Class-AB 2SC5200 / 2SA1943 (≈100 W)

Classic three-stage topology: differential input pair → VAS → emitter follower drivers (A940/C20733) → output pair (2SC5200/2SA1943). Thermally-compensated VBE multiplier mounted on the heatsink, separate speaker protection/zobel, and star-grounding for quiet operation.

Jump to Schematic BOM Bias & Setup

Specs

Power
≈100 W @ 8 Ω (±50–55 V)
THD
<0.02% (1 kHz, 1 W typical)
Bandwidth
10 Hz – 100 kHz (−3 dB)
Output Devices
1× pair 2SC5200 / 2SA1943
Drivers
A940 (NPN) / C2073 (PNP)
Rails
±42-46 V (4 Ω)

Overview

Long-tail diff pair (2× C1815 / A1015) (D667 / B647) with Miller comp (Cdom), then emitter-follower drivers (A940/C2073) into the emitter-resistored output pair (2SC5200/2SA1943). Bias is set with a VBE multiplier (e.g., C1815) bolted to the heatsink.

Rails: ±42-46 V for ~100 W/8 Ω; ±35–40 V for ~100 W/4 Ω (higher current, watch SOA/heat).

Schematic (inline SVG, 100 W class-AB)

Class-AB 2SC5200/2SA1943 100W Amplifier schematic
Functional schematic for a single-pair 2SC5200/2SA1943 stage.

BOM (core amplifier)

RefValue / PartNotes
Qin1,Qin2C1815 (NPN) matchedDifferential pair (low-noise)
Qmir1,Qmir2A1015(PNP)Current mirror
QvasD667 (NPN)VAS transistor
QvbC1815 VBE multiplier on heatsink; 1 k trimmer + 2×1 k
QdrvN/QdrvPA940 / C2073Drivers
QoutN/QoutP2SC5200 / 2SA1943Outputs; Re 0.33 Ω/5 W
Rf/Rg22 kGain ≈ 23× (~27 dB)
Cf68–100 pF NP0Feedback lead cap
Cdom68–100 pF NP0Miller comp
Zobel10 Ω/2 W + 100 nFOutput network to GND
Coil2–3 µH air core~10–15 turns, 1 mm Cu; // 5–10 Ω
Local decoupling100 nF per railNear VAS/drivers/outputs

Bias & Setup

  1. Power up with series lamp/variac. Input shorted, no load.
  2. Set bias trimmer low. Measure across one 0.22 Ω emitter resistor.
  3. Adjust to 11–15 mV (≈50–70 mA through the pair). Warm up and re-check.
  4. Check DC offset at output (<20 mV typical). Match diff pair if needed.
  5. Use a speaker-protection relay (delay + DC detect). Test with dummy load first.
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