Klark Teknik DN-504 (c. 1989)
Four channels of gain control in one rack space. The manual makes a good case for why you'd want that.
Overview
The DN504 - also written DN-504 - is Klark Teknik's quad compressor/limiter. It packs four channels of dynamics into a single unit of nineteen-inch rack space. The company's own manual opens by saying the DN504 was designed to give the studio or PA engineer "comprehensive and convenient automatic control over gain and level." It also argues that even at this packing density, the control on each channel exceeds what you'd find on many conventional two-channel compressor/limiters.
The trade press caught it on arrival. Studio Sound (August 1989, p.24) reported the DN504 as a recent addition to the Klark Teknik range. It was a 1U 19-inch rackmount with threshold, ratio, attack, release and output level on every channel, plus a hard/soft-knee switch per channel and LED metering of both gain reduction and output. The same item gives the noise figure Klark Teknik quoted: better than −94 dBm from 20 Hz to 20 kHz, unweighted. A later Studio Sound item (November 1990, p.22) added more detail - sidechain input, function bypass, variable-knee operation on the limiting side, and LED indicators for the dynamics functions.
This is a working engineer's tool, not a character box. The manual is candid about the design philosophy. The DN504 is meant to work as a near-invisible dynamic-range reducer when you want transparency, or as an outright effect when you want colour, and the controls are laid out so you can choose. The manual also calls out the metal chassis and the aesthetic presentation as aids to realising "full dynamic control over gain and level." That's a slightly grand way of saying the thing is built to be racked and patched.
Key Features
Four Channels in One Rack Unit
The headline is the count. Four compressors in a 1U chassis - the manual's phrasing is "four channels of compression ... in one unit of nineteen inch rack space." A conventional two-channel compressor/limiter of the period would use the same rack space for half the processing. The manual explicitly claims the per-channel control exceeds what those two-channel units offer. Owners posting internal photos on Gearspace have noted how little information circulates about the unit, which fits with how quietly it sat in the range compared to Klark Teknik's equalisers.
Per-Channel Threshold, Ratio, Attack and Release
Every channel carries its own threshold, ratio, attack, release and output-level controls. The front-panel section of the manual describes them as the means by which compression can be used "as an effect in its own right" rather than only as invisible gain management. Full manual control is there whenever you want it, and the auto mode is there for when you don't.
Hard and Soft Knee Switching
The per-channel knee switch dates the DN504 to a time when "knee" was becoming a user choice rather than a fixed characteristic. Studio Sound (August 1989, p.24) lists the hard/soft-knee switch as standard per channel. A later Studio Sound item (November 1990, p.22) notes variable-knee operation on the limiter side. In practice you can dial in a gentle, rounded onset of compression for material you want to keep natural, or a harder corner for material you want to sit down firmly.
Auto and Manual Attack/Release
The DN504 offers auto/manual control of the envelope. In auto, the attack and release controls are disabled and the unit sets its own timing based on the incoming signal. The manual claims this yields "almost undetectable" dynamic-range reduction. In manual, the rotary controls come into play and the compressor becomes a creative tool rather than a corrective one. Klark Teknik built the same idea into the DN500 and the rest of the dynamics line.
Stereo Linking
Channels can be linked in pairs for stereo operation, with channel 1 acting as master for channel 2 and channels 3 and 4 linking the same way. The manual says linking prevents image shift when compressing stereo material, because both channels of a pair respond to the same gain-reduction decision. On the slave channels the stereo switch is replaced by a "Stereo Slave" LED indicator.
Sidechain Inputs and Ducking
Four sidechain inputs are provided, one per channel, and the manual spends real space on what they're for. The worked example is ducking. Run stereo music through channels 1 and 2, patch a microphone into the sidechain of channel 1, and press the stereo link button. The music drops in level whenever the voice-over artist speaks, and rises again in the pauses. The manual notes that an open-circuit jack plug must be inserted in the sidechain input of channel 2 to break the jack socket's switch connection. Otherwise the music input to channel 2 would affect the degree of compression.
Metering and Indicators
Each channel carries a five-stage LED bar graph for gain reduction and a separate output-level meter. Studio Sound (August 1989, p.24) confirms LED metering of gain reduction and output on each channel. The later item (November 1990, p.22) adds that LED indicators cover the dynamics functions themselves. The manual describes the GR meter as indicating "in decibels by how much the signal is compressed at any instant."
Limiter Operation at High Ratio
Set the ratio above 20:1 and switch to auto attack/release, and the DN504 becomes a quad limiter - fast-acting, high-ratio compression that catches transients. The manual also describes cascading two channels, compression into limiting, for two-stage gain control: a mild 2:1 ratio above the first threshold, firm 50:1 limiting above the second.
Historical Context
The DN504 arrived in Klark Teknik's range in the late 1980s, reported by Studio Sound (August 1989, p.24) as a recent addition. The page title carries "c. 1989" as the only date-like figure, and no exact production start or end year appears in the available record. It sat in the Kidderminster line alongside the DN500 dual compressor/expander/limiter and the DN514 Quad Auto Gate. The manual itself points to the DN510 Advanced Dual Gate as the tool for more flexible ducking. No source explicitly states the DN504's position in the lineup relative to those models. A "DN504 Plus" variant exists, with its own operators manual, and a forum claim holds that the two differ only in front-panel colours and VCA branding. That's an owner's assertion, not a verified specification.
Collectibility & Value
The DN504 is not a collector's trophy the way Klark Teknik's graphic equalisers are, and the market reflects that. A current asking price of EUR 199.00 (approximately US $226.66) appears on an eBay listing for a four-channel unit. Dealer listings turn up in "fair-condition" states, tested to power on with no further functional testing performed. Owners who use them tend to be enthusiastic. One Reverb seller calls it "super easy to use and great sounding," and a Gearspace thread title reads "I absolutely like my KlarkTeknik DN504 for studio!!" There is no confirmed sold-price history, no documented common-failure pattern, and no maintenance or spare-parts information in the record. The one critical note in the sources, from a low-reliability page, calls the unit expensive and bulky. That contradicts the whole point of a 1U quad compressor, so treat it with caution.
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