A few Audio amplifier questions
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A few Audio amplifier questions
Hi there, I'm looking to design a board that amplifys an mp3 player or the like. I need 1 watt out @ 4 ohms, and that is where I have a question. I would use a 1 watt 4 ohm amplifier, but they are somewhat rare and expensive. I was thinking, "couldn't I just use a .5 watt 8 ohm amp?", since the speaker would draw twice as much power as the eight ohm. Is there anything wrong with this theory? I never had a class on audio, so that is why I'm asking. Any help is greatly appreciated.
- easternstargeek
- Posts: 347
- Joined: Mon Dec 13, 2010 1:39 pm
Re: A few Audio amplifier questions
You're basically on the right track. With audio amplifiers, there are a few things to consider:
1. The electrical power delivered to the speaker will be a function of the supply rail voltage of the amplifier and the speaker impedance. Other things being equal, doubling the speaker impedance will result in half as much power to the speaker. On the other hand, power goes as the square of the voltage, so doubling the voltage will quadruple the power (again, other things being equal).
2. Most amplifiers can only work reliably and safely within a limited range of load (speaker) impedance. In all cases, lowering the impedance of a single speaker, or wiring multiple speakers in parallel, will increase the amount of current delivered by the output stage. Some amplifier designs have protection circuits that will prevent damage to the finals in such a case. Others don't, and may fail outright while attempting to drive a very low load impedance.
Besides the risk of failure, most amplifiers rely on feedback to achieve better sonic performance. The presence of feedback makes the amplifier sensitive to the load impedance (this has more to do with the complex AC impedance characteristics, which are also a function of frequency). The bottom line is that some loads may drive an amplifier into instability. Better amplifiers are fairly insensitive to this phenomenon when used with conventional speakers.
In general, a complete amplifier spec will show the output power into a load of a given impedance. (1 watt, 4 ohm load). If you hooked up the same amp to an 8-ohm speaker, you'd only get 1/2 watt of power at the voice coil.
3. There's power, and then there's sound-pressure level (SPL). The two are, more or less, directly related. Double the wattage, double the SPL. Different speakers of the same impedance, however, may differ in their efficiency. A less efficient speaker won't sound as loud, other things being equal.
4. Finally, the frequency distribution of your audio has a huge impact on the instantaneous power delivered to a speaker, because the AC impedance is complex, and depends on the frequency. All speakers have a resonant peak- a frequency where the speaker is most efficient.
This has been a very cursory review of an extremely complicated topic. Much as been written about power amplification of audio signals, but in practical terms, the theory is pretty well-understood. It is only when you start going to the rarefied realms of boutique hi-fi and esoteric amplifier designs that you get into the weird stuff (and unfortunately, a great deal of pseudo-scientific hokum as well- so be careful- it's a highly contentious topic that is best avoided, especially for newbies)
Hope this helps, and have fun!
1. The electrical power delivered to the speaker will be a function of the supply rail voltage of the amplifier and the speaker impedance. Other things being equal, doubling the speaker impedance will result in half as much power to the speaker. On the other hand, power goes as the square of the voltage, so doubling the voltage will quadruple the power (again, other things being equal).
2. Most amplifiers can only work reliably and safely within a limited range of load (speaker) impedance. In all cases, lowering the impedance of a single speaker, or wiring multiple speakers in parallel, will increase the amount of current delivered by the output stage. Some amplifier designs have protection circuits that will prevent damage to the finals in such a case. Others don't, and may fail outright while attempting to drive a very low load impedance.
Besides the risk of failure, most amplifiers rely on feedback to achieve better sonic performance. The presence of feedback makes the amplifier sensitive to the load impedance (this has more to do with the complex AC impedance characteristics, which are also a function of frequency). The bottom line is that some loads may drive an amplifier into instability. Better amplifiers are fairly insensitive to this phenomenon when used with conventional speakers.
In general, a complete amplifier spec will show the output power into a load of a given impedance. (1 watt, 4 ohm load). If you hooked up the same amp to an 8-ohm speaker, you'd only get 1/2 watt of power at the voice coil.
3. There's power, and then there's sound-pressure level (SPL). The two are, more or less, directly related. Double the wattage, double the SPL. Different speakers of the same impedance, however, may differ in their efficiency. A less efficient speaker won't sound as loud, other things being equal.
4. Finally, the frequency distribution of your audio has a huge impact on the instantaneous power delivered to a speaker, because the AC impedance is complex, and depends on the frequency. All speakers have a resonant peak- a frequency where the speaker is most efficient.
This has been a very cursory review of an extremely complicated topic. Much as been written about power amplification of audio signals, but in practical terms, the theory is pretty well-understood. It is only when you start going to the rarefied realms of boutique hi-fi and esoteric amplifier designs that you get into the weird stuff (and unfortunately, a great deal of pseudo-scientific hokum as well- so be careful- it's a highly contentious topic that is best avoided, especially for newbies)
Hope this helps, and have fun!
Please be positive and constructive with your questions and comments.