Driving a Shure SE535 in-ear monitor from a 50-ohm source instead of a 1-ohm source shifted its frequency response by up to 7.5 dB (Sound On Sound, Phones and Ohms 2019), several times the 1 dB that independent testers treat as audible (section 1). The sources people actually plug into are far from uniform: Apple’s USB-C adapter measured 0.9 ohm while the second version of Google’s Pixel adapter measured 7.6 ohm (Audio Science Review, Apple vs Google USB-C adapters review 2019). At the other end of the cable, a multi-driver Shure SE846 dips to 5.5 ohms at 5 kHz (Archimago’s Musings, headphone impedance measurements 2021), which puts the “safe” source impedance for that earphone below 1 ohm. We aggregated data from Stereophile’s lab measurements, Reference Audio Analyzer, the IEC webstore, Audio Precision, independent measurement blogs and forums, and vendor documents labeled as such. For the commercial side of the same gear (sales, segments and dongle adoption), see our headphone DAC and amplifier market statistics; this page sticks to the electrical handshake between amplifier and headphone.
TL;DR
- A 50-ohm source moved a Shure SE535’s response by up to 7.5 dB versus a 1-ohm source (Sound On Sound, Phones and Ohms 2019).
- At a 1/8 source-to-headphone impedance ratio, the headphone receives 0.89 of the voltage, about 1 dB less (NwAvGuy, Headphone and Amp Impedance 2011).
- Apple’s USB-C adapter measured 0.9 ohm and Google’s second Pixel adapter 7.6 ohm (Audio Science Review 2019).
- A second lab put the same Apple adapter at 0.3 ohm averaged from 40 Hz to 15 kHz (Reference Audio Analyzer, Apple A2155 report).
- Hi-fi integrated amplifier headphone jacks measured 665 ohms (Rotel RA-6000) and 320 ohms (Marantz Model 40n), against 0.6 ohm for a Benchmark HPA4 (Stereophile measurements 2021 to 2023).
- The 2018 edition of IEC 61938 “adjusted” the recommended output source impedance (IEC Webstore, IEC 61938:2018).
- EN 50332 headphone safety tests require a signal source of 2.0 ohms or less (Audio Precision, TN143 Headphone Electroacoustic Measurements 2024).
- IEC 60268-7 requires a headphone’s impedance curve never to fall below 80% of its rated impedance (Audio Precision TN143 2024).
- A Shure SE846 in-ear monitor dips to 5.5 ohms at 5 kHz (Archimago’s Musings 2021).
- Balanced armature drivers turn from resistive to inductive at typically 100 to 300 Hz (Knowles WP03 white paper, vendor’s own document).
- A 2024 MacBook Air measured 1.25 V and 48 mW into 30 ohms (Audio Science Review forum measurements, May 30, 2025).
1. The Voltage Divider in Numbers: the 1/8 Rule and Damping Factor
Output impedance is the resistance an amplifier effectively places in series with the headphone. The two form a voltage divider: whatever share of the voltage the source keeps for itself never reaches the driver. Because a headphone’s impedance changes with frequency, that share changes with frequency too, and the result is an equalizer nobody asked for. At a ratio of 1/8 (source impedance one-eighth of headphone impedance), the headphone still receives 0.89 of the voltage, about a 1 dB drop (NwAvGuy, Headphone and Amp Impedance 2011), which is the arithmetic behind the “1/8 rule”.
