The Relentless Preamplifier represents the ultimate preamplifier design, playing a critical role in the reproduction stage when the music signal is particularly susceptible to compromise. The use of three separate chassis—a dedicated audio chassis for each channel separated by a power supply/control section—addresses these challenges and brings other benefits as well.
Positioned between the two audio channels, the power supply has nearly as much power supply capability as many power amplifiers. Extensive electrical and magnetic shielding keeps radiated interference out of critical preamplifier circuits. Internal line conditioning circuitry filters RF noise on the AC power and compensates for asymmetric power waveforms and DC on the mains. Power comes from two 150 VA toroidal transformers—one dedicated to the analog circuitry and one for the digital and control circuitry. These transformers drive an 8-amp bridge rectifier and 26,400μF of filter capacitance. The two regulators for the analog stages are scaled-down versions of those used in the Relentless Series amplifiers.
The input stage in each mono preamplifier is the result of a design experiment for a circuit that hadn’t been used in any previous D’Agostino preamplifier. The audio circuit topology uses a discrete differential FET input stage featuring a voltage input signal capability of an extraordinary 30 volts. With an input impedance of over 1 MOhm, the preamplifier’s front end is immune to source-related electrical issues. Developed specifically for the Relentless Preamplifier, a unique circuit maintains a stable bias so that temperature-related anomalies are offset to preserve performance during extended listening.
Featuring only balanced XLR inputs, the Relentless Preamplifier topology rejects common-mode input noise over an unusually wide bandwidth. All signal gain is realized in the current domain using proprietary multiple-output current mirrors with nearly 30 times the linearity of other designs. The use of four-layer circuit boards dramatically reduces distortion and propagation levels as compared to more commonplace, two-layer boards. The analog and digital circuit elements are isolated, including ground planes, from each other. Where necessary, digital control signals are optically coupled to analog circuits. The signal switching relays are hermetically sealed and use gold-plated contacts for maximum signal integrity. Utilizing no negative feedback anywhere, core D’Agostino circuit concepts include a signal path that is fully complementary and balanced from input to output, achieving a frequency response flat to 120 kHz. The overall result is a musically dense and dynamically rich signal presented to the balanced outputs.
