So the subject of power supplies in generally tailored to the subject area at hand. In our case – we are talking about power supplies for audio equipment. Yea, I know that your stereo receiver doesn.t need a power supply – it has a 120/120VAC power plug. But, that power plug feeds AC voltage to the stereo’s internal power supply.

Hence, you do In Fact Have A Power Supply after all.

Remember this -

If it is electronic it either runs on: Batteries (and may not need an actual power supply at all) , or:

A: It uses an external power adapter to either power up, or to keep the internal batteries charged. But, that wall wart (external Power adapter) is in fact a small power supply. All the theories and practices apply as they would an actual power supply.

B: It plugs into the AC Mains (110-125VAC in the US, 220 to 250V Europe and Asia). If it plugs into the AC Mains to get power – it definitely has a power supply.

So we must define the terms, Power Supply. For the sake of argument we will use one definition:

Power Supply, An electrical/Electronic assembly that converts Alternating current (AC Current, aka: AC Mains Voltage) to a Direct Current (DC Current). The device steps the AC Voltage up or down (For audio it is generally stepped down into multiple voltages) to the desired voltage (or voltages). It is then rectified, filtered and regulated (as required) to provide a noise free and useable DC voltage to power the many and varied electronic circuits that comprise the Stereo Receiver, Stereo Amp. Mixer, Cassette Deck (yea this item dates me – I admit it I’m old), CD Player or MP3 Player.

Since all electronic circuits are not created equal (but a late physics professor pondered in class one day – “Or are all transistors actually created equal? Or are they actually different? - Aren’t Transistors made from silicon or germanium, so a transistor is a transistor? Right”. I took the challenge and argued that it was a bad statement. He argued that it is essentially true. I pointed to the over 3,000 different transistor types and how they were each suited to a specific task. He countered that we all drive cars, and yes there are Fords, Chrysler, Dodge, Chevrolet, BMW, Mercedes, Fiats, Datsuns, Toyotas… Yet we all refer to them as “cars”. In my over all challenge I made the assumption that transistors are singularly suited to individual tasks. The professor asked me to bring one of my transistorized audio amps that I was working on building to the next class, oh yea – bring a few spare components, like resistor, transistors used in the amp, and don’t forget a rectifier or two and a power transformer. Last of all, he reminded me that I should also bring a speaker….... (obviously - for the amp?)....

The next day he challenged me again, and said he would prove his point. He picked up a 2N3055 Transistor, a TO-3 Device and asked what is this – I told him it was an audio amp transistor. He took a transformer, and two rectifiers and a capacitor, added the 2N3055 Audi transistor, and added three variable potentiometers (we didn’t have the exact resistors he was looking for) and he build a regulated power supply.The he asked the fateful question, Isn’t that an audio transistor, I didn’t build an amp – I built a power supply, and a regulated power supply. Look, my 8-track plays… If it is indeed an audio transistor, and transistors are singularly tasked – why does it work as a power supply?

I pointed out that it was a power transistor, and that it was designed to handle high current. He countered that I told him it was an audio transistor, not a power transistor. So how can a power transistors amplify audio?

The Point being, as a student I (along with all the others) thought we were smart and gifted. We thought we had it all figured out. But we didn’t. If retrospect it was pointed out that he didn’t select one of the T0-92 small signal transistors, to which he responded – “Give me one, I want to humiliate you some more”. I conceded. But stated in my defense that his demonstration was a show boat, and that it wouldn’t apply unilaterally to all devices. He closed with “ Physics is not absolute, just when you think it is – some one will prove you wrong. But the laws of Physics are definable, measurable, and predictable.. Just like the 2N3055 regulator, it can be an audio amplifier, or a regulator – the theory of operation is so similar to a transistor that it really doesn’t care. Granted, some transistors work better at specific currents, voltages, even frequency of the signals – but the theory is cemented in the physics of a semiconductor device. In the future power supplies will be more compact, less defined and more versatile. Because some one is going to build that “better mousetrap”, and then some one else will build a better one…. Act not fiction….

