Klark Teknik DN 27A
Twenty-seven oil-damped faders, a relay that passes your signal straight through when the power dies, and a company that built its reputation on the thing.
Overview
The DN 27A is a one-third-octave graphic equalizer, and the number in the name is literal: twenty-seven filters, with centers 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. Klark-Teknik's own manual calls it the successor to the widely acclaimed DN7. The company says it was built to be "simply the best." The build description backs that up. The inductors in each filter use precision ground air-gap ferrite cores with custom-wound coils, and the faders are oil-damped for smooth, positive operation. Every unit was tested electronically and subjectively before it left the factory.
Those 27 faders aren't short-throw trimmers. Each one travels 60mm, which gives high resolution of settings, and each has a center detent so you can find a true flat, 0dB position by feel. The input level control gives up to 6dB of system gain fully clockwise and full attenuation of the input signal anticlockwise. A front-panel "eq" bypass switch lets you A-B the dry and equalized output instantly. Center frequency accuracy is better than ±2% of nominal, calibration accuracy is ±0.5dB, and with the controls flat the frequency response holds ±0.5dB from 20 to 20KHz.
The electrical numbers explain why these units stayed in racks for decades. Output drive capability is +23dBm into a 600-ohm load, and the output clipping point is +23dBm (11V) into the same load. Distortion is quoted at less than 0.01% at 1KHz at +4dBm into 600 ohms, and less than 0.05% across 20Hz - 20KHz at +18dBm. Equivalent input noise is less than -95dBm, 20Hz - 20KHz unweighted, and dynamic range is greater than 115dB. Input impedance is 10K ohms nominal at any gain setting, and the input is A.C. coupled, which protects it from D.C. offset problems. The output source impedance is 56 ohms, short-circuit protected. Permissible input level is effectively unlimited thanks to the input attenuator. The manual does make the obvious point that high-level input signals should be attenuated enough to leave gain in the equalizer section without hitting the output clipping ceiling, and that transients need headroom too.
The power supply is where the DN 27A separates itself from a lot of its contemporaries. The rails run at higher voltage than previous units, which is what buys the +23dBm clipping point and the dynamic range figure. The supply includes output voltage regulators that are current-limiting and designed to shut down if overheated. Mains connection is via an IEC standard socket, with the nominal voltage indicated on a label beneath the inlet. Operation is internally switchable between 100-120V and 200-240V at 50-60Hz, drawing 5VA. The unit is fitted with a 3-pin socket, and the manual is blunt that the earth connection should not be removed for safety reasons.
The relay on the main p.c.b does two jobs that matter a lot in a live situation. First, delayed turn-on: the output socket stays isolated from the equalizer until power is switched on and all internal rails are stable, so DC or power-up transients 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. Losing power to the DN 27A does not mean losing signal. There's also an earth-lift switch on the rear panel that connects or disconnects signal ground to and from mains and chassis earth. The manual notes it is a quick and safe answer to earth-loop problems.
The rear panel carries complementary 3-pin XLR input and output sockets, a separate fuse holder, and the earth-lift switch. Inside, a 12-terminal connector carries the input and output lines and makes it easy to add line balancing on an optional plug-in board. Power rails are present on that connector too, so a plug-in p.c.b could house transformer and/or electronic balancing, or active filters. When no option board is fitted, a dummy p.c.b completes the input and output lines. Optional accessories listed by the manufacturer include input and output balancing transformers, Perspex security covers, a teak veneered case, and a heavy-duty wooden case. The chassis measures 482mm (19") wide, 137mm (5.4") high and 212mm (8.4") deep including the dust cover sleeve. The panel height is sized to mount in a standard G.P.O. rack.
Klark-Teknik built its reputation in this period on products like this one. Studio Sound (July 1982, p.85) said the company's commitment to research and development had earned it a worldwide reputation for precision professional audio products, and singled out the DN 22 and DN 27A graphic equalizers as having become standard tools for correcting room acoustics and complex equalization problems. That's a period publication calling it a standard tool, not the manufacturer. Earlier, Studio Sound (July 1978, p.110) ran the DN27 and DN22 under the headline "In a class of their own," arguing that a product of sufficient quality becomes a generic in its own right. The DN 27A is the revised version of that original DN27. The changes are delayed turn-on, equalizer and total system bypass, earth lift, and plug-in p.c.b options.
Before buying, note that the manual does not include servicing information and states that any maintenance or repairs should be carried out by a qualified engineer. Voltage selection changes carry the same warning. These are not units to open up casually. The supply rails run high, and the regulators and relay network are the reason the box behaves so gracefully, so they're the last thing you want to disturb with a hobbyist's soldering iron. The 27 faders are oil-damped and smooth when healthy. A unit that's been sitting in a damp warehouse for twenty years may need attention there before anything else.
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