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K+H O 300D & Pro C28

Active Monitors & Digital Controller By Hugh Robjohns
Published October 2004

K+H O300D & Pro C28Photo: Mark Ewing

Despite its small size, this new system leads the field when it comes to nearfield monitoring accuracy.

The German manufacturer Klein+Hummel — named after founders Horst Klein and Walter Hummel — have been making studio monitors and PA loudspeakers at their factory near Stuttgart since 1945, and have built an enviable reputation for reliability and quality. One of the company's smaller studio monitor systems — the O 100 and its O 800 subwoofer — was reviewed in SOS November 2002 and, a small niggle about the supplied handbooks being in German aside, I had nothing but praise for them.

The subject of this review is an altogether more serious studio monitor: the active three-way O 300D, along with an optional digital crossover unit called the Pro C28. Sadly, the handbooks supplied with the review equipment were still written entirely in German, which didn't impress, but with the aid of the Internet I was able to find an English-language version and eventually glean what I needed to know.

Infinite-baffle Design

The O 300D is a surprisingly small active three-way nearfield monitor, clearly intended for use on the meterbridge of a large-format studio console. It measures 253 x 383 x 290mm (hwd), weighs in at 13.5kg, and features full magnetic shielding, so placement near moving-coil meters and computer VDUs is not a problem. The sealed (infinite-baffle) cabinet is constructed very solidly from MDF finished in a charcoal-grey enamel as standard, although there is also a flock-finished version (recommended if the speakers are likely to be moved a lot), and various other special finishes are available to order. There are also various built-in options and kits for mounting the monitors in various ways if the console meterbridge is not appropriate. Each cabinet has an 8mm threaded socket on each side to which various optional mounting adaptors and brackets can be fitted. There are also heavy-duty brackets with quarter-inch holes fitted at the rear of the speaker for wall or ceiling mounting. As standard the monitors are not supplied with any kind of protective front grille, although a fine wire-mesh grille is available as an option, if required.

The driver complement comprises a 210mm (eight-inch) polypropylene-cone woofer, a 76mm [three-inch] soft-dome mid-range unit, and a 25mm [one-inch] titanium-dome tweeter. The tweeter is mounted above the mid-range unit — both being the same components as used in the company's larger O 500C monitor — and the pair are located beside the woofer, which occupies the remaining baffle space. The red K+H badge below the woofer illuminates when mains power is applied, and flashes if any of the amplifiers clip or if the protection limiters are triggered.

K+H prefer to use a three-way format even in modestly sized speakers like the O 300D because of the numerous performance advantages it affords — even though there is a corresponding cost penalty. Sharing the frequency range across three drivers reduces the demands on each individually, permitting much better control and accuracy. The bass driver no longer has to generate mid-range frequencies, avoiding problems with resonances, breakup modes, and harmonics, as well as reducing the problem of standing waves in the cabinet and Doppler distortions (mid-range frequencies being modulated by LF signals using the same cone). The mid-range and tweeter domes are housed in their own sealed enclosures and thus are completely independent of the woofer and the acoustic energy contained within the main cabinet.

The front baffle is made from a material referred to as 'Low Resonance Integral Moulding' or LRIM, and this can be machined to produce complex three-dimensional shapes such as the waveguides used for both the tweeter and mid-range drivers. These waveguides help control dispersion, as well as allowing the two drive units to be mounted in the correct vertical plane for accurate time alignment. An elliptically shaped waveguide is used for the tweeter to provide wider horizontal dispersion than vertical, and a modest horn shape is used for the mid-range dome. The narrower vertical dispersion is intended to minimise HF reflections from the console surface, which would otherwise cause detrimental comb-filtering effects. The wider horizontal dispersion ensures a large 'sweet spot', making it easier to hear the spatial imaging over a wider proportion of the console.

The O 300D monitors are 'handed', meaning that they are available as left- and right-hand models — K+H recommend placing the speakers with the tweeters on the outside edges when the speakers are used as a stereo pair. However, there is no dedicated centre version, and in 5.1 surround systems either a left- or right-hand monitor can be used for the centre channel. In this application it should be set up vertically, with the tweeter and mid-range driver at the top. For the LFE channel, one or two O 800 subwoofers should be used.

Amplifier & Electronics

The amplifier chassis at the rear of the monitor incorporates three discrete MOSFET power amplifiers providing 150W RMS (250W peak) for the woofer and 65W RMS each for the mid-range unit and tweeter. The mid-range amp can manage 75W peaks, while the tweeter amp can produce 110W peaks. Each amplifier channel has built-in protection limiters. The peak SPL measured at one metre is quoted as 112.8dB (for three percent distortion), although at more realistic listening levels (95dBSPL) the distortion is specified as being below 0.5 percent for all frequencies above 100Hz. The overall frequency response is given as 40Hz-20kHz (±2dB) and the supplied traces are remarkably flat. The crossover points are set at 650Hz and 3.3kHz, both with fourth-order (24dB/octave) slopes. There is also a protective subsonic filter which rolls off at 6dB/octave below 30Hz.

