HEADPHONES · YOUR NEXT FACE-OFF

Sennheiser HD 400 PRO vs Sennheiser HD 660S2

Real specs. Clear sources.Exact models · UK & US buying links

Compared products

YOUR FIRST CHOICE

Sennheiser HD 400 PRO

700047 · HD 400 PRO · wired

Full product profile Official image

Exact model & source notes

The HD 400 PRO (article 700047) is Sennheiser's open-back, around-ear wired studio headphone. It uses angled 120 Ω dynamic transducers with a polymer-blend diaphragm, velour ear pads and a detachable single-sided cable, and both a 3 m coiled and a 1.8 m straight cable come in the box. It weighs 240 g without the cable. Sennheiser lists a two-year warranty.

Specifications checked 27 Sept 2026.

Amazon links open search results. Check the exact variant, current price and availability with the retailer.

Manufacturer specifications

WHICH SUITS YOU?

Choose the HD 400 PRO for

Lower stated nominal impedance: 120 ohm vs 300 ohm

Choose the HD 660S2 for

Higher stated nominal impedance: 300 ohm vs 120 ohm

Built from verified specifications that differ. Not a rating or a recommendation.

See the 12 verified differences

THE CONTENDER

Sennheiser HD 660S2

700240 · black with bronze accents · 300 ohm · 1.8 m balanced and unbalanced cables

Full product profile Official image

Exact model & source notes

The HD 660S2 is an open-back, over-ear headphone with a 38 mm dynamic driver rated 8–41,500 Hz (−10 dB) at 300 ohms. Sennheiser revised its acoustics for stronger sub-bass, and it ships with 1.8 m 6.35 mm unbalanced and 4.4 mm balanced cables plus a 3.5 mm adapter.

Specifications checked 11 Sept 2026.

Amazon links open search results. Check the exact variant, current price and availability with the retailer.

Manufacturer specifications

HD 400 PRO UK & US retailers

UK & US price history: no verified listing yet

HD 660S2 UK & US retailers

UK & US price history: no verified listing yet

Search links open retailer results, not a matched listing; eBay listings are checked daily. As an Amazon Associate I earn from qualifying purchases. As an eBay Partner, we may be compensated if you make a purchase.

THE DIFFERENCES, MADE VISIBLE

Visual facts

Differences first. Select a topic for its graphic, sources and conditions.

How to read this comparison

Manufacturer facts for the exact models shown. Not a quality rating or a test result.
“Different” means the known values differ, not that one is better.
Missing or conflicting evidence is marked and never counts as a difference. Source chips hold each fact’s sources and conditions.

FEATURES SIDE BY SIDE8 topics

Yes and No describe the listed feature. Missing evidence stays unverified.

SELECTED TOPIC

01 / 08

Impedance

Nominal electrical load, not sound quality or loudness. Output capability and headphone sensitivity matter together; impedance alone cannot decide whether an amplifier is needed.

Sennheiser HD 400 PRO120 Ω

Source 1 Source 2

Sennheiser HD 660S2300 Ω

Manufacturer specification · Nominal impedance; check that your amplifier or audio source can drive this load.

Source 1

Explore the full specifications

Start with your listening environment. Open and closed earcups suit different surroundings. A closed back is not the same as active noise cancellation. Check the exact cable and headphone output. We do not turn impedance, driver size or unpaired lab results into a sound-quality score.

WHAT CHANGES BETWEEN THEM?

Key differences

Full specifications ↓

12 verified differences, most decision-relevant first. 1 is also drawn in Visual facts above. 1 feature is the same under different published names, grouped below. Missing information stays separate.

Wired / wireless connections

Sennheiser HD 400 PROWired; detachable single-sided cable: 3 m coiled cable with 3.5 mm jack plug and a 1.8 m straight cable supplied, plus a 3.5-to-6.3 mm adapter

Sennheiser HD 660S2Wired analogue stereo; 1.8 m cables with 6.35 mm unbalanced and 4.4 mm balanced plugs; included 3.5 mm adapter

Sources

Sennheiser HD 400 PRO

Manufacturer specification · Replacement coiled and straight cables are listed as spare parts.

Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0)

Sennheiser HD 660S2

Manufacturer specification

Sennheiser HD 660S2 — exact model specifications Sennheiser HD 660S2 — operating guide and specifications

Check the exact configuration and source conditions for your use.

Nominal impedance

In Visual facts

Sennheiser HD 400 PRO120 ohm

Sennheiser HD 660S2300 ohm

Sources

Sennheiser HD 400 PRO

Manufacturer specification

Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0)

Sennheiser HD 660S2

Manufacturer specification · Nominal impedance; check that your amplifier or audio source can drive this load.

Sennheiser HD 660S2 — operating guide and specifications

Check the exact configuration and source conditions for your use.

Driver construction

Sennheiser HD 400 PROAngled 120-ohm dynamic transducers with a polymer-blend diaphragm

Sennheiser HD 660S238 mm open dynamic transducer; aluminium voice coil and vented magnet system

Sources

Sennheiser HD 400 PRO

Manufacturer specification · Sennheiser does not state the transducer diameter on the pages and documents read.

Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0)

Sennheiser HD 660S2

Manufacturer specification · Sennheiser lists 38 mm as the transducer size.

Sennheiser HD 660S2 — exact model specifications

Check whether the diameter describes the complete transducer or only its diaphragm. Different labels and driver sizes do not establish better sound.

Cable arrangement

Sennheiser HD 400 PRODetachable, single-sided cable; coiled 3 m and straight 1.8 m cables supplied

Sennheiser HD 660S2Detachable 1.8 m unbalanced and 4.4 mm balanced cables included; XLR-4 cable optional

Sources

Sennheiser HD 400 PRO

Manufacturer specification

Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0)

Sennheiser HD 660S2

Manufacturer specification · Optional cables are sold separately; balanced cables need a balanced headphone output.

Sennheiser HD 660S2 — exact model specifications Sennheiser HD 660S2 — operating guide and specifications

Check the exact configuration and source conditions for your use.

Supplied plug and adapter

Sennheiser HD 400 PRO3.5 mm jack plug with 6.3 mm adapter

Sennheiser HD 660S26.35 mm unbalanced cable; 4.4 mm balanced cable; 6.35 mm-to-3.5 mm adapter

Sources

Sennheiser HD 400 PRO

Manufacturer specification

Sennheiser HD 400 PRO — product specification sheet (EN, V1.0)Sennheiser HD 400 PRO — instruction manual (09/2022)

Sennheiser HD 660S2

Manufacturer specification · The 4.4 mm balanced cable needs a balanced headphone output; an XLR-4 cable is optional.

Sennheiser HD 660S2 — exact model specifications Sennheiser HD 660S2 — operating guide and specifications

Check the exact configuration and source conditions for your use.

Rated total harmonic distortion

Sennheiser HD 400 PRO<0.05% at 1 kHz, 90 dB SPL

Sennheiser HD 660S2≤0.04% at 1 kHz, 100 dB SPL

Sources

Sennheiser HD 400 PRO

Manufacturer specification

Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0)

Sennheiser HD 660S2

Manufacturer specification · Rated at 1 kHz and 100 dB SPL; other models use different test levels, so compare with care.

Sennheiser HD 660S2 — exact model specifications Sennheiser HD 660S2 — operating guide and specifications

These published limits can use different input voltages, loudness levels and standards. They cannot establish lower distortion in matched listening conditions.

Show 6 more verified differences

Ear pads

Sennheiser HD 400 PROVelour; replaceable

Sennheiser HD 660S2Velour ear pads; replaceable

Sources

Sennheiser HD 400 PRO

Manufacturer specification

Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0)

Sennheiser HD 660S2

Manufacturer specification · Replacement procedure shown in the manual.

Sennheiser HD 660S2 — exact model specifications Sennheiser HD 660S2 — operating guide and specifications

Check the exact configuration and source conditions for your use.

