Looking for advice for low power audio amp

Discussion in 'The Projects Forum' started by Jack.Straw, Jul 9, 2013.

  1. Jack.Straw

    Thread Starter New Member

    Jul 15, 2010
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    Hi. I've been building LM386 amps for a while now. They work fine, but i'm wondering if there is a better option. I'm using 8x rechargeable AA batteries (9.6 volts) for power. The amp powers a single speaker, so I am merging the left and right channel (stereo to mono) before it goes into the amp. Is there another chip that might be a bit louder using the same power source?

    Also, a couple other related questions:

    1) When salvaging used speakers I've been looking for 4"-5" that are at least 8 ohms. I've ignored the watt rating. If i wanted to achieve the best possible sound from a lower powered amp, what would be the optimal ohm/watt rating i should look for?

    2) The input jack is 1/8" (headphone). When doing stereo to mono, are resisters really needed or can i just splice the two signals together? If so, what is the recommended value that won't reduce the volume too much?

    Thanks in advance,
    -Scott
     
  2. #12

    Expert

    Nov 30, 2010
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    You could "bridge" a pair of 386 chips, one running inverted, to double the voltage to the speaker. There are tons of audio power chips. Use the search engine at a vendor site like mouser, jameco, digikey.

    When the voltage supply is low, a lower ohm speaker will allow more amps to flow at the same peak voltage.

    Do not just short your two input channels together. It shorts out the source of the signals to each other so the internal amplifiers fight.
     
  3. BobTPH

    Active Member

    Jun 5, 2013
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    The most you are going to get out of a non-bridged amp into 8 Ohms at 9.6V is about 1.4W and that would be only if the output could swing rail-to-rail. So an LM386 is probably about as much as you will get that way.

    You can get twice as much power into a 4 Ohm speaker at the same voltage.

    A bridged amp will give you 4 times the power at the same voltage. So look for an IC that as a bridged amp, or make your own out of two of them.

    You cannot simply connect the wires together to combine stereo into mono, or they would be fighting each other. Use a 10K resistor from each. Turn up the gain if you need to.

    Bob
     
  4. Jack.Straw

    Thread Starter New Member

    Jul 15, 2010
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    Thanks guys, I appreciate the replies.

    So a 4 Ohm speaker would be optimal? I thought i read somewhere that the 386 required something 8 Ohms or higher, but that was years ago and I could be remembering wrong. Would using a 4 Ohm speaker cause the chip to get too hot?

    Thanks, I will look into that now!

    I tried a 10k resistor and the volume seemed significantly reduced. This amp is intended for MP3 player / phone audio use... wouldn't increasing the gain introduce distortion?

    Thanks for your help and advice,
    -Scott
     
  5. #12

    Expert

    Nov 30, 2010
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    Distortion is not directly related to gain.
    The entire world of audio amplification (except for musicians) is about how to amplify with very little distortion.
    It has been accomplished millions of times.
     
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Ebay has lots of little class-D amplifier modules. They are extremely energy efficient from battery use, compared to something like a pair of LM386 amps.

    Like this 3W+3W stereo amp, ready built for about $12;

    [​IMG]

    It runs fine from +5v DC, so you need less batteries in total than a LM386 setup.
     
  7. Jack.Straw

    Thread Starter New Member

    Jul 15, 2010
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    Hmm.. i looked at the class D amps on ebay. There are a lot of 12v versions that are saying 10w-15w per channel. I'm going to be driving a single 4" full-range speaker. If i step up my battery pack from 8 to 10 1.2v AA batteries (12 volts), would you say that the 12v versions would be significantly louder when considering a 4" speaker?

    Also, most of the 12v class d amps i'm finding are stereo, offering 10-15 watts to each channel. The sellers specifically state that "it is not acceptable to connect the - outputs of the L & R speakers together." If i was to take my audio source from stereo to mono before going into the AMP i could use a single channel, but feel like I would be giving up half of my available power. I would prefer for the stereo to mono splice to happen after the amp if possible. Is that possible?


    Most of the 12v class D amps i'm seeing on ebay have the volume control and 1/8" input jack mounted next to each other facing forward. That setup will not work for my application as it will be mounted on a slightly curved surface. That leaves me with the following options:

    [​IMG]
    TDA2030 Mono 15W
    This is the only mono option I see. The picture says "Audio input - stereo or mono", but i'm not sure if it's really a stereo jack or not.


    [​IMG]
    10w per channel, IC not listed




    [​IMG]
    10w per channel, PAM8610 IC




    Which of those looks best to you guys?