O 300D rear panel.O 300D rear panel.Photo: Mark Ewing

There are two audio inputs available: analogue and digital. The Lundahl transformer-balanced (and floating) line-level analogue input is connected via an XLR socket and has a nominal sensitivity of +6dBu — although there is also an input attenuator which ranges from zero to -24dB to accommodate higher levels. Digital signals can be connected in either the conventional AES3 format via the same XLR socket used for the analogue input, or via a BNC socket in either AES3id (unbalanced) or S/PDIF formats. The balanced AES signal is routed through a transformer, the secondary of which is wired in parallel with the BNC socket. This enables the latter to be used as a loop-through if required (to convey the stereo digital input signal on to the other stereo speaker, for example), or to connect a 75Ω termination either directly or via a BNC 'T' piece, as appropriate. The handbook provides comprehensive instructions about how to correctly connect inputs of various formats. A screwdriver-operated rotary switch selects the type of input signal in use: analogue, left digital channel, right digital channel, or a mono sum of both left and right digital channels. The digital input is handled with a 24-bit delta-sigma converter that can accept any standard sample rate between 32kHz and 96kHz.

As with most active monitors, the O 300D features a range of room-correction equalisation facilities, all adjusted through recessed screwdriver-operated controls. The bottom end can be tamed to compensate for placement in free space, near a wall, in a corner, or in a corner near the ceiling — each successive setting introducing 3dB more attenuation to the woofer's output. There is also a mid-range equaliser which attenuates a broad band of frequencies extending from about 50Hz up to 1kHz to compensate for reflections from a console when mounted on the meterbridge. The options provide from zero to 6dB of attenuation in 2dB steps. Finally, there is a high-frequency level control to accommodate the variabilities of control-room HF absorption and reflection. This provides level trim from +1dB to -2dB in 1dB steps, and it affects frequencies from about 2kHz upwards.

The handbook provides a complete set of traces and measurements for the O 300D, which appear very impressive. One plot is of the system group delay, and this is interesting because it shows negligible delay for any frequency above about 100Hz, and only a gently rising delay below that to about 10ms at 50Hz. This is linked to the use of a sealed enclosure, and results in a very tight and fast bass response with good transient accuracy.

Other rear-panel facilities include a ground-lift switch to separate signal and chassis grounds, a mains rocker switch, and an IEC mains inlet socket. One particularly interesting feature is a seven-pin XLR socket and its associated slide switch. The socket provides direct inputs to each of the three power amplifiers, bypassing all of the internal input handling, crossover, and protection circuitry (apart from a peak limiter for the bass amp). The slide switch disables this internal circuitry and enables the external three-channel discrete inputs, the aim being to drive the monitors from the optional Pro C28 digital controller unit. This unit provides input-signal handling, digital conversion, crossover, sophisticated room equalisation, and protection limiters, all performed in the digital domain with high-resolution processing — see the 'Pro C28 Digital Controller' box for more details.

Pro C28 Digital Controller

The Pro C28 is a rackmounting stereo digital loudspeaker controller which occupies 2U of rack space. It is primarily designed for use with the O 300D studio monitor — although it can also be configured to operate with a wide range of monitors and speakers systems — and creates up to four audio bands per channel to drive a tweeter, mid-range driver, woofer, and subwoofer. The crossover uses specially developed Finite Impulse Response (FIR) filtering algorithms for precise and independent adjustment of the overall system's amplitude and phase responses, and the system is similar to that built into the company's large O 500C monitors. The linear-phase FIR filter characteristics allow much more accurate in-room response and transient behaviour than is normally possible with conventional analogue crossovers and electronics.

Pro C28 rear panel connections.Pro C28 rear panel connections.Photo: Mark Ewing

In addition to the crossover FIR filters, the unit also incorporates delay facilities for each input channel (up to 1000ms) and an Infinite Impulse Response (IIR) filter section that enables real-time variable equalisation. This caters for both standard boundary correction and very sophisticated room-correction EQ — the latter being accommodated with ten bands of fully parametric EQ per input channel.

To protect amplifiers and drivers, the C28 controller incorporates separate digital limiters for each output, and these take into account both transient peak and sustained RMS levels, with a look-ahead facility to ensure every transient is caught. The software is also able to model the power amplifier's load performance to ensure effective protection, and can anticipate overheating and over-excursion of each driver by modelling their voice coils.

