Behind The Scenes (Mar. 1975)

LAST MONTH I reported on the audio press junket arranged by Philips and AKG, and covered some of the interesting new developments we were shown at the Philips facilities in Eindhoven. The second half of our trip was through the courtesy of the AKG company, and early on a Sunday morning we flew from Amsterdam to Vienna where AKG is headquartered.

Once installed in our hotel, those who had the stamina were free to explore the historical sights and to soak up the gemütlicheit of the living monument that is Vienna. The last time I was in Vienna just happened to be the day after the Russian occupation of the city ended, and a tearful, joyful populace was celebrating.

There was still quite a bit of damage from the bombing and shelling of the war, but as was apparent to our press group on the occasion of this visit, a major proportion of Vienna's historical buildings and landmarks had miraculously escaped destruction.

I must say that AKG really went "all out" in entertaining us. Early Sunday evening a group of AKG executives met us at our hotel and invited our press group to climb into an assembly of 18 horse-drawn open carriages, and then this procession was given a tour through the streets and byways of Vienna. Of course, I gave my most gracious "Queen Elizabeth 2nd" hand waves to the people lining our route, as we wended our way into the cobblestone courtyard of the Palais Schwarzenberg. Our group was escorted into a reception room, a great high-ceilinged place aglitter with crystal chandeliers, the gleam of parquet floors, hung with rich tapestries and paintings, and in one corner, standing fully 8-ft. tall, one of those incredible blue and white porcelain stoves so reminiscent of the Hapsburg era. A bar was at one end of the room and while we enjoyed our champagne and orange juice (very "in" and very European!), we were formally introduced to our hosts.

Feeling that we had somehow be come a part of a typical Viennese operetta, we made our way into an opulent dining room, and as we started on our delicate smoked sturgeon with whipped cream/horseradish sauce, we heard the Third Man Theme, played on the zither by none other than the composer, Anton Karas. At the conclusion of our superb dinner, Dr. Gorike, the head of AKG gave us a welcoming speech and an outline of our activities in the technical aspects of our visit.

AKG--Akustische and Kino-Gerate GmbH--was founded some 29 years ago by Dr. R. Gorike and Herr E. Pless for the production of film projectors (that's the Kino in the company name). In their work a need arose for a microphone which would record in one direction while discriminating against the unwanted background noise of the projector. Thus in 1953, they marked the AKG D-12 cardioid dynamic microphone which quickly established AKG's reputation as a manufacturer of high-quality microphones.

From that point on, microphones became the principal product of AKG. Mass production methods were developed to produce high-quality dynamic microphones at a reasonable price and their types D-11, D-19, D-24 and D-119 became a familiar sight in recording studios throughout the world. Early on, AKG did pioneering work in the manufacture of condenser microphones, one of the most famous being the C-12, which featured remote control change of pickup patterns. As manufacturers of electro-mechanical transducers, it was only natural that AKG would undertake the production of headphones, and many high-quality types emerged, including back in 1959, the first "open-aire" types, which are now becoming so popular.

In the ensuing years, AKG has become one of the best-known manufacturers of dynamic and condenser microphones. Their latest dynamic microphone, the D-200, has created great interest because of its unusual design. It is known as a "two-way" cardioid as it employs separate high frequency and low-frequency diaphragms and a crossover network, as in many loudspeaker systems. In the field of condenser microphones the model C-414 is quite versatile with patterns switchable from omnidirectional, cardioid, hyper-cardioid, and figure eight. The C-451 is a whole system of condenser mikes with the mike handle containing an FET preamplifier, and accepting seven interchangeable "capsules" of different pick-up patterns. The same system has now become available with electret capsules.

With growth has come diversity, and AKG is in the fields of digital delay, artificial reverberation, and ultrasonic transducers. The company holds over 600 patents in electro acoustics. More than 750 people are employed by AKG, and they have expanded to facilities in Munich, London, and Zurich. As mentioned last month, AKG products are marketed in the U.S. by North American Philips Co.

On a Monday morning, a half-hour bus ride brought our group to the AKG factory. There we met a number of the AKG engineers and scientists, including Chief Engineer W. Fidi, and inspected some of the extensive research facilities. Then with the aid of AKG marketing director Herr H. Schnabel, Herr Fiki, and several other engineers, we all sat down to a sort of AKG products symposium and "interchange of ideas" conference.

With AKG's involvement in electromechanical technology, it wasn't too surprising when it was announced that they were entering into the manufacture of phonograph cartridges. Sometime during 1975, a full line of magnetic cartridges will be introduced, including a special model for the CD-4 quadradisc. A thorough briefing on the special qualities of various AKG microphones followed, in preparation for the next day's inspection of the production facilities.

Following this we were shown the AKG digital-delay system, which is somewhat similar to the Blesser and Eventide units in the U.S. The main thrust here is in retaining the performance of the device, while making a most determined effort to reduce the cost of the unit. There is a very compelling reason for this, as you will see a bit later on. After a break for lunch at the charming medieval-style inn, not far from the AKG plant, it was back to our conference.

The afternoon was devoted almost entirely to a discussion and demonstration of the AKG BX-20 reverberation device, including its use in some unusual applications. At the risk of offending some of the AKG microphone people, this BX-20 demonstration was, in my opinion, the highlight of the AKG visit.

Let me preface this description of the BX-20 by stating that it is in essence a coil spring reverb device, even if it is rather euphemistically called a "torsion transmission line" or TTL. Such devices have been absolute anathemas to me, as any I have ever heard have been plagued by the curse of the characteristic "boinngggg" coloration of sound they produce.

