Two-channel valve microphone preamplifier, takes a mic or instrument on a combo XLR/jack, runs it through an input transformer and three valve stages, and puts it back out balanced through an output transformer — into a screw terminal for the main output plus two JST headers that feed an ADAT converter board and a gain meter. I'd like a second set of eyes on it before I order boards.
**Signal flow (per channel):**
Combo XLR/TRS in → P48 phantom feed (2 × 6.8k, switched) → −20 dB pad (820R / 182R / 820R, switched) → polarity reverse switch
→ input transformer (Neutrik nte10-3) → 12AY7 gain stage (68 k plate, bypassed cathode) → coupling cap → 100 k gain trimmer → 12AY7 second stage (150 k plate) → DC-coupled → 12AU7 White cathode follower → output transformer (off-board) → balanced out ×3
The jack tip is normalled through the connector, so with nothing plugged into the 1/4" side the grid sees the mic transformer, and inserting a plug swaps it over to a high-impedance instrument input. Roughly 60 dB of gain in the valve stages on top of whatever the input transformer contributes. The White cathode follower is there specifically so the output transformer gets driven with real current capability rather than a plain follower.
**Power:**
both rails come in from external supplies on screw terminals, nothing rectifies on this board. B+ is 225 V DC regulated, with 2 × 22 µF / 400 V of bulk on-board and a 4.99 k / 10 µF RC per channel for the gain stages. Heaters run on 50.4 V DC with all four valves in series (4 × 12.6 V @ 150 mA); the chain is ordered so both cathode-follower valves sit at the top of the string, which keeps the worst-case heater-to-cathode voltage around 98 V.
**Main components:**
- U1 / U3 — 12AY7 / 6072A, input and second gain stage (one per channel)
- U2 / U4 — 12AU7 / ECC82, White cathode follower output stage
- J1 / J11 — Neutrik NCJ10FI-H, combo XLR + 6.35 mm switching jack input
- SW1 / SW4 — +48 V phantom, Alps SPUJ190900 latching DPDT
- SW2 / SW5 — −20 dB pad, same switch type
- SW3 / SW6 — polarity reverse, same switch type
- R12 / R31 — PT01-B120D-A104, 100 k gain trimmer
- C17 / C18 — 22 µF / 400 V, B+ reservoir
- J4 / J14 and J3 / J13 — input transformer (NTE 10/3), off-board on JST
- J6 / J16 and J5 / J15 — output transformer (UTM3510), off-board on JST
- J8 / J18 — balanced main output, screw terminal
- J10 / J20 and J17 / J23 — balanced output to ADAT converter and to gain meter
- J21 — B+ 225 V in, screw terminal
- J22 — heater 50.4 V in, screw terminal
**Board:**
180 × 200 mm, 2-layer (top signal + ground pour / bottom signal + ground pour), 1.6 mm FR4, 35 µm copper, through-hole only, single-sided assembly. Ground pour on both layers with 161 stitching vias. Custom clearance rules for the high-voltage nets: 1.25 mm for everything at or above ~167 V, per IPC-2221 table 6-1 case B2. DRC is clean — no errors, no unconnected nets.
Any feedback welcome — layout, grounding, the high-voltage side, component choices, anything I've missed.