One particular advantage of the FIR approach is that the crossover slopes can be as steep as 96dB/octave, and so the crossover region spreads over a much smaller region than is possible with analogue filter designs. This is claimed to benefit the speaker's overall dispersion and directivity characteristics. The maximum SPL of the speaker is also increased because of the accuracy of the amp and voice-coil modelling and the precise control exerted by the digital limiters.

The Pro C28 can either be operated from its front panel, with an optional RC55 infrared remote control, or from a PC via RS232 or MIDI ports. In complex surround-sound systems several Pro C28 units can be linked together in one control group using MIDI. The front panel provides an input level meter, overload/clip lights for each output, and a two-line 24-character alphanumeric display. The unit can store up to 70 different parameter sets — which can also be transferred to/from a PC — and settings can be password protected to prevent unauthorised tampering.

The unit's internal flash memory is pre-programmed with the relevant FIR settings for a range of K+H monitoring systems, enabling a very fast and hassle-free basic installation. Once the unit is hooked up to the monitors via supplied seven-pin XLR cables, and the input signal connected to the controller, the user simply has to select the appropriate monitor type and adjust the room EQ as necessary. The IIR EQs can then be used to provide more complex and sophisticated room correction, ideally based on the results of accurate room analysis and measurement equipment. Once established, K+H can also translate a customer's IIR filter settings into FIR parameter sets, to maintain the linear phase attributes of the overall system, if required.

One of my concerns with the digital input on the O 300D is the potentially reduced resolution which occurs if the input signal is attenuated in the digital domain. To address this issue, the Pro C28 uses a 'stacked configuration' for its 24-bit delta-sigma A-D and D-A converters, with a 'gain ranging' technique which is claimed to provide a usable dynamic range of more than 130dB.

The two analogue XLR inputs are electronically balanced (an input transformer can be switched into circuit if required), and can accommodate peak levels of +24dBu. The four balanced analogue XLR outputs per channel can be switched to operate with nominal levels of +16dBu, +10dBu or +4dBu. Each channel is also equipped with a seven-pin XLR for single-cable connection to K+H monitors. Internal digital signal processing is performed with 48-bit precision, but can only operate at sample rates of 44.1kHz or 48kHz. The unit is also equipped with a separate digital input and output, which can be configured to operate as an insert point if you like.

Using The O 300D

I'm a firm believer in the advantages of three-way loudspeakers myself, and use the rather large passive three-way PMC IB1s as my main reference monitors. So it was with great enthusiasm that I rigged the O 300Ds alongside to let battle commence! I hooked the K+H speakers up using the balanced analogue inputs, checked that all the EQ settings were flat, switched them on, and started playing some familiar reference CDs.

I have to say my initial reaction was one of shock! These are modestly-sized nearfield monitors, yet they were producing an enormous sound — not only were they capable of going loud effortlessly, but there was also a deep bass output and a huge stereo image. I was so surprised, I quickly checked that I wasn't inadvertently driving the PMCs at the same time! I wasn't... these monitors really do produce a staggeringly big sound — and the fact that they do it from such small boxes is astounding.

Given the cabinet size, the amount and depth of bass is very impressive — as is the naturalness of it. I can quite believe the claim that the response is 2dB down at 40Hz, and because of the sealed cabinet the response falls gently, so that even the lowest musical octave is still conveyed into the room. There is nothing hyped about the bass at all, either. It is very accurate and believable, without any of the boomy, one-note quality from which so many small-box ported designs suffer. It is also very tight and well controlled — there is no obvious overhang, and kick drums and bass guitars are delineated perfectly, allowing you to hear exactly who is playing what and when.

The most obvious strength of all well-designed three-way systems is the mid-range clarity — the result of having a bass driver that only has to worry about bass — and in the O 300D the clarity and resolution of the mid-range is superlative. The dispersion of each driver through the crossover regions is also very well matched, so that the entire system is phenomenally well integrated — both on and off axis. The top end is as well balanced as everything else, although I found that in my room I preferred the balance with the HF trimmer set to -1dB.

An advantage of a small cabinet, and having the mid-range and tweeter mounted in waveguides, is that the imaging is excellent and stable over a wide area. Simple coincident mic recordings produced lifelike imaging with huge width and depth information, seeming to fill the room with the soundstage. My ever-challenging spoken-voice recordings were handled with great aplomb, delivering a totally believable and extremely neutral character. A variety of singing voices were also handled superbly, with excellent natural dynamics and every little detail laid bare for inspection — but without any hint of the slightly shouty or overly forward character that many monitors seem to employ to appear 'revealing'. The O 300D comes across as a really high-class, accurate monitor in every way, despite its diminutive size. For its intended nearfield applications, there is plenty of monitoring level available, and the sound doesn't harden with increasing levels until you get close to near-deafening limits.