Damping factor is the same ratio turned upside down: headphone impedance divided by source impedance. A 1/8 rule therefore means a damping factor of 8. The rule is not about perfection, it is about keeping the worst-case tonal shift near the edge of audibility. The rows below show how quickly real earphones leave that zone once the source climbs into double-digit ohms.
| Metric | Value | Source |
|---|---|---|
| Voltage reaching the headphone when source impedance is 1/8 of headphone impedance | 0.89 of the unloaded voltage, about 1 dB lower | NwAvGuy, Headphone and Amp Impedance 2011 |
| Minimum damping factor suggested for headphones (headphone impedance divided by source impedance) | 8 or better | NwAvGuy, Headphone and Amp Impedance 2011 (author comment) |
| Frequency response variation of an Ultimate Ears SuperFi 5 IEM driven from a 43-ohm source vs from a 10-ohm source | 12 dB total vs 6 dB | NwAvGuy, Headphone and Amp Impedance 2011 |
| Maximum response deviation of a Shure SE535 IEM, 50-ohm source vs 1-ohm source | 7.5 dB | Sound On Sound, Phones and Ohms 2019 |
| Response change treated as the threshold of audibility in independent amplifier tests | 1 dB | Archimago’s Musings, headphone impedance measurements 2021 |
Context: the SuperFi 5 and SE535 figures are the clearest public before-and-after measurements of this effect we found, but they are older than three years. Most recent available data: the 2011 and 2019 measurements above. The physics has not changed; the earphone models have.
2. Phones, Dongles and Portable Players: Measured Output Impedance
Portable sources split into near-ideal and good-enough-for-some. Apple’s USB-C adapter measured 0.9 ohm while the second version of Google’s Pixel adapter measured 7.6 ohm in the same review (Audio Science Review, Apple vs Google USB-C adapters review 2019). Reference Audio Analyzer, a second independent lab, put the Apple adapter lower still at 0.3 ohm averaged from 40 Hz to 15 kHz (Reference Audio Analyzer, Apple A2155 report); the gap reflects test method rather than a different product and is logged in the Data watch note.
Whether a number is “low” depends entirely on the earphone. Apply the 1/8 rule to the SE846’s 5.5-ohm minimum from section 5: 5.5 divided by 8 is about 0.69 ohm. By that arithmetic, the Apple adapter passes on one lab’s figure (0.3) and misses on the other’s (0.9), and every other source in this table misses. A 20-ohm dongle is in another league: with a 32-ohm nominal earphone the source impedance equals 62% of the load (20 divided by 32), more than four times the 1/8 limit.
| Metric | Value | Source |
|---|---|---|
| Output impedance, Apple USB-C to 3.5 mm adapter vs Google Pixel USB-C adapter (second version) | 0.9 ohm vs 7.6 ohm | Audio Science Review, Apple vs Google USB-C adapters review 2019 |
| Same Apple adapter (model A2155), output impedance averaged from 40 Hz to 15 kHz | 0.3 ohm | Reference Audio Analyzer, Apple A2155 amplifier report (accessed 2026) |
| Soditer “fourth generation” USB-C adapter (Realtek ALC4042 chip), output impedance | 20 ohms | Archimago’s Musings, Soditer USB-C adapter measurements 2020 |
| Smartphones measured by Sonarworks, a headphone calibration company | 170 devices, most between 1 and 10 ohms | Sound On Sound, Phones and Ohms (page 2) 2019, Sonarworks data |
| Sony smartphones in the same Sonarworks sample | Output impedance in the teens of ohms | Sound On Sound, Phones and Ohms 2019 (Sonarworks data) |
| Apple iPhone 3G headphone jack, output impedance | 3.1 ohms across the audio band | Stereophile, JH16 Pro measurements 2011 |
Context: in the Sonarworks smartphone sample, the article reports that Apple and Samsung phones did well and that the weakest results came from less common brands. Most recent available data for smartphones: 2019, so treat the phone row as a historical baseline rather than a picture of current models.
3. Desktop Amps, Interfaces and Hi-Fi Headphone Jacks
Desk-bound gear spans more than three orders of magnitude, from below 0.3 ohm to 665 ohms. The Rotel RA-6000’s headphone jack measured 665 ohms “at all audio frequencies”, against 0.6 ohm for the Benchmark HPA4 dedicated headphone amplifier (Stereophile, Rotel RA-6000 measurements 2023). That is a ratio of more than 1,000 to 1 between two products a buyer might place on the same shelf.
