The Spark Plug User Guide

The Spark Plug is a three-band harmonic saturation processor for adding weight, presence, and texture without turning your whole mix into one broad color. Shape the low, mid, and high ranges independently, then bring the result back together with global level and mix controls.

It is not a model of a particular analog circuit or piece of hardware. Its four Character modes are purpose-built saturation behaviors — each designed for a different musical response.

Quick Start

1. Start with Init, or choose a factory preset as a starting point.
2. Pick the band you want to affect — Low, Mid, or High — and choose a Character.
3. Raise Drive until the color becomes clear, then use that band's Mix to place it in the sound.
4. Turn on Auto Gain when comparing tones, so extra level does not make one setting seem better by default.
5. Use Output Trim to set the final level, then compare with Bypass before committing.

For focused work, use Solo to hear one or more bands on their own. While Solo is active, the global dry blend is temporarily disabled so you hear the processed band material clearly.

The Three Bands

The Low, Mid, and High sections are split by two adjustable crossover points. Each band has its own Character, Drive, Mix, and Solo controls, so a low-end thickening move does not have to add edge to the vocal range or top end.

The crossovers use smooth 24 dB/octave transitions rather than hard boundaries. The divider readouts between the bands are the controls: drag them to set the handoff points.

• Low/Mid crossover: 60–900 Hz; default 250 Hz.
• Mid/High crossover: 1.1–12 kHz; default 3 kHz.

Character

Each band can use one of four saturation characters.

• Soft is the cleanest starting point. It stays subtle until you supply enough Drive, then rounds peaks smoothly.
• Warm adds gentle asymmetric harmonic weight that becomes more apparent as the signal is driven harder. It is useful for thickening without obvious edge.
• Punchy reacts more strongly to attacks than to sustained material, bringing out transient bite and forward movement.
• Vintage combines a biased, tape-inspired saturation shape with progressively softer high frequencies and a small low-frequency bloom as Drive increases.

These names describe musical behavior, not modeled hardware. Choose with your ears: a Character may be right for one source and wrong for the next at the same settings.

Drive

Drive pushes that band into its selected Character. Its range is 0–24 dB, with a default of 0 dB. More Drive generally means more harmonics, more density, and more level.

Band Mix

Each band's Mix blends its saturated signal with its clean signal, from 0–100%. It is a useful way to make a bold Character choice but keep only the useful amount of it. The default is 100%.

Solo

Solo lets you audition one or more frequency bands while you work. Use it to find the part of the spectrum you want to shape, then turn it back off to judge the full result.

When any Solo is active, DRY BLEND OFF appears below Global Mix. This is intentional: the global dry path is suspended while soloing, so the audition remains clear.

Global Controls

Auto Gain

Auto Gain is off by default. When enabled, it estimates the level change created by the saturation processing and compensates for it gradually. This makes it easier to compare Characters and Drive settings by tone rather than by loudness.

Auto Gain is not LUFS matching, and it does not change the Output Trim value you set. It also pauses its adjustment during very quiet material rather than chasing silence.

Output Trim

Output Trim is the final manual level control, ranging from −24 to +24 dB and defaulting to 0 dB. Use it to place the processed signal where you want it in your session or against a reference.

Auto Gain works independently of Output Trim. If you need an exact level relationship to another track, set that deliberately with Trim and your DAW's own meters or reference tools.

Global Mix

Global Mix blends the fully processed result with a phase-matched version of the original three-band signal. At 100%, you hear the full effect; at 0%, you hear the unprocessed tonal balance through the plug-in's own matched signal path.

Global Mix is an excellent creative wet/dry control, especially when several bands are being driven. For a true untouched comparison, use Bypass rather than setting Global Mix to 0%.

Bypass

Bypass is the host-visible bypass control. It makes a click-free, latency-aligned transition to the dry input, so it is the right choice for an honest before/after check.

The processing engine continues running underneath Bypass, allowing a seamless return when you re-engage it.

Meters and Clip Indication

Input and output meters show stereo peak level from −48 to 0 dBFS, with held peak markers. A red cap at the top of a meter indicates clipping; click the cap to clear it.

The output path includes fixed safety protection against overs. It is not a limiter and has no threshold or ceiling control. If it is working hard enough to light the clip indication, back down Drive or Output Trim unless that audible distortion is part of the sound you want.

Presets, A/B, and Undo

The factory bank includes Init plus five voiced starting points:

Preset
Presence
Warmth
Impact
Patina
Full Charge

You can also save your own presets. Factory presets cannot be renamed or deleted; user presets can be saved, renamed, and removed from the preset menu.

A/B holds two complete settings states for fast comparison. Switching slots recalls that entire state immediately and clears the current Undo history. Undo and Redo treat a single drag, typed entry, or reset as one editing step.

Useful Starting Points

Add low-end weight without clouding the mix

Start in the Low band with Warm or Vintage. Add a modest amount of Drive, then pull Band Mix back until the weight arrives without making the kick or bass feel bigger than the track can support.

Bring out vocal or instrument presence

Use the Mid or High band with Punchy for articulation, or Warm for density without as much attack. Solo while finding the range, then turn Solo off before deciding how much Drive and Mix belong in the full arrangement.

Give a full mix a finished texture

Start from Patina or Full Charge, turn on Auto Gain, and make smaller adjustments than you think you need. Use Global Mix to set the final intensity. This is a color decision, not a substitute for corrective EQ, compression, or final level management.

Parallel-style saturation

Drive one or more bands more assertively, then reduce Global Mix. This retains the character of the saturated version while keeping more of the original signal in the final result.

Interaction Notes

For Drive, Mix, Trim, Global Mix, and the crossover controls:

• Drag to adjust.
• Hold Shift while dragging for finer control.
• Right-click for exact numeric entry.
• Double-click to reset to the default.
• Use arrow keys to nudge; press Return to enter a value directly.

The plug-in window is resizable from 80% to 140% of its native size. It opens at its native size each time.

Formats, Demo, and Support

The Spark Plug is available as AU and VST3 for macOS. It includes a fully functional 14-day demo. A license covers two computers.

Click the logo to open the About overlay, where you can access this User Guide, the product page, the Mainline Audio website, support, and the tooltip preference.