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
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
- High performance design – very low THD+N
- Five on-board RCA inputs
- Passive-switched inputs maintain signal integrity
- Switched headphone amplifier output
Frequency response flat from 10Hz to 20kHz, -1dB @ 82kHz (see
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.