The Story of the "Otala" Amplifier

A story of the legendary 25W “Otala” amplifier as seen by Terje Sandstrøm, one of the original designers.

Also see “The People Involved”, for other designers and people who contributed.

Introduction — the birth of the Otala amplifier

At an AES conference in 1973, Dr. Matti Otala presented a paper describing the design of a TIM-free audio amplifier. Present at this conference was Svein Erik Børja, a Norwegian record and broadcasting producer, and a great audio enthusiast. Svein Erik Børja was also one of the greatest Golden Ears of his time — he was able to hear even the slightest imperfection in an audio component. Having been dissatisfied with the sound of the transistorized audio amplifiers of that day, he saw an opportunity here for an audio amplifier of a new generation. Dr. Otala’s talk about TIM also explained the imperfections Svein Erik himself had noted in audio amplifiers.

He brought the paper to a friend of his who was running his own audio company in Norway, Per Abrahamsen of Electrocompaniet. Per decided to give the amplifier a try, to see what it might bring. They got the help of Nils Jørgen Kjærnet at Nera in Oslo, who did the circuit board design and also, to my knowledge, helped with the mechanical design of the amplifier.

They first made just a couple of amplifiers, but the sound was so good, fulfilling all their expectations, that they decided to manufacture a series of these amplifiers.

I started showing up at Electrocompaniet in the autumn of ‘74, and started to work for Per in the spring of ‘75. One of my first jobs was to assemble the first production series of the Otala amplifiers — a series of 10 amplifiers, based on the same PCBs as the original two prototypes. During the summer I started to look at the design, having been an electronics hobbyist since my early teens. At that time neither Per nor I knew too much about high-end audio design. That may well have been the factor behind the amplifier’s success, as it evolved over the next 5 years — we didn’t know how it should be done, so we worked it out from the framework given by the Otala design. We may well have been the first audio amplifier designers of the new school — the TIM-free designers, so to speak.

The first Otala amplifier in “production”

The first amplifier series used the same printed circuit boards as the first two prototypes. The PCB was a two-layer design — we called it a T-board, due to its shape. The power transistors were mounted onto the cooling fin from both sides of the stem of the “T”. One layer of the board was a ground plane (Nera being an RF company, that wasn’t so strange). The ground plane caused us a lot of mounting problems, because the component legs often got stripped when they were squeezed through the holes in the PCB, causing small metal pieces from the legs to curl up and make sweet, nearly invisible, shorts to the ground plane. We did see smoke …

In the autumn of ‘75 we started to get some attention, and a visit to Matti Otala at the VTT (Valtion Teknillinen Tutkimuskeskus — the Technical Research Centre of Finland), where he worked as a professor, became necessary. There were several reasons:

We had named the amplifier “Otala-Lohstroh” after its inventors, and Otala wanted to see the amplifier before he and Lohstroh could possibly allow us to use his name.

At the same time, we started to experience problems with the amplifier — it didn’t quite meet the specifications stated in the AES paper. We needed his help.

As it turned out, the amplifier was renamed to “The 2-channel audio power amplifier”, which was the name used for the rest of the amplifier’s lifetime. The relationship with Otala was established, and he visited the company on several occasions. There is still good contact between Otala and Electrocompaniet.

Period of confusion, start of revision

It was early 1976. We started to invest in measuring equipment. The company slowly turned into an audio amplifier company. It lost several of its old customers; the loudspeakers it sold became more neglected. We worked from early morning into the late nights. That would be the standard for the next 4 years. An amplifier brought to the US by Svein Erik blew up at the first listening test — embarrassing. We used the night to invent a new type of short-circuit protection. We didn’t want anything that could affect the sound, so we simply used a high-impedance circuit to sense the current through the output transistors, and then used a relay to switch off the voltage to the output transistors. It worked!

