Усилители мощности стерео — Margules U-280 SC 30th Anniversary

The U-280 SC 30th Anniversary is a limited edition valve stereo power amplifier from Margules of Mexico City, marking three decades of the U-280 line. The Margules family has worked in audio since the early twentieth century, when the European engineer Jacob Margules started it, and the present business was set up in the early 1980s with third and fourth generation members still involved. The amplifier is handcrafted in Mexico, the run is limited to 30 signed units with newly designed circuits, and the price at launch in 2023 was USD 12,000.

Key specs that matter

In stereo the amplifier gives 25 W per channel in triode mode and 50 W per channel in ultralinear mode. Bridged as a monoblock it delivers 55 W in triode and 100 W in ultralinear. The output stage uses four KT88 valves as supplied and accepts 6550, KT90, KT120 and KT150 types, while the driver section uses four 12AU7 valves that should not be substituted. Frequency response is 20 Hz to 40 kHz at minus 3 dB and THD is below 0.08%. The chassis is 45 cm wide, 22 cm high and 35 cm deep and weighs 30.5 kg. It runs on 120 V mains and draws up to 585 W.

Engineering and use context

The output stage operates in pure class A in both modes. Margules' Active Servo Bias, listed as patent pending, adjusts bias continuously from the signal instead of from the characteristics of each valve, so the owner never sets bias by hand and can change between the listed power valve types without adjustment. The circuit uses no global negative feedback. In its place a Back Electromotive Force Sensor system reacts to the voltage returned by the speaker and adjusts the output impedance and damping to suit the load. Each channel has its own shunt-regulated power supply with no control loop circuit. Triode or ultralinear operation is chosen by a switch for each channel on the top plate and can be changed during playback, and a separate selector sets stereo or mono use, where both channel settings must match. An internal timer holds the amplifier in standby for about 3 minutes after power-on, and LEDs by the power valves show when it is ready. Warranty is five years on parts and labour and 45 days on valves.

Pairing

Gain potentiometers on each channel let a single source with gold-plated RCA outputs feed the amplifier directly without a preamplifier. Five-way binding posts with switchable 4 or 8 ohm taps cover most passive speakers, and in triode mode sensitive designs make the most of the lower output.

Alternatives

The standard Margules U-280SC shares the valve complement, switchable modes and stereo or monoblock operation.