Beyond the Basic Cathode Follower
White cathode follower, SRPP, and mu-follower — trading complexity for performance
The basic cathode follower has gain slightly less than unity (A_v = μ/(μ+1)), low output impedance (Z_out ≈ 1/gm), and high input impedance. But it has a fundamental asymmetry: the tube can only source current (pull up) actively — current sinking depends on the passive cathode resistor. This limits output swing, increases distortion, and raises the output impedance for negative-going signals.
Cathode Follower Topology Comparison
Standard vs White vs SRPP vs Mu-follower
Output Impedance vs Frequency
Cathode Follower Variants
From simple buffer to audiophile output stage
| Topology | Z_out | Distortion | Use |
|---|---|---|---|
| Standard CF | 1/gm ≈ 370Ω | 0.5-2% (asymmetric) | Buffer, line driver |
| White CF | < 50Ω | < 0.1% (symmetric) | Headphone amp, cable driver |
| SRPP | 2rp/(μ+2) | < 0.3% (push-pull) | Headphone amp, phono stage output |
| Mu-follower | 1/gm | < 0.05% | High-end DAC output, preamp |
| Aikido | 1/gm | < 0.1% | PSRR-optimized buffer |
Building Advanced Cathode Followers
1. White Cathode Follower
The White CF adds a second tube as an active current sink. When the upper tube sources more current (positive swing), the lower tube detects the increased voltage across Rk and sinks more current. This creates push-pull action with dramatically lower output impedance. Use matched 6SN7 sections. Typical values: Rk = 10kΩ, B+ = 300V, Ip ≈ 10mA per tube. Output impedance drops to < 50Ω — excellent for driving long cables or headphones.
2. SRPP (Shunt-Regulated Push-Pull)
The SRPP stacks two triodes: the lower as a common-cathode amplifier, the upper as a cathode follower. At the output node (junction), both tubes contribute current — one pushes, the other pulls. Optimal when the load impedance equals 2×rp. With a 6SN7: rp ≈ 7.7kΩ, so optimal load ≈ 15kΩ. Higher loads favor the mu-follower instead. SRPP is ideal for headphone amplifiers (32-600Ω loads need the low Zout).
3. Mu-Follower
The mu-follower replaces the passive cathode load with a pentode or MOSFET CCS. The CCS provides infinite (AC) load impedance, so the triode follower operates with maximum linearity and voltage swing approaching B+ - Vak_min. The key: the CCS must have output impedance >> rp of the follower tube. An EF86 or cascode MOSFET CCS works well. This topology achieves the lowest distortion of any single-ended tube buffer.
Testez vos connaissances
What is the fundamental limitation of a standard cathode follower?
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.