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The Musical Fidelity kW Hybrid preamplifier offered a sensible maximum gain of 12.5dB, this sourced from a very low impedance of less than 1 ohm across the audioband (including the impedance of 6' of interconnect). The preamp preserved absolute polarity. The input impedance was lower than specification at a moderately high 41.5k ohms at low and middle frequencies, dropping slightly to 36.5k ohms at 20kHz.

The Volume control demonstrated excellent channel matching at the top of its range, but at 12:00, where it would more likely be used, there was…

Turning to the Musical Fidelity kW750, I preconditioned the amplifier by running it at 250W into 8 ohms for about 35 minutes, by which time my dummy load was too hot for safety. (It was also when the amplifier's illuminated feet changed from orange to blue.) The kW750's chassis and heatsinks were very hot, implying that they are only just large enough to pass the FTC heat test. The THD+noise at this power level was 0.02% with the amplifier cold, dropping to 0.013% after five minutes, 0.01% after 15 minutes, and returning to 0.017% after 25 minutes. The THD…

Pass's Supersymmetry Balanced Single-Ended class-A…

Description: Solid-state monoblock power amplifier. Maximum output power (1% THD, 1kHz): 1000W into 8 ohms. Maximum output voltage: ±45V. Maximum output current: ±30A. Frequency response: -0dB at DC, -3dB at 60kHz. Input impedance: 20k ohms balanced. Damping factor: 60 ref. 8 ohms nominal. Voltage gain: 30dB. Slew rate: >40V/µs. Output noise: 500µV unweighted, 20Hz-20kHz. Random noise floor: approximately 2µV. Dynamic range: 155dB (random noise floor to peak output). Balanced Common-Mode Rejection Ratio: 85dB 1kHz. DC offset: <100mV. Power consumption:…

No negative feedback circuits might be more accurately described, in most cases, as local feedback only designs. Some might argue that local feedback is still feedback, but according to Nelson Pass, "there is always some amount of feedback locally around any gain device by the nature of the device." He thinks that "no feedback" describes a circuit in which negative feedback doesn't extend further than a single stage or gain device.

Conventional negative feedback, local or otherwise, is used to make the circuit's output look more like its input.…

I drove the X1000s with a variety of front-ends. This included running direct from the analog outputs of the dCS 972 Sample Rate Converter and Elgar D/A processor at 24/192, and from the balanced outputs of the fully differential BAT VK-5i/VK-P10 preamp/phono combo. (The X1000 manual is quite explicit that the output impedance of the associated preamp is of little consequence, and that balanced is better.) I also used the new Classé Omega line-level preamp, from both its balanced and single-ended outputs. I got fabulous sound with the single-ended Nagra PL…

Because of the Pass X1000's power output and the restrictions of our test load, I couldn't perform our usual 1/3-power preconditioning test. Instead, the amplifier was warmed up for one hour at 50W into 8 ohms before any measurements were taken.

The X1000's input impedance measured 23.7k ohms. The output impedance measured between 0.18 and 0.20 ohms (the higher values were at 20kHz)—slightly higher than normally experienced with solid-state amplifiers, but not enough to be a serious concern. The voltage gain was 30.14dB, and pin 2 of the (balanced) input…

Since the Andra's debut in Stereophile there have been…

After more than a year…