Modern Class-A output stages drive low-impedance speakers directly, burning enormous quiescent current. But before the transistor age — and still today in tube guitar amps and high-end audio — designers used a transformer to couple the output stage to the load. This trick lets a high-impedance, low-current device efficiently drive a low-impedance load like an 8 Ω speaker, while also providing DC isolation and a path for the collector current to flow through a near-zero DC resistance.

The key insight is impedance reflection. A transformer with turns ratio N = N_p/N_s reflects the secondary load impedance Z_L back to the primary as:

• Z_primary = N² × Z_L

So an 8 Ω speaker on a 20:1 transformer looks like 400 × 8 = 3.2 kΩ to the transistor — a perfectly reasonable collector load.

Real-world example: a single-ended 6V6 tube amp. The 6V6 wants to see roughly 5 kΩ plate load for maximum undistorted power output. With an 8 Ω speaker, you'd choose a turns ratio of √(5000/8) ≈ 25:1. The plate sits at +250 V DC, but because the primary's DC resistance is only ~200 Ω, the tube dissipates power only on the signal swing — not in heating a resistor.

The DC load line is nearly vertical; the AC load line is the slope that matters. The quiescent operating point sits at (V_CC, I_Q), but the signal swings symmetrically around it along the AC load line of slope −1/Z_primary. The peak voltage swing can actually exceed V_CC — the transformer's inductive kick lets the collector swing up to roughly 2×V_CC on the positive half-cycle. This is why a 250 V plate can swing to nearly 500 V peak.

Efficiency rule of thumb: a transformer-coupled Class-A stage tops out at 50% efficiency, versus 25% for resistor-coupled Class-A. The transformer eliminates the DC dissipation in the load resistor — that's where the doubled efficiency comes from.

Watch out for:

• Core saturation at low frequencies — the primary inductance must be large enough that ωL ≫ Z_primary at your lowest signal frequency (typically L > Z_primary/(2π·f_low)).
• Leakage inductance causing high-frequency rolloff and ringing — interleaved windings help.
• DC magnetization from the quiescent current pushing the core toward saturation — single-ended designs need an air gap; push-pull cancels this naturally.

For a 5 W output stage, expect a transformer with primary inductance of 10–30 H and a gapped EI core sized for ~10 mA of DC bias without saturating.