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Thread: Audio Amplifier

  1. #1
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    Default Audio Amplifier

    Hi, I plan to make a 4 chanel audio amplifier. Im not going too fancy, no volume control etc. I just want a PCB with 4 phono inputs and 4 speaker outputs. The inputs will be from a PC's soundcard and the speakers i have i think are about 5W. The speakers are for my pinball machine (yep, back on that project for a while).

    Ive not really done anything with audio before. Google finds a lot of schematics but they are all different and im not sure what would be best so my question really is where do i start learning about these kind of circuits?

  2. #2
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    IMO - LM386 (or maybe LM383)...simple, capable of a few watts, popular, cheap...
    I've also used NE5532, same deal, not quite as powerful.

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    Ive just been looking at the Rapid website and ive found something interesting. Aparently its a 20W audio amplifier inside what looks like a 5 pin triac. There are a few circuits in the datasheet. Could it really be as simple as the diagrams on pages 2 and 3? I hope the one on page 6 (labeled page 5) is whats inside this thing and not the circuit i would have to build.

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    Sorry, was replying at the same time as you.

    Rapid actually have LM386M-1 and LM386N-1 on the same page as the one ive just been looking at and it seems to be pretty much the same thing but cheaper The diagrams actually look simpler too.

    What exactly is the gain? Is that how many times more you get out than what you put in? Is it Volts or Amps or both?

    Edit: It appears that the N and M determine the package type. Looks like i best get N. Ive already made that mistake once :P
    Last edited by The Master; - 18th August 2008 at 15:11.

  5. #5
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    Quote Originally Posted by The Master View Post
    There are a few circuits in the datasheet. Could it really be as simple as the diagrams on pages 2 and 3? I hope the one on page 6 (labeled page 5) is whats inside this thing and not the circuit i would have to build.
    You'll notice the schematic on page 2 is a double ended supply (+v, -v, grnd) whereas the one on page 3 is a single ended supply (+v and grnd).

    And yes, the one on page 6 is the internal schematic.

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    This suddenly seems a lot easier. Everything i found while searching google looked like that huge schematic and used a lot of transistors that Rapid doesnt stock.

    I just read in the datasheet for the one you suggested that its "Voltage gain"

    The one i was looking at before said you have to use a heatsink because it can overheat without even doing anything. I dont see any mention of heatsinks for this one at all though. Are they needed?

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    Quote Originally Posted by The Master View Post
    This suddenly seems a lot easier. Everything i found while searching google looked like that huge schematic and used a lot of transistors that Rapid doesnt stock.

    I just read in the datasheet for the one you suggested that its "Voltage gain"

    The one i was looking at before said you have to use a heatsink because it can overheat without even doing anything. I dont see any mention of heatsinks for this one at all though. Are they needed?
    You'll find that these cheaper op-amp's aren't really brain surgery. You can beat them up quite a bit. They'll take it.
    Don't worry too much about the gain. That's what the volume knob is for! When the speaker distorts, better back off.
    And yes, heatsinks...Don't run without at least something on there. If I remember right, the chip has a thermal overload shutdown, but don't count on that saving the chip.

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    Ok good point, the first one i looked at had internal thermal cutout on it so this one probably does too. Ill make sure to get heatsinks just incase though.

    Volume control isnt really a problem. I want the amp to be turned up as high as possible (without causing distortion) then turn the volume up and down on the PC that controls everything.

    Are these chips safe to run directly from a PC? The diagrams for the LM386 show a variable resistor between the input and the chip. Since i dont want volume control can i just remove that completely or does there need to be some kind of resistor or protection device in place?

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    Hi,

    (Philips Semiconductors)

    TDA1558Q
    2 x 22 W or 4 x 11 W single-ended car radio power amplifier

    Datasheet: (See page 9).
    http://www.nxp.com/acrobat_download/...558Q_CNV_2.pdf

    Digi-key price: USD 5.50
    Quantity Available: 552

    http://search.digikey.com/scripts/Dk...=568-3508-5-ND

    * * *

    UK RS Components Ltd

    RS Stock No. 182-7834
    RS price: 3.76

    Best regards,

    Luciano
    Last edited by Luciano; - 18th August 2008 at 17:10.

  10. #10
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    Do you know what the output impedance of your sound card is? Does it really matter?
    Are you going to use AC coupling or DC coupling?
    If you don't know what any of the above, you might want to get to some researching before you blow something else up! (ie. sound card outputs, amp inputs, etc).
    Other than that, put the volume controls in there, play with them, get it all working, play with it after that, make it work good for you, remove what you think you don't need and go with it. You really can't go wrong starting small and working your way up.

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    Luciano, the TDA1558Q looks like it would be a good choice. At first glance it looks like it doesnt need any external components either. It is a little more expensive but the main thing is that Rapid doesnt seem to stock it. Ill keep it in mind as a backup plan but i think im going to stick with the LM386. Thanx anyway

    skimask, no i dont know what those things mean. I do have a few soundcards from old PCs just laying around doing nothing so i guess you could call them expendable. I will still go with your idea and add the volume control in to start with though. Ill be making it all up on a breadboard first anyway and im sure i have some variable resistors spare too.