Needless to say, such spring reverb units have not found favor with professional recording engineers. However, it turned out that the BX-20 was a new breed of spring reverb, since in several hours of listening to it, none of us could detect even a smidgin of the "boinnggg" coloration.

The BX-20 is designed to be a portable unit and consists of three main parts... the electro-mechanical reverb unit proper, the electronic circuits, and the elastic support.

The BX-20 is a two-channel unit with independent control of decay time of each channel. Thus, it can be used for stereo or mono recording.

The length of the spring is 47.2 inches as calculated for the desired delay time. To make the spring fit into a reasonable size enclosure, the spring has been bent twice. At each end of the spring are moving coil systems.

Each moving coil consists of two coils which have a rigid mechanical connection between them and which vibrate in a strong magnetic field. The signal to be reverbed is fed into one half of the coils and picked up by the other. There are mechanical dampers inserted at specially calculated intervals along the coil spring. On plug-in printed circuit boards are an input amplifier, output amplifier, and two attenuation amplifiers for each channel. The two springs with the magnet systems and various supporting systems are mounted in a rigid cardboard tube which is filled with porous foam material. The tube is mounted on a single-point pendulum suspension, whose natural resonance is below 1 Hz. The whole unit is mounted inside a strong wooden box, with additional sound insulating material.

The "dry" signal to be reverbed is fed in phase to each moving coil half on the ends of the spring. The reverberated signal is picked up by the two remaining coil halves, amplified, and connected in opposite phase. Motional feedback is used in the attenuation amplifiers permitting the variation of decay continuously from 2 to 4.5 seconds in each channel. Because the amplifiers are controlled by a d.c. voltage, remote control is possible.

Input and output levels of the amplifiers are plus 6 dB, with an input impedance of 1 k ohms and an output impedance of 50 ohms.

Okay...so what is different about these spring reverb units that prevents the "boinnggg" coloration of the signal? The secret is in the preparation of the spring wire itself. By a special "denting" process, the wire is "etched" and thus has thousands of discontinuities along its length. As much as 20 percent of the wire material is removed during this process which reduces the mass of the wire, which is an aid in the propagation of higher frequencies. Below 1 kHz, the individual turns of the spring are deformed by bending parts of the coils toward the spring axis. Selection of the turns that are to be deformed is done statistically, so that along the length of the spring some coils are quite stretched and elongated, while at other intervals, some coils are considerably compressed. All of this is in aid of maximum non-homogeneity of the surface of the wire, which permits a high degree of statistical diffusion in frequency and time domains. In other words, for the production of as non-coherent a sound as possible. The system works and not only is the "boinggg" eliminated but the reverb itself has an exceptionally natural character akin to a good chamber.

Having briefed us on the BX-20, the AKG engineers took us into an adjoining room, where there was a typical stereo system set up in front of several rows of chairs. However, in addition to the pair of Tannoy speakers up front, there was another pair in the rear of the room, facing forward.

The demonstration began with the playing of a record of Bruckner's 7th Symphony, in straight stereo through the front speakers. Nice enough, but nothing spectacular. Then the same recording, plus 30 milliseconds of delay through the AKG digital delay unit to the rear speakers. Aha! A very considerable improvement in acoustic perspective. Next, the same recording, sans digital delay, but with about 2.5 seconds decay through the BX-20 unit to the rear speakers. A different perspective, but interesting and much preferable to straight stereo. Finally, the same recording, plus the 30 mS digital delay, plus the 2.5 seconds decay through the BX-20. WOW!! This was sensational. It is by now a cliché, but the walls of the room did indeed seem to "fall away," giving a tremendously enhanced "concert hall" perspective to the sound. This was pseudo-quadraphony of a very high order, and it was hard to believe that a stereo recording could be processed to achieve such a thrilling and very desirous sound.

Why, this could revitalize even the oldest of stereo recordings.

Ah, well! A great sound, but the digital delay unit is expensive (and now you know why AKG is so anxious to reduce its cost) the BX-20 is over $3500, and that makes the whole thing "blue sky," right? Well, friends, not entirely. After this demonstration, the AKG engineers flipped us by handing around a very junior version of the reverb spring, and calmly stating that it was the prototype of a consumer version, to sell for "around $300." To be sure, it would have a restricted decay, on the order of 1.8 to 2.4 seconds, but that would be enough to do the trick. There already exists a 6X10, with essentially the same performance as the larger unit but simplified and with some frills removed...and at half the price! expect to have one of these AKG reverb units before long, which I will combine with my UREI Cooper Time Cube acoustic delay line (At least I hope it can be used!), and see if I can come up with that great sound I heard in Vienna.

The following day we visited the microphone production facilities at AKG and were impressed with how many parts such as diaphragms, moving coil assemblies, miniature transformers, etc. are made with such precision, many on proprietary machines designed for these fabrications. It was apparent that mass production of high-quality micro-hones requires very specialized facilities.

That final evening, we were driven over 80 miles north west of Vienna, to the medieval town of Durnstein, where we enjoyed cocktails on a bluff overlooking the Danube, and then a superb dinner in the Refktorium, amid much wine and good fellowship. The people of AKG were great hosts, who did themselves proud in the best tradition of Viennese hospitality. We learned a great deal, under the most pleasant auspices.

(Source: Audio magazine, Mar. 1975; Bert Whyte)

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