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Neumann KH420

Active Monitors By Hugh Robjohns
Published September 2015

Neumann are known for producing outstanding microphones, and their new flagship monitors are no less impressive!

The latest additions to Neumann’s KH monitor range are the KH420s, three-way midfield or main speakers that are, in essence, updated versions of the K+H O410 monitors that I reviewed in 2009 ( In that review I praised the O410s for their superbly accurate and clean mid-range, beautifully open and detailed top end, well controlled and extended bass, flexible room EQ tools, and optional digital input. All those plaudits remain just as valid for the KH420s, but the new model boasts numerous incremental refinements and upgrades that raise the standard even higher. Impressively, though, the new, improved KH420s are actually better value today than when I reviewed their forebears in 2009: after taking UK inflation into account, my calculator tells me the KH420s are roughly six percent less expensive than their predecessors!

Neumann KH420

Apart from the illuminated white Neumann badge, the differences between the old O410s and new KH420s are visually subtle. The cabinet is exactly the same size and shape as on the previous model, measuring 645 x 330 x 444mm (HWD), although it is 1kg lighter than previously, at a still-hefty 35kg. Maintaining the cabinet shape means that the comprehensive array of mounting hardware options all remain fully compatible. The grey paint finish is new, though, as is the internal construction, which has been improved to reduce cabinet resonances even further. Although the cabinet still sports its familiar pair of triangular ports either side of the bass driver, the internal construction has been revised to provide higher capacity, reducing dynamic compression at high listening volumes as well as reduced turbulence and distortion.

Perhaps the most visual changes are to the waveguides around the drivers, which have all been subtly reshaped and re-optimised to further control and smooth the off-axis response, making the speaker easier to integrate into different acoustic spaces. Behind the waveguides, all three (magnetically compensated) drivers are new compared to the O410. Developed with Neumann’s modelling and optimisation tools, the 25mm tweeter is the same as that used in both the KH120 and KH310 monitors, while the 75mm mid-range dome is also borrowed from the KH310s. Using the same core components obviously helps to maintain a consistent family character and performance, as well as simplifying manufacture. The 250mm long-throw bass driver is a brand-new design made exclusively for the KH420s, with considerable design effort to ensure very low distortion levels. Compared to the previous O410 model, the KH420s’ new drivers achieve about 5dB lower harmonic distortion in the mid-range and upper bass regions, while mid-range intermodulation distortions are also improved by 5-10 dB.

The back panel offers a  wealth of EQ options.The back panel offers a wealth of EQ options.Refining the performance specifications of a low-end product is relatively easy, but taking an already superb product and improving its distortion levels by 5dB or more is extremely challenging, and the Neumann team is to be commended on an extraordinary achievement. It’s all about attention to detail, of course, and I was intrigued by another example of this: the bass driver’s chassis design was carefully optimised to improve air flow around the basket specifically to minimise turbulence noise.

As with the O410s, the baffle sub-panel carrying the mid-range and tweeter drivers (and the illuminated logo) can be removed and rotated to allow the speaker to be used horizontally in left- or right-handed forms, if required. Here as on other Neumann monitors, the brightness of the logo badge can be adjusted via a rear-panel switch (100, 60 or 30 percent, or off), and it flashes or lights up red if the elaborate protection circuitry is activated; this happens even if the normal logo illumination is deactivated.

Tower Of Power

Completely invisible to the casual onlooker, but a very significant upgrade is that the KH420 features not one, but two, universal (switched-mode) power supplies, using the same basic design as the KH310. Consequently, there’s no external mains voltage selector, and the speaker accepts any voltage between 100-240V AC, making life simpler. One of the two internal supplies powers just the bass amplifier section, while the other looks after the tweeter and mid-range power amplifiers — a configuration that is claimed to improve the overall transient response.

The power amplifiers and their sophisticated protection systems are also improved over the O410s, being the same modular Class-AB design as used in the KH120 and KH310 speakers. (I’m told the Neumann engineers still consider Class-D amplifiers to be ‘not quite there yet’!). The previous O410 model used a ‘Class-H’ amplifier design, which is basically Class-AB but with the power rails for the output devices changing voltage depending on the signal level. This clever technique minimises power consumption and heat dissipation, but is prone to distortion artifacts when the rails switch voltage. While I didn’t notice any problems in the O410s, apparently the KH420s’ improved loudspeaker drivers revealed some issues, and so the amplifier system was replaced, again maintaining consistency with the other KH models. Although the new amplifier pack brings welcome sonic improvements, the down side is a small increase in idling power consumption — but this is the only specification that has not improved over the previous model! The KH420s’ idling power consumption is 60W (as opposed to 36W in the O410s) but, on the upside, the full-power consumption is reduced to only 800W when the O410s consumed 1300W. This improvement is largely down to the use of far more energy-efficient switched-mode power supplies.