We now understood that we had to start changing the amplifier. Svein Erik’s golden ears became even more important than ever before. We were in uncharted territory; the old measurement methods could not be trusted. We had to rely on our own methods, our own interpretations of the measurement results. Now began an active period of theory, practice, and reading articles (Svein Erik provided us with articles en masse, on all aspects of audio design and electronics design in general).

We found that the compensation scheme used in the amplifier did not work correctly. As we measured the bandwidth to be far less than specified, we needed to redesign these filters. The amplifier used shunt feedback — one of its really strong points — with an input lag compensation; at each of the three amplifying stages, a lead-lag compensation was introduced. These did not match the actual poles we measured. One of the reasons for this, we assumed, was that the original design used “fresh,” newly developed Philips transistors. As both Otala and Lohstroh were working at the Philips labs in Eindhoven at the time of the design, we assumed they had access to better transistors. (I don’t remember if Otala ever confirmed this to be the case.)

In any case, a period followed where we changed the compensation back and forth with little or no effect. We did achieve something, but no significant improvement.

We had also now measured the distortion (having just bought our first distortion measurement set and spectrum analyzer). It wasn’t pretty — the distortion was far worse than specified, and performance was far below even our own expectations. Again we didn’t manage to do anything significant at first.

A major change came first when we started to change the quiescent current of the transistors. The design was in fact done as a mix between the old way and the new way. Older design books teach that you should use low current in the first stage due to noise. This was also done here (see “Noise Optimisation” for an explanation of why that’s not actually the case). We knew this theory was wrong, so we increased the quiescent currents, which decreased the resistance and again dramatically increased the bandwidth of the amplifier. The distortion also went down — not dramatically, but enough to make a significant change for the better in the sound (see “Transistors, Resistors, Current and Distortion” — not yet written).

The Audio Critic

At this time we started to hear rumours about a test in the US on our amplifier. One day our mail started to overflow, and a few days later we got hold of the test ourselves. It was in a magazine named The Audio Critic, and the test was fabulous. It started:

Audio freaks - Eat your hearts out: This is the worlds best sounding amplifier, and One: You can’t buy one in this country (The USA!), and Two: (not surprisingly - the only slightly negative in the whole test) It is too low powered to be counted.

Suddenly we had more requests for amplifiers than we could hope to handle. Sales boomed. It was mid-‘76.

We worked all summer, mornings, days, evenings, and into the night — Monday to Friday, Saturday and Sunday. Per’s wife, Anne, was not always too happy about this. Per had two small boys, and she had to take care of them a lot by herself. Anyway — her “sacrifice” made building these amplifiers possible, and the nice thing about it all is that both Per’s and Anne’s sons now work at Electrocompaniet! So in the end it turned out to be an investment in their own future as well!

The great change

Dr. Otala’s theorems on TIM very often came out as an attack on high-feedback amplifiers. Although it is correct that high-feedback amplifiers are more prone to TIM than low-feedback amplifiers, there is no magic here. The main goal of any amplifier is to reproduce the incoming music as perfectly as possible, neither adding nor subtracting anything. TIM is just one type of distortion; swapping one type for another doesn’t help. It is true that some types of distortion are harsher to the ear than others, but there still is a balance to be struck. If 0.1% of TIM equals 1% of THD in audibility, then an amplifier with 2% THD and 0.07% TIM will sound worse than an amplifier with 1% THD and 0.1% TIM.

So, we realised that the balance of distortions was the essential factor to consider. Not only do you have to balance THD against TIM, but also low-frequency distortion against high-frequency distortion, frequency and phase response against non-linear distortion in general, and so on.

This insight triggered the Great Change:

One night (it always happened at night!) we increased the feedback by 10 dB, for a total of 30 dB of feedback. The sound improvement was staggering — and contrary to common belief in our own community!

After this we only adjusted the amplifier slightly. It had found the form it should have for its remaining life at Electrocompaniet. And the amplifier was a success!

On marketing

We did brochures! The first one was rather technical. The second one was more professional. The rest of the marketing was done by word-of-mouth. The fact that the amplifier and the company were both hard to get hold of stimulated the market. Here was a mysterious company with a killer amplifier. Audio people love such stuff!

