Предусилители — Lamm Industries L2.1 Reference

The L2.1 Reference is the hybrid line stage at the top of Lamm's solid-state range, made in Brooklyn. It replaced the L2 Reference, which ran for 15 years, becoming available in October 2016. It is a two-chassis design, with a solid-state control and audio unit and a separate valve power supply.

Key specs that matter

There are three single-ended line inputs and one tape or home theatre processor loop. One XLR and one RCA output pair feed the power amplifiers. Input sensitivity is 125 mV. On its predecessor, the L2, maximum voltage gain was 14.87 dB, input impedance 41 kohm and output impedance 130 ohms, with response of 4 Hz to 140 kHz at -3 dB. The L2.1 adds an output phase switch, protection circuitry and remote on and off for Lamm power amplifiers.

Engineering and use context

The audio circuit uses selected high-voltage super-linear MOSFETs and runs in pure Class A from input to output, with no overall feedback at any stage. All stages, including the high-current output buffers, are single-ended. The high current capability lets the preamplifier drive long cables and difficult loads. Volume is set by a TKD 41-step potentiometer. The separate power supply uses a choke-input filter, a full-wave valve rectifier and a high-speed valve voltage regulator operating at +350 V, which Lamm uses to suppress hum and buzz. The L2.1 changes over the L2 include greater immunity to radio frequency interference arriving through signal lines and mains, modified protection and time-delay circuits, improved signal-to-noise ratio, circuit boards with gold-plated traces and through-holes, and new polypropylene capacitors in place of electrolytics in the valve regulator. A protection circuit mutes the outputs until the preamplifier stabilises after switch-on, allows muting during use and mutes automatically if mains voltage drops. Parts include military-grade Dale/Vishay metal film resistors, RCD wire-wound resistors and Bourns multi-turn potentiometers. Each unit is built by hand in Brooklyn. The control unit and power supply sit in separate chassis and need two shelves.

Pairing

The high-current output drives Lamm's M1.2 Reference hybrid monoblocks or valve amplifiers directly, and the remote switching integrates with them. The processor loop lets a surround system share the main speakers.

Alternatives

Lamm Industries' LL1.1 Signature is the four-chassis all-valve dual-mono line preamplifier of the Signature series.