Whereas the KH120s use one amplifier module for the tweeter and a second for the bass/mid-range, and the KH310s have one for the tweeter, one for the mid-range, and two in a bridged configuration for the bass driver, the more powerful KH420s have a bridged pair for the tweeter, another bridged pair for the mid-range, and two bridged pairs in parallel (ie. four in total) for the bass driver. These provide continuously rated power outputs of 130, 130 and 295 W, for the tweeter, mid-range and bass driver, respectively. At full output they generate a maximum SPL of 116dB at one metre for 3 percent THD when working in full space.

Panel Show

The amplifier chassis panel at the rear of the cabinet features unusual diagonal heat-sink fins that radiate heat very efficiently whether the speaker is either vertical or horizontal. All of the configuration controls can be found at the bottom of the panel, along with the integrated mains on/off switch and IEC inlet, and an XLR accepting an electronically balanced analogue input signal. (An optional digital input module is also available — see box.) The input-sensitivity and basic room EQ options are exactly the same as those provided on the KH310s, starting with a rotary attenuator covering a 0 to -15 dB range. An output level switch allows nominal 94, 100, 108 or 114 dB SPL to be generated from a 0dBu input signal.

Room equalisation facilities are provided by an ‘acoustical controls’ section, where you can reduce the bass output by 0, 2.5, 5 or 7.5 dB (measured at 50Hz but with the roll-off starting from about 150Hz). A mid-range switch introduces a broad response dip of 0, 1.5, 3 or 4.5 dB between 50 and 500 Hz, centred at about 180Hz, while an HF shelf provides +1, 0, -1 or -2 dB above 2kHz. The superbly detailed handbook provides very constructive advice about the most appropriate settings for different speaker locations and applications.

In addition, a single parametric-EQ section is also provided to aid with compensation for strong room modes, offering adjustable gain (-12 to +4dB), Q (1 to 8), and centre frequencies. The last parameter has three switchable ranges of 25-80 Hz, 50-160 Hz or 100-320 Hz, plus an off position. The O410 monitors had a similar parametric EQ facility, but the revised version in the KH420s has a wider range and is easier to use.

Specs Appeal

By introducing new bespoke drivers, dual SMPS power supplies, employing the amp modules from sibling KH models, and making numerous incremental improvements to the waveguides and other aspects of the design, the new KH420s have better specifications across the board compared to their predecessors. For example, they boast a flatter frequency response (26Hz-22kHz ±3dB and 28Hz-20kHz ±2dB), with greater bass extension (26Hz vs 30Hz). Additionally, the KH420’s dispersion in both the vertical and horizontal planes is better controlled than its predecessor, thanks to the revised waveguides.

Interestingly, despite their considerable cabinet volume, the KH420s’ bass extension is only 8Hz lower than that of the KH310s (26Hz vs 34Hz) — but the real benefit of the larger cabinet and, especially, the larger bass driver becomes apparent when comparing maximum SPL figures. The KH420 can deliver over 6dB more SPL than the KH310 (which itself delivers 6dB more SPL than the diminutive KH120). The KH420 also manages over 0.5dB more SPL than the old O410.


To evaluate the new KH420 monitors on a subjective level, I set them up alongside my trusty (but getting long-in-the-tooth, now) passive three-way PMC IB1s powered by a Bryston 4B amplifier. The mid-field IB1s have a comparable cabinet volume and also feature a 75mm mid-range dome, and I know them very well. I also know the Neumann KH310s very well, since I use a pair as nearfield monitors every day.

With all the KH420s’ EQ switches at their zero positions, and with the sensitivity set to the 100dB SPL option, the Neumann monitors matched the level of my IB1 setup quite closely, and only a modest trim of the input attenuators was required to get perfect A-B level matching.

My immediate impression, as I started through my usual collection of critical auditioning tracks, was of a transparent neutrality almost indistinguishable from the KH310s. The very low end has a slightly different character, though, with a tad more weight, depth and power, but still very tight and controlled and, importantly, with no imposing overhangs on bass transients, which is a very worthwhile improvement over the old O410s.

