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SA400 Semi-Digital Power Amplifier

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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 DC, not at AC 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 DC 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 RF 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 anamplifier 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 that have impedance values ranging 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 (two channel) amplifier, but by flicking a switch on the back panel, it is converted into 'bridge mode' where it then operates as a monoblock (single channel) amplifier capable of delivering over 1000 watts into 8 ohms.

Throughout the design, the component quality has been chosen to suit the job being performed (there has been no attempt to over specify components for marketing reasons), and ensure that the amplifier can be built at the lowest cost commensurate with the extremely high performance for which it was designed, and operate reliably over a long period.



Specification

 Power Output (Continuous Average, 1kHz, Both Channels Driven) :-

Stereo8 Ohms >300Watts
4 Ohms >540Watts
Mono Bridged8 Ohms >1100Watts
4 Ohms >1350Watts
Distortion (1kHz, 8 Ohms, -1dB below full Power) :-<0.2%
Input Sensitivity (100Watts, 8 Ohms, 1kHz) :-High 0.5 Volts
Med. 1.0 Volts
Low 1.4 Volts
Separation (2Hz - 20kHz) :- >60dB
Frequency Response (-3dB) :-<1Hz - >100kHz
Input Impedance :-Phono 20k Ohms
XLR (Per Leg) 10k Ohms
Damping Factor :-8 Ohms >100
4 Ohms >50
S+N/N Ratio (IEC "A") :->100dB
Absolute Phase :-Phono Zero Deg.
XLR Zero Deg., Pin 2 Hot
XLR CMRR (100Hz - 10kHz) :->60dB
Dimensions (Including Terminals) :-D 330, H 95, W 445 mm
Weight :-Net 16kg
Packed 18kg


E. & O.E. The policy of Alner Hamblin Electronics is that of continuous design and development.
We reserve the right to change specifications without prior notice.


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