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Buzz Audio DBC-20

Diode-bridge Compressor
Published January 2019
By Hugh Robjohns

Buzz Audio DBC-20

Buzz update a characterful concept to create a compressor that's capable of both vintage and modern sounds.

Of all the different forms of dynamic processor on the market today, the diode-bridge compressor is generally considered to be one of the most characterful, While these became popular in the late 1960s and early 70s with the rise of solid-state electronics (the vintage Neve 2254 model is probably the most famous example, a design that lives on in the current Neve 33609), the very first diode-bridge compressor is claimed to be the Telefunken U13, which was introduced in the 1940s and used a valve rectifier. One of the reasons that the diode-bridge compressor became so popular was its very fast response — much faster than optical compressors, for example — and that made it very attractive in broadcasting applications, in particular, but its intrinsic sound character is also usefully different from its peers.

Now, New Zealand-based manufacturers Buzz Audio have produced their own take on the theme, in the form of the DBC-20 dual‑channel diode-bridge compressor.

The Bridge

At the heart of every compressor is some active device that provides variable attenuation, the amount being determined by a control signal of some sort. Fundamentally, a diode allows current to flow in one direction only, but there is a small region of its 'transfer curve' where its conductance effectively varies in proportion to the voltage applied across it, and that allows its use as a variable voltage-controlled attenuator in a diode-bridge compressor. Unfortunately, this operating mode isn't very linear and results in a fair amount of distortion, but by using pairs of closely matched diodes in a 'balanced bridge' configuration, a lot of that distortion can be cancelled out.

In the case of the DBC-20, Schottky barrier signal diodes are employed, as these were deemed to not only deliver the right sound character — more 'punchy' than from standard silicon types — but their tight manufacturing tolerances mean it isn't actually necessary to match them in pairs, and this helps to save costs and alignment time during production.

That 'bridge configuration' I mentioned typically places the diodes in a diamond array, with an audio signal applied across two opposite corners, and the control signal across the other two. However, this arrangement requires the audio signal to be in a balanced format (so that the control signal doesn't create a hefty DC offset at the output), and so diode-bridge compressors typically have fully balanced signal paths throughout, often with transformers to couple the signal in and out of the bridge. Typically, the remaining distortion from this arrangement is predominantly the odd harmonics, and that creates an harmonically rich sound character which tends to make the processed sounds appear to move towards the front of the mix — often...

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Published January 2019