Vintage distortion vs modern high gain: Distortion+, RAT and a Metal Zone-style circuit compared in ngspice

What "vintage" and "modern" mean here

Vintage distortion here means circuits like the MXR Distortion+ from the 1970s, the ProCo RAT (1978) and the Boss DS-1 (1978). They amplify once with an op-amp (the DS-1 also has a transistor booster), clip with diodes to ground and trim the highs with a simple tone control.

As the representative of modern high gain we take the Boss MT-2 Metal Zone from 1991. Boss describes it as a dual-stage gain circuit with seven filters before and after the distortion. After that comes a three-band active EQ with a sweepable mid.

Test conditions: a palm-muted riff added

Guitar synthesis, pickups (single coil and humbucker), the ngspice setup and loudness matching are the same as in Overdrive, distortion and fuzz circuits.

This time there is one more phrase: a 7.4-second riff of palm-muted sixteenth notes on the low E and A strings (120 BPM), with G5, A#5 and A5 accents, ending on a held E5. Fast chugging on the low strings shows most clearly how each circuit treats the bass before it clips.

Vintage: one gain stage, then diodes

The MXR Distortion+ is just a non-inverting 741 stage and two germanium diodes. 4.7 kΩ + 47 nF goes from the inverting input to ground and Distortion (1 MΩ) is in the feedback, so the gain is up to 1 + 1M ÷ 4.7k = 214 (46.6 dB). The 47 nF lowers the gain below 720 Hz. The output goes through 1 µF and 10 kΩ to germanium diodes to ground, with 1 nF taking off a little treble.

The RAT has the same shape with more gain from the LM308 (67 dB at full Distortion, limited by GBW in practice) and silicon diodes (see Figure 4 in the previous article). The DS-1 puts a transistor booster in front of the op-amp to cut lows before amplifying. All of them clip only once.

With a single clipping stage, the harmonics of whatever band was amplified and the intermodulation of chords go straight to the output. The Distortion+ clips at germanium's 0.3 V, so its output is also small.

Modern: clip twice, filter in between

The Metal Zone-style circuit follows Electric Druid's analysis. The input goes through a 15 nF + 100 kΩ high-pass (106 Hz). The first op-amp has 220 kΩ ∥ 100 pF in the feedback and a resonance near 1 kHz on the ground side, made with a transistor gyrator. The simulation replaces the gyrator with a series RLC tuned to the analysis's figure (a 36 dB gain peak).

After the first stage, a 10 kΩ / 10 kΩ divider and a capacitor take off highs, and a 33 nF + 100 kΩ high-pass (48 Hz) feeds the second stage. Its gain is 1 + (1k + Distortion) ÷ 1k, from ×2 to ×252 as the Distortion pot (250 kΩ) turns. Then 2.2 kΩ feeds silicon diodes to ground, followed by a high shelf of up to −10 dB.

The first stage alone has 36 dB of gain near 1 kHz, so even a 0.3 V input hits the op-amp's swing limit (±3.2 V). The first clipping therefore happens in stage one; the inter-stage low-pass removes the upper part of the harmonics it creates, and stage two clips again. The low-string fundamental (82 Hz) reaches only about 1 V at the first stage and is not clipped yet. What clips first in stage one is the mids and above; the bass is carried intact to stage two.

Small-signal gain up to the diodes is 65 dB at most with the clip setting (Distortion 100 kΩ). That is about the same as the RAT at full Distortion (64 dB): the modern circuit does not simply have more gain (Figure 2).

Gaps between chugs: more gain, smaller gaps

Figure 3 shows the output level over the first second of the riff (eight palm-muted sixteenth notes). Taking the ratio of the lowest to the highest level within each sixteenth note gives 0.09 for the clean signal, 0.61 for the Distortion+, 0.83 for the RAT and 0.84 for the Metal Zone-style circuit. The closer to 1, the less the output drops between notes, so the gaps between the chugs fill in.

How much they fill in follows the amount of gain, not the number of clipping stages. The Distortion+, with the least gain (46 dB at most), keeps its gaps; the RAT and the Metal Zone-style circuit, with about the same gain (around 65 dB), fill in about equally. The leftover vibration of the muted strings and the brief tail before the next note are lifted to the level of the loud notes. To make chugs tight with high gain, the circuit is not enough: players rely on muting technique and a noise gate that shuts the signal off between notes.

EQ: carving the mids after clipping

The MT-2's Low and High are ±15 dB and its Mid is ±15 dB too, with the mid centre sweepable from about 200 Hz to 5 kHz. Low is a gyrator around 105 Hz and Mid is a Wien-bridge band-pass. Because the EQ comes after the clipping, cutting or boosting here changes only the tone, not how the circuit distorts.

Boosting lows and highs and cutting the mids (a "scooped" setting) is the classic high-gain choice. The clips use Low +8 dB (105 Hz), Mid −12 dB (600 Hz) and High +6 dB (above 3 kHz) (Figure 4). It sounds huge on its own, but the core of a guitar's sound is in the mids, so it tends to disappear in a band mix.

All riffs side by side

The same riff through five circuits and settings. Try switching the pickup as well: the humbucker's larger output increases the distortion in every circuit.

What the simulation leaves out

• The Metal Zone-style circuit is a simplified version: it uses the structure and the component values given in the analysis, and the rest were chosen to match the analysis's numbers. The transistor gyrator is replaced with a series RLC, and the two fixed gyrators after the clipping (4.9 kHz and 105 Hz) are left out.
• The EQ is linear, so it was applied after ngspice with digital filters of the same type (peaking and shelf). The curves differ slightly from the real Low, Mid and High controls.
• The op-amps are models with only GBW, slew rate and output swing (741: 1 MHz, 0.5 V/µs). Noise is not modelled. In real pedals, gains above 60 dB amplify pickup hum and op-amp noise just as much, which is why high-gain rigs often add a noise gate.
• No guitar amp or speaker is included. The box under each player applies a speaker-like band limit.