The Gyrator
Real inductors are heavy, expensive, and magnetically noisy. A gyrator uses a capacitor and an active element to simulate an inductor — same impedance behavior, zero magnetic pickup, adjustable Q.
Why Simulate an Inductor?
Real inductors are impractical at audio frequencies — gyrators solve this
Real inductors at audio frequencies are large, heavy, and lossy. A 10H inductor for a tone stack can weigh over a kilogram and picks up magnetic interference from transformers and mains wiring. The gyrator offers an elegant alternative.
A gyrator uses a capacitor combined with a gain element (op-amp, transistor, or tube) to present an inductive impedance at its input. It "rotates" the capacitor's impedance by 90°, transforming 1/jωC into jωL.
The equivalent inductance is given by:
With a 100kΩ resistor and a 100nF capacitor, you get L_eq = (100k)² × 100n = 1000H — an impossibly large value for a real inductor, yet trivially achieved with a gyrator.
Ref: Horowitz & Hill, "The Art of Electronics" 3rd Ed. §2.4.1 — Impedance transformation via gyration
Gyrator Designer
Set R and C to compute the equivalent inductance
The gyrator converts capacitance to inductance: L_eq = R² × C. Adjust the sliders below.
Tube Gyrator Circuit
Using a triode to simulate inductance
A triode (like one half of a 12AX7) can serve as the active element in a gyrator. The plate load is a capacitor instead of a resistor. The cathode resistor sets the bias. The circuit presents an inductive impedance at the input.
Where R_k is the cathode resistor, R_p is the plate resistance (r_p of the tube), and C_p is the plate capacitor. The tube’s gain converts the capacitive impedance at the plate into an inductive impedance looking into the circuit.
Typical values: R_k = 1.5kΩ, r_p = 62.5kΩ (12AX7), C_p = 100nF gives L_eq ≈ 9.4H — perfect for a mid-frequency tone stack inductor replacement.
Where Gyrators Shine
Four key applications in tube audio
Tone Stacks
Replace heavy inductors in Fender/Marshall mid-scoop EQ circuits. A gyrator gives identical frequency response in a fraction of the space.
Active EQ
Parametric and graphic EQ circuits using gyrators for each band. Adjustable center frequency, Q, and gain.
RIAA Phono
RIAA equalization with gyrator-simulated inductance. Avoids large, expensive inductors while maintaining accuracy.
PSU Filtering
Electronic choke: a gyrator in the power supply filter replaces a heavy iron choke, providing superior ripple rejection.
Real Inductor vs Gyrator
| Parameter | Real Inductor | Gyrator |
|---|---|---|
| Size | Large (>100cm³) | Tiny (<1cm³) |
| Weight | 0.5–2 kg | < 5 g |
| Magnetic pickup | Yes (shielding needed) | None |
| Q factor | Fixed by construction | Adjustable via R |
| Cost | $10–$50+ | $1–$5 |
| Frequency range | Wide (DC to RF) | Audio (20Hz–20kHz) |
| Power required | No | Yes (B+ supply) |
Testez vos connaissances
What does a gyrator circuit simulate?
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.