Technical things

See the schematics. Also see some of the calculations that were done.

The People Involved

Several people were involved in the development of the 25W amplifier. This chapter takes the history from the perspective of the people involved, in chronological order.

A Finnish professor named Matti Otala (also here). On sabbatical at Philips in Eindhoven, together with Jan Lohstroh he designed a TIM-free amplifier. It was one step in Otala’s quest against the new distortion called TIM, and later DIM.

Svein Erik Børja, a Norwegian broadcast and record producer, heard the presentation given by Dr. Matti Otala at an AES conference in Copenhagen in 1973. Together with Nils Jørgen Kjærnet at Nera he built two units based on the Otala/Lohstroh schematics.

Per Abrahamsen was contacted by Svein Erik, who knew him from his Mojo Blues days, and also knew that Per had an audio/electronics company. Per was mostly working with PA equipment, but the arrival of the Otala amp turned his company, Electrocompaniet, into an audio company.

At this time several others were involved with the company, but I don’t remember all of them, mostly because they were before my time, and also because they slowly disappeared as the company changed from a PA company to an audio company. Among these were Petter Hjerpseth and Klaus Væthe Jr. The latter later became a good friend of mine, and we tried to make an audio company together in 1983, with the help of Paal Rasmussen, my former partner at NRF — more on that later. Petter disappeared during 1975; Klaus stuck with the company on and off until sometime in 1977/78.

I (Terje Sandstrøm) first turned up in late 1974, and started to visit EC on a monthly basis. After half a year of this, Per told me we were both better off if I started to work for him, instead of just spending his time. In the beginning I mostly did assembly work, but during 1975 I worked myself up the ladder, and later I did more and more of the theoretical work on the amplifier, calculations and so forth. I stayed until 1979, but did some part time work and consulting for EC until 1982.

It is important to note that Svein Erik stayed with the company as its Golden Ear for a very long time. In a way this was his hobby!

Peik Borud entered the company in 1976. He was an M.Sc. engineer in electronics, and an audio freak. He stayed throughout 1978, and was, together with Per and me, one of the basic designers of the 25W amplifier in this period. Peik did a lot during his time at EC.

Paal Rasmussen entered the company in 1977 and stayed on and off for slightly less than a year. He came from Seas AS, a loudspeaker company, and before that he had worked in England with several of the famous designers there. He and I were two of the three who started The Norwegian Radio Manufacturing Company in 1979 — more on this later.

Kjell Winther was our record provider, and got amplifiers in return. He was also a hi-fi nut, working in a record store, and provided us with the latest and best-sounding records for our sound tests.

Nils Kvam was a record producer who also worked with listening tests and also provided opportunities for the company sales-wise. He is still working with EC.

Øistein Klevhus helped me with the Engineering High School (I was rarely present …), and I got him a part-time job at EC from early ‘78 (or late ‘77) until spring ‘79. We did the School Amplifier together, spring ‘79.

Two of the people in production who really stayed for a while were Knut Arne Jacobsen and his then-girlfriend Aud. There were also a couple of girls who came from Tandberg to work in production — (their names are not remembered). These also stayed for quite a while.

There were also some sales and managing people (Helge …, Jan Richter, among others) involved from period to period. I’m working on re-remembering their names. Any help from others is appreciated.

Others who contributed included, for example, Svein Erik’s brother, Bjørn Børja, from Seas AS, who designed a cute little moving-coil amplifier that EC manufactured for some time.

During 1979 the company moved, I started my way out, other people came in, and I don’t recall their names, sorry. However, the first era of the Otala amplifier ended sometime in 1979-1980, and the new Electrocompaniet appeared. It moved to Skårer, where it is still located.

This text was originally written some time in the late 1990’s, and minor updated around 2005.

Protection Networks

Protection networks, power supplies, and other stuff not so interesting, but still needed.