Rated frequency range

Sennheiser HD 400 PRO6–38,000 Hz (−10 dB)

Sennheiser HD 660S28–41,500 Hz (−10 dB)

Sources

Sennheiser HD 400 PRO

Manufacturer specification

Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0)

Sennheiser HD 660S2

Manufacturer specification

Sennheiser HD 660S2 — operating guide and specifications

Range endpoints need their stated threshold. They do not describe response flatness or prove better bass, detail or sound quality.

Operating temperature

Sennheiser HD 400 PRO−15 °C to +55 °C

Sennheiser HD 660S2−10 °C to +55 °C

Sources

Sennheiser HD 400 PRO

Manufacturer specification · Relative humidity up to 90%.

Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0)

Sennheiser HD 660S2

Manufacturer specification

Sennheiser HD 660S2 — operating guide and specifications

Check the exact configuration and source conditions for your use.

Voltage-referenced sound pressure level

Sennheiser HD 400 PRO110 dB SPL (1 kHz, 1 Vrms)

Sennheiser HD 660S2104 dB SPL at 1 kHz, 1 V rms

Sources

Sennheiser HD 400 PRO

Manufacturer specification · The specification sheet states 110 dB (1 kHz / 1 V RMS). The UK page technical table labels the same conditioned figure Max. sound pressure level; this record preserves the voltage-referenced claim without independently certifying a maximum. The separate 120 dB feature statement gives no reference and is not assumed to use these test conditions.

Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0)

Sennheiser HD 660S2

Manufacturer specification · Rated at 1 kHz with 1 V rms; not directly comparable with dB/mW ratings.

Sennheiser HD 660S2 — operating guide and specifications

Keep the frequency and voltage or power reference attached. Sensitivity describes output for an input, not sound quality; dB/V and dB/mW are different measures.

Serviceability

Sennheiser HD 400 PROReplaceable ear pads and components; ear pads, coiled cable and straight cable listed as spare parts

Sennheiser HD 660S2Detachable cable and replaceable ear pads

Sources

Sennheiser HD 400 PRO

Manufacturer specification

Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data

Sennheiser HD 660S2

Manufacturer specification · Replacement procedures are in the manual; spare-part availability varies.

Sennheiser HD 660S2 — operating guide and specifications

Check the exact configuration and source conditions for your use.

Weight

Sennheiser HD 400 PRO240 g

Sennheiser HD 660S2260 g

Sources

Sennheiser HD 400 PRO

Manufacturer specification · Without cable, per the specification sheet and manual; the product page gives the same 240 g without stating a basis.

Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0)Sennheiser HD 400 PRO — instruction manual (09/2022)

Sennheiser HD 660S2

Manufacturer specification · Approximate, without the cable.

Sennheiser HD 660S2 — operating guide and specifications

A size or weight trade-off; it does not establish the better product.

Same feature, different names (1)

The sources publish different names for the same feature. Both names and their sources are shown; they are not counted as differences.

Wearing style

Sennheiser HD 400 PROOver-ear (circumaural), open-back wired studio headphones

Sennheiser HD 660S2Over-ear (circumaural), open-back dynamic headphones

Sources

Sennheiser HD 400 PRO

Manufacturer specification

Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0)

Sennheiser HD 660S2

Manufacturer specification

Sennheiser HD 660S2 — operating guide and specifications

Both sources state the same feature under different published names. This is not a verified difference; check the exact feature you need.

Missing or non-comparable evidence (12)

Active noise cancellation

Sennheiser HD 400 PRONot verified

Sennheiser HD 660S2Not verified

Sources

Sennheiser HD 400 PRO

The complete exact sources establish an open dynamic wired design but do not explicitly negate active noise cancellation. Open construction and omission of ANC do not establish a false capability. · Manufacturer specification

Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0)

Sennheiser HD 660S2

The reviewed exact sources do not explicitly establish active noise cancellation absence. Preserve separately sourced acoustic design and passive attenuation; they cannot turn omitted ANC into a false capability. · Manufacturer specification

Sennheiser HD 660S2 — operating guide and specifications

Evidence is missing or conflicting on at least one side. This is not a verified difference.

