Impedance Matching Basics — Headphone and Amp Compatibility
The fundamental principles of impedance matching between headphones and amps, and how to identify compatible pairings.
What Is Impedance?
Impedance is the resistance to alternating current, measured in ohms. Headphone impedance typically ranges from 16 to 600 ohms, and this value significantly affects compatibility with amplifiers.
The 1/8 Rule
The fundamental guideline for headphone-amp compatibility is the “1/8 rule”:
Headphone impedance / Amp output impedance >= 8
When this ratio (the damping factor) is 8 or higher, the amp can properly control the headphone driver.
Examples
- HD600 (300 ohms) with amp output impedance of 10 ohms: 300 / 10 = 30 — OK
- IEM (16 ohms) with amp output impedance of 10 ohms: 16 / 10 = 1.6 — Not OK
Driving a low-impedance IEM from a high output impedance amp causes frequency response deviations, pulling the sound away from the intended tuning.
What Happens with High Output Impedance
Frequency Response Shifts
Headphone impedance varies with frequency (especially in dynamic drivers). When the amp’s output impedance is high, these variations translate directly into audible peaks and dips in certain frequency bands.
Reduced Bass Control
A low damping factor means the driver is poorly controlled, resulting in loose, undefined bass. This is most obvious with bass-reflex speakers but affects headphones too.
Impact on Multi-BA IEMs
Balanced armature IEMs use multiple drivers divided by crossover networks across frequency bands. High output impedance disrupts crossover behavior, significantly altering tonal balance. Campfire Andromeda is a well-known example of this sensitivity.
How to Check an Amp’s Output Impedance
Most manufacturers list output impedance in their spec sheets. When it’s not published, here are general tendencies:
- Desktop headphone amps: Generally under 1 ohm (modern units)
- Portable amps: Around 0.5–5 ohms
- Tube amps (OTL): 30–120 ohms (designed for high-impedance headphones)
- Smartphone/PC headphone jacks: Roughly 1–10 ohms
Guide by Headphone Impedance
Low Impedance (16–32 ohms)
Most IEMs and portable headphones fall here. Choose an amp with low output impedance — ideally under 1 ohm.
Suitable amps: Topping A90 (0.3 ohms), FiiO K7 (< 1 ohm)
Mid Impedance (32–150 ohms)
Many full-size desktop headphones occupy this range. Most amps handle them without issue, giving you the widest selection.
Suitable amps: Nearly any desktop headphone amp
High Impedance (250–600 ohms)
Represented by the Sennheiser HD600 series (300 ohms) and beyerdynamic DT series (250/600 ohms). These are voltage-driven, so an amp with ample voltage output is needed.
Suitable amps: Dedicated desktop headphone amps. OTL tube amps are also a great match.
Practical Advice
- Always check an amp’s output impedance before buying
- If you switch between multiple headphones, a versatile amp with output impedance under 1 ohm is the safest bet
- Treat tube amps (OTL) as dedicated tools for high-impedance headphones
- For IEM users, accessories like the iEMatch attenuator can also help
Summary
Impedance matching is a critical factor that directly affects sound quality. Use the 1/8 rule as your baseline and verify compatibility between your headphones and amp. IEM users in particular should pay close attention to amp output impedance.