A jack measuring hundreds of ohms hands the frequency response to the headphone’s impedance curve, exactly the voltage-divider effect described in section 1, and a headphone-amplifier maker describes a fixed series resistor as the most common output circuit (see the last row). The dedicated headphone amplifiers and converters in this table range from below 0.3 ohm to around 10 ohms, and the 10-ohm units sit in a gray zone: 10 ohms is 1/30 of a 300-ohm headphone, comfortably inside the 1/8 rule, but nearly twice the SE846’s 5.5-ohm minimum (section 5).
| Metric | Value | Source |
|---|---|---|
| Benchmark HPA4 headphone amplifier, output impedance | 0.6 ohm at 20 Hz and 1 kHz, 0.83 ohm at 20 kHz | Stereophile, Benchmark HPA4 measurements 2021 |
| RME ADI-2 Pro FS converter, headphone output impedance | Below 0.3 ohm across the spectrum | Archimago’s Musings, headphone impedance measurements 2021 |
| Topping DX3 Pro V2 and ASUS Xonar, headphone output impedance | Around 10 ohms | Archimago’s Musings, headphone impedance measurements 2021 |
| Audio Research I/50 integrated amplifier, headphone output impedance | 48 ohms from 20 Hz to 20 kHz | Stereophile, Audio Research I/50 measurements 2023 |
| Marantz Model 40n integrated amplifier, headphone output impedance | 320 ohms | Stereophile, Marantz Model 40n measurements 2022 |
| Rotel RA-6000 integrated amplifier, headphone output impedance | 665 ohms at all audio frequencies | Stereophile, Rotel RA-6000 measurements 2023 |
| Benchmark HPA2 headphone amplifier, maker’s stated output impedance | About 0.1 ohm | Benchmark Media, Headphone Impedance and Sensitivity 2014 (vendor’s own claim) |
| Most common headphone output circuit, according to a headphone-amplifier maker | Op-amp driver followed by a 30-ohm series resistor | Benchmark Media, The 0-Ohm Headphone Amplifier 2001 (vendor’s own claim) |
Context: the same magazine’s buying guide describes the Rotel jack’s “very high source impedance” as not optimal for low-impedance headphones (Recommended Components 2024 Edition: Integrated Amplifiers). The HPA4 and RME figures are older than three years; most recent available data: 2021 measurements.
4. Standards: IEC 61938, the 120-Ohm Legacy, and Test Rules
The formal headphone source-impedance target cited across these sources is a legacy one, and it has since been revised. The third edition of IEC 61938, published January 12, 2018, lists among its changes that the “recommended value of output source impedance is adjusted” (IEC Webstore, IEC 61938:2018). The new value sits behind the paywall, so any article quoting a specific replacement figure without the standard in hand should be treated with caution.
Measurement standards went the opposite way from the old 120-ohm idea. Test labs want a source that is close to a perfect voltage source, so the test reflects the headphone and not the amplifier: EN 50332, the European standard that caps the maximum sound pressure level headphones can produce, requires a test signal source of no more than 2.0 ohms (Audio Precision, TN143 Headphone Electroacoustic Measurements 2024). The same note adds that some consumer devices with headphone jacks it has tested have a much higher output impedance.
| Metric | Value | Source |
|---|---|---|
| IEC 61938 first introduced, and its headphone output recommendation | 1996, source impedance of 120 ohms | Sound On Sound, Phones and Ohms 2019 |
| Current edition of IEC 61938 | Edition 3.0, published January 12, 2018, replacing the 2013 second edition | IEC Webstore, IEC 61938:2018 |
| Headphone-relevant change in the 2018 edition | Recommended output source impedance “adjusted” (value not in the public abstract) | IEC Webstore, IEC 61938:2018 |
| Value newly recommended in the 2018 edition for impedance-defined loudspeaker systems | 6 ohms | IEC Webstore, IEC 61938:2018 |
| Maximum output impedance of the test signal source under EN 50332 | 2.0 ohms | Audio Precision TN143 2024 |
| IEC 60268-7 rule for a headphone’s rated impedance | Impedance curve minimum must be at least 80% of the rated value at every frequency | Audio Precision TN143 2024 |
| Frequency range over which IEC 60268-7 requires headphone impedance to be measured and displayed | 20 Hz to 20 kHz | Audio Precision TN143 2024 |
Context: the 120-ohm figure dates from 1996 and never caught on with amplifier designers, according to the same article. The public preview of the 2018 edition lists the change but does not print the new headphone value.
