Dear Sam,
Thank you for the tutorial and other tutorials. One important factor that should be taken into account is the power supply. Depending on the power output of the amplifier, the power consumption of the amplifier is just as important. For example if you have a class D amplifier with 25W per channel, you may need a power supply that handles high current.
But where does one get a power supply? Certainly your RPi will not be able to supply enough current for a high power amplifier so you need to find another source of power.
Perhaps the easiest way but not the only way is to use the power supply/chargers used on laptops. There is no need to build a power supply from scratch. It is certainly safer than building one. Not any old power supply, but one that can supply enough current to the amplifier before it clips.
A lot of laptop power supplies have a voltage of 19.5Vdc. Unfortunately the MAX9744 sold by Core-Electronics will not operate at 19.5V because its operating ) voltage range is between 4.5 and 14V.
Two examples will be shown, one for 19.5V and 14V which is used for the MAX9744.
Example for the current requirements for a class D amp which can operate at 19.5Vdc
So the power delivered to each channel = V^2/R. If the speaker is 8 ohms, then the maximum power is 19.5^2/8 = 47.53W per channel. Total power = 95W, total current = P/V = 95/19.5 = .4.8A
That is if your amplifier can operate at 19.5V, the maximum power that can be delivered into 8ohms is 47.5W. The input current max is 4.8A.
Example for the current requirements for a class D amp which can operate at 14Vdc suitable for the MAX9744
So power per channel = V^2/R. Suppose the speaker is 8ohms, then P = 14^2/8 = 24.5W. That’s good because the MAX9744 delivers 20W per channel. Now the total power is 24.5 * 2 = 49W.
Therefore you will need a 14V supply that can supply P/V = 49/14 = 3.5A. Don’t go for 5V supplies as they require a higher current and are hard to find.
Speaker protection: - you may consider purchasing a speaker protection kit which prevents spikes on initial powering up of amplifier and to prevent DC going to the speakers.
POWER SUPPLIES - PURCHASE LOCALLY - purchase power supplies from Australian retailers and/or suppliers which should have these power supplies approved by the Australian electrical authorities.
Conclusion: Know your class D amplifier, check its suitable operating voltage. For the MAX9744, the maximum voltage is 14V. There are other class D amplifiers which can operate at 19.5V. But always use E = IR, P = EI, or P = I^2 R or P = V^2/R. Make sure that the total output current of the supply always exceeds the total power requirements of the speaker. For example we saw for the MAX9744 for a 14V supply, that if the power supply’s input current is AT LEAST 3.5A, then the audio should not distort.
Thank you
