The Highland Dynamics Delta 4-7 MK.2 is a dual-channel, transformer-balanced, hand-wired microphone preamp and D.I. unit inspired by a revered 1960s British circuit, built in a limited run of 20 units using premium components and delivering a distinctly three-dimensional, valve-driven sonic character.

Design and Build Quality

Highland Dynamics constructs each Delta 4-7 MK.2 unit by hand using point-to-point wiring, solid-core copper wire, and oversized film capacitors for robust amplifier power delivery. The signal path features paper and oil capacitors from West-Cap, Vitamin Q, and NYT, along with 2-watt carbon composition resistors. Precision 1% resistors maintain stability in non-audio circuits. Grayhill rotary switches govern both gain and bass cut functions, ensuring durability and repeatability. The preamp is transformer-balanced at both input and output, employing an EF86 tube for gain and a 6DJ8 for output drive, enhancing clarity and tonal warmth.

Input and Output Flexibility

Users can select between 150 and 37.5 ohm input impedances via transformer taps, offering tonal variation and ideal matching for various microphone types. The Delta 4-7 MK.2 includes a variable input pad, positioned after the input transformer and D.I., which allows seamless adjustment from no attenuation to line-level input—particularly beneficial for fine-tuning mic or instrument signals. The output pad, situated post-output transformer, enables users to reduce hot output levels without altering the character of the signal.

Gain Control and Tonal Shaping

The unit features four gain stages—Low, Medium, High, and Open—with gain controlled by varying the amount of negative feedback applied to the EF86 tube. The Low setting delivers the most hi-fi, controlled sound, while the Open mode applies no feedback for maximum gain and pronounced harmonic colouration. The variable input and output pads allow users to maintain preferred tonal settings while managing levels. Additionally, the bass cut filter offers three high-pass options—roughly 60 Hz, 150 Hz, and 300 Hz—placed before the gain stages to refine low-end response early in the signal path.

Advanced Routing and Overdrive Potential

Each channel of the Delta 4-7 MK.2 can be cascaded into the other via a routing switch, enabling console-like tube saturation. Setting the second channel to line level and gradually disabling the pad introduces dynamic overdrive, suitable for creative tone shaping and saturation. The D.I. input caters to direct instrument recording, while the THRU output allows users to integrate effects pedals between channels—ideal for achieving stereo clean and stereo effected signals simultaneously.