Microphone

In Visual facts

Sennheiser HD 400 PRONot verified

Sennheiser HD 660S2Not verified

Sources

Sennheiser HD 400 PRO

The complete exact sources list the headphone and accessories but do not explicitly establish absence of a microphone. An omitted microphone and a stereo headphone plug alone cannot become a sourced false capability. · Manufacturer specification

Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0)

Sennheiser HD 660S2

The reviewed exact sources do not explicitly establish microphone absence. Omission from a package or technical inventory, a stereo audio connector, and a without-remote statement do not establish microphone=false. · Manufacturer specification

Sennheiser HD 660S2 — operating guide and specifications

Evidence is missing or conflicting on at least one side. This is not a verified difference.

Battery life

Sennheiser HD 400 PRONot verified

Sennheiser HD 660S2Not verified

Sources

Sennheiser HD 400 PRO

The retained exact sources do not explicitly establish battery life or that no battery is required for operation. Do not infer a not-applicable operating requirement from analogue/dynamic construction or a battery-included No row. · Manufacturer specification

Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data

Sennheiser HD 660S2

The reviewed exact sources do not explicitly establish battery life or that no battery is required for operation. A wired/dynamic audio path, omitted charger or Battery included N is not an explicit operating-power requirement. · Manufacturer specification

Sennheiser HD 660S2 — operating guide and specifications

Evidence is missing or conflicting on at least one side. This is not a verified difference.

Published acoustic changes

Sennheiser HD 400 PRONot verified

Sennheiser HD 660S2Revised 300-ohm coil, more compliant surround, 70 Hz resonance and revised transducer airflow

Sources

Sennheiser HD 400 PRO

No source recorded for this side.

Sennheiser HD 660S2

Manufacturer specification · Sennheiser describes these changes as the S2 revision of the HD 660S.

Sennheiser HD 660S2 — exact model specifications

Evidence is missing or conflicting on at least one side. This is not a verified difference.

Coiled cable rated length

Sennheiser HD 400 PRO3 m

Sennheiser HD 660S2Not verified

Sources

Sennheiser HD 400 PRO

Manufacturer specification · Separate supplied coiled cable. The source states 3 m but does not establish its relaxed/extended measurement basis. A 1.8 m straight cable is also supplied.

Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0)

Sennheiser HD 660S2

No source recorded for this side.

Evidence is missing or conflicting on at least one side. This is not a verified difference.

Rated contact pressure

Sennheiser HD 400 PRONot verified

Sennheiser HD 660S2Approximately 6.0 N ±1.0 N

Sources

Sennheiser HD 400 PRO

No source recorded for this side.

Sennheiser HD 660S2

Manufacturer specification · Nominal clamping force; individual fit and comfort can differ.

Sennheiser HD 660S2 — operating guide and specifications

Evidence is missing or conflicting on at least one side. This is not a verified difference.

Frequency equalisation

Sennheiser HD 400 PRONot verified

Sennheiser HD 660S2Diffuse-field equalised

Sources

Sennheiser HD 400 PRO

No source recorded for this side.

Sennheiser HD 660S2

Manufacturer specification

Sennheiser HD 660S2 — operating guide and specifications

Evidence is missing or conflicting on at least one side. This is not a verified difference.

In the box

Sennheiser HD 400 PROHD 400 PRO headphones, 3 m coiled cable with 3.5 mm jack plug, 3.5 mm to 6.3 mm adapter, 1.8 m straight cable, quick guide, safety guide

Sennheiser HD 660S2Not verified

Sources

Sennheiser HD 400 PRO

Manufacturer specification

Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0)

Sennheiser HD 660S2

No source recorded for this side.

Evidence is missing or conflicting on at least one side. This is not a verified difference.

Rated long-term input power

Sennheiser HD 400 PRONot verified

Sennheiser HD 660S2Maximum 500 mW under EN 60-268-7

Sources

Sennheiser HD 400 PRO

No source recorded for this side.

Sennheiser HD 660S2

Manufacturer specification · Rated to EN 60-268-7; the maximum input the headphones can handle, not the amplifier power they need.

Sennheiser HD 660S2 — operating guide and specifications

Evidence is missing or conflicting on at least one side. This is not a verified difference.