Incidentally I got an A in his course. Learned a lot too. His humbling demonstration has served me well in supplying a fundamental thought pattern – “Why Not?” This is the phrase that drives innovation. Without it, we might as well go camping and cook by the camp fire.

I like the 2N3055 NPN Power Transistor, I also like the MJE2955, and the TIP-41C. Great regulators for those 10Amp to 35-Amp Power supplies. Properly heat sinked and used, they will provide for durable and reliable power supplies for just about any project you may conceive of.

Now we have the 78XX and 79XX family of 3-pin voltage regulators. This is definitely a marvelous age that we live in. The 78XX series give us positive 1.0 amp (yea I know most are rated at 1.5 Amps – but that is ideal, and also subject to high failure rates when used at the extreme power limits of the devices). The 79XX series are negative voltage regulators. Using the 7XXX series you can easily cobble together a functional Positive and Negative Voltage regulator. Using a transistor as a shunt or current controlling device you can supplement or magnify the regulated voltage current capacity. Or you can spend more and get an LM338 (Plus 2.7V-32VDC Regulator rated at 5.0 Amps), an LM338T (T0-220 Device which can handle 3.0 Amps). Most regulators come in varied packaging, ie: TO-220 Devices, TO-3 Power Devices, and even TO-92 Low power devices. The current trend is to move into surface mount devices, so you will find regulators in several Surface Mount Device (SMD) Configurations as well.

The proliferation of and ready supply of cheap 3-Pin regulators has transformed the Power supply category in engineering. But have they eliminated the engineering challenges of designing, building and marketing High Out put Power Supplies? In some ways, yes - in others - not so much. Granted an LM338K (T0-5 Version of the LM338T, 5.0Amp 3-Pin Regulator) will set you back about $30.00 and some change. The LM396K (10.0Amp Variable Voltage Regulator - Only available in TO-3 Package), will set you back a whopping $98.00 or more, if you are lucky.But the LM317 can be had for about $0.85 to $3.99 a pop. Using Power Transistors will not only make your power supply durable and dependable - it will lighten the drain on your pocketbook as well. Face it, the WOW! factor of a power supply with all those power transistors looks really awesome. So you can have a) Durability, b) Dependability, c) Economy and be d) Awesome?... Makes you wonder why we don't have more EEs.


The LM317T is a variable 1.5A Regulator
The LM350T is a variable 3.0A Regulator
The LM338T is a variable 5.0A Regulator
The LM3396K is a variable 10.0A Regulator
Most of the 7XXX Regulators are fixed voltage, and limited to 1.0 to 1.5A.

DIY Audio has an excellent section that discusses these regulators. I strongly suggest you visit their site:
http://diyaudioprojects.com/Technical/Voltage-Regulator/
I know it is a Tube-Amp site, but hey, even tube amps need a power supply to work.
Don't forget to open the link in a new browser, otherwise, you will loose your link to my site.

Just remember, if you are going to build a reliable and efficient bench supply, or even a supply to power that power hungry amp – use a transistor as the load regulator. A simple LM317T will easily control up to 50-Amps, if the proper Power Transistors are used and configured to. In the following web pages I hope to give several examples that will prove this very point. In one example an LM317T is used to control the base’ of tree TIP-41C power transistors. The LM317 doesn’t actually directly control any power in the power chain – it just controls the transistors that control the power. In another, an LM338T is used along with an MJ2955 Power Transistor. In this circuit the Power Transistor controls the bulk of the power supplied to the power supply output terminals, but the regulator (the LM338) also shares a portion of the power. This is a shunt regulator. Think of the transistor as a meter shunt, and the regulator as a meter. But in our case the regulator is actually controlling a small portion of the power and the power transistor is mirroring the regulating action, essentially pulling more current through the power supply than the 3-Pin regulator IC is capable of handling.

Most of all - Do Not Forget: Your power supply is going to supply the power that makes "your project " work! So spend a little time and effort and design or build a power supply that will let your project do the job you designed it for. Simple as that.

Good Luck....
Dave R. Mason Electronic Design Systems
High Forrest Technologies (aka: All About Mixers).