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The design features of Ampliwire 250 DMB
After the Electrocompaniet 25 watt amplifier had established a new standard for transistor
amplifiers, research was undertaken to find ways to make the amplifier even better, and to
extend its highly musical sound quality to more powerful amplifier designs. The engineers
at Electrocompaniet were not satisfied by only reducing the commonly recognised types of
distortion to low levels. They recognised that distortion appears in many forms, and that
distortion was still audible in listening tests even when conventional categories of distortion
were at astonishingly low levels.
Traditionally, designers increased feedback to make a larger portion of the output signal
control the amplifiers response. Our listening tests showed us that simply applying more
feedback was not the answer. In fact, as one kind of distortion went down, other parame-
ters would be adversely affected, leading to an overall degradation of sound quality. We
knew that the other conventional design approach of eliminating feedback completely was
not the answer either, because this would cause high distortion levels, and as a result
would produce a “woolly” sound.
The answer to the dilemma was found in a novel approach to feedback theory. We
developed a feedback concept that allowed local feedback to be applied around individual
stages of the amplifier circuit. This approach allowed us to avoid the sonic disadvantages of
overall feedback from output to input. The concept was further developed to reduce phase-
and interphase distortion between stages of the amplifier as well. We were able to concent-
rate the loop feedback on the stages of the amplifier where it resulted in audible
improvement.
Stability margins were also expanded because feedback no longer affected the frequency
response. The use of this concept of individual gain blocks - complex in design but simple
in function - allowed us to reduce distortion to minute values in all the products.
The amplifier is divided into two separate sections or gain blocks. The input block is a
transconductance amplifier without overall feedback. This avoids large output current being
fed back to the input, and mixed with the minute input signal. The output block is a
transresistance amplifier with parallel feedback. This is done to prevent higher frequencies
than the feedback loop can handle, from entering the loop. An approach like this will
prevent Transient Inter modulation Distortion (TIM) and Slewing Induced Distortion (SID),
eliminating the need for an extremely wide bandwidth.
All stages work in Class A with an efficiency of less than 0,1%. The power supply of the AW
250 DMB consists of two 650 VA toroidal transformer. Furthermore, the power supply
consists of a 120.000 micro farad reservoir divided into twelve 10.000 micro farad
capacitors in parallel with 4,7 and 0,1 micro farad polycarbonate- and polypropylene
capacitors.