PEDAL BENCH
by Stompbox Labs
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Modulation1974· Electro-Harmonix (USA)

Electro-Harmonix Small Stone

OTA-based four-stage phaser with a wide, watery sweep and an iconic Color switch.

⏳
Schematic pending verification

We don't ship reference schematics until they've been hand-authored against authoritative sources (manufacturer service docs, ElectroSmash analyses, GEOFEX writeups) and reviewed in the simulator. Until then, the BOM, signal chain, controls and mod notes below are accurate, and you can still add this pedal to a project — it just won't include a circuit yet.

About

Where the Phase 90 uses JFETs, the Small Stone uses CA3094 OTAs as voltage-controlled resistors in its four all-pass stages. The 'Color' switch changes the feedback path, sharpening the notches for a more pronounced, vocal sweep.

Signal chain preview

5 stages
Input
Input buffer (op-amp)
4× CA3094 OTA-based all-pass stages
Summing amp (dry + phased)
Triangle LFO sweeps the OTAs
Color switch toggles feedback around stages 3-4
Output

Quick facts

  • OTA-based phasing has a wider, lusher sweep than JFET phasers like the Phase 90.
  • The Color switch toggles a feedback path — on = sharper, more 'vowel-like' phasing.
  • Famously heard on Frusciante's 'Scar Tissue' clean tone (often paired with a DD-3).
  • Three eras: V1 (1974, CA3094), V2 (~76), V3/V4 Russian Sovtek ('90s) — all sound subtly different.

Controls

  • Rate (1M linear pot)
  • Color (SPST switch)

References