Complex orchestral recordings were handled with the same consummate skill as a wide range of rock, pop, and jazz: quality recordings stood out as such, while mediocre recordings were revealed for what they were. Bass-heavy tracks were delivered without any sign of strain, and even after being glued to these monitors for an entire day I felt no hint of fatigue at all. The O 300D is without question one of the very best active three-way monitors I have ever used — and undoubtedly the best monitor bar none for a cabinet as small as this. It is quite simply astonishing.

Switching briefly to the digital input, I felt the quality of the internal converter was pretty good, but it didn't quite match the degree of resolution afforded by my Apogee PSX100, and became the quality 'choke point' in an otherwise exceptional signal path. Part of the quality reduction is probably due to the need to control the monitoring level in the digital domain — thus, at modest listening levels several of the most significant bits remain unused and resolution is inherently compromised. Using the analogue input, the converter was always working at optimum resolution and the analogue signal attenuated to suit, with minimal loss of quality in the process.

Consequently, I would prefer to see the digital input provided as an option rather than a standard feature — hopefully with a corresponding reduction in the price of the monitor (albeit a small saving). I would also like to see built-in and switchable 75Ω terminations provided for the BNC socket, as many users won't realise the importance of correct termination (despite the clear instructions in the handbook) or have suitable BNC 'T' pieces and terminations available.

Pro C28 Comparisons

With the Pro C28 controller configured and hooked into the O 300D monitors, and with all the IIR equalisers set flat, I restarted my listening tests using the same material. Initially, I was hard pressed to tell any difference, although when I moved on to material with a fast-moving and dynamic bass content I became aware of a slightly greater sense of reality and, if possible, crisper transient edges to most instruments. I didn't notice any significant improvements to the stereo imaging, and as I wasn't driving the system to ear-splitting levels any improvements in the overload protection were wasted on me!

While I did perceive some incremental improvements in overall quality — particularly at the bottom end — I have to say that I don't think I heard more than £2500-worth of improvement! In 'difficult' control rooms, the Pro C28 would certainly allow precise and elaborate equalisation to be applied, potentially enabling a far more linear response to be obtained than would ever be the case with conventional analogue electronics. However, sophisticated measuring equipment (and the experience to interpret the results) are required to optimise the filter settings and it wasn't practical to assess this aspect of the system for this review. However, I have heard the improvements such equalisation can bring to a system when used properly (during my review of the similarly equipped Tannoy Ellipse iDP monitors), so I have no doubt that the Pro C28 would be a very powerful and beneficial tool in the appropriate circumstances.

I have some concerns over the limited range of sample rates — many potential users might well prefer to work with 96kHz or even 192kHz digital sources, for example. The balance of cost to benefit is a major concern of mine too, and it is not helped by the fact that even if an O 300D installation uses the Pro C28, the customer still has to buy a complete set of utterly redundant I/O, crossover electronics, and limiters within each O 300D monitor. Perhaps K+H should consider supplying a stripped-down, lower-cost version of the monitors for exclusive use with the Pro C28 controller, and update the DSPs and interfaces to accommodate the higher sampling rates that are becoming commonplace in many studios now.


Even without the Pro C28, these K+H O 300D monitors are expensive, but I found them astonishingly good and very easy to work with. For a similar financial outlay in the UK, the obvious competition would include the ATC Active 20, Dynaudio Air 15, Genelec 1032AM, JBL LSR6328P, and Tannoy Ellipse 10. These are all good reference-quality monitors, but all apart from the Tannoy Ellipse are also two-way designs, and even in the Tannoy you don't have a traditional woofer/mid-range/tweeter complement such as in the K+H design. The resolution and detail of the O 300D, especially through the mid-range and upper bass, is exceptional, as is the quality and quantity of bass. Both easily justify the professional price tag, before you even consider the compact dimensions, which make the monitor ideal for location work or when working in unfamiliar studios. When the time comes to upgrade my nearfields these K+H monitors will be at the top of my auditioning list, and I strongly recommend you put them at the top of yours too!


  • Superlative sound quality and performance.
  • Extremely compact dimensions.
  • Comprehensive room-tailoring EQ.
  • Flexible mounting options.
  • Facility for external digital crossover option.


  • Digital input probably unnecessary for most users.
  • Pro C28's relatively high cost and limited sample-rate capability.


The O 300D is an active three-way nearfield monitor which, despite its unfeasibly small box, produces a huge sound in every regard. It provides an extremely clean and neutral sound across the entire spectrum, with a very high degree of resolution and transparency. However, the Pro C28's restricted sample rates and very high cost will limit its attractiveness, given that it will mostly provide only incremental improvements to an already very fine monitor.


O 300D monitors, £3090.26 per pair; Pro C28 controller, £2719.10. Prices include VAT.

Beyerdynamic +44 (0)1444 258258.