Nagra Amp II-S

The AMP II-S is a stereo power amplifier in Nagra's Series II. It joins the PREAMP II-S, which Nagra describes as its natural partner. It succeeds the Classic AMP, introduced in 2015. 

Nagra's engineers revisited the power supply, input stage, PCB layout, thermal management and mechanical construction, drawing on the development of the HD and Reference electronics.

Power supply

A new linear power supply is built around a custom 800 VA potted transformer for greater energy reserves and voltage headroom. The transformer sits on a suspension that absorbs 99% of vibrations between 50 and 100 Hz to prevent hum. Transformers are optimized for different mains voltages.

Circuit

The AMP II-S is a Class AB design with 10 W of Class A. Each channel uses a single pair of MOSFET transistors. Rail voltage has been increased for greater headroom and improved transient capability. A new input board, derived from the Reference AMP, improves left/right channel separation. The input stage is hand-wired to the power amplifier, the internal cabling is new, the PCB layout has been optimized and the circuitry rerouted. Controls and front panel are optically isolated for lower noise.

Construction and connections

A CNC-machined heatsink minimizes unwanted vibration and provides thermal inertia for stable operation. The front panel carries a Modulometer and a rotary selector. The rear panel has RCA and XLR inputs with a type switch, a 1 V / 2 V sensitivity switch per channel, remote in and out, and one pair of speaker terminals per channel. Nagra spikes and new spike cups are included.

Specifications

• Class: AB
• Power: 120 W rms into 8 Ω, per channel
• Bandwidth: <10 Hz to 75 kHz, +0 / -3 dB
• Crosstalk: >90 dB
• Signal-to-noise ratio: typically 110 dB, measured in ASA A
• THD+N: <0.05 % @ 120 W
• Input impedance: >100 kΩ
• Output impedance: 60 mΩ
• Dimensions (L x W x H): 461 x 280 x 174 mm overall (18.15 x 11.02 x 6.85 in)
• Net weight: 24.5 kg / 54 lb