Transformer Design
The output transformer is the heart of every tube amplifier. Its design determines bandwidth, distortion, power handling, and ultimately the amp's sonic character. Master core selection, winding techniques, and frequency response analysis.
Why Transformers Matter
Output and power transformer role in tube amplifiers
The output transformer is the most critical and expensive component in a tube amplifier. It bridges two worlds: the high-impedance plate circuit (typically 3k–10kΩ) and the low-impedance loudspeaker (4–16Ω). Without proper impedance matching, power transfer to the load would be negligible.
The impedance transformation follows the square of the turns ratio. This fundamental relationship governs all transformer design:
For a typical single-ended triode amplifier driving an 8Ω speaker with 5kΩ optimal plate load, the turns ratio is √(5000/8) = 25:1. The power transformer provides B+ (250–450V), heater voltages (6.3V or 12.6V), and sometimes negative bias supply.
Ref: Horowitz & Hill, "The Art of Electronics" 3rd Ed. §6 — Radiotron Designer's Handbook 4th Ed. Ch.5
Impedance & Turns Ratio Calculator
Enter your requirements to calculate transformer parameters
The turns ratio is determined by the impedance match. From there we derive currents, voltages, and core size. The minimum primary inductance sets the low-frequency limit — use L_p ≥ Z_p / (2π × f_low × 5) for less than 1dB loss at the lowest frequency of interest.
Core Types Compared
Choose the right core geometry for your application
| Core Type | Cross-section | Bandwidth | Weight | Cost | Notes |
|---|---|---|---|---|---|
| EI Lamination | 1–10 cm² | 30Hz–15kHz typical | Heavy | Low | Most common in vintage amps. Easy to wind, gap adjustable via shims. Higher leakage than C-core. |
| C-Core | 0.5–8 cm² | 20Hz–25kHz typical | Medium | Medium | Grain-oriented steel, lower core losses. Better HF due to tighter coupling. Used in hi-fi output transformers. |
| Toroidal | 0.3–6 cm² | 15Hz–40kHz typical | Light | High | Lowest leakage and stray field. Compact and efficient. Difficult to wind, no air gap (problematic for SE amps with DC bias). |
Winding Design & Interleaving
Primary/secondary calculations and leakage reduction techniques
1. Primary Winding
The number of primary turns is determined by the required inductance, core area, and permeability: N_p = √(L_p / (A_L × 10⁻⁹)) where A_L is the core's inductance factor in nH/turn². For a given core, more turns means more inductance but also more winding resistance and capacitance.
2. Wire Gauge Selection
Wire gauge is chosen for a current density of 3–5 A/mm² (conservative for continuous duty). The primary carries DC bias current plus signal current. For a 5kΩ primary at 50mA DC bias, AWG 32–34 (0.20–0.16mm) is typical. The secondary carries higher current at lower voltage — AWG 18–22 depending on power rating.
3. Interleaving for Reduced Leakage
Leakage inductance is the enemy of high-frequency response. By splitting the primary and secondary into multiple sections and interleaving them (P-S-P or P-S-P-S-P), leakage drops by 1/n² where n is the number of interleaving sections. A simple P-S has the most leakage; a 3-section P/2-S-P/2 cuts it by 4×.
4. Insulation & Layer Practice
Between primary and secondary, use 3 layers of insulation tape rated for the B+ voltage (typically 400–500V). Between primary layers, a single layer of interleave tape prevents voltage breakdown. The primary-to-secondary insulation must withstand the full B+ plus any transient spikes at power-off.
Frequency Response Limits
Low-frequency rolloff from primary inductance, high-frequency rolloff from leakage and capacitance
Low-Frequency Rolloff
At low frequencies, the primary inductance becomes comparable to the source impedance (plate resistance). The transformer acts as a high-pass filter. The −3dB point is where the inductive reactance equals the total resistance in the circuit:
Where R_p is the plate resistance and R_L' is the reflected load impedance. For a 12A triode (r_p = 800Ω) with 5kΩ reflected load and 10H primary, f_low = (800 + 5000) / (2π × 10) ≈ 92Hz. To reach 20Hz, you need approximately 46H of primary inductance.
High-Frequency Rolloff
At high frequencies, the leakage inductance and distributed winding capacitance form a resonant circuit that creates a peak followed by steep rolloff. The −3dB point is approximately:
Typical values: L_leakage = 10–100mH, C_winding = 200–1000pF. With 30mH leakage and 500pF capacitance, f_high ≈ 41kHz. The resonant peak can cause ringing on square waves — a Zobel network (R-C across the secondary) damps it.
Testez vos connaissances
How does impedance scale with the turns ratio of a transformer?