These days, audio power amplifiers that produce low noise and distortion and cost only a few hundred dollars are relatively easy to find. In fact, they’re built into many of the latest multi-channel home theatre systems. Much of this gear is based around hybrid amplifier modules, which typically produce distortion levels in the 0.02% realm.

Those serious about their audio will demand a much higher level of performance than can be found in these mass-produced units, which explains why the discrete power amplifier projects described in SILICON CHIP are so popular.

For example, the 15W Class-A Stereo Amplifier described in July and August 1998 still gets a high ranking, as does the 100W "Ultra-LD" class-AB design described more recently. These amplifiers are expensive to build but offer performance that typically costs many times more in comparable commercial units.

Having built one of our high-performance amplifiers, many readers have also asked us for a matching preamplifier design. And so our design brief was simple: a minimalist approach, focused on achieving ultra-low noise and distortion, but with enough gain (with the "wick" wound right up) to overdrive any of our audio power amplifiers, including the big 350W and 500W units. So, what were our options?

Features & Performance Main Features High performance design – very low THD+N Five on-board RCA inputs Passive-switched inputs maintain signal integrity Switched headphone amplifier output Measured Performance Frequency response flat from 10Hz to 20kHz, -1dB @ 82kHz (see Fig.5) Maximum input signal 2.9V RMS (9.5V RMS output) Input impedance ~90kΩ Output impedance 100Ω Harmonic distortion typically <.0005% (see Fig.7) Signal-to-noise ratio -102dB unweighted Channel crosstalk -96dB @ 1kHz, -73dB @ 10kHz Source crosstalk -110dB @ 1kHz, -93dB @ 10kHz Note: all measurements were performed at the maximum volume setting with the output driving a 50kΩ load. Input signal amplitude was 600μV RMS (2V RMS output). For crosstalk measurements, non-driven inputs were back-terminated into 600Ω.