Multi-stage error amps, feed-forward null tricks for sub-mV ripple.
When the loop in chapter 5 runs out of headroom — Zout stuck at 1 Ω, ripple stuck at 1 mV — two patterns add gain without compromising stability: cascade (more loop gain) and feed-forward (parallel cancellation path).
ConceptCascade — multiplying µ
Two error-amp stages in series. The first compares Vout · β to Vref; the second adds gain before driving the pass tube. Effective µ ≈ µ1 × µ2. A 12AX7 cascode followed by another 12AX7 reaches µeff > 5000, bench Zout < 0.5 Ω, ripple atten 65 dB.
ConceptFeed-forward — cancelling before the loop
The error amp can only correct what reaches it through R1. A spare tube samples Vraw ripple directly via a Cffcoupling cap, scales and inverts it, and injects the correction at Vout through Riff. Done right, the cancellation pushes ripple atten to 95 dB on the bench.
The hidden cost: feed-forward has two sweet spots — one for AC ripple (trim ≈ 35 %), one for DC line drift (trim ≈ 65 %). With a single physical pot you can only nail one. Most builds optimise for the AC null because that's where the audible noise lives.