Static magnetic field at the surface

Sennheiser HD 400 PRONot verified

Sennheiser HD 660S24.5 mT

Sources

Sennheiser HD 400 PRO

No source recorded for this side.

Sennheiser HD 660S2

Manufacturer specification · Static magnetic field at the headphone surface.

Sennheiser HD 660S2 — exact model specifications Sennheiser HD 660S2 — operating guide and specifications

Evidence is missing or conflicting on at least one side. This is not a verified difference.

Advertised SPL without stated reference

Sennheiser HD 400 PRO120 dB SPL

Sennheiser HD 660S2Not verified

Sources

Sennheiser HD 400 PRO

Manufacturer specification · UK page feature statement. Frequency, input voltage/power and measurement reference are not stated; the 120 dB claim is not labelled maximum. Do not compare it numerically with the specifically conditioned 110 dB at1 kHz/1 Vrms claim or use it as a comparable sensitivity/maximum-output input.

Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data

Sennheiser HD 660S2

No source recorded for this side.

Evidence is missing or conflicting on at least one side. This is not a verified difference.

Manufacturer sub-bass claim

Sennheiser HD 400 PRONot verified

Sennheiser HD 660S2Double the sound pressure at the lowest frequencies

Sources

Sennheiser HD 400 PRO

No source recorded for this side.

Sennheiser HD 660S2

Manufacturer specification · Sennheiser does not give the frequency range or comparison model for this claim.

Sennheiser HD 660S2 — exact model specifications

Evidence is missing or conflicting on at least one side. This is not a verified difference.

2 shared fields have the same recorded value.

YOUR HEADPHONES. YOUR RULES.

Check against your must-haves

Feature matching for the features you select. It doesn’t rate sound or comfort.

Start with your listening setup.

Choose a starting point, then fine-tune your must-haves.

FINE-TUNE THE DETAILS

Your essentials.

These shortcuts select the features shown above. Check the fit against your own equipment and listening space.

A feature match describes the selected requirements. It doesn’t rate sound or comfort.

Open-back designFor a quiet listening spaceClosed-back designAn enclosed outer earcupDetachable cableUnplug the cable at the headphonesBuilt-in microphoneFor calls and voice chatActive noise cancellationNot the same as a closed back

YOUR CHOICE

Sennheiser HD 400 PRO

Select a feature to check your fit.

YOUR CHOICE

Sennheiser HD 660S2

Select a feature to check your fit.

Feature matching, not a sound-quality score. The specification table links to the evidence for each feature.

WHAT EACH CHOICE ASKS OF YOU

What it brings and what to check

Sennheiser HD 400 PRO

What it brings

Two cables come in the box — a 3 m coiled cable and a 1.8 m straight cable — plus a 3.5-to-6.3 mm adapter, so desk and studio setups are covered without buying accessories.
Manufacturer source Manufacturer source
Angled 120-ohm dynamic transducers use a polymer-blend diaphragm, and the headphones weigh 240 g without the cable.
Manufacturer source Manufacturer source
Ear pads and both cable types are listed as replaceable spare parts.
Manufacturer source

What to check

It uses an open acoustic principle, and Sennheiser publishes no ambient-noise attenuation figure for it.
Manufacturer source Manufacturer source
The specification sheet gives 110 dB SPL at 1 kHz / 1 V RMS. A separate feature statement gives 120 dB SPL without a measurement reference, so the two figures cannot be treated as the same test. The transducer diameter is not stated.
Manufacturer source Manufacturer source
The supplied cables use a 3.5 mm plug, with a 6.3 mm adapter included in the box.
Manufacturer source Manufacturer source

Sennheiser HD 660S2

What it brings

Both a 4.4 mm balanced and a 6.35 mm unbalanced cable are included with a 3.5 mm adapter, and the velour ear pads are replaceable.
Manufacturer source Manufacturer source
A revised 300-ohm voice coil, more compliant surround and vented magnet system are designed for more sub-bass, and sensitivity is rated at 104 dB SPL at 1 V.
Manufacturer source Manufacturer source

