Audio Amplifier Power Supply Circuit

As you've probably gathered from my previous posts, I'm working on building myself a high-power audio amplifier.  I've found that both the two books I own and the internet share a common method of documenting amplifier builds. This method showcases the circuitry that revolves around the amplification, and often glosses over building a power supply for said circuitry. I mean, go figure right?  What I've wanted for a while now, not just with this project, is a reliable method for building an amplifier power supply.  In the past, I've looked at schematics for audio amplifiers and realized how they function for the most part, and then felt stuck when I saw rails for power. After spending some time reading and searching the internet, I've come to a few conclusions, and feel that I have a few new tools at my disposal.

When it comes to building a power supply, the first thing you need to do is find out what supply voltages the amplifier circuit can run off of, as well as what you want your max output power (in watts) to be.  In my case, I'm using solid state amplifier IC's that require positive and negative voltage sources, as well as a ground connection. So along with providing the right amount of output power, I'll also need a supply that can put out both a positive and negative voltage. You'll be able to perform all of the necessary calculations to build your power supply by knowing the required values of these voltage sources.

For example, say I want to build a stereo amplifier that can put out 60 watts RMS per channel running off of a supply voltage of +/- 20-45 volts.  In order to calculate what I need for a power supply, I'll first use the equations Power = Amps * Volts [P=IV] and Volts = Amps * Resistance [V=IR] to get P = (V^2) / R, or V = sqrt(RP) to calculate the RMS voltage I'll need from my supply. Using my desired output of 60 watts through 8 ohms:

           
 V = sqrt(60*8) = sqrt(480) = 22 volts.     (1)

I then convert this value from RMS voltage to standard voltage by multiplying it by sqrt(2), or 1.414.

1.414 * 22 = 31.1 volts = 31 volts.     (2)

Next, I need to account for any drops in voltage that will occur from my power supply circuit.  This includes the approximate values of a 5 volt power supply droop if running at full power, a 2 volt drop from the amplifier circuit itself, and a 1 volt drop from the power supply circuit's rectifier, bringing the total drop to 8 volts. I also need to convert my voltage back to an RMS value. 

(31 volts + 8 Volts) * 1/sqrt(2) = 39 volts * .707 = 28 volts RMS.     (3)

This is the number I've been looking for! In order to provide the output power that I want, I'll need a transformer with +/- 28 volt dual secondary rails.  In order to buy the correct transformer, I also need to know its VA rating.  This is simply volts*amps, and should be specified with the transformer's specs.  Using ohm's law and our voltage value from (1) and Ohm's law, we can calculate that:

22 volts / 8 Ohms = 2.75 Amps.     (4)

We then need to increase this value by 20% to account for approximate losses, and then double our answer because we'll be powering two channels at 60 watts each.

2.75 Amps * 1.2 * 2 channels = 6.6 Amps.     (5)

Next, we multiply this number by the voltage value that we obtained through (3) in order to get the desired VA rating of our supply.  In case 4 ohm speakers are used with the amplifier, this rating should be doubled.

6.6 Amps * 28 Volts = 184.8 VA * 2 = 370 VA.     (4)

You now have the power requirements for your transformer! 370VA, with +/- 30V dual secondaries.  I'd recommend getting a toroidal transformer in order to keep electromagnetic noise to a minimum.

In order to complete your calculations for making a power supply circuit, you'll need to figure out what values you want to use for your reserve capacitors (CR). You can do so using the rule of thumb of 1000 microfarads per 10-15 watts output. In the case of my above example, I'd use capacitors rated for (120 watts / 15 watts/microfarad) * 1000 = 8000 microfarads at minimum.

As a side note, based on my schematic, CE is used to reduce external noise, and the four CS capacitors are used to reduce noise from bridge rectifier BR. Both the CE and CS capacitors are ceramic.