An Open Source Hi-Fi Amplifier

Part 1. Introduction

First of all, I'll explain what i mean by “Open Source”. If you browse the audio forums on the internet, you will find that most people who post designs for audio amplifiers on these forums do one or more of the following things with their designs:

  1. Insert “easter eggs” in the design.
  2. Use obsolete or exotic components.
  3. Show a schematic, but never a PCB layout.
  4. Making performance claims that are almost impossible to duplicate.

All four of these things are intended to make you buy a kit of parts from them. There are advantages to buying the kits, and indeed many companies throughout the history of electronics have done extremely well. But the companies that do well, did not do most of what is listed above. As a matter of fact, one company that did quite well for a very long time, Heathkit, offered their assembly manuals separately (and very inexpensively), and you could buy the manual, gather your own materials if you wished, and build the equipment from scratch. That was before the internet became a convenient way of getting information. Before I get into describing the idea of an “open source” amplifier, i will expand a little on the list above…

Easter Eggs

An “easter egg” is an intentional mistake. Some of these take the form of an incorrect component value that's critical to the performance of the amplifier. This is the most subtle form of sabotage. The circuit works, but is far from working to the advertised performance (if the spec is a realistic one). This would take the form of a resistor value being 10k, instead of 1k. In the right place, this type of error would not stop the circuit from working, but might increase the distortion to audible levels. Only somebody with a lot of experience or an engineering degree might notice this error. Another “easter egg” could be a transistor or other polarized component drawn backwards in the schematic. Something like this can cause catastrophic failure of the circuit, resulting in a mass of burned components. Another intentional spoof is to leave a wire out of the schematic or a trace missing from the PC board artwork. Another, more effective method of obfuscation, is to leave out the component values from the schematic. While this is commonly done in engineering texts to show how a circuit works, it presents a huge obstacle for somebody trying to build a working circuit.

Obsolete or Exotic Components

When somebody publishes a design for an amplifier, they may design it with obsolete or rare components if they are intending to sell kits. The kit maker may have a warehouse full of these parts, and they are available nowhere else. Even if there aren't “easter eggs” in the design, substituting other, more commonly available parts presents another engineering challenge in order to get the circuit working properly. The difference in electrical characteristics may require adjusting resistor or capacitor values, or differences in pin assignments may require changing the circuit board. Again, the intent is to make the amplifier difficult to duplicate, or to force a builder to buy parts from the kit maker. The prices of existing stock of these parts elsewhere, and the proliferation of counterfeits of these components make them difficult to obtain (except, of course by buying from the kit maker).

Omission of a PC Board Layout

Some causes of distortion in amplifiers relate to the physical placement of components and proper layout of the circuit board traces. Without a copy of the (hopefully) correct circuit board layout, any hobbyist's attempt at creating one's own circuit board may cause performance issues.

Impossible Performance Claims

There are actually two causes of this. The first is benign…. the designer used simulation software, and never actually built and measured the real world performance. There will always be losses of performance between a simulation and reality. The second is not so benign. Somebody builds a copy of the circuit, measures it's performance, and it is an order of magnitude poorer than the claims made. The builder contacts the kit maker, and asks why the performance is so poor, and the kit maker claims that in order to get that level of performance, they must buy the kit.

What I Mean by Open Source

Like software code, circuit design uses many “building blocks” that are fairly standard. I will cover several common building blocks, and work out an amplifier design using them. The design will use inexpensive and commonly available components, and where component substitutions are likely, the methods of adjusting the circuit to compensate will be explained. The circuit board layout will be done on single sided PC board to simplify fabrication. The performance claims will be reasonable, so that even if the construction isn't perfect, the performance will be reasonably close to these goals. The tools used for producing the schematics and PCB layout will be Kicad, which is an open source software suite. Kicad is available on Linux, Windows, and MacOS platforms. The design will be scalable. While I will optimize the design for 75-100 Watts per channel, the method of scaling the design up to about 300 Watts will be explained. The final construction (i.e. chassis, power supply, etc… ) will be discussed, but not “set in stone”, but left up to the builder. Hopefully, by the time this project is complete, experimenters who have followed this article will have enough knowledge to “roll their own”, and possibly make their own improvements. A few “add-ons” will also be discussed, protection circuits, signal and clipping indicators, balanced input converters, and even a method of tailoring damping factor (and maybe a few more as they come to mind). One of the possibilities with having this information available, is that somebody may want to start a company to build amplifiers. I might even put down on paper (figuratively speaking) an idea I have had bouncing around in my head for a few years now…. a 100% Analog 5.1 Channel Surround Processor.

Why Build Amplifiers?

To listen to audio, you need to do something that takes a lot of energy, and that's to move air. If you like realistic surround sound when you're watching a movie, or like to feel like you're at a concert, you need to move a LOT of air. Air has a density of 1.225 kg/m3 at sea level. A 12' woofer with an excursion of 1/2“ displaces almost a liter of air, so that's about 1.225g of air. It may not seem to be a lot, until you consider that most speakers have a conversion efficiency of about 1% (electrical to acoustic energy). The subject of speaker efficiency will be covered later. It takes the same amount of energy to move 1 gram of air, as it might take to move an efficient mechanical actuator with a mass of 100 grams the same 1/2 inch.1)

In this modern age of multichannel sound, and home theater systems, an amplifier is required for each channel, although the power requirements for each channel may differ. The Subwoofer channel will require more power than any of the other channels, and the Center channel usually requires twice the power as any of the other remaining channels. We will focus on a design for an amplifier that can be duplicated as many times as needed2), and is compact enough to fit multiple channels in a standard chassis. Another thing to explore, is the use of “multipurpose” circuit board layouts, and that will be covered in a later chapter (By “multipurpose”, I mean that while I may not use all the “building blocks” discussed, the circuit board layout can include traces etc… for more complex circuits. With a simple amp, these areas are “jumpered out”. for the more complex circuits, these areas are used as laid out. 3)

As the demand for home theater systems increases, the demand for high performance amplifiers increases. Many current, name brand amplifiers perform well, but an amplifier needs to perform one job, and do it well, and that is to drive speakers, which in turn move air. An amplifier, basically should act like a “piece of wire with gain”. The amplifier should introduce as little in the way of distortion and noise as possible, and should closely control the speaker's voice coil motion in order to prevent the speaker enclosure from coloring the sound.

This is the first of a series of articles. The intent of this series is to provide educational information on how amplifiers work, and how they are built, and also to provide an open source design, that anybody with sufficient financial backing can put into production with little or no R&D cost.

Engineering Electrical_Engineering

Efficient actuators have limitations that make them very bad for driving speaker cones. It has been tried before.
Did you want 5.1 channel sound, or 12.2 channel sound, or just old fashioned 2.0 stereo?
This is a common practice in commercial audio manufacture, where model “A” may be a simple 8 transistor amplifier, but model “B” has 6 additional transistors. The same circuit board mask is used for both models. this reduces tooling costs over making two different boards.

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