The School Amplifier

Øistein Klevhus, now at FHI (the Norwegian Institute of Public Health), and myself did a project together at the end of our engineering school. I am greatly in debt to Øistein, because he was the one who made it possible for me to go through with that school — I was far too busy with EC, and had far too little time for school. Øistein came over to me at EC with schoolwork tasks and other stuff I had to do, and in that way it was possible for me to get through the school without nearly being present. I am forever grateful for that!

The amplifier: it was a very special design, fully complementary, and used matched single field-effect transistors at the input, along with some very interesting output transistors that had a very low turnover point.

More info on the matching of the input transistors will follow, but for now: they required manual matching, and it wasn’t easy to find a pair that worked. Note also the very low emitter resistors on the output stage — this matches a later paper I did for the AES; see the AB distortion paper.

The Ultimate Cooling Fins

These cooling fins may be the best-engineered fins ever made for an audio amplifier. They are the result of a strong interest in thermodynamics I had at the time. The fins are much thicker at the base than ordinary fins, and are gradually made thinner outwards. This was done to improve heat transfer through the fins, while at the same time ensuring a well-defined air flow and an even surface temperature — thus the lowest overall heat-transfer resistance.

The fins were very carefully designed, and every aspect was calculated to give optimal performance. The compromise in size and weight was found acceptable for the performance gained.

(The overview photo of the finished fin, CoolingFin1.jpg, was referenced on the old site but the image file itself was never found in the archive — only the close-up below survived.)

If you take a closer look at a fin:

Note the zig-zag pattern — this was done to increase the overall surface area of the fin by 40%. Not bad, huh?

We made a few dozen of these fins, and they performed as intended. However, none ever made it into a commercial amplifier.

AES Paper: AB Distortion in Output Stages

My Audio Engineering Society journal article on class AB distortion in power amplifier output stages, 1983 — referenced throughout the Schematics and Theory sections.

“Distortion in Class AB Power Amplifiers”, Terje Sandstrøm (Institute of Physics, University of Oslo). Presented first as an AES convention paper, then published in the Journal:

• AES Convention 71, Paper Number 1870, presented 6 March 1982 — AES E-Library entry
• Journal of the Audio Engineering Society, Vol. 31, Issue 11, pp. 858–861, November 1983 — AES Journal Forum entry

This was one of two papers I presented at AES conventions in the early ’80s; this is the one that made it into the Journal. I haven’t been able to identify the other one — if you know its title, let me know via the about page.

The May '78 Preamplifier

Details on this preamplifier will be added over time. First, the line stage — the specifications achieved are shown below:

The design was made very similar to the “Otala” designs. It was a single-power-supply design — note the input and output capacitors. It was designed to replace our old, completely single-ended preamplifier, which was never a big hit.

At Electrocompaniet this preamp was named “Model II”. It was designed in the period January to July 1978 — I don’t remember why it ended up being called the May ‘78 preamp.

Later Designs

Preamplifier of 1982

This preamplifier was designed in 1982, and a prototype made in 1983. Only one unit exists, and it sounds wonderful — by far the best-sounding preamplifier I have ever made. Still today I’m rather pleased with the design. It is divided into several gain blocks. In the accompanying schematics section I’ve shown the block schematics and one of the gain stages, which is used in two places. More details on this design will follow later.

Power amplifier of 1982

This design was also made as one prototype, but it never made it into a fully working version, although it worked partly. The design was rather good anyway, and had a lot of interesting features. The cooling fins, for example, used new heat-transfer knowledge to its best — I have never seen anything similar, either before or after.

Other Designs

There were several EC designs made between ‘75 and ‘80, in that first period. Several of these never made it into production. On the pages in this section, details of these designs are given, as far as we have been able to recover them: some schematic details, some of the story behind them and the reasons for their development, and some technical goodies — and possibly a hint or two.

Among these: the “Regulated 25W” amplifier, which had regulated power supplies for both the pre- and power stages — it did away with all the ripple, and provided high-speed power to the amplifier. It sounded good! But the heat …

Also the “May ‘78” preamplifier, and the “School Amplifier”, which wasn’t really an EC design — it was a school project sponsored by EC, using EC components and instruments.

The “Krinken” preamplifier

In 1977 we were approached by the Norwegian Broadcasting Corporation — Norsk Rikskringkasting (NRK) — and asked if we could develop a professional preamplifier for studio work. It was to have differential input and output. To us, of course, that meant no transformers. The electronics were completely differential all the way through. The preamp had passive equalization, a lot of transistors, and sounded very good.

(The schematics for this one were never relocated — if they turn up, they’ll be added here.)

The Perfect 25W Amplifier

How should it be done?

Base it on the Special Version schematic. That should be the starting point. Don’t attempt to modify an existing amp — it should be built from scratch!

Then follow the points below:

1. Change the power supply as follows:
2. Increase the pre-stage voltage from 30 to 35V.
3. Place regulators on the pre-stage voltage supply, allowing 5 more volts to drop here, taking the unregulated DC voltage up to 40V.
4. Add a cascode stage on the 3rd amplifier stage.
5. Replace the output transistors with modern Japanese types, e.g. the … (never specified — my notes don’t give a part number).
6. Refine the other stages, as shown in the suggested schematics.
7. Design a new PCB layout, following the PCB rules detailed below.
8. Use heavy-gauge wire on all supply lines and loudspeaker outlets. If possible, use steel/copper bars between the electrolytics.
9. Keep all wires close to the chassis, glue them onto the metal.
10. Place emitter followers before the 3rd stage.
11. Use shielded wires from the input signal jacks to the board. Use two signal wires, one for ground and one for active, with the shield connected only at one end.

PCB layout rules:

1. Keep the input stage separated from the output stage.
2. Place series resistors as close to the base/gate of the receiving transistor as possible.
3. Keep EVERYTHING SYMMETRICAL.
4. Short leads everywhere.
5. The higher the network impedance, the shorter the leads.
6. Separate the input and output stage at the connection between the 3rd-stage emitter followers and the 3rd-stage common-emitter stage.
7. Use THICK traces on all parts of the output stage. KEEP IT SYMMETRICAL. (Any asymmetrical trace routes here will cause an imbalance, with a corresponding lack of distortion cancellation — you won’t want that, will you?)
8. Don’t use a ground plane, but guard rings may be useful (never tried them, though). A ground plane adds capacitance between all traces and ground, reducing high-frequency performance. Remember, this is not a radio — the signals do NOT depend on RF reflections and things like that. Capacitance is MUCH worse!

Then you’ll probably have some questions. Before you get in touch, here are some anticipated ones, answered up front:

FAQ

Can I sell amplifiers based on this schematic? No — they are intended for your personal use. And for your close friends, if that helps you finance the thing. If you are a company and want to make money on this, contact me via the about page before my lawyers contact you!

Can you provide PCBs or components? I can’t provide PCBs or mechanics, but I may have some suitable electronic components — I’ll put up a list of these later. Check back.

Can I increase the power output? Well, you can increase it slightly, up to perhaps 50W. All output stages including the 3rd stage should have a corresponding voltage increase, and perhaps you should add more output transistors in parallel. Note, however, that the more transistors you add in parallel, the more capacitive load you introduce — 3, or max 4, in parallel.

What quiescent current should I use? The quiescent current should be calculated from each transistor’s Hfe-versus-Ic curve. It should be set to no more than slightly less than half the current at which Hfe has its maximum. As for the lower limit — don’t push it too far down. Note also that more transistors in parallel means more power loss.