5. Headphone and IEM Impedance Curves: Why Nominal Ratings Mislead
A nominal impedance is one number standing in for a curve, and the curve is what the amplifier sees. Sonarworks measured impedance on 797 headphone models and found that 57% have some variance of impedance with frequency (Sound On Sound, Phones and Ohms 2019). For the remaining flat-impedance models, a higher source impedance mostly lowers the volume; for the 57%, it reshapes the tonal balance.
Multi-driver balanced armature in-ear monitors are the worst case. A balanced armature is a miniature driver type used in many IEMs, and it turns inductive (its impedance rises with frequency) at unusually low frequencies, so impedance climbs through the midrange; passive crossovers that split the signal between drivers then add dips and peaks. The JH Audio JH16 Pro shows why rating rules matter: rated at 18 ohms, it averages about 13 ohms in the bass, which is 72% of its rating (13 divided by 18), below the 80% floor that IEC 60268-7 sets for a rated value (see section 4). For scale, these electrical swings can rival the sample-to-sample differences covered in our headphone unit-to-unit variation statistics.
| Metric | Value | Source |
|---|---|---|
| Share of 797 measured headphone models whose impedance varies with frequency | 57% | Sound On Sound, Phones and Ohms 2019 (Sonarworks data) |
| Shure SE846 IEM, minimum impedance | 5.5 ohms at 5 kHz | Archimago’s Musings, headphone impedance measurements 2021 |
| Ultimate Ears SuperFi 5 IEM, impedance range | 10 to 90 ohms | Archimago’s Musings, headphone impedance measurements 2021 |
| JH Audio JH16 Pro IEM (rated 18 ohms), measured impedance | About 13 ohms in the bass and lower midrange, maximum 35 ohms at 2 kHz | Stereophile, JH16 Pro measurements 2011 |
| Frequency at which balanced armature drivers change from resistive to inductive | Typically 100 to 300 Hz | Knowles, WP03 The Science of Premium Sound white paper (vendor’s own document) |
| Knowles RAI-33651-000 balanced armature driver, nominal impedance | 25 ohms DC resistance, 36 ohms at 500 Hz, 60 ohms at 1 kHz | Knowles RAI-33651-000 datasheet (vendor spec) |
Context: dynamic drivers swing too, but mainly upward around their resonance, where the Phones and Ohms article notes impedance “can rise to more than 1.5 times its nominal value”, typically around 100 Hz. The same article shows the SE535 dipping from a nominal 32 ohms to 9 ohms, which explains its 7.5 dB swing in section 1. The JH16 Pro data is older than three years; most recent available data: 2011 measurements.
6. Voltage, Power and Sensitivity: What a Source Must Deliver
Low output impedance is half the job; the source also has to swing enough voltage. A 2024 MacBook Air headphone jack measured 1.25 V and 48 mW into 30 ohms, and 2.97 V and 29 mW into 300 ohms (Audio Science Review forum, MacBook Air 2024 headphone output measurements, May 30, 2025), which lines up with Apple’s own stated 1.25 Vrms below 150 ohms and 3 Vrms from 150 ohms to 1k ohms (Apple Support, Use high-impedance headphones with your Mac, vendor’s own claim). In other words, the same port changes its maximum voltage depending on the headphone attached.
