Also see 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 layouts 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 first changes to the frequency compensation. The changes were done to improve the frequency response and slew rate of the amplifier.
Referring to the above schematics 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 amplfier brought to the US, a non-intruding (sound-wise) protection network was added to the amplifiers.
Of protection networks and power supplies
Several intermidiate steps are missing, but at the end the design looked like shown in the next schematics, which applies for amplifiers with serial numbers above 100. These were heavily EC modified. See the History article.
The real Electrocompaniet schematics serial no's approx 100 ->
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 EC designs, but in my own company I did two new modifications, one known only as the NRF Mod., the other known as "The Special Version".
The modifications were only further improvements, naturally following from the EC time.
The "Special Version" schematics are shown here:
SCHEMATIC 5 is coming later
Note the heavily reduced values of the output stage emitter resistors, they're down to 0.33 ohm from 1 ohm ! This reduces the AB nonlinearity as described in my 1982 AES paper.
Note also the changed input stage, with a JFET differential pair as a source follower. It was found that the bipolar input stage added distortion caused by its nonlienar input base current acting on the input resistors. (Which again shows : When we reduced the input resitors from 6k8 to 2k2 we heard a sound improvement and believed it was caused by improved frequency response behaviour above 20kHz. It may have been the distortion reduction by 10dB which we heard !) To reduce this distortion further the JFET pair was added. Further, a cascode pair was added to eliminate the Miller effect and the nonlinear voltage modulation of the critical first stage.
The gain distribution was also changed, the 3rd stage gain was increased by increasing the load resistors from 2k2 to 3k3. More gain was needed to increase the overall feedback to 40 dB. It was found the the increase on this point (3rd stage) was the less critical. However, we got sligthly more relative modulation effect on the 3rd stage than we wished.
All in all approximately 100 amplifiers were modified, and an additional 50 Special Versions were made.
By the end of the 80's and the beginning of the 90's I tried a few more changes to the amplifier design, which never made it beyond the lab bench. Only one amp exist with these modifications.
They included emitter followers added before the 3rd stage to reduce the modulation effect mentioned above.
Also see some of the later calculations made on this amplifier.
What above some thoughts on the "Perfect 25W" amplifier ?
Or the perfect preamplifier, which was realized, but.....
Any thing you are missing, or any comments are appreciated : tsandstr@online.no