Sam just shared a new tutorial: "How to Use Speakers and Amplifiers with Your Project"
A popular question that we get all the time is ‘How do I choose a speaker or amplifier to go with my project?’. It’s a good question, and if you’re not familiar with the terminology, it can seem a bit daunting to know whether …
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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
Hi Anthony,
It’s so good to see community engagement like this, thanks for your insight and thoughts. The purpose of the tutorial was to simplify things that are often complicated for beginners and lower that entry level into using audio in projects.
It’s a great point about power supplies though, one which could be its own tutorial.
Hello Anthony,
In order to get louder performance from a speaker, so far I’m told increasing the voltage would make it louder. Say 16 Volts.
But I’d like to know how does the current (A) effect the performance of a speaker? If the voltage is lowered to 5V and higher current is supplied to achieve the speakers 30W. What would be the difference in performance of the speaker?
I’ll be using a 30W speakers 6 ohms.
Amplifier rated at 45W.
Thank you
Depending on speaker efficiency 30W can be a lot of noise. You don’t HAVE to supply them with 30W, that is what the speaker can handle safely without damage.
Cheers Bob
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Dear Ross165876,
Merely increasing the voltage may not necessarily increase the loudness. My reply in 2017 was an exercise in the application of Ohm’s law and the power equations.
In fact, increasing the voltage of the MAX9744 sold by Core to 19.5Vdc may well damage/wreck the unit. The MAX9744 operates between 4.5V and 14V.
That is because the RPi does not provide enough current to power an amplifier, even if it operates at 5V, you need an external supply.
A laptop supply which operates at 19.5 will damage it.
Therefore according to the datasheet, you can safely operate the MAX9744 between 4.5V and 14.4V. Operate at less than 4.5V, you’ll get distortion or other operational issues. Operate at greater than 14V, you may wreck the MAX9744.
So don’t just increase the voltage thinking that it will increase the volume.
Thank you,
Anthony of Sydney
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Hi Ross
Most self respecting Class D amps I have seen provide a bit of a chart listing expected maximum power at different voltage levels with corresponding current requirements. So this sort of info should be readily available for commercial units but if you are going for a home grown amp this could be anything. Also it would appear the required speaker impedance is quoted. Any deviation from this requires an output inductor value change. This info is also provided with commercial units I have seen.
Incidentally I don’t believe these “Class D” amps are that new. They smell a bit like the old “Sinclair” amps of some 50 years ago. These also had some form of PWM pulsed drive to the speakers. The difference now of course we have dedicated ICs to do all the hard work for us.
Cheers Bob
Edit: Just googled “Sinclair audio amplifiers”. Interesting. The first one apparently hit the deck in 1964. Old hat eh!!!
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Hi Ross et. al.
To my knowledge, most good Class D amps will double their power with half the impedance (In contrast to AB and other amps that usually won’t)
Anthony is right with the spec recommendations, make sure you use the right supply! Something like this might be a good fit.
Good luck with your project!
-James
Hi James
A lot of this is dependant on the source impedance of the amp. It does not matter much what class it is if the source impedance is down near zero or 0.5Ω or less this will approximately apply and is usually the current capability of the power supply which can be the limiting factor. Class A would probably be the worst in this regard and is not very efficient but I thinks rarely seen these days. Maybe some purists could have such an amp.
Public address and other systems with long speaker cable runs use a 100V or 70V distribution system to cut down on I^2R losses in the cable itself.
I think high end audio and public address are a speciality on their own and to come down to the nitty gritty requires some pretty involved math.
As far as class D is concerned there is an inductor and possibly a capacitor value along with the speaker impedance involved in recovering a good audio signal and it seems that any change to the speaker impedance requires a change in one or both of these values for optimal results.
Cheers Bob
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