Hazelrigg Industries D Series

Is the Hazelrigg brothers’ first foray into the 500‑series format as impressive as their rack gear?

If you read our SOS 2022 reviews of Hazelrigg’s rackmount VNE compressor and VLC mic pre and EQ, you’ll know about the company’s close ties with DW Fearn: essentially, Hazelrigg manufacture and distribute Fearn’s products. But owners George and Geoff Hazelrigg design and build their own products too, and all combine high‑quality sound with simple, intuitive control. Their new D Series comprises a trio of 500‑series modules, based on the circuits in Hazelrigg’s and Fearn’s revered mic preamps, inductor‑based equalisers and PWM compressors.

The DVA Preamp

The DVA microphone preamplifier, a valve‑based design with the plate running on 150V.

The DVA microphone preamplifier’s input circuitry matches that of Hazelrigg’s VLC and DW Fearn’s VT‑2 preamps, and as with those models, both the input and output are transformer balanced. The input stage utilises a 6072A valve, a low‑noise twin‑triode type designed primarily for use in the low‑level stages of audio‑frequency amplification. The A suffix indicates a special‑quality valve, intended for use in critical industrial and military applications, and it differs from a standard 6072 (12AY7) in having controls on interface resistance (the build‑up of an oxide layer between heater and cathode) and a higher resistance to inter‑electrode leakage.

Unlike on the VLC and VT‑2, the output stage isn’t valve‑based, but instead employs a solid‑state signal path with low harmonic distortion and a high output. This hybrid valve/solid‑state configuration was necessary partly because of the 500 series’ physical size constraints, but also the format’s official power supply specifications. In the DVA, the 6072A valve’s plate voltage is set at 150V... which happens to be the value specified by the valve’s original manufacturer, General Electric, for the valve’s use in low‑level audio‑frequency amplification stages.

The DVA’s sparsely populated silver‑coloured front panel hosts a black gain control, similar in size to the mic/instrument combo XLR/jack socket below. (Inserting a jack switches the host chassis’ rear input out of circuit.) Between these, a column of four toggle switches activate 48V phantom power, a ‑20dB pad, polarity reverse and a gently sloping high‑pass filter, with a corner frequency of 150Hz to counter the proximity effect. Single LED indicators display the output level (green/red) and the presence or otherwise of 48V phantom power.

In terms of measured performance, the DVA delivers a frequency response of 20Hz‑20kHz (±2dB); maximum gain of 60dB; SNR of 79.5dB; EIN of ‑129.5dBm; THD+N of 0.04%; maximum input level of ‑20dBm (0.5% THD) / 0dBm with ‑20dB pad; and a maximum output level of +23dBm.

The DLC Inductor EQ

The DLC inductor EQ, which once again includes a valve‑based input stage.

The DLC is based on the passive inductor EQ circuit of Hazelrigg’s VLC, itself a design first seen in DW Fearn’s VT‑5. Although this new circuit uses the same capacitors and hand‑wound inductors as the VLC, the audio path isn’t identical; the differences give the DLC a slightly more linear performance. The audio input circuitry is the same as that of the DVA, again utilising a 6072A valve. The output gain stage also features a low‑THD/high‑output solid‑state signal path, and compensates for the gain loss that’s typical of passive filter stages. As in the VLC, the input and output are transformer balanced.

Being a two‑band EQ with separate boost and cut controls for...