Chapter 30

Theory

The theory of the Otala amplifiers, and thus the basis for the Electrocompaniet amplifiers, has been detailed in many papers, mostly by Matti Otala, but also by several others. At Electrocompaniet these theories were extended, and a theoretical framework for our amplifiers was established. The following pages give you an insight into both the original theories by Dr. Matti Otala, and also the theories built up in the first years of Electrocompaniet. Some of the thoughts are solely mine, however — don’t assume that Electrocompaniet of today will vouch for all I say here :-)

This is the original outline for the Theory section. Most items below were only ever sketched as topic headings on the old site and never actually written up; they’re kept here as a record of the plan, marked (not yet written).

On TIM, DIM, nonlinearity and distortion

TIM — this was the starting point of Otala’s theories, and his own idea from the beginning. In the early ’70s many people had already noticed that the sound of the new transistor amplifiers was inferior to the sound of tube amplifiers. How could that be, when the transistor amps had less distortion than the tubes?
DIM — what it is, and how it relates to TIM. This was when everybody tried to find measurement methods. (not yet written)
The debates and the quarrels — but Otala was right! (not yet written)
“All distortions are equal, but some are more equal than others.” That was the feeling at Electrocompaniet at that time, and the beginning of an understanding of how these distortion mechanisms interacted, and how they affected the sound — and that the basis of all distortion is nonlinearity. The nonlinearity is the source, the distortions the symptoms. (not yet written)

A theoretical framework for building good-sounding amplifiers

A theory of single stages — a single stage is the basis for all amplification, and at Electrocompaniet a model for the single stage, and “our way”, evolved.
A theory of multiple stages — multiple stages put together do not behave as N times a single stage; they interact, and in many cases even counteract. (not yet written)
A theory of feedback — many people are either pro-feedback or fully against feedback. At Electrocompaniet we learned to live with feedback in all forms, local and loop, and learned how to get it as a friend and not an enemy. (not yet written)
Output stages — that current, what does it do? (not yet written)

How do we calculate the nonlinearity

Calculations of input-stage nonlinearity — this is simple, once you know how; a table summarises everything.
Calculations of the nonlinearity of multiple stages. This is more complicated, but by following a set of rules, and abiding by the general model we use, it is not that difficult. (not yet written)
Calculations of distortions — one thing is the nonlinearity, another is how much distortion of the different types your amp will measure. (not yet written)
Designing for lowest distortion. Note that there very seldom exists one optimum point — there are just too many variables, so you have to use some creativity and a feel for what’s going on. (not yet written)

Frequency response, rise times, slew rate

What’s the difference?

Open and closed loop frequency response
How the frequency plane and time plane interconnect (not yet written)
Time-response behaviour (not yet written)
Poles and zeroes (not yet written)
Compensation of an amplifier (not yet written)

Other factors

The damping factor — or output impedance, what is it? (not yet written)
Why there are no simple solutions (not yet written)
Power-supply interaction (not yet written)
RFI — what can it do (not yet written)
The importance of the components (not yet written)