diff --git a/README.md b/README.md
index 27cd5c5c..89141a22 100644
--- a/README.md
+++ b/README.md
@@ -19,7 +19,7 @@ board), so these drawings post-date the Rev. 0/1 boards they describe.
 | **1401** | Buttons board | Rev. 0 | 2× UCN5821A lamp drivers, 2× 74HCT165, 74HCT164, 74LS00/05 |
 | **1402** | Keypad board | Rev. 1 | 2× 74HCT164, CD74HCT165, 74LS00/05 |
 | **1408** | Display board ("RIO external display") | Rev. 1 | Intersil **ICM7228B** 8-digit driver, 8× LTS313A 7-segment |
-| **1413** | Amplifier board | *(schematic only, not photographed)* | 4-channel speaker filter/check board, 12 VDC 5 A |
+| **1413** | Quad Amplifier Board | Rev. 1 | **Pyramid PB-150P "Pro Plus"** car amp (bolt-on) + passive 3-way crossovers |
 
 Plus the cockpit position sensors: **HP HEDS-5700** two-channel quadrature
 optical encoders (option code **I** = 512 counts/rev; the photographed unit is
@@ -98,10 +98,22 @@ hardware addressing is what the 0x00–0x6F logical input map in
   printer port"** header with series resistors and per-line LEDs — a
   diagnostic tap so a PC parallel port can exercise/observe the display bus.
   Silkscreen reads "P/N 140B" but the schematic title block confirms **1408**.
-- **1413 Amplifier board** (`AMP_PLT`) — four channels (front/back ×
-  left/right): LC crossover filtering (820 µH/180 µH), 4 Ω 5 W loads, 6-pin
-  woofer/midrange/tweeter speaker connectors, a 12 VDC 5 A supply input, and
-  a green/red "speaker outputs checking circuit" per channel.
+- **1413 Quad Amplifier Board** (`AMP_PLT`, silkscreen "Quad Amplifier
+  Board … P/N 1413 REV. 1") — the actual amplification is a stock
+  **Pyramid PB-150P "Pro Plus"** 4-channel 12 V car-audio amplifier bolted
+  to the board as a module; the schematic only drew the VWE-designed
+  passive circuitry around it. The board feeds the Pyramid's `B+12V / GND
+  / REMOTE` screw terminals and speaker-level `HIGH INPUT` connectors, and
+  its four outputs (`1CH/L`…`4CH/R`) return through terminal strips into
+  per-corner 3-way passive crossovers: Dale IHB-3 inductors (820 µH woofer
+  low-pass, 180 µH midrange, date-coded 9531), MMP 100 VDC film capacitors
+  (2.2 µF/10 µF), and Dale CP-5 2 Ω 5 W pad resistors. Corner speaker
+  connectors J1–J4 (front/back × left/right) are 6-pin: `1 woofer+,
+  2 midrange+, 3 tweeter+, 4 woofer−, 5 midrange−, 6 tweeter−` (silkscreen,
+  matching sheet 5). Power enters on J5 (4-pin: `+12VDC ×2, GND ×2`) with a
+  green POWER ON LED; the per-channel red LED pairs across the outputs are
+  the schematic's "speaker outputs checking circuit". PCB etch NT 1-0A 3695
+  — the same fab batch series (week 36 '95) as the buttons board.
 
 ## GAL dump — [`gal/GAL20v8a_5764.JED`](gal/GAL20v8a_5764.JED)