Analog · 2026·04·02
Vactrol Phase 90 Clone
AbstractA four-stage phaser built around LDR/LED vactrols, with an ATtiny85 running a bare-metal PWM LFO.
[ FIGURE 1 — HERO PHOTO ]
drop a board / object shot in here
drop a board / object shot in here
Figure 1.— Vactrol Phase 90 Clone.
The sweep comes from homemade optocouplers — an LED and an LDR heat-shrunk together in the dark — driven by a triangle LFO the ATtiny synthesises in software. Matching the vactrols by eye and by ear was most of the work.
An LM358 oscillated under capacitive load until the compensation was sorted; a good reminder that the cheapest op-amp in the drawer has opinions.
Specification
| Topology | Four all-pass stages, vactrol-swept |
| LFO | ATtiny85, PWM-driven LED |
| Optocouplers | Homemade LED/LDR pairs |
| Enclosure | 1590B, hand-drilled |
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