Instruction manual

2
SA400 Semi-Digital POWER AMPLIFIER
Introduction
In 1999, we designed and built the first prototype of this amplifier using the original
Tripath evaluation board. The intention then was to compare other emerging digital
technologies to decide which was best. To ensure that the comparison was as valid as
we could make it, we designed and used a much larger power supply than required
and made the amplifier have sufficient gain to not need a preamp. The sound quality
of that original prototype was so good that we built a further five for selected users to
evaluate for an extended period in their own systems. After that successful evaluation,
the amplifier was put into production.
The SA400 is the result of everything learnt in the subsequent years and uses a new
board we designed to take the latest Tripath 105A chips and to incorporate extra
facilities to lift the earth (to prevent earth loops) and be able to vary the amplifier gain
to match other system sensitivities. The original massive power supply has worked so
well in providing effortless sound reproduction that it has been retained without
change. It uses twin transformers, which supply separate full wave positive and
negative supply lines. The Zero volt reference is then made at D.C., not at A.C. via
the usual centre tap on the transformer, together with a Bi-Phase Rectifier. This
approach improves the stability of the Zero volt line by ensuring that each supply line
is separate from the other during high load current demands. Other advantages include
a reduction in the Power Supply Pumping Effect (common to All Switched Output
stages, which use an Inductor for Digital to Analogue Reconstruction), and also helps
to remove D.C. from the Transformer, which can be responsible for increased noise
(hum) from the transformer especially at switch on.
We have also opted for a Linear Power Supply rather than an r.f. or Switched Power
Supply because no matter how good the Switched Power Supply, extra noise
(extending up to 100MHz) will be present, and hence the possibility of emission,
(both Radiated and Conducted) into the Amplifier Itself, which in turn would produce
more noise at the Output. Our objective was to produce an Amplifier with as wide a
Dynamic range as possible.
A major benefit of the Tripath digital technology is the high efficiency in performing
the amplification, around and above 90%. As a result, the power supply has even
more headroom than if used in a conventional class A or AB design, and very little
heat is generated. The amplifier barely gets warm at any power level, and ensures that
components will not be subject to the heat that causes deterioration and earlier failure.
The amplifier operates at its optimum all the time, no need to wait for it to come up to
operating temperature.
The amplifier has been designed to drive loudspeakers available in the marketplace
today, with impedance values that range from 3 to 9 ohms. The massive power supply
ensures sufficient current is available at the lower impedance values. The amplifier
normally operates as a stereo amplifier, but by flicking a switch on the back panel, it
is converted into “bridge mode” where it then operates as a monoblock amplifier