What to check

At 300 ohms with a rated 104 dB SPL at 1 kHz / 1 V rms, check that your amplifier or audio source can drive the headphones properly.
Manufacturer source Manufacturer source
The open-back design leaks sound and lets outside noise in, and the supplied cables are 1.8 m long. An XLR-4 cable is sold separately.
Manufacturer source Manufacturer source

COMPARE THE DETAILS

Full specifications

How we verify

Differences only

27 of 27 specifications Sources in each row

Source-backed specifications for Sennheiser HD 400 PRO and Sennheiser HD 660S2
Specification	Sennheiser HD 400 PRO	Sennheiser HD 660S2

Wearing style Sources Sennheiser HD 400 PRO Manufacturer specification Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0) Sennheiser HD 660S2 Manufacturer specification Sennheiser HD 660S2 — operating guide and specifications	Over-ear (circumaural), open-back wired studio headphones	Over-ear (circumaural), open-back dynamic headphones
Acoustic design Sources Sennheiser HD 400 PRO Manufacturer specification Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0) Sennheiser HD 660S2 Manufacturer specification Sennheiser HD 660S2 — operating guide and specifications	Open-back	Open-back
WeightDifferent Sources Sennheiser HD 400 PRO Manufacturer specification · Without cable, per the specification sheet and manual; the product page gives the same 240 g without stating a basis. Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0)Sennheiser HD 400 PRO — instruction manual (09/2022) Sennheiser HD 660S2 Manufacturer specification · Approximate, without the cable. Sennheiser HD 660S2 — operating guide and specifications	240 g	260 g
Ear padsDifferent Sources Sennheiser HD 400 PRO Manufacturer specification Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0) Sennheiser HD 660S2 Manufacturer specification · Replacement procedure shown in the manual. Sennheiser HD 660S2 — exact model specifications Sennheiser HD 660S2 — operating guide and specifications	Velour; replaceable	Velour ear pads; replaceable
ServiceabilityDifferent Sources Sennheiser HD 400 PRO Manufacturer specification Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 660S2 Manufacturer specification · Replacement procedures are in the manual; spare-part availability varies. Sennheiser HD 660S2 — operating guide and specifications	Replaceable ear pads and components; ear pads, coiled cable and straight cable listed as spare parts	Detachable cable and replaceable ear pads
Rated contact pressure Sources Sennheiser HD 400 PRO No source recorded. Sennheiser HD 660S2 Manufacturer specification · Nominal clamping force; individual fit and comfort can differ. Sennheiser HD 660S2 — operating guide and specifications	Not provided	Approximately 6.0 N ±1.0 N

Wired / wireless connectionsDifferent Sources Sennheiser HD 400 PRO Manufacturer specification · Replacement coiled and straight cables are listed as spare parts. Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0) Sennheiser HD 660S2 Manufacturer specification Sennheiser HD 660S2 — exact model specifications Sennheiser HD 660S2 — operating guide and specifications	Wired; detachable single-sided cable: 3 m coiled cable with 3.5 mm jack plug and a 1.8 m straight cable supplied, plus a 3.5-to-6.3 mm adapter	Wired analogue stereo; 1.8 m cables with 6.35 mm unbalanced and 4.4 mm balanced plugs; included 3.5 mm adapter
Supplied plug and adapterDifferent Sources Sennheiser HD 400 PRO Manufacturer specification Sennheiser HD 400 PRO — product specification sheet (EN, V1.0)Sennheiser HD 400 PRO — instruction manual (09/2022) Sennheiser HD 660S2 Manufacturer specification · The 4.4 mm balanced cable needs a balanced headphone output; an XLR-4 cable is optional. Sennheiser HD 660S2 — exact model specifications Sennheiser HD 660S2 — operating guide and specifications	3.5 mm jack plug with 6.3 mm adapter	6.35 mm unbalanced cable; 4.4 mm balanced cable; 6.35 mm-to-3.5 mm adapter
Cable arrangementDifferent Sources Sennheiser HD 400 PRO Manufacturer specification Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0) Sennheiser HD 660S2 Manufacturer specification · Optional cables are sold separately; balanced cables need a balanced headphone output. Sennheiser HD 660S2 — exact model specifications Sennheiser HD 660S2 — operating guide and specifications	Detachable, single-sided cable; coiled 3 m and straight 1.8 m cables supplied	Detachable 1.8 m unbalanced and 4.4 mm balanced cables included; XLR-4 cable optional
Cable length Sources Sennheiser HD 400 PRO Manufacturer specification · Supplied straight cable. A separate 3 m coiled cable is also supplied; the source does not specify whether its length is relaxed or extended. Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0) Sennheiser HD 660S2 Manufacturer specification · Length of the supplied cable. Sennheiser HD 660S2 — operating guide and specifications	1.8 m	1.8 m
Coiled cable rated length Sources Sennheiser HD 400 PRO Manufacturer specification · Separate supplied coiled cable. The source states 3 m but does not establish its relaxed/extended measurement basis. A 1.8 m straight cable is also supplied. Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0) Sennheiser HD 660S2 No source recorded.	3 m	Not provided