Voltage sensitivity, not power sensitivity, is the figure that predicts whether a source is loud enough. Benchmark’s table shows a 300-ohm Sennheiser HD 650 and a 22-ohm Audeze LCD-X land within 0.5 dB of each other per volt even though their per-milliwatt ratings differ by 11 dB (Benchmark Media, Headphone Impedance and Sensitivity 2014, vendor’s own table). Low-sensitivity planar headphones raise the voltage bar further, a design trade-off discussed in our planar magnetic vs dynamic driver distortion statistics.
| Metric | Value | Source |
|---|---|---|
| Sensitivity gap between the AKG K1000 (78 dB/V) and the Astell and Kern AKR03 Roxanne Max (140 dB/V) | 62 dB, about 1258 times more voltage needed by the K1000 for the same loudness | Sound On Sound, Phones and Ohms 2019 |
| Sennheiser HD 650 (300 ohms) vs Audeze LCD-X (22 ohms), sensitivity per volt and per milliwatt | 112 vs 112.5 dB SPL at 1 Vrms; 107 vs 96 dB SPL at 1 mW | Benchmark Media, Headphone Impedance and Sensitivity 2014 (vendor’s own table) |
| Loudness of a Sony MDR-V600 (119.5 dB SPL at 1 Vrms) vs a Sennheiser HD 650 (112 dB SPL at 1 Vrms) driven with the same voltage | About 7.5 dB louder | Benchmark Media, Headphone Impedance and Sensitivity 2014 (vendor’s own table) |
| Typical power requirement of earphones | A few milliwatts to 500 milliwatts | Audio Precision TN143 2024 |
| Apple’s stated Mac headphone jack output by headphone impedance | Up to 1.25 Vrms below 150 ohms; 3 Vrms from 150 ohms to 1k ohms | Apple Support, Use high-impedance headphones with your Mac (vendor’s own claim) |
| 2024 MacBook Air headphone jack, measured into 30 ohms and into 300 ohms | 1.25 V and 48 mW (52 mW ideal); 2.97 V and 29 mW | Audio Science Review forum measurements, May 30, 2025 |
| Apple A2155 USB-C adapter, maximum output before clipping | 0.5 Vrms and 32.1 mA | Reference Audio Analyzer, Apple A2155 amplifier report (accessed 2026) |
Context: the 52 mW “ideal” is what 1.25 V into 30 ohms works out to (1.25 squared divided by 30); the measured maximum came in at 48 mW, about 92% of that (48 divided by 52).
Summary: Headphone Amplifier Output Impedance Matching by the Numbers
| Metric | Value | Source |
|---|---|---|
| Maximum response deviation of a Shure SE535 IEM, 50-ohm source vs 1-ohm source | 7.5 dB | Sound On Sound, Phones and Ohms 2019 |
| Voltage reaching the headphone at a 1/8 source-to-headphone impedance ratio | 0.89, about 1 dB lower | NwAvGuy, Headphone and Amp Impedance 2011 |
| Ultimate Ears SuperFi 5 response variation from a 43-ohm source | 12 dB total | NwAvGuy, Headphone and Amp Impedance 2011 |
| Apple USB-C adapter vs Google Pixel adapter (second version), output impedance | 0.9 ohm vs 7.6 ohm | Audio Science Review 2019 |
| Apple A2155 USB-C adapter, average output impedance 40 Hz to 15 kHz | 0.3 ohm | Reference Audio Analyzer (accessed 2026) |
| Soditer USB-C adapter, output impedance | 20 ohms | Archimago’s Musings 2020 |
| Headphone output impedance, Benchmark HPA4 headphone amplifier vs Rotel RA-6000 and Marantz Model 40n integrated amplifiers | 0.6 ohm vs 665 ohms and 320 ohms | Stereophile measurements 2021 to 2023 |
| IEC 61938 third edition (January 12, 2018), headphone-relevant change | Recommended output source impedance “adjusted” | IEC Webstore, IEC 61938:2018 |
| Maximum test source impedance under EN 50332 | 2.0 ohms | Audio Precision TN143 2024 |
| IEC 60268-7 floor for a headphone’s impedance curve | At least 80% of rated impedance | Audio Precision TN143 2024 |
| Shure SE846 IEM, minimum impedance | 5.5 ohms at 5 kHz | Archimago’s Musings 2021 |
| Balanced armature drivers, resistive-to-inductive transition | Typically 100 to 300 Hz | Knowles WP03 white paper (vendor’s own document) |
| Knowles RAI-33651-000 balanced armature driver, impedance at 500 Hz vs 1 kHz | 36 ohms vs 60 ohms | Knowles RAI-33651-000 datasheet (vendor spec) |
| Sennheiser HD 650 vs Audeze LCD-X, sensitivity at 1 Vrms | 112 vs 112.5 dB SPL | Benchmark Media 2014 (vendor’s own table) |
| 2024 MacBook Air, measured into 30 ohms | 1.25 V and 48 mW | Audio Science Review forum, May 30, 2025 |
Methodology and Sources
- Sound On Sound: Rudolfs Putnins, Phones and Ohms, October 2019 and page 2, including Sonarworks impedance data on 797 headphone models and 170 smartphones.
