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An Open Source Hi-Fi Amplifier Part 2 Reference Designs

Reference Designs

Reference designs have played a large part in the electronics industry over the years, both for engineers and hobbyists. Other technical literature such as engineering periodicals and hobbyist magazines also “fueled the fire” of innovation in the electronics industry. The electronics industry has always been a good example of the concept of “open source”. Large corporations, wanting to see their ideas proliferate throughout the industry would publish “reference designs” in their data books. Of course, if you wanted to build whatever you saw in the reference design, you would buy the vacuum tubes or transistors listed in the schematic, and that component was usually the product of the company publishing the design. RCA was well known for including reference designs for everything from simple amplifiers, to complete color TV sets in the reference design section of their data books. National Semiconductor, and other semiconductor companies produced books that were a compilation of “application notes” (another name for reference designs, but with a lot more information on how the circuits worked). Engineering periodicals have almost always published things like “Design Ideas” where an engineer comes up with an interesting circuit and publishes it in an engineering periodical. Hobbyist magazines often would glean from these sources, and publish a project based on these sources (or in some cases, the writer of the article would come up with something completely new, and share it with his audience). Or a hobbyist, tinkering at home would come up with an idea, and sell it to one of the large corporations, send it in to the engineering periodical, or the hobbyist magazine, and thus keep the whole process flowing. It was this process that created the “computer revolution”.

How Hobbyists Built an Industry

The process of progress in electronics has always been a mix of educated engineers, hobbyists, and tinkerers providing new ideas to each other. The tinkerer or hobbyist today is often the engineer of tomorrow. Open literature describing new ideas and technology fuels the cycle. Originally, the primary focus of the electronics industry was radio communication. While many engineers drove the industry, there were many more hobbyists that built “homebrew” radio equipment. These hobbyists were regular people who found the radio phenomenon interesting. Eventually, there were enough of them, that governments found it necessary to regulate and license them as “amateur radio operators”. Many of these “amateurs” eventually became so expert in this new field, that they went to school to become engineers. In the US, Hiram Percy Maxim formed the American Radio Relay League, an organization of amateur radio operators. Most amateur radio operators joined the organization, but even if they didn't, there were publications from the ARRL that they could buy, most notably QST magazine, and the annually published Radio Amateur's Handbook. Both publications had articles containing schematics and construction details for all kinds of electronic equipment. Many amateur radio operators were also engineers, and the “give-and-take” between the tinkerers and engineering community propelled a huge creative engine. In essence, this was the original “open source” model for an industry.

Where Does Audio Enter The Picture?

Reginald Fessenden invented Amplitude Modulation of radio signals in 1906. At the time, most radio transmitters used spark gaps as the method of generating RF energy. This method did not create a continuous RF carrier wave, which made it unsuitable for transmitting voice signals. There were other methods that did meet the criteria for sending audio, and the earliest was the Alexanderson Alternator. This radio transmitter used a huge motor coupled to an alternator that produced the RF energy directly. Morse code was sent by turning the field coils of the alternator on and off with a relay controlled by the morse key. Fessenden's innovation was to control the field coil with an analog current, using a carbon button microphone. the result was an amplitude modulated radio carrier. It was crude (even cruder were early experiments where the carbon microphone was inline with the antenna wire…) but it worked. Later, when transmitters began using vacuum tubes, the audio modulator became an audio amplifier with it's output injected into the supply rail for the final RF amplifier. In order to get 100% modulation, the modulator had to have as much audio power output as the transmitter itself. Also, along the same lines, the receiving apparatus changed. In the early days of radio, the audio came directly from a diode detector to a very sensitive set of headphones. As vacuum tube technology entered the picture, first the headphones could be driven by a vacuum tube, but then the idea of using a loudspeaker driven by more powerful tubes began to be the norm. People could now hear news and music broadcasts in their home. The listeners wanted clearer audio from the broadcasts they received, and the radio engineers improved the amplifiers used for modulating the signal, and the engineers at the radio factories improved the amplifiers and speakers for the radio receivers. All the while, the “give and take” process between engineers and hobbyists continued to fuel the creative process. But now there were two “flavors” of enthusiasts, the radio amateurs, and the “audiophiles”. The science of electronics was beginning to specialize. Audio amplifiers were finding new uses. Phonographs, which were originally purely mechanical, could now be built with electronic amplifiers. Instead of using a passive “megaphone”, a microphone, amplifier, and speakers were much more effective. Audiophiles began to tinker and experiment with the same fervor as their “cousins” in the radio amateur community. Engineers often traded ideas with the experimenters, and the state of the art, of both radio and audio, advanced by leaps and bounds, even to the point where television was invented, with several forms, from a mechanical system, to several totally electronic systems. But world events would soon put a damper on this open cooperation between engineers and experimenters, because the majority of the electronics industry would be called to war, including the bulk of the tinkerers and amateurs.