You’ve read the section on how this came about? If not, go back and do so! Otherwise, take a look at the world’s first protection network that did not affect the sound AT ALL. Yes — it is the first one!

The design is very simple: passive sensing of current, then triggering a relay which removes the supply voltage from the output stage. As you probably know, the EC design uses one power supply for the voltage-amplifying stages and one for the power output, where all the large currents go. This separation made this protection scheme work. If not, big bangs would be heard …

The power supply also needed some care.

However, we did not make it difficult. The major point here was the use of small capacitors instead of the sluggish big ones everybody else used at that time. We understood that small capacitors in parallel were a better choice than single big ones.

The cabling was another issue — more on that later.

The Audio Critic Test

The Audio Critic test, from 1976, that triggered the flood of orders for the amplifier:

Marketing Brochures

We did brochures! See the Otala story for the context.

Our first brochure

It was rather technical.

The two remaining pages are still waiting to be scanned.

Our second brochure

More professional this time :-)

Norsk Radiofabrikk

Norsk Radiofabrikk — The Norwegian Radio Manufacturing Company

The company was founded in 1979 by Espen Evensberget, Paal Rasmussen and myself. More info on this will follow later, but for now, see what the press said at the time below.

The company also put out its own press release, “Tre Faser i en Utvikling” (“Three Phases in a Development”), about the story of the Otala power amplifier including the NRF modifications, written in 1979 — that document hasn’t survived, or hasn’t been found again yet.

Press coverage, 1980/81

Danish High Fidelity no. 2, 1981

Hi-Fi & Elektronikk no. 2, 1980

Hi-Fi & Elektronikk no. 5, 1980

Musikkavisen PULS no. 2, 1980

Musikkavisen PULS no. 4, 1980

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 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 on from the EC time. See the NRF modifications page.

The “Special Version” schematic is shown here:

Note the heavily reduced values of the output-stage emitter resistors — 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 nonlinear input base current acting on the input resistors. (Which again shows: when we reduced the input resistors from 6k8 to 2k2, we heard a sound improvement and believed it was caused by improved frequency-response behavior above 20kHz — it may have been the 10dB distortion reduction that 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 raising the load resistors from 2k2 to 3k3. More gain was needed to increase the overall feedback to 40dB. It was found that increasing this particular point (3rd stage) was the less critical of the options. However, we got slightly more relative modulation effect on the 3rd stage than we wished for.

All in all, approximately 100 amplifiers were modified, and an additional 50 Special Versions were made.

By the end of the ’80s and the beginning of the ’90s, I tried a few more changes to the amplifier design, which never made it beyond the lab bench. Only one amp exists 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 about some thoughts on the “Perfect 25W” amplifier? Or the perfect preamplifier, which was realized, but …

Anything you find missing, or any comments, are appreciated — reach out via the about page.

Original 1973 Otala Schematics

Note the hand-drawing on the schematic. First attempts at traditional current limiting can be seen — this was soon discarded. Note also the extra resistors near the output: the loudspeaker was to be coupled between the output terminal and the small 0.1 ohm resistor to ground. This was a later idea (I don’t remember when it was introduced) which would give some current feedback in addition to the traditional voltage feedback. The effect would be to make the loudspeaker “invisible” to the amplifier.

Note that you can see the prices on the transistors here: NOK 4.70 in qty 100 for BD203/204. Cheap transistors!

The Real Electrocompaniet Schematics

The schematics of the real EC 25W amplifiers, shown below, are taken from a service manual, with full acceptance from Per Abrahamsen of Electrocompaniet. He approved of, and even supported, the idea of making these schematics public through this site, in order to cover the history of Electrocompaniet.

Note that the schematic layout is very close to the original, except for the changes to the compensation networks (look at the emitters on the differential voltage-amplification pairs). The major changes can be found by looking at the component lists below, which apply for amplifiers with serial numbers below 275. The component list for serial numbers above 275 is covered in an addendum further down. You’ll need both lists, cross-checked, to get the full picture.

