Усилители мощности стерео — Circle Labs Audio M500

Circle Labs Audio builds the M500 as a hybrid stereo power amplifier in Cracow, Poland, combining a valve input stage with bipolar transistor output stages, and it can be bridged into a fully balanced mono amplifier. It is the larger of the brand's two power amplifiers, above the M200, and its companions in the catalogue are the P300 preamplifier, the V1000 phono stage and the A200 and AS100 integrated amplifiers. Fuses are rated T10A for 230 V and T20A for 120 V mains, and the shipping weight is 36 kg.

Key specs that matter

In stereo the specification table gives 140 W into 8 ohm and 340 W into 4 ohm per channel. Bridged to mono, the M500 delivers 600 W into 8 ohm and 1200 W into 4 ohm. Frequency response is 2 Hz to 800 kHz at -3 dB. Input sensitivity is 0.85 V for full power, and gain is 35 dB, with a three-step selector that reduces it to 32 or 27 dB. Input impedance is 47 kOhm. The amplifier measures 430 x 230 x 360 mm and weighs 31 kg. Consumption is below 0.1 W in standby, 100 W at idle and 1400 W maximum.

Engineering and use context

At the input sits an ECC88 or 6922 valve in an SRPP circuit, a single-valve stage with only one resistor and a Mundorf Silver Gold Oil coupling capacitor, together with ultra-low-noise Burr-Brown JFET input stages. The output stages, which Circle Labs calls Circle Power, use high-current Sanken bipolar transistors in a topology the company describes as close to single-ended. The power supply is dual mono, with separate supplies for each channel from toroidal transformers totalling 1200 W, a DC blocker against mains DC and a total capacitance of 880,000 µF, and the valve section has its own shielded EI transformer. The very large capacitance acts as an energy store for peaks, which supports the rise in power into lower impedances. The gain selector lets the amplifier match both sensitive and less sensitive loudspeakers and preamplifiers with different output levels, reducing noise when full gain is not needed. Bridging two channels into one gives a fully balanced mono amplifier for systems that need more power.

Pairing

The Circle Labs P300 preamplifier is the natural partner. In stereo the M500 drives most loudspeakers, and a pair of bridged units provides the full bridged power for large, low-impedance designs.

Alternatives

The Circle Labs M200 is the smaller power amplifier with 160 W into 8 ohm in stereo and a weight of 20 kg.