Audio Research Ref 330M Mono Blocks

The Reference 330M is unique, combining new thinking, new design, and new parts with the best concepts from other Reference amplifiers. All gain is accomplished using vacuum tubes; transistors are only used for impedance buffering.

The new front end borrows its architecture from the 610T, using an ECC83 tube for voltage gain. The ECC83 output is buffered using special bipolar transistors that have tubelike qualities. Bipolar buffers allow for maximising the input stage headroom and preserve the best sonic qualities of the ECC83.

The second stage is the classic circuit used in the 610T and 250, with a 6H30 tube differential pair biased with a DMOS current source. The 6H30 plates are bootstrapped by the output stage.

To drive the output tubes, they use new Silicon Carbide (SiC) JFETs. These devices allow them to automatically bias each output tube to an ideal quiescent current, and they also maximize driver headroom and help give us better bottom end control.

The output drivers are also bootstrapped by the output, providing even more headroom during large signal swings.

Output tubes are the new KT170 type, which have increased plate voltage margin, peak cathode current, and plate dissipation. They sound wonderful, with great musicality. In a properly designed circuit, the KT170 provides an elevated level of performance that is unequaled.

The power supply features solid state regulation for the front end and tube regulation (6H30 and 6550) for the output tube screens and driver stage. It also has a Pi filter using a large choke, reducing output stage supply ripple.

The 330M is a fully balanced amplifier, from input to output. Operation is Pentode only because this offers the highest performance. The screen and plate voltages are chosen for best linearity.

The rear panel includes balanced XLR and single-ended RCA inputs, 8Ω and 4Ω output taps, 12V Triggers, RS-232 control, tube hour meter, 20A IEC, and fan speed adjustment.