Build a 50 Watt/Channel Stereo Power Amp
Now you can have a low cost, high quality, quiet, easy to build, and reliable 50 watt per channel power amp that compares very favorably with any power amp on the market. The secret of this high performance lies in hybrid power amp technology.
A hybrid amp is a solid state device that includes a 30dB gain preamp, all power driver transistors, and integral heat sink in one compact, sealed package (sort of like a big op amp). The heat sink can be bolted onto a larger heat sink if necessary. These hybrid amps come in many shapes, sizes, and power levels from several different manufacturers, but this article will concentrate on the Sanken 50W power amp modules since these are widely available and inexpensive. Each module only needs a 1 Volt peak-to-peak input signal to deliver its rated output power at 1KHz, with a maximum distortion of 0.5%.
Referring to the schematic, the amp is powered by a bipolar power supply which not only saves us a few parts compared to a single polarity supply, but allows for direct coupling to the speaker (no coupling caps) for better sound quality. To obtain +33 Volts DC we use a 117V to 50V center tapped AC transformer (T1) and a full wave bridge rectifier (D1), which converts AC to raw DC. S1 is the main power switch, L1 is a neon bulb (such as an NE-2) pilot light, and F1 is the main line fuse. C1 and C2 minimize any residual AC ripple from the supply.
The modules are Sanken S1-1050G 50W RMS power amps, which are designed to drive 8 Ohm loads and, unlike some of the older Sanken modules, offer built-in current limiting protection. For driving 4 Ohm loads, the recommended module is a Sanken S1-1050GL. F2 and F3 protect the speakers and modules in case of malfunction, while R1/C3 and R2/C4 suppress any unwanted oscillations. C5/C6 provide additional power supply bypassing, while C7/C8 are feedback gain compensation capacitors.
On the input side, C9/C10 are input coupling capacitors that decouple DC between the amp and the source. Log potentiometers R3/R4 adjust the volume levels.
When you pick up your module, get the matching plug-in socket and spec sheets too. The spec sheets give you additional information on heat sinking, single supply operation, and operating at reduced power. The amp module heat sinks should bolt securely to a metal chassis (adding a little heat sinking compound never hurts), and all grounds should connect to one common point to prevent unwanted oscillations (important!). Use shielded cable for the inputs, but connect the shield to ground at one end only.
Mount the bypass and input capacitors as close to the modules as possible (you should be able to mount these right across the socket pins) and connect the R1/C3 and R2/C4 RF noise suppressors at the speaker output terminals. Keep any AC power wires routed away from the amps, and keep the inputs away from the outputs. F1, F2, and F3 can be panel mounted fuse holders; if L1 is a standard neon bulb you will need to use the indicated dropping resistor, however some neon bulb assemblies already have the dropping resistor built-in in which case this part won't be necessary. I'd also recommend using a 3 conductor AC power cord, with the ground wire connected to the chassis at the single-point power supply ground.
Before turning on power, check between both supply lines and ground with an Ohmmeter - if you get a short circuit, start looking for wiring errors. Double check your wiring and the polarities of all capacitors; then, without any speakers connected, apply power with the volume control all the way down and check whether any voltage is present at the speaker terminals. If this voltage is either very low or non-existent, chances are all is well - turn off power, wait a few minutes for the capcitors to discharge, hook up you speakers, and you're ready to go.
You might consider adding a few options, such as a time delay speaker connection circuit or a LED power meter. But no matter how simple or complex you make this amp, by carefully following the precautions given above you should have an easy to build and rewarding project for under $100. So, get building and enjoy!
Robert isn't kidding when he says these are high quality modules. I've built several hybrid power amps; one of these amps even put a very highly touted audiophile power amp to shame - better transient response, more open sound, and lower noise. Whether for hi-fi, studio monitor, or instrument amp, these modules give a good account of themselves. About the only caution I would add is to beef up the heat sinks a bit if you plan to use these amps for live performance.
Feature by Robert Rafuse
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