The KH420s are ruthless at exposing the technical inadequacies of recordings — which is what monitors should do, of course — and subtle distortions, imaging and balance issues all stood out very clearly. It was easy to hear progress in correcting balance and EQ issues too, which is also a vital element in a set of high-end monitors. The old O410s enjoyed very low distortion and were superb at extracting inner details, but the KH420s seem even better, sounding even cleaner and more transparent. The stereo imaging proved wide and very stable over a large listening area, with a very good sense of depth so that a vast sound stage was created in front of the listening position.

I didn’t feel a need to adjust the speaker’s room EQ facilities at all, which is unusual — I often find myself turning down the tweeter level of most ‘monitors’ by a decibel or so — but the response of the KH420s seems perfectly balanced, to my ears, without the mid-range hype of so many speakers. This is one of those ‘Marmite’ aspects of monitor design: some people expect and need a forward mid-range, while others prefer a more neutral presentation. I suspect preferences stem from whatever style of monitors were employed at the start of a career, but for me, with my BBC background, neutral and natural win, and the KH420s suit that bias. The well-designed on-board EQ facilities do allow a wide degree of tonal shaping and in-room tailoring, though, and the provision of a tuneable parametric EQ is a real boon in rooms with troublesome modes.

As usual, I auditioned a very eclectic range of music from orchestral, chamber and choral, to jazz, pop, rock, R&B, and electronic, as well as a number of my own recordings and mixes, both at elevated and very modest listening levels. Every genre was portrayed with great precision, accuracy, control and resolution at every listening level; the good material sounded spectacular, and the less good recordings revealed their weaknesses clearly but without becoming unpleasant (mostly!). Mixing on the KH420s is a real pleasure simply because of the clarity: small changes of EQ or dynamics settings or fader moves are immediately obvious and unambiguous, reducing fatigue and confusion. Not surprisingly, there is a very strong family character amongst all the KH monitors, and I often forgot I was working with the KH420s because the sound presentation was so similar in quality, resolution and scale to my own KH310s.

Under Neumann’s stewardship, Klein+Hummel’s highly regarded speakers have evolved into a stunningly good range of extremely high-quality studio monitors, with superb technical specifications and real-world usability, and extraordinarily attractive pricing. The KH120, KH310, and KH420 form a beautifully designed trio offering progressively greater bass extension and maximum SPL capabilities, while delivering impressive detail, resolution, and tonal accuracy. The KH420 serves as a very apt flagship to the range, and I’d strongly recommend an audition for anyone seeking a high-end mid-field monitor.


Broadly comparable models worth considering at a slightly lower price point than the a pair of KH420s include Adam’s S5X, Unity Audio’s Boulder, ATC’s SCM25A and Eve Audio’s SC3012. The PSI Audio A25M, Barefoot Micromain 27, Geithain RL903k and ATC SCM45 SL Pro are all a little more expensive.

Factory Tour

The Sennheiser factory in Tullamore, Ireland.The Sennheiser factory in Tullamore, Ireland.I was invited last year to visit Sennheiser’s factory in Tullamore, Ireland, where most of the brand’s headphones are made. Their first Tullamore plant was set up in 1990, initially building aviation headsets, but it expanded into other models rapidly and moved to its current larger premises in 2002. On entering the factory floor, the vista is filled with several enormous automated lines of baffling but ingeniously designed and interlinked robotic machines constructing a wide range of moving–coil headphone transducers. Different lines create different models, from the tiny 6mm units employed in Sennheiser’s top-of-the range HD800 earphones, through to the very large transducers in the HD700 headphones. The complexity and precision involved is quite astonishing, and these machines operate continuously, 24/7, to ensure reliability. Apparently a new drive unit of one type or another is manufactured every five seconds or so!

It was fascinating to watch each stage of the process, with machines winding almost invisibly thin wires on formers, creating stress-relieving bends on the terminal leads, applying and curing glues holding sub-assemblies together, and so on. Automatic testing of every drive unit featured at several different stages as construction progressed down the line, and the remarkably small number of discovered rejects was carefully set aside for scrutiny so that the systems can be fine-tuned as necessary.

Further back on the clean, spacious, and very modern factory floor, workers sit at long benches assembling most of the company’s hi-fi headphone products, as well as many of the professional versions, including the ever-popular HD25s (but the range-topping HD800s are apparently currently built in Germany). Assembled headphones also go into a soundproof booth for final high-level soak-testing before being packaged in their retail boxes and dispatched.

Amongst all the Sennheiser products, a relatively small area of the floor is allocated to assembling and test the Neumann monitor range, working in batches of different models at different times to match sale orders, rather than for stock — when I was there they were producing KH310s, but KH120s and KH420s are also built in the same way on the same lines.