Driver constructionDifferent Sources Sennheiser HD 400 PRO Manufacturer specification · Sennheiser does not state the transducer diameter on the pages and documents read. Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0) Sennheiser HD 660S2 Manufacturer specification · Sennheiser lists 38 mm as the transducer size. Sennheiser HD 660S2 — exact model specifications	Angled 120-ohm dynamic transducers with a polymer-blend diaphragm	38 mm open dynamic transducer; aluminium voice coil and vented magnet system
Rated frequency rangeDifferent Sources Sennheiser HD 400 PRO Manufacturer specification Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0) Sennheiser HD 660S2 Manufacturer specification Sennheiser HD 660S2 — operating guide and specifications	6–38,000 Hz (−10 dB)	8–41,500 Hz (−10 dB)
Nominal impedanceDifferent Sources Sennheiser HD 400 PRO Manufacturer specification Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0) Sennheiser HD 660S2 Manufacturer specification · Nominal impedance; check that your amplifier or audio source can drive this load. Sennheiser HD 660S2 — operating guide and specifications	120 ohm	300 ohm
Voltage-referenced sound pressure levelDifferent Sources Sennheiser HD 400 PRO Manufacturer specification · The specification sheet states 110 dB (1 kHz / 1 V RMS). The UK page technical table labels the same conditioned figure Max. sound pressure level; this record preserves the voltage-referenced claim without independently certifying a maximum. The separate 120 dB feature statement gives no reference and is not assumed to use these test conditions. Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0) Sennheiser HD 660S2 Manufacturer specification · Rated at 1 kHz with 1 V rms; not directly comparable with dB/mW ratings. Sennheiser HD 660S2 — operating guide and specifications	110 dB SPL (1 kHz, 1 Vrms)	104 dB SPL at 1 kHz, 1 V rms
Rated total harmonic distortionDifferent Sources Sennheiser HD 400 PRO Manufacturer specification Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0) Sennheiser HD 660S2 Manufacturer specification · Rated at 1 kHz and 100 dB SPL; other models use different test levels, so compare with care. Sennheiser HD 660S2 — exact model specifications Sennheiser HD 660S2 — operating guide and specifications	<0.05% at 1 kHz, 90 dB SPL	≤0.04% at 1 kHz, 100 dB SPL
Advertised SPL without stated reference Sources Sennheiser HD 400 PRO Manufacturer specification · UK page feature statement. Frequency, input voltage/power and measurement reference are not stated; the 120 dB claim is not labelled maximum. Do not compare it numerically with the specifically conditioned 110 dB at1 kHz/1 Vrms claim or use it as a comparable sensitivity/maximum-output input. Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 660S2 No source recorded.	120 dB SPL	Not provided
Frequency equalisation Sources Sennheiser HD 400 PRO No source recorded. Sennheiser HD 660S2 Manufacturer specification Sennheiser HD 660S2 — operating guide and specifications	Not provided	Diffuse-field equalised
Rated long-term input power Sources Sennheiser HD 400 PRO No source recorded. Sennheiser HD 660S2 Manufacturer specification · Rated to EN 60-268-7; the maximum input the headphones can handle, not the amplifier power they need. Sennheiser HD 660S2 — operating guide and specifications	Not provided	Maximum 500 mW under EN 60-268-7
Published acoustic changes Sources Sennheiser HD 400 PRO No source recorded. Sennheiser HD 660S2 Manufacturer specification · Sennheiser describes these changes as the S2 revision of the HD 660S. Sennheiser HD 660S2 — exact model specifications	Not provided	Revised 300-ohm coil, more compliant surround, 70 Hz resonance and revised transducer airflow
Manufacturer sub-bass claim Sources Sennheiser HD 400 PRO No source recorded. Sennheiser HD 660S2 Manufacturer specification · Sennheiser does not give the frequency range or comparison model for this claim. Sennheiser HD 660S2 — exact model specifications	Not provided	Double the sound pressure at the lowest frequencies

