Klark Teknik DN27

Twenty-seven oil-damped faders on one-third-octave centres, and a bypass relay that keeps the show alive when the mains don't.

Overview

The DN27 is the equaliser that made "Klark Teknik" a shorthand for a certain kind of British studio correctness. Its own manual describes it as the successor to the widely acclaimed DN7, a "totally professional unit" that carried over the ease of use and reliability of its predecessor while adding new features. The DN7 built the reputation, and the DN27 is where the company doubled down on it. Klark Teknik's advertising in Studio Sound (July 1978, p.110) was blunt about the ambition. It put the DN27 and DN22 graphic equalisers "in a class of their own" and said a product of this quality "becomes a generic in its own right."

There are 27 filters, with centres at 40, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1K, 1.25K, 1.6K, 2K, 2.5K, 3.15K, 4K, 5K, 6.3K, 8K, 10K, 12.5K and 16KHz. That's roughly one-third-octave spacing across the audio band, which is why this box shows up in room-voicing and monitor-tuning work and not just as a tone-shaping toy. Modern Recording (April 1977, p.62) drew the line sharply. It said the DN27's operation points up the difference between "home type" graphic equalizers and those genuinely intended for professional recording and room-voicing. The reviewer singled out the number of adjustable bands, organised at approximately one-third-octave intervals, and their extremely accurate and precise action. That is what permits fine adjustments beyond what domestic units allow.

Each of the 27 faders has a travel length of 60mm, which gives genuinely high resolution of settings. A centre detent lets you find a true flat, 0dB position by feel rather than by eye. The faders are oil-damped, a detail the manual is proud of, for smooth, positive operation. Centre frequency accuracy is better than ±2% of nominal, and calibration accuracy is ±0.5dB. Together those two numbers mean the band you reach for is the band you get, and the scale on the panel means what it says.

The electrical side is just as deliberate. Frequency response is ±0.5dB from 20Hz to 20KHz with the controls flat. Distortion is specified at less than 0.01% at 1KHz at +4dBm into a 600 ohm load, and less than 0.05% from 20Hz to 20KHz at +18dBm into the same load. Equivalent input noise is less than -95dBm from 20Hz to 20KHz unweighted, and dynamic range is greater than 115dB. The output clipping point is +23dBm - 11V - into 600 ohms, and the output source impedance is 56 ohms, short-circuit protected. Input impedance is nominally 10K ohms at any gain setting. The manual notes the permissible input level is virtually unlimited thanks to the input attenuator, and A.C. coupling protects against D.C. offsets.

The manual does offer one caution. High-level input signals should be attenuated enough to leave gain in the equaliser section without exceeding the output clipping level, and transients need adequate headroom to be processed accurately. The DN27A will take whatever you feed it, but it won't forgive you for feeding it something already hot and then asking for boost on top.

The input level control provides input/output system gain of up to 6dB when fully clockwise, and full attenuation of the input signal when turned fully anti-clockwise. A preset potentiometer inside is adjusted for unity gain with the panel level control at mid position, which is a sensible factory starting point. Input and output connections are made via complementary 3-pin XLR sockets on the rear panel, and mains comes in through an IEC standard 3-pin socket. A separate holder carries the main fuse, the serial number label is on the unit, and the 3-pin mains socket carries the usual warning that the earth connection should not be removed for safety reasons. The nominal mains voltage is indicated on a label under the inlet socket, and the unit is internally switchable for other voltages. The manual specifies 200-240V A.C. with 100-120V internally switchable, 50-60Hz at 5VA, and warns that voltage selection changes should be made by a qualified engineer with the correct fuse fitted.

What separates the DN27 from a lot of rack EQ is what happens when things go wrong. The main board carries a relay and drive network performing two functions. First, delayed turn-on: the output socket is isolated from the equaliser until power is switched on and all internal power rails are stable, so DC or transients from powering up never reach downstream equipment. Second, total system bypass: if power is lost or a power-related failure occurs during operation, the input is electronically connected directly to the output. Loss of power does not mean loss of signal. There's also a front-panel equaliser bypass switch for instant A-B comparison between dry and equalised output - the manual calls it an "eq" bypass - and a separate system bypass via gold contact relay during power down or when the unit is switched off.

The power supply uses output voltage regulators designed to shut down if overheated, and they're current-limiting. The supply rails run at higher voltage than in previous units, which is what allows the +23dBm output clipping level and the wide dynamic range in the first place.

Two more practical touches. On the rear panel there's an earth-lift switch, which connects or disconnects signal ground to and from the mains and chassis earth. The manual is straightforward that this may prove invaluable if earth-loop problems are encountered, and may provide a quick and certainly safe solution. On the main board there's a 12-terminal connector through which the input and output lines travel, permitting easy addition of line balancing on an optional plug-in board. Power rails are present on that connector too, so a plug-in card could house transformer and/or electronic balancing and could also include facilities such as active filters. When no such option is fitted, a dummy p.c.b is plugged in to complete the input and output lines. Optional accessories listed for the unit include input and output balancing transformers, Perspex security covers, a teak veneered case, and a heavy-duty wooden case.

Physically, it's a 19-inch rack unit: 482mm wide, 137mm high, 212mm deep, with the dimensions in the manual including the dust cover sleeve and a panel height intended to mount in a standard G.P.O. rack. The construction is described as completely dependable and sturdy, with every unit thoroughly tested both electronically and subjectively before leaving the factory. The inductors in each filter are made from precision ground air-gap ferrite cores combined with custom-wound coils.

The manual is candid that it does not include servicing information and that maintenance or repairs should be carried out by a qualified engineer.

Klark Teknik's period advertising leaned hard on the company's self-image. Studio Sound (October 1979, p.104) ran copy stating that by then the company had a reputation for manufacturing the ultimate in signal processing products, with each one representing the very best in its specific technology. It described substantial research followed by alignment, burn-in period, and finally performance verification using a music source. The same advertisement appeared in the magazine again in December 1979 (p.104) and March 1980 (p.132), and a similar version ran in August 1979 (p.88).

No production years are documented here, so dating an individual DN27 means going by the unit itself rather than the calendar. The manual is explicit that the DN27A is the successor to the DN7, which gives a rough ordering within the line but not a date range.