Evolution of the Schematics

Also see the later schematics

The first design was taken directly from the paper by Otala and Lohstroh, AES 1973. To it was added a power supply only. The PCB layout and the mechanical design were done at EC.

Original Otala — first-series schematics

Only 10 amplifiers were made using these schematics. None exist today.

The next series was done after early 1976, and incorporated the first changes to the frequency compensation. The changes were made to improve the frequency response and slew rate of the amplifier.

Referring to the schematics above, the fixes were done on the first- and second-stage lead network (the RC’s between the emitters of the differential pairs); the lag networks on the collector sides were removed, and the input lag network was redimensioned.

After a breakdown of the first amplifier brought to the US, a non-intrusive (sound-wise) protection network was added to the amplifiers.

Several intermediate steps are missing, but at the end the design looked like what’s shown in the next schematics, which applies for amplifiers with serial numbers above 100 — these were heavily EC-modified. See the History section.

The real Electrocompaniet schematics, serial numbers approx. 100 and up

Note the reduced values of all input-stage resistors, indicating the increased quiescent current in these stages.

From serial no. 275, the Great Change was applied to the amplifiers.

This was the last of the true EC designs. After leaving Electrocompaniet I did further modifications of my own — the NRF Mod. and “The Special Version” — in the years that followed; see NRF Modifications for those (they follow on from the EC design, but aren’t EC work themselves).

See Later Designs for what came after.

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