A Veil Of Secrecy

During World War 2 many of the engineers in the radio industry were asked to design radio (and other) equipment for the military. They were often sworn to secrecy, so they could no longer collaborate with their “civilian” counterparts. The amateur radio community was essentially “shut down” for the duration. In the US, the FCC issued Order 87, which essentially took amateur radio operators off the air1). Amateur periodical publications continued to be published, but with a definite “wartime” bent. Many amateur radio operators and audiophile experimenters ended up in the military, either operating or repairing electronic equipment because of their expertise. Military electronic equipment, even though it may be something as mundane as a public address amplifier, often became classified as Secret, or Top Secret, and the manual issued with the equipment would require that the equipment be totally demolished in case of enemy capture 2). Many engineers and experimenters (as well as engineering students) worked under a veil of secrecy on such things as RADAR, Radio Direction Finding, SONAR, Gun Director Systems (an outgrowth of which was the Operational Amplifier), and the worlds first digital computer. The Pre-war “open source” atmosphere had gone away, but the spirit of experimentation and ingenuity that had been fostered by it, kept it's momentum, and the engineers and (former) hobbyists alike came up with many new technologies and many ingenious solutions to what may have seemed insoluble problems.

Postwar, the Veil is Lifted

After the war, many things returned to almost the way things had been before the war. Many of the new technologies invented during the war began to trickle down to consumer items. Some of the new technologies were adopted in the civilian world as portions of them were declassified. One engineer, who had discovered the heating effect of microwaves (and could not share his discovery at the time because the magnetron tube he was testing was highly classified) went on to invent the microwave oven 3). Many returning veterans, who either went to war with prior technical know-how, or picked it up in the military created a new wave of experimenters and tinkerers. Much of the electronic equipment they had used became available in a huge surplus electronics market. The electronics hobby exploded in the postwar years. While equipment was usually bought to use, many experimenters bought surplus equipment also as a source of parts for other projects. Amateur radio operators would quite often do both, as much military radio equipment included the capability of operating within the bands assigned to amateurs. Many audiophiles would buy surplus radio modulators and public address amplifiers to tinker with for a sound system. The hobbyist publications that survived the war were now bulging with articles, on not only reusing surplus equipment, but construction projects using common components from surplus equipment. Radio and general electronic parts stores often had huge bins of these components 4). Companies like RCA, GE, and Raytheon, published data books for their vacuum tubes (as mentioned above), with often up to half of the book devoted to “reference designs”, and in the growing hobbyist market, these reference designs were gold for the tinkerer with spare time, and a fair stock of parts. The reference designs often used easily available off-the-shelf components. For the audiophile, this meant he could go to the local parts store and get everything he needed to build the 50 watt amp in his RCA tube manual. Often, an experimenter would say “how can I get more bass out of this” or “how can I get a better frequency response”?, and they would begin testing ideas on the reference design. If they were successful, they might publish their modification, as well as how they arrived at it in an audiophile magazine. Remember that many of these experimenters were also engineers, and would learn something useful, or even publish their own modifications and analysis. The “give and take” was again propelling the whole electronics industry forward.

Another invention was on the horizon that would literally launch the industry into space, and it was nothing but a grain of sand, but it would eventually solve many of the problems inherent in the use of vacuum tubes.

WARNING: Many circuits shown in the following lists of publications use “hot chassis” designs. This means one side of the power line is connected to the chassis. If the power cord is plugged into an outlet with the chassis connected to the “hot” side of the line, instead of the “neutral” side, the chassis is at line voltage potential, and is a severe shock hazard. This warning is in case anybody gets curious and wants to build a project using these older designs.

Reference Design Publications (from the vacuum tube era)

Reference designs were published by manufacturers of vacuum tubes. The most prolific and popular were published by RCA. Hobbyists were free to build the projects, but manufacturers, while allowed to experiment with the designs, had to either come up with a version that didn't infringe any RCA patents, or license the design from RCA if the equipment went into production. After the patents expired, the reference design was “fair game”, and many manufacturers borrowed from these reference designs at will. In fact, many of the “5-tube radios” from the 1960's were of almost identical design, even though there were many manufacturers. Following are examples of publications containing reference designs from the era of vacuum tubes:

Examples of Radio Hobby Publications

Audio Hobbyist Publications

Dedicated audio hobbyist publications were not as common, but many audio hobbyists published their work in “general” electronics magazines. Publishers of these magazines (Hugo Gernsback being the largest) would also print compilations of audio projects presented in their magazines. Below are a few examples:

Engineering Electrical_Engineering

1) In WWI amateur radio operators not only were taken off the air, but had to completely dismantle their equipment. In WWII however, they were not only allowed to keep their equipment, but many were allowed to operate as part of the WERS (Wartime Emergency Radio Service).
2) Often, this was required, if for no other reason, the design might be slightly more efficient in power consumption. For a military unit, this translates to fuel usage (military units often do not have the luxury of just plugging into the nearest wall outlet), and fuel consumption is, in wartime, a very high priority.
4) Radio Shack began as an amateur radio store that stockpiled surplus parts.

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