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Headphone Amplifiers

Amplifier Design · Interactive

Headphone Amplifier Design

OTL and transformer-coupled topologies for driving headphones with vacuum tubes. Interactive calculators for impedance matching, power requirements, and circuit selection.

Select a tube — calculators update live

6SN7Legendary musicality. Holographic.

μ20

rp7.7kΩ

Pd max5W

Va max450V

01 — The Case for Tubes

Why Tube Headphone Amps?

Intimacy, harmonics, and the OTL possibility

Headphones are the most revealing transducers most listeners own. They expose every detail of the signal path, making them the ideal partner for tube amplification. The second-harmonic character of triodes adds a subtle warmth that complements the analytical nature of headphone listening.

The power requirements are dramatically lower than loudspeakers — typically under 100 mW for comfortable listening. This opens up the possibility of Output Transformerless (OTL) designs, eliminating the most expensive and sonically compromising component in a tube amplifier: the output transformer.

With headphones, you listen in a private acoustic space where micro-details, spatial cues, and tonal nuances are laid bare. Tubes deliver these with a natural, three-dimensional quality that solid-state designs struggle to match.

Typical power

1-100 mW

vs 1-100 W for speakers

OTL possible

300-600 Ω

high-Z headphones

THD character

2nd harmonic

even-order, musical

02 — Architecture

OTL vs Transformer-Coupled

Two philosophies for driving headphones

OTL — Output Transformerless

The tube drives headphones directly through a coupling capacitor. No output transformer means no core saturation, no winding capacitance, and wider bandwidth.

Topologies: Cathode follower, SRPP, White cathode follower.

Best for: High-impedance headphones (300-600 Ω) where the tube output impedance is a small fraction of the load.

BandwidthExcellent

CostLow

Impedance matchingLimited

Transformer-Coupled

An output transformer matches the high plate impedance to the low headphone impedance. Works with any headphone, but the transformer quality determines the sonic ceiling.

Topologies: Single-ended, push-pull. Standard amplifier topologies scaled down.

Best for: Low-impedance headphones (32-150 Ω) or when you want maximum flexibility across different headphones.

BandwidthTransformer-limited

CostHigher

Impedance matchingUniversal

OTL works when Z_load ≫ Z_out — aim for damping factor > 4

OTL Cathode Follower

Transformer-Coupled

03 — Calculator

Impedance Matching

See why OTL works best with high-impedance headphones

Headphone impedance

Tube rp7.7kΩ

Tube Zout (cathode follower)367Ω

Damping factor0.8(300Ω / 367Ω)

Voltage for 1mW548mVrms

Current for 1mW1.83mArms

Damping factor below 2 — the tube output impedance is too high for this headphone. Consider a transformer-coupled design or a lower-rp tube like 6AS7G.

Damping factor across impedances (cathode follower)

Headphone Z	Damping	Verdict
32Ω	0.1	Poor
80Ω	0.2	Poor
150Ω	0.4	Poor
300Ω	0.8	Poor
600Ω	1.6	Poor

04 — OTL Topologies

Cathode Follower, SRPP & White CF

Three output stages with increasing complexity and performance

Simple Cathode Follower

The simplest OTL topology. Unity voltage gain with very low output impedance. 100% negative feedback through the cathode resistor provides excellent linearity.

The output impedance is the plate resistance divided by (mu + 1), making high-mu tubes less useful here — you want moderate mu with low rp.

Z_out = r_p / (μ + 1)

Gain ≈ μ / (μ + 1) < 1

6SN7 — Cathode Follower into 300Ω

B+250V

Output Z367Ω

Voltage gain0.95×

Damping factor0.8

Max swing (est.)100Vpk

Max power into load16667mW

05 — Power Calculator

How Much Power Do You Need?

Most headphones need surprisingly little — calculate your requirements

Sensitivity103 dB/mW

Target SPL100 dB

HP impedance300Ω

Extra dB needed-3dB above 1mW

Power required0.50mW

Voltage required388 mVrms

Peak voltage548 mVpk

Current required1.29mArms

Very low power — even a simple cathode follower will deliver this with ease. Focus on low noise and good linearity rather than raw power.

P_required = 10^{(SPL - Sensitivity) / 10} mW

V_rms = √(P × Z) | I_rms = √(P / Z)

Typical headphone sensitivity ranges

IEMs / earbuds

105-120 dB/mW

<1 mW

Efficient dynamics

100-108 dB/mW

1-10 mW

Average dynamics

95-102 dB/mW

10-50 mW

Planar magnetics

90-98 dB/mW

50-200 mW

06 — Reference Designs

Recommended Circuits

Proven topologies matched to common use cases

6SN7Cathode Follower

Target: 300Ω+ headphones

Simplest build. Excellent with Sennheiser HD600/650. One dual-triode per channel.

B+150-250V

Zout~350Ω

12AU7SRPP

Target: 32-600Ω (versatile)

Good all-rounder with voltage gain. Works with most headphones. Easy to source tubes.

B+200-300V

Zout~200Ω

6AS7GOTL (cathode follower)

Target: 32-150Ω headphones

Power tube with very low rp. Drives low-impedance headphones directly. Runs hot — needs ventilation.

B+100-150V

Zout~30Ω

6080White Cathode Follower

Target: 32-300Ω

Dual-triode power tube. White CF topology achieves very low output impedance. Premium performance.

B+150-200V

Zout~15Ω

07 — Practical

Design Considerations

Critical details for a successful headphone amp build

Volume Control

Place the volume pot at the input, before the tube. A 10-50kΩ logarithmic pot works well. Consider a stepped attenuator for better channel matching at low volumes — critical for headphone listening.

Channel Matching

Headphones reveal channel imbalance ruthlessly. Match tubes within 5% for plate current. Dual triodes like 6SN7 and 12AU7 share a cathode structure which helps, but measuring and selecting is still recommended.

Hum & Noise

Headphones are 20-40 dB more sensitive than speakers to hum. Use DC heater supplies for driver tubes. Keep signal wiring short and away from power transformers. Star grounding is essential.

B+ Requirements

Headphone amps need far less B+ than speaker amps — 150-250V is typical for OTL designs. This means simpler, smaller power supplies and safer builds. A good opportunity for a first tube project.

Coupling Capacitors

OTL designs need an output coupling capacitor. Size it for the headphone impedance: C = 1/(2π × f × Z). For 300Ω headphones and 20Hz cutoff, about 27μF. Film caps preferred for quality.

Headphone Jack Wiring

Use a switching jack that disconnects the output when headphones are removed. This prevents the coupling caps from slowly charging to B+ through the load resistor. A 1kΩ bleeder resistor across the output is good practice.

08 — Reference

Key Equations

Essential formulas for headphone amplifier design

Output impedance

CF: Z_out = r_p / (μ + 1) | SRPP: Z_out ≈ r_p / (μ + 2)

Damping factor

DF = Z_load / Z_out

Power into headphones

P = V_rms² / Z_load | P = I_rms² × Z_load

Required power from sensitivity

P(mW) = 10^{(Target SPL - Sensitivity) / 10}

Coupling capacitor sizing

C = 1 / (2π × f_-3dB × Z_load)

Cathode follower gain

A_v = μ / (μ + 1) — always less than unity

Quiz de synthèse

Test Your Knowledge

Validate your understanding of tube headphone amplifier design.

Question 1 / 7

Why are tube headphone amps an ideal first tube project?

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