Advanced Passive Components
Resistors, capacitors, and wire — the humble passives that shape your amplifier’s voice. Component selection in tube circuits is measurable engineering, not mythology. This module covers the physics behind the choices.
Why Component Selection Matters
Carbon comp vs metal film, capacitor dielectrics, wire-wound inductors
In tube circuits operating at high voltages (250–450V) and low currents (1–10mA), passive component choices directly affect sound quality, reliability, and noise floor. A coupling capacitor's dielectric absorption can smear transients. A resistor's excess noise adds hiss. Wire routing creates ground loops.
Carbon composition resistors were standard in the tube era but generate 3–6dB more noise than modern metal film types. In a high-gain preamp with A_v = 60, that excess noise is amplified to audible levels. Component selection is not audiophile mythology — it is measurable engineering.
Every resistor generates Johnson-Nyquist noise proportional to its resistance, temperature, and bandwidth. At 25°C, a 1MΩ resistor produces ~4µV RMS over a 20kHz audio bandwidth. Reducing plate load resistance lowers noise but also reduces gain — a fundamental trade-off.
Ref: Horowitz & Hill, "The Art of Electronics" 3rd Ed. §1.2 — Johnson noise and resistor selection
Resistor Thermal Analysis
Noise voltage, temperature drift, and power derating for audio resistors
Adjust the resistor value, operating temperature, and temperature coefficient to see real-world effects. Carbon comp resistors have TC of 200–500 ppm/°C; metal film types are 50–100 ppm/°C. The V_n formula assumes 20kHz audio bandwidth.
Capacitor Dielectric Comparison
Film, ceramic, electrolytic, mica, oil — ESR, voltage, temperature stability, audio suitability
| Type | ESR | V max | DA | Temp Stability | Audio Use |
|---|---|---|---|---|---|
| Film (PP) | <0.01Ω | 630V | 0.001–0.02% | Excellent | Best for coupling & RIAA |
| Film (PS) | <0.005Ω | 200V | 0.001–0.01% | Excellent | Premium RIAA, low V only |
| Paper-in-Oil | 0.05–0.5Ω | 1000V+ | 0.01–0.05% | Good | Classic tone, HV coupling |
| Ceramic C0G/NP0 | <0.01Ω | 500V | <0.01% | Excellent (0 ppm) | Small values, bypass only |
| Ceramic X7R/Y5V | 0.05–1Ω | 100V | 2–5% | Poor (−15/+22%) | Avoid in signal path |
| Mica | <0.01Ω | 500V | 0.001% | Excellent | Precision, small values |
| Electrolytic (Al) | 0.1–10Ω | 500V | 5–15% | Poor | PSU filter only, bypass with film |
Coupling Capacitor Dielectrics
Polypropylene vs polystyrene vs paper-in-oil for audio coupling
1. Dielectric Absorption (DA)
When a capacitor is charged, then briefly discharged, the voltage partially recovers due to trapped charge in the dielectric. This DA effect smears transients and adds memory distortion. In a coupling cap passing audio, DA creates a subtle form of intermodulation distortion proportional to the signal level.
2. Polypropylene (PP)
The gold standard for tube audio coupling caps. DA of 0.001–0.02%, low ESR, available up to 630V. Wima MKP, Mundorf MCap, and Vishay MKP1837 are common choices. Metalized PP types are self-healing — a major reliability advantage in HV circuits.
3. Polystyrene (PS)
The lowest DA of any common dielectric (0.001–0.01%), and the negative temperature coefficient (−150 ppm/°C) can compensate for positive-TC resistors in RIAA networks. Limited to ~200V and small values (≤10nF typically), so mainly used in phono stages and precision filters.
4. Paper-in-Oil (PIO)
The original tube-era coupling cap. DA of 0.01–0.05% — higher than PP but still good. Available in very high voltage ratings (1000V+). Prized for warm, rich tone in guitar amps. Modern reproductions (Jensen, ASC) offer improved reliability over NOS units. The oil impregnation provides excellent insulation resistance at high voltages.
Wire Gauge & Skin Effect
Hookup wire selection, skin depth at audio frequencies
1. Wire Gauge Selection
For point-to-point tube amp wiring, 22 AWG (0.64mm) solid-core copper is the workhorse gauge. It handles the low currents (1–50mA) with negligible resistance, is stiff enough for turret or eyelet boards, and fits standard tube socket pins. Use 20 AWG for heater wiring (higher current), and 18 AWG for power supply runs where voltage drop matters.
2. Skin Effect at Audio Frequencies
At high frequencies, current crowds toward the wire surface. The skin depth in copper is:
At 20kHz, skin depth is δ ≈ 0.47mm. A 22 AWG wire has radius 0.32mm — well within the skin depth. Skin effect is negligible at audio frequencies for any standard hookup wire. Claims of audible differences from exotic conductors at audio frequencies are not supported by physics.
3. Practical Wiring Tips
Use PTFE (Teflon) or silicone-insulated wire for high-voltage runs — PVC insulation can break down above 300V. Keep signal wires short and dress them perpendicular to heater wiring to minimize hum pickup. Twist heater supply wires tightly (1 twist per cm) to cancel their magnetic field. Use shielded cable only for long signal runs; the added capacitance can roll off high frequencies in high-impedance tube circuits.
Testez vos connaissances
Which capacitor dielectric has the lowest dielectric absorption (DA)?