s germanium character is specifically part of its sonic identity; it’s not simply a vintage material choice but a fundamental tonal difference.
Is the M104 a boost or a distortion?
Both, depending on how you set it. At low Distortion with high Output, the M104 functions primarily as a clean boost with mild germanium coloring — useful for pushing a tube amp’s input stage into natural saturation. At higher Distortion settings, it’s a genuine hard-clipping distortion pedal producing significant gain on its own. This dual character — boost-or-distortion depending on knob position — is one of the M104’s most practical features and distinguishes it from pedals that do only one or the other well.
How does the M104 compare to the MXR M78 Custom Badass ’78 Distortion?
The M78 is explicitly based on the M104 circuit with the most significant addition being a Tone control that the M104 lacks. If you’ve played an M104 and found yourself wishing you could roll off some top end or add brightness, the M78 is the answer. The M104’s fixed character is its design philosophy: commit to the circuit, adjust everything else around it. The M78 offers more practical flexibility at a higher price point ($125.99 MSRP vs. $99.99 for the M104). Both are legitimate choices — the right one depends on whether you want the Distortion+ sound with adjustment options or the pure, non-adjustable original.