Electrocompaniet AW60FFT - Manual (Page 3)

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 parameters 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 concentrate 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 60 FTT
consists of a 575 VA toroidal transformer. Furthermore, the power supply consists of a 40.000
microfarad reservoir divided into four 10.000 micro farad capacitors in parallel with 4,7 and 0,1 micro
farad polycarbonate- and polypropylene capacitors.