    Thanks!
    -Scott
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    First, you did not say what the application is, so we can't really say if something will be "loud" enough.

    Also that top photo of yours is not a class-D amp it's a TDA2030 amp chip with poor energy performance.

    A 10W amp is pretty loud! Regarding number of batteries, type of amp etc etc we really need to know your application.

    Also, why do you want mono? To allow one speaker only, to keep the size down?

    That 3W+3W amp chip in my post is very energy efficient and quite loud. You can get them on ebay much cheaper (and smaller!) as just an amp PCB without the pots and connectors for $3.99;

    [​IMG]
     
  9. tracecom

    AAC Fanatic!

    Apr 16, 2010
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  10. THE_RB

    AAC Fanatic!

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    All stereo class-D amps are mono too, you just use one channel and ignore the other. When they are assembled and tiny, at $3.99 complete it can be stereo or mono just as you like. ;)
     
  11. Jack.Straw

    Thread Starter New Member

    Jul 15, 2010
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    Well, my application's constraints are due to novelty. I've been building these beer keg amps for a couple years:

    [​IMG]

    Up until now i've been making a LM386 circuit and powering it with 6x AA NiMH batteries, with a built-in charger. That setup has been acceptable since i usually use my personal model in the kitchen, the garage, or while camping. The battery life is exceptional. Recently i decided i wanted to make one that is bluetooth. I bought a couple inexpensive bluetooth receivers on ebay, both of which run on USB power (5v). I was going to use an LM7805 to drop the voltage down to 5v to feed the bluetooth receiver, but was worried 7.2 volts might not be enough so I upped my batter pack to 8x (9.6v). Once i was up to 8 batteries i thought to myself.. hell, i may as well add 2 more batteries and give a 12v amp a try. I'd like to see how good I can get one of these to sound, given my obvious constraints. The keg is 6" wide, 10" tall, and has a 1" hole near the bottom where the tap used to be. It is what it is, but I would still like to see how far i can push the envelope :)

    At someone's advice i've dropped the LM7805 in favor of a switching regulator (LM2575) to avoid wasted heat.

    What kind of speaker would you guys recommend for the 12v version?

    edit: i just purchased a couple different 5v 3w amps on ebay too... just for testing :)

    Thanks for the replies,
    -Scott
     
    Last edited: Jul 12, 2013
  12. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Clever idea. I like it.
     
  13. THE_RB

    AAC Fanatic!

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    Awesome! And it's nice too to see what someone is making as well as just answering their questions. :)

    If you are using a 5v Bluetooth receiver then the little 5v 3W class-D amp is ideal, especially for battery life. As for sound power it will be limited to about 1.5W without clipping, but 1.5W is reasonably loud with a good 4" speaker.

    Your can might be damping the speaker too much and reducing sound efficiency, is the can ported?
     
  14. Jack.Straw

    Thread Starter New Member

    Jul 15, 2010
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    Yes, the can has a 1" hole near the bottom where the tap used to be. I measured my speaker wrong though, I'm actually using 5.25" speakers. Considering I'm used to something like 0.5w from the lm386, the 3w may be the way I go for the sake of battery life. I have a 12v amp coming too though..
     
  15. THE_RB

    AAC Fanatic!

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    I think the "3W" is a little optimistic. The class-D amp is digital and has two push-pull PWM outputs running from a regulated 5v DC.

    So max audio sinewave out is 10v p/p, or 1.56W into 8 ohms. An audio squarewave could put about 3.12W into 8 ohms.

    Either way it would be louder, better quality (less clipping) and use much less power than a LM386. :)
     
  16. Jack.Straw

    Thread Starter New Member

    Jul 15, 2010
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    What is an audio squarewave??
     
  17. wayneh

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    Sep 9, 2010
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    It's what most cheap guitar tuners generate. But seriously, it's just a square wave in the audible frequency range.
     
  18. #12

    Expert

    Nov 30, 2010
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    It's a guitar into a 9 volt preamp, driving a Fender Champ turned all the way up.:D
     
  19. wayneh

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    Sep 9, 2010
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    Yeah, babeee! The twang is the thang!
     
  20. THE_RB

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    I added the term "audio" to describe the sinewave and squarewave result the amp makes into the speaker.

    As the amp is digital and runs from 5v DC, it's entire output consists of modulated 0-5v PWM "squarewaves", on two complimentary pins. Those PWM 0-5v squarewaves cause a result of a 10v p/p "audio sinewave" into the speaker, or max power to the speaker is delivered as a 10v p/p "audio squarewave" (as differentiated from the 0-5v "PWM squarewaves" that come out of the amp's output pins. :)
     
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