(This scan is a bit rough — a better one may come later.)

Above 275

Component lists for serial numbers above 275, the Great Change:

So — this covers it. No further change was done to these amplifiers by Electrocompaniet. Well, not quite true — I did some further modifications after I left Electrocompaniet; see the NRF modifications page.

I’m not quite sure how many amplifiers were made, but by mid-‘77 we had reached serial no. 275, which took 2 years to reach. From then I believe approx. 50 units were manufactured each month, until approx. 1981-82, when Electrocompaniet introduced the Ampliwire series, which replaced the 25W amplifier. This means there could be some 2000 amplifiers around. Perhaps Per knows more.

See also more information on transformers, wiring, mechanical assembly and troubleshooting.

1982 Preamplifier Schematics

This is the phono stage, handling both MM (Moving Magnet) and MC (Moving Coil) pickups. The design uses active RIAA compensation, which was untraditional for the EC design — up to this time it used passive high-pass and active low-boost. The gain stages are designed to handle the active feedback, and note (!!!!!), the active RIAA compensation used shunt feedback. A tough job to calculate, but when that was done, it worked wonders!

The line-stage section is more traditional, with a normal series-feedback line stage. Note the extra buffer on the output.

For both block schematics above, note the roman numbers inside the amplifiers, denoting the particular gain block used.

This drawing shows gain blocks 3 and 4. Note the completely symmetrical design. The other gain stages also used an ingenious method of utilizing all the current from the last differential stage. Also note the current generators — ever seen something like that? Well, it works!

Details on compensation and other design choices will be described further in a later write-up. Don’t try to build the gain stage as shown above without that context — smoke will arise. Be patient.

Transformers, Wiring & Mechanical Assembly

This page is intended to help you if you’re going to service your amplifier. Electrocompaniet still services their old amplifiers, and I still service those I modified between 1979 and 1983 — however, the process may be slow (too much other stuff to do), so if you’re good with a soldering iron and have some instruments to help you along, this page together with the schematics may be what you need.

The transformers were also special:

And the wiring harness:

We thought we ought to have those exploded mechanical-assembly-view drawings, like the Japanese manufacturers did. What do you think about this?

Then onto the troubleshooting pages.

Troubleshooting Your Amplifier

This page contains info on troubleshooting the power amplifier. For some info on the preamplifier, see Troubleshooting the Preamplifier.

The following are excerpts from the original service manual of the EC 25W amplifier.

If you have specific problems, get in touch via the about page, and if I know the answer I’ll follow up — and the case will also be added here. Also see more drawings on the transformers, wiring & mechanical assembly page.

Troubleshooting the Preamplifier

I will put up schematics and other material on the preamplifier as well. However, first a few tips, based on real cases I’ve had in recent years.

“Scraping” in the volume control

This is due to DC leakage through the coupling capacitors, out from the RIAA stage and into the line stage. Replace the RIAA output electrolytics with 2.2 µF polyester, and the line-stage input capacitors with 0.68 µF polyester. Remove the 100k loading resistors.

The NRF Modifications

After I left Electrocompaniet in 1979, I tried to start up a new audio company in Asker together with two good friends, Paal Rasmussen and Espen Evensberget. We called the company “Norsk Radiofabrikk” (in English, the Norwegian Radio Manufacturing Company). We wanted it to sound old-fashioned, and really had a lot of good fun with this. However, our own original designs never made it into reality. In 1980, or thereabouts, we started to modify EC amplifiers. We modified between 50 and 100 amplifiers, and a somewhat lesser number of preamplifiers. The sound improvement was very good — not too surprising, since I knew these amplifiers in and out. Later we did some 50 amplifiers from the ground up, called the Special Version — the modification taken to its logical end. The components for these amplifiers came partially from Electrocompaniet. I don’t think Per liked it, but I also don’t think he really minded — if he had, he wouldn’t have sold us components, like cabinets and so on.

