Audio Amplifier Power Basics

Discussion in 'The Projects Forum' started by Alicec63, Jun 20, 2013.

  1. Alicec63

    Thread Starter New Member

    Jun 20, 2013
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    I am new to audio amplification so forgive my inexperience here... And I may be wayyyy off track... But from looking through datasheets of Audio Amplifiers (looking to use the TPA2012), obviously as the supply voltage decreases, so does the output power capability.

    I guess I have 2 questions really. My first is, to amplify an audio signal from an mp3 player/ipod that is connected to a set of 8 ohm speakers, what sort of power output would be necessary to get the output to a 'reasonablly loud' state - i understand that is very vague. But currently the speakers connected are very quiet - so would just like to boost the volume!

    Secondly, if the amp is powered via batteries, obviously the voltage of the battery is going to drop when they are 'running out' so will this limit the output power of the amplifier as its supply voltage is dropping? Meaning this would have a direct impact of the maximum volume outputted from the speakers. If so, ideally I wouldnt want the output to get quieter as the batterys are dying?

    Hopefully that all makes sense. Any help on this would be great.

    Thanks!!!
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    The datasheet says 2.1W into 4ohm speakers @5v supply, it will work at 3.6v but at only 0.7w.
     
  3. Veracohr

    Well-Known Member

    Jan 3, 2011
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    Your speakers should have a power capability rating, and your amp should approximately match that.
     
  4. #12

    Expert

    Nov 30, 2010
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    This is all just a very typical problem. "My MP3 player can't be heard from 10 feet away. I wish I could hear it well at 15 feet away, and the batteries never get tired".

    You need an amplifier, which you have already figured out, and good speakers make a world of difference, but batteries get tired, and that's all there is to it. My personal experience says that good speakers make a world of difference. That's where your money should go. I don't mean pretty pink speakers with unicorns and flowers on the case. I mean good efficiency in decibels per watt. If the speakers don't tell that number, it's because they are miserable speakers that have nothing to brag about.

    Amplifiers of 1 or 2 or 5 watts are not a problem at all. Build one, buy one, they are all good enough for an MP3 player. The problems with getting good quality sound from a small amplifier were taken care of when integrated chips were invented.

    As for the batteries, either don't use batteries or buy good speakers. Think about it...where does the power get used up? In the speakers. I've seen speakers that were 100 times as efficient as some other speakers. Buy the good speakers or watch 99% of your battery power turn into heat instead of sound.

    So, this turns into a shopping expedition.
    My living room speakers produce 105 decibels from one watt of power at 3 feet (one meter) of distance, but they weigh 65 pounds each and cost $800. If you could find speakers that produce 95 decibels per watt, you would be doing very well!

    One book (Sound System Engineering by Don and Carolyn Davis) tells me that long term, conversational level, sound loudness, is 70 db. If you buy speakers that can produce 70 db/watt-meter, the sound you would get would resemble a person talking to you from 3 feet away while you pay 1 watt of power. If you can buy speakers that produce 80 decibels per watt-meter, you can get the same loudness for a tenth of a watt. 90 decibels sensitivity, and you get the same loudness for 1% of a watt. Get the picture?

    Put your money in the speakers or put your money in the batteries. The big difference is, you only have to buy the speakers once.

    And, while we're at it, there is no such thing as a 3 inch Bass speaker. You have to move some air to get Bass. Listen to what you are considering for a purchase, and listen at the power level you expect to use. Any speaker can sound loud with a Crown 400 watt amplifier. Try your suspected speakers with a battery operated amplifier, then think about whether you want to carry AA batteries or D cells.

    This is what I call swapping brains for money. Pick one. You can throw a lot of money at crap or you can spend some brains on the job.

    And that is my opinion.
     
  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Good speakers are very important for sound efficiency.

    Decent 8" drivers in cabinets (speaker boxes) will produce a pretty loud result for room listening, even at 1W or 2W RMS.

    Crappy little 2" speakers glued into the side of a plastic appliance will not.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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  7. Alicec63

    Thread Starter New Member

    Jun 20, 2013
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    Thanks a lot for your input. It's backing up what I already thought, so I'm glad I'm on the right track.

    "Your speakers should have a power capability rating, and your amp should approximately match that. "

    In terms of quote above, how close should these be rated? Is it better to have an amp that can provide more power than the speaker, or vice versa? Say a 1.5W amp for a 1A speaker? Would this cause damage to the speaker if at full 'whack'?

    As far as picking speakers go, I am pretty limited due to the fact they are to be intergrated into a product. And also need to be pretty cheap! The guy also doesn't really want the speakers changed either! So trying to make the best out of what we've got!
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    The power rating of the speaker should match or exceed that of the amp.
     
  9. Alicec63

    Thread Starter New Member

    Jun 20, 2013
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    Initially that is what I thought, but I have read a few things on the contrary to this i.e.

    "Over-powered
    Run the risk of blowing speaker drivers by physically driving the elements too far
    in the enclosures...

    Underpowered
    Run the risk of driving power amp too hard to get desired SPL, causing clipping
    Clipping causes more THD (total harmonic distortion)
    Sends more signal to tweeters
    Risk blowing tweeters due to thermal overload
    In general, a good rule of thumb is to choose an amp that has a calculated power rating
    1.6-2.5 times the power rating of the speaker(s) at the specified impedance. See the​
    section on the following page for specifics."
     
  10. #12

    Expert

    Nov 30, 2010
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    Try to keep your amps and watts in order.

    My decision is to use speakers that are rated for twice the power the amplifier can produce. Simple reason: A sine wave (musical signal) fully distorted into a square wave, will deliver twice as much power as an equal voltage delivered as a sine wave.

    There are exceptions, but you are talking about AA batteries and $2 speakers. The high tech esoterica barely matters at that level of performance. Just get the speakers rated for twice the amplifier power and expect the owner to crank it as high as it will go. It will sound like crap, but it won't smoke. After he replaces the batteries, it will still work.
     
  11. Alicec63

    Thread Starter New Member

    Jun 20, 2013
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    Ha yea I meant 1W speaker not 1A! ...Long week!

    Ok I will stick to using a speaker with a higher rating than the amp! Just so it doesn't go bang!
     
  12. wayneh

    Expert

    Sep 9, 2010
    12,155
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    I might not demand a 2X safety factor but I sure as heck wouldn't recommend a system where a mere turn of the volume dial "guarantees" damage.
     
  13. Alicec63

    Thread Starter New Member

    Jun 20, 2013
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    Thanks again for your help.

    Out of curiosity, if I have an amplifier that is capable of 1.5W and a speaker rated at 1W. Is there a way of limiting the amplifier so that it only runs at about 0.75W?
     
  14. MrChips

    Moderator

    Oct 2, 2009
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    You could put a fuse in series with the speaker.
    I wouldn't worry about it.
     
  15. Alicec63

    Thread Starter New Member

    Jun 20, 2013
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    If I know the input voltage to the amp and the load resistance, can't I just select a suitable gain to limit the output power?
     
  16. MrChips

    Moderator

    Oct 2, 2009
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    No. The output signal is the input signal multiplied by the gain. You need to limit the input signal.

    High power speakers can be protected from overload by placing a suitable incandescent light bulb in series with the speaker.
    As the current increases the bulb lights up and its resistance increases thus limiting the current. Works like a compression circuit.

    For a 1W speaker with a 1.5W amp I wouldn't worry about it. You will know the speaker is being overloaded from the severe audible distortion.
     
    Last edited: Jun 26, 2013
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