- NwAvGuy: Headphone and Amp Impedance, February 9, 2011, source of the 1/8 rule arithmetic and the SuperFi 5 measurements used here.
- Archimago’s Musings (independent measurement blog): headphone impedance, sensitivity and amplifier output impedance, March 27, 2021 and Soditer USB-C adapter measurements, April 25, 2020.
- Audio Science Review: Apple vs Google USB-C headphone adapters review, 2019 and MacBook Air 2024 headphone output measurements by a forum member, May 30, 2025.
- Reference Audio Analyzer: Apple A2155 USB-C to 3.5 mm adapter amplifier report (test date not shown on the page).
- Stereophile (John Atkinson’s lab measurements): Rotel RA-6000, Marantz Model 40n, Audio Research I/50, Benchmark HPA4, JH Audio JH16 Pro, and Recommended Components 2024 Edition: Integrated Amplifiers. Years come from the issue references in Stereophile’s Recommended Components listings (Vol.44 No.2 for the HPA4, Vol.45 No.8 for the Model 40n, Vol.46 No.2 and No.9 for the RA-6000 and I/50).
- IEC: IEC 61938:2018 Multimedia systems, guide to the recommended characteristics of analogue interfaces, edition 3.0.
- Audio Precision: TN143 Headphone Electroacoustic Measurements, 2024, by Joe Begin, summarizing EN 50332 and IEC 60268-7 requirements.
- Vendor documents (labeled as vendor claims wherever used): Benchmark Media, Headphone Impedance and Sensitivity 2014 and The 0-Ohm Headphone Amplifier 2001; Apple Support, Use high-impedance headphones with your Mac; Knowles WP03 white paper and Knowles RAI-33651-000 datasheet (neither Knowles document prints a date).
- Data watch: the two independent labs disagree on the same Apple USB-C adapter (0.9 ohm at Audio Science Review vs 0.3 ohm averaged from 40 Hz to 15 kHz at Reference Audio Analyzer); both are cited, neither is averaged. Sound On Sound reports a 7.5 dB maximum SE535 deviation while Sonarworks’ own help-center article describes a 7 dB worst case for distortion of the response; we use the published 7.5 dB figure caption. Peer-reviewed literature on source impedance and headphone response is thin, so this roundup leans on independent measurement labs, and much of the core data (NwAvGuy 2011, Stereophile 2011, Sound On Sound 2019, Archimago 2020 and 2021) is older than three years. The headphone source impedance value in IEC 61938:2018 is not public, and the IEC 60268-7 80% rule is cited through Audio Precision’s 2024 summary rather than the standard text, so it should be rechecked against the current edition of IEC 60268-7.
Last updated: November 7, 2026. We update this roundup quarterly, and the next refresh is expected when Stereophile publishes its next Recommended Components edition.