I have had some trouble locating all the schematics and component lists — don’t really know where I have (mis)placed them. However, I have redrawn the schematic for the modified amplifier below. Please be aware that the schematic is not to be used for commercial purposes without my permission. All private use is encouraged.

Components with red values are changed, components with blue values are new, otherwise equal to original values. Please get in touch regarding proper frequency compensation of this amplifier.

I’ve also included some pictures of the modified circuits below, so that you can see for yourself how we did the practical part of the modifications. Note that none of the traces were cut on the circuit board — all new components were wire-mounted (“bird’s nests”). Not the prettiest thing, but it sure works.

The modification significantly reduces the distortion of the amplifier, bringing much more clarity to the midrange, a tighter bass, and a smoother, less harsh top.

(A later note also mentioned a further schematic variant that popped up — apparently a later version that also never made it into reality, with component values and general setup equal to the modified ones described here, but with an extra double pair of output transistors and emitter followers added before the third stage. That particular schematic image itself hasn’t survived.)

Modification photos

(Captions translated from the original Norwegian file names.)

Calculations on the 25W Amp

The following image is the first of an analysis done on the 25W amplifier. The date, 4 July 1980, indicates that this was done after I left EC (which was in ‘79, at least on a full-time basis — after a short break in ‘80 or so, I returned on a consultancy basis to do more work for them until sometime in 1982).

I will upload the whole analysis. For those technically inclined, it should be interesting — lots of similar analysis was also done earlier. I got the feeling that many people believed we were only some kind of non-serious hippies. Well, in a way we were outside the establishment, but we did our mathematics! Also note the point made at the end of the note, saying that feedback below 20dB is no good idea when it comes to distortion.

The Regulated 25W Power Amplifier

We noted early on that the power supply had a significant effect on the sound. We decided to implement regulated power supplies for the amplifier. I’m not quite sure when we did this, but it might have been sometime in ‘76, possibly ‘77. We did it straightforward — no switching, no nonsense, only linear regulators. Hot? Yes, indeed!

I’ve had some trouble locating the schematics for the regulators, but I believe I’ve found them. It’s possible that the ones shown here are preliminary schematics — the design never made it into production, so I’m not quite sure if any other version ever existed.

Only a couple of these amplifiers were ever made, and one of them was returned to me for service a couple of years ago. Sadly, it was by then beyond repair.

The amplifier has two sets of supplies, one for the pre-stages and one for the output stages, so two sets of regulators were needed.

Note the Q27 emitter — it should be coupled to the unregulated pre-stage power supply. Q27 acts as a current generator and needs some voltage headroom. Note also that there are no current limiters, so if the regulated output was shorted to ground, blue smoke was the result!

The transistors are listed with only numbers: 139/140/203/204 are BD, and 413 is BC. All BD transistors were heatsinked — in fact, they were mounted onto the bottom plate of the amplifier!

A Theory of Single Stages

The general idea

When you design something, you’re moving your ideas and abstract thoughts into the real world. Using electronic components, they will never act exactly equal to your ideas. So the best approach is to approximate their behavior, and make them work as close to your ideas as possible. Further, if you choose your models skillfully, you will get a behavior from the device which very closely mimics what you’re after.

Therefore, the sequence is: Model ⇒ Design ⇒ Measurement ⇒ Listening — and you circle this sequence until you’re happy :-)

The walkthrough below is rather detailed, to show the general outline of the procedure. The other derivations are less rigorous.

A model for a single stage

We model a single stage as a voltage-controlled current source. In our ideal world this means we want an active device working as a transconductance device. A transistor (and also a tube) is very close to this ideal. But one has to further improve the circuitry around the active device, in order to make it behave as closely as possible to this ideal.

What this means is that a perfect transistor, and thus a perfect stage, will have infinite input impedance, no reverse coupling from output to input, infinite output impedance, and a finite and constant (with respect to both the signal and the environment) transfer conductance.

