Headphone and Amp Impedance (2011)
Headphone and speaker performance depends heavily on how an amplifier’s output impedance interacts with the load, with engineers contrasting the old “Zout = Zin for max power transfer” idea from RF theory against the modern audio goal of very low output impedance for accuracy and efficiency. Commenters examine the popular “1/8th rule” (amp output impedance ≤ 1/8 of headphone impedance), generally arguing that lower is almost always better with today’s designs and that most modern DACs and amps already meet or exceed audible transparency at low cost. The conversation also touches on audiophile “snake oil,” practical gear recommendations, and edge cases like using 3.5 mm jacks for low‑power data and power links without damaging headphones.
Impedance matching and the 1/8 rule
- Central topic: how low amplifier output impedance should be relative to headphone impedance.
- Many argue the 1/8 rule (Z_out ≤ Z_load/8) is conservative and that “as low as possible” is generally better today.
- Some skepticism: 1/8 still allows relatively high source impedance, which can matter with long cables or tricky loads.
Power transfer vs sound quality
- Classic “Z_out = Z_in for maximum power transfer” is discussed.
- Several comments stress that audio amps are usually voltage sources; efficiency and accurate voltage at the load matter more than maximizing power transfer.
- Matching impedances is important in RF for SNR and reflection control, but for speakers/headphones you generally want low source impedance and high load impedance.
RF vs audio, transmission lines
- At audio frequencies and normal cable lengths, lines behave as simple wires; impedance matching for wave propagation isn’t needed.
- Rules of thumb: transmission-line effects only matter when cable length is a significant fraction of a wavelength, which for audio is hundreds of meters.
Car audio and practical matching
- For car speakers, recommendations focus on:
- Low amp output impedance.
- Adequate amp power above speaker handling, to avoid clipping and tweeter damage.
- Low THD, especially at higher output and higher frequencies.
Headphone amp design and evolution
- Older or tube/Class A–AB designs may have higher output impedance; modern solid-state and class D headphone amps typically have very low output impedance.
- The 1/8 rule is seen as largely a historical or worst‑case guideline given modern designs.
Digital correction and modeling
- Some ask why sources don’t measure and pre‑EQ driver+cable+headphone combinations.
- Replies: each headphone is different, measurements require specialized equipment, and speaker/headphone distortions dominate over amp-induced effects.
- Interest in SPICE/equivalent-circuit models for headphones and speakers is expressed.
Using 3.5 mm jacks for power and data
- One subthread explores repurposing TRS/TRRS jacks for low‑power devices.
- Concerns: accidental headphone connection, plug shorting during insertion, current limiting, and connector choice (3.5 mm vs RJ‑style vs USB‑C).
- Various protection schemes (high source impedance, current limiting, mechanical switches, detection schemes) are debated; simplicity vs safety is a recurring tension.
Audiophilia, measurements, and gear choices
- Several comments push a measurement‑driven, anti‑snake‑oil stance: modern cheap DACs and modest amps can already be audibly transparent.
- Others describe the “rabbit hole” of continually upgrading headphones, amps, DACs, and even clocks, and the tension between subjective enjoyment and objective accuracy.
NwAvGuy blog persistence
- The blog is praised as a resource for new engineers.
- Commenters note the author’s long absence, discuss Blogspot hosting, inactive-account deletions, and domain renewals, but the situation remains unresolved/unclear.