Need help matching speakers for MAX9744

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marine_hm
 
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Need help matching speakers for MAX9744

Post by marine_hm »

I don't know much about watt, ohms ratings for audio. I have the MAX9744, want to hook up 2 1.5" 10W full range speakers into a project. There are two versions; 4 and 8 ohm. My main concern is to not max out the amp, causing it to overheat.
Operating voltage: 12V

One more question: If I wanted audio input from an Ipod or Iphone; I would just cut off one end of an audio cable and strip back the insulation and connect that to the input on the MAX9744?

Thanks in advance. Can't wait to show off my project once completed.

Nick

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adafruit_support_mike
 
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Re: Need help matching speakers for MAX9744

Post by adafruit_support_mike »

For speakers, 'ohms' are a term of convenience. The actual relationship between voltage and current (which we define as 'resistance') is complicated, so we just sort of wing it and say, "on average, the speaker behaves about like an R ohm resistor."

The relationships between voltage, current, and power are straightforward, but you have to run the numbers a few times before they start to feel comfortable.

The voltage-to-current relationship is Ohm's Law, most commonly written V=IR, with V being voltage, I being current, and R being resistance. If you have any two of those values, Ohm's Law tells you the third.

The power equation is P=IV, with P being power and IV being current and voltage again.

You can combine the basic power equation with Ohm's Law two ways: P=I(V=IR)=I^2R, and P=V(I=V/R)=V^2/R

Those are useful when it comes to estimating the current through a speaker.

The thing about power that takes some getting used to is the increase in both voltage and current.. if you send 1 volt through a 1-ohm resistor you get 1 amp of current, and doing so uses 1 Watt of power (P=1V x 1A = 1W). If we double the voltage, we have 2v across the 1-ohm resistor, and that will produce 2A of current.. no big surprise, since Ohm's Law tells us the current can be treated as a function of voltage and resistance. That means both terms in the power equation have doubled though, and P=2v x 2A = 4W of power.

The relationship between I and V means that doubling either one quadruples the power. More generally, power is proportional to the square of the voltage or current.

So.. to run an 8-ohm speaker at 10W, we estimate the current by starting with P=I^2R and plug in the values we know: 10W=I^2 x 8-ohm, which morphs into I=sqrt( 10W / 8-ohm ) == 1.118A

To estimate the voltage across the speaker we start from P=V^2/R and munge things around to get V=sqrt( 10W x 8-ohm ) == 8.9v

You'll need at least a 9v power supply capable of delivering 1.2A to drive one 8-ohm speaker at 10W. That isn't too bad.

You can crunch the numbers the same way to find the current and voltage requirements for a 4-ohm speaker, but there's a shortcut: the extended power equations tell us that voltage and current are proportional to the square root of the resistance. If we drop the resistance by a factor of 2, we lower the resistance by sqrt(2) and raise the current by sqrt(2).

Those values are roughly 70% and 140%, so dropping the resistance of an 8-ohm speaker to 4-ohm drops the voltage from 8.9v to about 6.3v, and raises the current from 1.12A to about 1.6A. The 4-ohm speaker will give you the same volume from a lower supply voltage, but will draw more current.


So overall, you want a 2.4A power supply for the 8-ohm speaker and a 4A power supply for the 4-ohm speaker. Those values will give you full output power, but will keep the speakers from seriously overloading.
marine_hm wrote:One more question: If I wanted audio input from an Ipod or Iphone; I would just cut off one end of an audio cable and strip back the insulation and connect that to the input on the MAX9744?
Yep, that will work.

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ImaginaryAxis
 
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Re: Need help matching speakers for MAX9744

Post by ImaginaryAxis »

If you want 20W total from a 12V, you will need to use the 4 ohm speakers.

Without going through the math, take a look at pg. 8, bottom right graph, "Output Power vs Supply Voltage." This graph is specifically uses 4 ohm impedance. At 12V and at 10% THD+N, you will reach 17W total. You push the output power to 20W , but the distortion will be higher than 10%. What it will be is anyone's guess, since Maxim did not provide that type of graph. You will also need to ensure your speakers can handle the power of your application.

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marine_hm
 
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Re: Need help matching speakers for MAX9744

Post by marine_hm »

You answered both of my questions. AND MORE Thanks

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marine_hm
 
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Re: Need help matching speakers for MAX9744

Post by marine_hm »

My latest Bluetooth speaker with the MAX9744:
Inspired by the payphone on Adafruit.com
phone.JPG
phone.JPG (78.34 KiB) Viewed 191 times
Lift the handset to turn it on. Turn it off by putting it back on the hook. You guessed it... The dial is the volume!

Love it. Thanks Adafruit!

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adafruit_support_mike
 
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Re: Need help matching speakers for MAX9744

Post by adafruit_support_mike »

Looks good.. thanks for posting the photo!

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