Fix opto-sat amplitude attenuation: R10 too low + U2B gain too high

[NeoRecasata]

This is the open rev 0.2 TODO: "Fix amplitude difference on opto sat output. The opto stage attenuates the signal from the transformer too much."

Root cause

Two factors compounding:

1. U2B has gain = −68× (R8 / R6 = 680Ω / 10Ω). Designed-in massive gain to drive the vactrol LED into compression.
2. R10 = 330 Ω is the LED current limit. The schematic already documents the other vactrol's (R18) appropriate values: VTL5C3 → 47K, Silonex 325R3 → 470Ω, handmade vactrol → 47K. R10 = 330 Ω is too low for any of these vactrol types.

The combination slams the LED hard at any non-trivial input level, the LDR drops to minimum resistance, and most of the signal shunts to GND.

Three fix options (in order of preference)

Option A — Replace R10 with a trim pot (recommended)

Replace fixed R10 (330 Ω) with a 10K trim pot in series with 1K fixed. Adjustable range: 1K (close to current behavior, very aggressive saturation) to 11K (gentle saturation).

Pros:

• Per-build calibration accommodates any vactrol type
• Builder tunes by ear or scope
• No other circuit changes needed

Cons:

• Adds one component
• Needs a calibration step

Option B — Reduce U2B gain

Change R6 from 10 Ω to 100 Ω → gain drops from −68× to −6.8×. Vactrol still works but doesn't slam.

Pros:

• One-component change
• Predictable, no calibration

Cons:

• Hardcodes the saturation depth
• Different vactrol types still need different R10 values

Option C — Add a make-up gain stage

After C3 (the AC coupling to "Wet" net), add a TL072 or NE5532 stage with gain ~3–5× to recover lost amplitude.

Pros:

• Keeps the heavy saturation character
• Independent calibration

Cons:

• Adds an op-amp + extra components
• Doesn't address the root cause

Recommendation

Option A. Adjustable, calibratable per-build, accommodates the documented vactrol variations.

Pair with Issue [#2 — document vactrol type guidance for R10] below.

Action

• Pick A / B / C
• Apply in rev 0.7 (schematic) → rev 0.2 (PCB)
• Update calibration procedure in README accordingly
• Bench-test with at least one vactrol type to confirm the fix works

[NeoRecasata]

Root cause confirmed by comparing against the Colourupter source

I pulled the DIYRe Colourupter schematic v1.7 and traced it against this opto-sat stage.

Colourupter:

• IC1a: input buffer
• IC1b: signal amp with modest gain (~1–3×), LDR shunts the feedback network
• IC1c + IC1d: SEPARATE parallel amp chain that drives the vactrol LED, independent of the signal path

Multi-Sat (this design):

• U2A: input buffer ✓
• U2B: combined signal amp + LED driver with gain −68×
• No separate LED-drive chain — same amp output goes to BOTH the wet output AND the LED

The divergence: Colourupter intentionally separates signal amplification from LED drive into two parallel amp chains. Multi-Sat collapsed them. To get useful LED current from the combined amp, U2B's gain had to be cranked to −68×. That same gain over-amplifies the signal path before the LDR can attenuate it — the LDR has to compensate for huge gain, so it always sits near its minimum resistance. No graceful compression "shoulder."

Revised fix recommendations

Option D (new, most faithful to Colourupter): Restore the parallel architecture. Add a buffer + LED-drive amp chain in parallel to the main signal path. Estimated parts cost: 1 op-amp section + 4 resistors + 1 cap. Returns the design to the Colourupter's intended behavior.

Option B (revised, still good): Reduce U2B gain. Per Colourupter, IC1b's gain ranges 1–3×. Change R6 from 10Ω to 220Ω → gain becomes −680/220 = −3.1× (matches Colourupter). The vactrol still works in feedback, but signal isn't over-amplified.

Option A (original recommendation): R10 as a trim pot. Still good for vactrol-type calibration but doesn't address the gain-amp imbalance. Stack with A or B for full fix.

My updated recommendation: B + A together — reduce U2B gain to ~3× and make R10 a trim pot. That gives Colourupter-faithful behavior plus per-vactrol-type calibratability.