In the box Sources Sennheiser HD 400 PRO Manufacturer specification Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0) Sennheiser HD 660S2 No source recorded.	HD 400 PRO headphones, 3 m coiled cable with 3.5 mm jack plug, 3.5 mm to 6.3 mm adapter, 1.8 m straight cable, quick guide, safety guide	Not provided

Operating temperatureDifferent Sources Sennheiser HD 400 PRO Manufacturer specification · Relative humidity up to 90%. Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0) Sennheiser HD 660S2 Manufacturer specification Sennheiser HD 660S2 — operating guide and specifications	−15 °C to +55 °C	−10 °C to +55 °C
Static magnetic field at the surface Sources Sennheiser HD 400 PRO No source recorded. Sennheiser HD 660S2 Manufacturer specification · Static magnetic field at the headphone surface. Sennheiser HD 660S2 — exact model specifications Sennheiser HD 660S2 — operating guide and specifications	Not provided	4.5 mT

Show unverified (3)Not verified, not provided or conflicting for both products. Kept for completeness; never counted as a difference.

Specifications without verified values for either product
Microphone Sources Sennheiser HD 400 PRO The complete exact sources list the headphone and accessories but do not explicitly establish absence of a microphone. An omitted microphone and a stereo headphone plug alone cannot become a sourced false capability. · Manufacturer specification Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0) Sennheiser HD 660S2 The reviewed exact sources do not explicitly establish microphone absence. Omission from a package or technical inventory, a stereo audio connector, and a without-remote statement do not establish microphone=false. · Manufacturer specification Sennheiser HD 660S2 — operating guide and specifications	Not listed by the manufacturer	Not listed by the manufacturer
Active noise cancellation Sources Sennheiser HD 400 PRO The complete exact sources establish an open dynamic wired design but do not explicitly negate active noise cancellation. Open construction and omission of ANC do not establish a false capability. · Manufacturer specification Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 400 PRO — product specification sheet (EN, V1.0) Sennheiser HD 660S2 The reviewed exact sources do not explicitly establish active noise cancellation absence. Preserve separately sourced acoustic design and passive attenuation; they cannot turn omitted ANC into a false capability. · Manufacturer specification Sennheiser HD 660S2 — operating guide and specifications	Not listed by the manufacturer	Not listed by the manufacturer
Battery life Sources Sennheiser HD 400 PRO The retained exact sources do not explicitly establish battery life or that no battery is required for operation. Do not infer a not-applicable operating requirement from analogue/dynamic construction or a battery-included No row. · Manufacturer specification Sennheiser HD 400 PRO (article 700047) — official UK product page and technical data Sennheiser HD 660S2 The reviewed exact sources do not explicitly establish battery life or that no battery is required for operation. A wired/dynamic audio path, omitted charger or Battery included N is not an explicit operating-power requirement. · Manufacturer specification Sennheiser HD 660S2 — operating guide and specifications	Not listed by the manufacturer	Not listed by the manufacturer

First product manufacturer source Second product manufacturer source

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Each record represents an explicitly checked model and configuration. Original import: 5 Sept 2026; this is not its verification date. This is not a list of current offers or a popularity ranking.