    I kind of get what AC/DC coupling means but im not sure what i would go with. Arnt sound cards line-level (i believe that means 0V and below) so i would assume DC since the voltage should never cross 0V.

    Do you know what the output impedance of your sound card is? Does it really matter?
    Erm, I wouldnt have thought so but since you mention it i guess it might. I didnt think the sound card would matter because its inputting to the chip. I have heard that the impendance of the speaker matters though. These speakers are the type that only have a part number on them and no other usefull information. A quick google search says they are 4 ohms. One of the 2 (LM386 or the first one i was looking at) said it can be used for both 4 and 8 ohm speakers

  12. #12
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    Quote Originally Posted by The Master View Post
    I kind of get what AC/DC coupling means but im not sure what i would go with. Arnt sound cards line-level (i believe that means 0V and below) so i would assume DC since the voltage should never cross 0V.
    AC coupling - basically thru a capacitor, output ends up swinging + to - around ZERO.
    DC coupling - basically thru a resistor, output follows the input but with less 'driving' capability (ie. less current because it's going thru a resistor/current-limiter).

    Impedance - Yes, come to think of it, all of these op-amps have very high input impedance, so you're right, for the most part, it doesn't matter.
    Speaker impedance does matter though. Try to connect a 2-ohm woofer to an 8-ohm output and you probably won't be impressed.

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    Just to check, can the output impendance be less than the speaker impendance? I know it cant be more because it works the oposite way to watts (a 2W amp wouldnt work well with an 8W speaker etc).

    Now im not so sure about the coupling. Is this actually something in the sound card or is it the way the circuit connects to it? If its part of the soundcard then how do i tell the difference?

  14. #14
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    Quote Originally Posted by The Master View Post
    Just to check, can the output impendance be less than the speaker impendance? I know it cant be more because it works the oposite way to watts (a 2W amp wouldnt work well with an 8W speaker etc).
    Watts don't matter much... A 2W amp will drive an 8W speaker just fine, all other things being equal.

    Impedance - Google it, Wiki it...
    If the output impedance is LOW, then that means the output's load can draw MORE.

    Now im not so sure about the coupling. Is this actually something in the sound card or is it the way the circuit connects to it? If its part of the soundcard then how do i tell the difference?
    Both...

    Just hook the thing up to something (mp3 player, whatever) and start playing around. Put a small resistor inline (to limit the current in case you accidentally short something with something else that wasn't meant to be shorted) with the inputs and start playing if you're that worried about it. Reduce the resistor value (parallel a few or something) until it starts working.

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    Sounds like a good idea. Ill order some of those chips with my next order from Rapid and start messing with them.

    Thanx for your help

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    The LM386 is a low power chip (0,25w@8 ohm, 1w@32 ohm) and has many flavors according to the drive load impedance and power supply. Choose carefully, according to the speakers ohms. Do not choose an LM386 driving 32 ohms if your speakers are 8! You will loose many milliwatts.

    If you finally select this one, provide adequate earth on the PCB as this is what cools the chip.

    I believe it is not suited for the case... Choose a stronger chip as Luciano suggested.

    Ioannis

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    Hmm, looks like this is about as difficult as finding the correct PIC chip :P

    I went back to Rapid's site and found a TDA2030H 14W HI-FI POWER AMPLIFIER (RC). The datasheet says "12W on a 4W load" so im sure that would be enough. The datasheet even gives a PCB layout design (I can read those better than schematics).

    Just incase you think theres something better heres the list i have to choose from Audio power ICs. There are more powerfull things on there but i dont want to waste money buying 75W amplifiers for 5W speakers. 20W is probably about right because im not sure exactly what the speakers are

  18. #18
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    I had my numbers backwards the whole time.
    I really meant to indicate the LM383 (8W), not the LM386 (which is more suited for driving headphones).
    Yes, LM383. The datasheet has a schematic in there for setting up a pair of them in a 'push-pull' configuration to get a 16W amp.
    But, as stated earlier, make sure you have a decent heatsink on it, for that matter, any chip you chose, otherwise you'll spend the bulk of your time in 'thermal overload shutdown' and eventually you'll start killing chips.

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    I think the best thing to do is order a few of the chips on that page and all the external components i need for each one so i can experiment a bit. I should probably dig out some old speakers and an old mixer too so i have better control over the volume and if something breaks it doesnt matter much.

    Its a good job i know about the heat part. Normally if a chip even gets slightly warm it means ive done something wrong. This chip is obviously different and heating up is normal.

    How do i put a heatsink on a chip? Is it as simple as just putting it ontop and securing it to somewhere else on the PCB to hold it on? Ive got some thermal paste for CPU heatsinks, should i put some of that on? What size heatsink should i use (roughly). Will one thats about twice the widthe of the chip, a little longer and about 1cm tall do or would i need really big ones? Theres going to be a fan or 2 in with it anyway so that should help

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    Go with your own suggestion of the TDA2030... it's a standard TO-220 'style' package and Rapid have stacks of Heatsinks that will do the job.

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