Real transistors are not quite as good as this, but we can improve on the transistor in order to make it behave more like this ideal. Doing this will normally make the stage perform better in all respects, but keep in mind that all rules will turn back on you at a certain stage. There is no such thing as a free lunch.

If such a stage is voltage-driven, we will reduce the nonlinearity from all “leakages” back to the input, be it input impedance or reverse coupling. The dominating nonlinearity will then be the transconductance, which is easy to control.

A transistor in a common-emitter coupling is the starting point. It has, in principle, the behavior described above. The following rules exist:

Linear behavior:

Transfer conductance, given by:

gm = Ie/Vt

Ie is the emitter DC current and Vt is the voltage equivalent of temperature, normally equal to 25 mV — the exact formula is kT/q, where k is Boltzmann’s constant, q is the charge of an electron, and T is the absolute temperature in Kelvin.

The inverse of the transconductance is called the dynamic resistance, called re.

Current amplification: Hfe = Ic/Ib, derived for a particular current hfe = ic/ib, which applies for small-signal currents around a quiescent point Ic.

Input impedance: rin = hfe*re

The dominating nonlinear mechanism lies in the transconductance. Since this is a single stage (not a differential stage) it will generate a smooth series of harmonics (if stimulated with a pure sinusoid).

If the input signal is given as x (where x can be e.g. sin(wt)), then the output y will be:

And x is a relative parameter which must always obey |x| < 1 in order for the series to converge. If |x| < 1 then it follows that |y| < 1. The output current is ie and the output parameter is then ie/Ie. The input signal generating a current of ie is uin, from the formula ie = uin*gm. It then follows from the transconductance formula that the input parameter we seek is uin/Vt.

So, given the input x, defined as uin/Vt, and the output y, defined as ie/Ie — what do these things mean?

If uin/Vt exceeds 1, then the varying part of the current exceeds the quiescent current Ie, and the stage is clipping. When the stage is clipping, our formula breaks down. If we want to find the distortion when the stage is clipping, we’ll have to resort to Fourier analysis.

To make this into a practical case: assume a transistor running at a current of 1mA. The gm is then 1/25 siemens (the inverse of ohm, the unit for transconductance, although “mhos” — ohm reversed — is also used). An input signal of 1 mV will then generate an output current of 1/25 mA = 40 µA. But now note: this is the first-order approximation. As can be seen from the series expansion above, we also have second- and third-order components. The first-order coefficient should be pretty close to gm, but what are the other two coefficients?

The real equation relating input voltage to output current is Ie = Is*exp((Ube/Vt)-1), called the Ebers-Moll equation. Is is the “leakage” current, but don’t bother about it — we’ll soon enough get rid of it. We are interested in finding the equation for the behavior around the quiescent point. We do this by adding small deviations ie and ube to the equation above. Resolving this, we get the much simpler equation: ie/Ie = exp(ube/Vt)-1, and its inverse: ube/Vt = ln(1+ie/Ie). These equations are called the signal equations, and will be used to get the coefficients for the series expansion above. We’ll first make a series expansion of the first equation, and we get:

The efficiency parameter

We now introduce the efficiency parameter ni. The point of introducing this parameter is to generate simpler formulas for calculating the distortion, and to gain a better understanding of how the distortion and other transistor parameters are coupled.

The parameter is defined as:

ni = ie/Ie

For a bipolar stage without local feedback, as the stage discussed above, ni is equivalent to ube/Vt, where we only consider the linear part of the series expansion. The equation above can then be written as:

The second-order distortion is defined as the second-order term divided by the first-order term, and the third-order distortion in the same manner:

2nd = ni/2

3rd = ni²/6

If the input signal is a sinusoid, then the following equations hold (ask via the about page if you’d like the proof), where 2ndh is the second-order harmonic distortion, and 3rdh is the third-order harmonic distortion:

2ndh = 2nd/2

3rdh = 3rd/4

Putting it together:

2ndh = ni/4

3rdh = ni²/24

…and remember, ni = uin/Vt.

