Star Grounding & Ground Loops
Why the ground wire is the most important wire in your amplifier
Ground is not a magical zero-voltage reference — it's a wire with resistance, inductance, and current flowing through it. When multiple circuits share a ground wire, the current from one circuit creates a voltage drop that appears as a signal in another. This is the ground loop, and it's the #1 cause of hum and noise in tube amplifiers.
A 10cm ground wire has approximately 100nH of inductance. At 50Hz, this is negligible. But the power supply return current contains harmonics up to 10kHz+, where 100nH represents 6mΩ of impedance. With 100mA of ripple current, that's 0.6mV of noise injected into every circuit sharing that ground path. For a 12AX7 preamp with 60dB gain, this becomes 600mV at the speaker — clearly audible.
Magnetic Shielding Effectiveness
How thick must the shield be to attenuate transformer hum?
Materials: Steel μr≈200 σ=6.99, Mu-metal μr≈20000 σ=1.6, Copper μr=1 σ=58.7, Aluminum μr=1 σ=37.7
Star Ground Implementation
Shielding Materials Compared
Magnetic vs electric field shielding — choosing the right material
| Material | μr | σ (MS/m) | δ @ 50Hz | Best for |
|---|---|---|---|---|
| Mild Steel | 200 | 6.99 | 3.8mm | Transformer covers, chassis |
| Mu-metal | 20,000 | 1.6 | 0.56mm | Phono preamps, input stages |
| Copper | 1 | 58.7 | 9.4mm | RF shielding, Faraday cages |
| Aluminum | 1 | 37.7 | 11.6mm | Chassis, lightweight shields |
| Permalloy | 80,000 | 1.1 | 0.27mm | Tape heads, critical shielding |
| Silicon Steel | 4,000 | 2.0 | 1.1mm | Transformer laminations |
Grounding & Shielding Cookbook
1. Heater Wiring
For low-noise preamps, DC heater supply is essential — it eliminates the 50/60Hz magnetic field from heater wires. For power stages, AC heaters with a center-tapped or artificial center-tap (two 100Ω resistors) are adequate. Twist heater wires tightly (one twist per cm) and dress them away from signal wiring. Elevate the heater reference to +50-70V DC to prevent cathode-heater leakage noise.
2. Star Ground Implementation
Use a single bolt through the chassis as the star point. Run separate ground wires from: (1) input stage, (2) driver/phase splitter, (3) output stage cathodes, (4) power supply filter caps (heavy current), (5) heater center-tap. The chassis bolt connects to safety earth. Never use the chassis as a signal ground return — the sheet metal has too much resistance and picks up magnetic interference.
3. Cable Shielding
Shielded cable blocks electric field coupling (capacitive crosstalk) but does nothing against magnetic fields at audio frequencies. For magnetic field immunity, use balanced (differential) signal transmission or twisted pairs. Ground the shield at one end only to prevent shield current from creating a ground loop. Exception: very long runs (> 10m) may need both ends grounded with a ground-lift resistor (10Ω).
4. Transformer Orientation
Mount the power transformer and output transformer at 90° to each other to minimize magnetic coupling. The power transformer's stray field is strongest along its core axis. Keep input tubes (12AX7, 12AT7) as far as possible from the power transformer — even a few centimeters make a measurable difference. Mu-metal tube shields on input tubes reduce hum pickup by 20-30dB.
Testez vos connaissances
What is the primary purpose of star grounding in a tube amplifier?
References
- Paul Horowitz & Winfield Hill, The Art of Electronics, 3rd ed., Cambridge University Press, 2015. ISBN 978-0521809269Canonical reference for analog design — covers tubes in Ch. 2.4 & Ch. 3.
- Morgan Jones, Valve Amplifiers, 4th ed., Newnes, 2012. ISBN 978-0080966403Modern engineering treatment of tube audio design.