Electrocompaniet REC4.5 - Manual (Page 3)

The design features of Preampliwire EC4½
After the Electrocompaniet 25 watt amplifier had established a new standard for transis-
tor 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 indivi-
dual 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 frequ-
ency 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 various amplifier stages are divided into two separate sections or gain blocks. The
input block is a transconductance amplifier without overall feedback. This avoids large
output voltage 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
EC-4 1/5 consists of a 50 VA toroidal transformers , and 10.000 micro farad reservoir
divided into ten 1000 micro farad capacitors, with separate regulator for each stage
and channel. EC 4 1/2 also features a separate powersupply for the digital circuitry( the
remote section), to avoid intereference between the analoge and digital circuitry.