FET stages

It is interesting to do the same exercise for FET transistors. The result is a simpler series, with only first- and second-order components. By inserting the efficiency parameter, one gets the following equation for the FET’s second-harmonic distortion:

2ndh = ni/8

Half the amount of the bipolar transistor. Some people have argued that the FET is a much more linear device than the bipolar. This equation shows that to be only a partial truth — there is only a 6dB improvement.

Local feedback

It is well known that local series current feedback (read: inserting an emitter resistor) reduces the distortion. The feedback factor can be written as:

D = 1 + gm*Re = 1 + (Ie*Re)/Vt

and the resulting efficiency/distortion equations are then:

2nd = ni/(2*D)

and

3rd = ni²/(3*D)

or for the harmonic distortion:

2ndh = ni/(4*D)

3rdh = ni²/(12*D)

Note that the efficiency parameter ni is still defined as ie/Ie, but the input version is now ni = uin/Vth, where Vth = Vt*D.

Differential stages

There is a similar set of equations for the differential pair.

If the stage is completely in balance (which of course rarely happens), all second-order components will be cancelled.

The no-feedback solution will have a basic transconductance of gm = Ie/(2Vt), which gives ni = uin/2Vt.

The corresponding distortion is then equal to the single stage, except for a mismatch parameter m and a common-mode signal factor c:

3rd = ni²/3

and

2nd = (m+c)*ni/2

If the current source feeding the emitters has infinite output impedance, c approaches 0. The formula for c is c = ik/(2ie). More information on this is in my AES paper (not yet ported to this site). There used to be an ActiveX Single Stage Calculator here as well — it’s retired along with the rest of the site’s old ActiveX controls.

Summary

Open and Closed Loop Frequency Response

The open-loop frequency response is the frequency response of the amplifier with no feedback — before feedback, or with the feedback network deliberately broken.

The closed-loop frequency response is the frequency response of the amplifier with feedback.

These two are closely related. The theoretical closed-loop frequency response is equal to the open-loop frequency response times the amount of feedback. If you have 40dB (100 times) of feedback, and an open-loop response of 1kHz, the closed-loop frequency response is 100kHz.

The formula relating these two is:

where D is the feedback factor and Aol is the open-loop gain. The total denominator expression is what we call feedback.

The open-loop frequency response is determined by the internal compensation (intended or not) of the amplifier. Many amplifiers are designed with one stage having a very high output impedance, so the stray capacitance of that stage’s output determines the open-loop frequency response. For integrated circuits the open-loop frequency response is either specified, or you can see it graphically as a function of gain — in the latter case, look at the maximum gain, which means zero feedback.

Just to remind you: the closed-loop gain Acl is related to the open-loop gain Aol in exactly the same way as the frequency response, although inversely.

At Electrocompaniet the thinking favored a large open-loop bandwidth. This is also my opinion, but I feel it shouldn’t be larger than necessary. There is always a tradeoff, and if you go for too high an open-loop bandwidth, you reduce the possible amount of feedback you can have. My thinking is that as long as the open-loop bandwidth is high enough, you should use the rest of your gain for feedback. This will give you a more optimal design, because the overall distortion will be reduced.

What determines the open-loop bandwidth

Mostly it is determined by the last voltage-amplification stage. The collectors of this stage (assuming transistor amplifiers, which these articles are all about :-)) are connected to the bases of the drivers of the output stage. The input impedance of these drivers is normally very nonlinear, and strongly frequency-dependent. This means you can very well get a major pole here which varies strongly with signal level and the load (loudspeaker and cables) of the output stage. The solution to this is to voltage-drive the output stage, thus loading down the amplification stage. This will also have the effect of pushing up the cutoff frequency at this point. The EC amplifiers have this pole around 500kHz. The benefits of a voltage-driven output stage are described elsewhere on this site, including in an AES paper (not yet ported to this site).

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.

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.