Monitor construction started in Tullamore in 2006 shortly after Sennheiser acquired the K+H brand, but the original production process was fundamentally redesigned under Neumann’s stewardship to improve quality and consistency, as well as ease of build and serviceability. In terms of production numbers, monitor production has grown substantially in recent years as the brand’s reputation has spread, but there is considerable capacity in Tullamore to increase much further.

Drive units, cabinets, and sub-assemblies are all shipped in from various European manufacturers, and the assembly process starts with individual components being inspected and checked, and then collated into construction kits. Everything carries a bar-coded serial number which is scanned and logged at each stage of manufacture. On the manual assembly line, the drivers are installed on the front baffle, then the cabinet is attached and acoustic materials installed before finally fitting the pre-assembled amplifier chassis (which is built in Germany).

Once assembly is complete, each unit is tested for electrical safety and then installed in a bespoke panel on the back wall of a fairly large anechoic chamber.


An optional digital input module (the DIM 1) is available, if required, and can be fitted in place of a blanking plate at the bottom of the rear panel. The DIM 1 accepts AES3 or SPDIF signals on either XLR or BNC connectors, but encoded formats (MP3, DTS, AC3, DSD and so on) are not supported. A BNC output is provided to pass the digital audio stream to another speaker, and a rotary switch allows selection of the required audio source: digital subframe A (left), B (right) or A+B (mono sum), or the analogue input.

Additional input modes route the selected signal through an adjustable delay line, which can be used to compensate for positional errors in surround speaker arrays (for instance, if the centre speaker is too far forward, or the rear channels too far back), or to re-establish lip-sync with display screens. Three rotary switches determine the delay time, each with a 0-15 multiplier for the associated range of 0.1ms, 1.6ms or 25.6ms. The switches can be used in combination to achieve any desired delay value up to a maximum of 409.5ms.

Preaching Tolerance

Neumann’s consistency graph, taken from a  sample of 2500 monitors.Neumann’s consistency graph, taken from a sample of 2500 monitors.KH monitors are sold individually, but guaranteed to be ‘pair-matched’ with any other KH monitor of the same type — an engineer adjusts six parameters on the amp chassis to align the drive units to Neumann’s very tight tolerances. The complete test process takes about five minutes and the reject rate is a remarkably low 0.3 percent, despite the required precision. Although development speaker models are soak-tested for 24 hours at high levels at the factory, to check reliability and long-term conformity to specification, the production units are not ‘run in’ as their performance does not appear to change during initial use.

One aspect of the KH production that particularly impressed me was the remarkable consistency between units, although no loudspeaker manufacturer currently reports its manufacturing consistency. I understand that there are some efforts underway to introduce this measure (or something similar) into the international standards to encourage reporting, and to that end Neumann have submitted a technical paper on the subject for presentation at the AES Convention in October. The graph below is drawn from that paper, and was created from the test measurements of 2500 KH monitors manufactured at Tullamore. It shows how the frequency response deviates away from the average, and in a perfect world all the lines should be on the 0dB axis. However, every process has tolerances so deviations are expected, with inconsistencies (a wider vertical spread in the lines) coming from things like inappropriate (cheap) parts, wider tolerance components, poor design techniques, inappropriately designed or adjusted production alignment, and uncontrolled production processes. Conversely, a tighter vertical spread is likely to be due to employing well designed parts and narrower tolerance components, sophisticated design and modelling techniques, and well-defined production processes controlled using statistical techniques.

The graph shows that, between 40Hz and 12kHz, 50 percent of the 2500 tested KH monitors are within ±0.2dB of the median (cyan), and 80 percent are between ±0.35dB (blue). These are astonishing results of precision and consistency. Even the remaining ‘wayward’ 10 percent of production units are still within ±0.9dB (magenta). From these results it seems easy to justify Neumann’s claim of every KH monitor being ‘pair-matched’ to every other monitor of the same type within very tight tolerances!


  • Very low-distortion drivers provide supremely neutral and clean sound presentation.
  • Drive units and amplifiers shared with other KH family models.
  • Well controlled and extended frequency extremes.
  • Comprehensive room EQ facilities.
  • Stunningly small and extremely consistent manufacturing tolerances.
  • Digital input and compensating delay unit option.
  • In real terms, the KH420s are six percent less expensive than their predecessors!


  • Monitors of this quality are still expensive!


Large three-way active monitor speakers with superb transparency, neutrality, and precision, boasting impressive technical specifications which surpass those of their predecessors in almost every department.


£7680 per pair including VAT.

Sennheiser UK Ltd +44 (0)1628 402200

$9799 per pair.

Neumann USA +1 860 434 9190

Published September 2015