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Amplifier supply current.

  1. Aug 5, 2013 #1
    So I want to make a tda2050 amlifier with 4 of these chips, and after posting a lot of questions about different audio chips here, the chips always turn out to draw a few amps EACH as said in the datasheet.
    On this chip it says a max of 5 amps and people always tell me to supply them with a transformer what can supply that much for each chip (the max). but I don't see why because I have been powering the tda2050 with a 400ma transformer reading that the chip only draws about 200 MAX when playing audio with a 4 ohm load. So why can't I just use like a 2A transformer to power 4 of these chips?
     
  2. jcsd
  3. Aug 5, 2013 #2
    CORRECTION I am only reading about 50ma!
     
  4. Aug 5, 2013 #3
    What is your power supply voltage? And it lil we symmetrical or single supply?
     
  5. Aug 5, 2013 #4
  6. Aug 5, 2013 #5

    jim hardy

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    Wait'll it hits a booming bass note....
     
  7. Aug 5, 2013 #6

    sophiecentaur

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    The point about class B amplifiers is that they only demand a lot of current when they are actually delivering a lot of signal power. 12W of audio at 12V will need 1A at any frequency - it's just that most music doesn't involve maximum amplitude sinusoids. Amplifier designs (even down to the size of Heat Sinks used) rely on this.
     
  8. Aug 5, 2013 #7
    I made sure I was using a song with a ton of bass, and evan when I cranked the Ipod on full to the point where I was hearing a ton of distortion out of the amplifier I still had like 100ma going through.

    ok, so would it be okay to run four of these amplifiers as a 2 channel amp (bridge mode,) with like a 3 or 4 amp transformer?
     
  9. Aug 6, 2013 #8

    jim hardy

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    The circuit you linked is for split supply,
    that is three wires: +18, -18, and common ('ground') .

    You say your supply is "a regulated 18v..." - is it just two wires +18 and common, or is it three wires +18 -18 and common?

    Regulators are typically current limiting. How much current can your regulated supply deliver?

    next question in next post.
     
  10. Aug 6, 2013 #9

    meBigGuy

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    The peak to average ratio of music is, what, 5 to 1? maybe more. Your meter is only reading the average. You WILL saturate to the supply voltage at times, so calculate accordingly. You may be OK with the voltage dropping when you saturate, but be sure you understand it.

    So assume you drive an +-9 volt squarewave into a 4 ohm speaker? (is it actually +- 9?) The peak current is 9/4 - 2.25 amps from 1 side then 2.25 amps from the other side. If you limit the maximum drive somehow, you can adjust accordingly. Power is 9*2.25 = 20.25 watts.
     
  11. Aug 6, 2013 #10

    jim hardy

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    If you have only 1/10 amp at 18 volts you cannot get more than 1.8 watts , but that is enough to make quite a bit of noise.

    Given this amplifier's impressive capability I do not believe you are getting anywhere near full power from it.

    What is measured power supply voltage at point you begin to notice distortion?

    What kind of speakers are you using? What is their DC resistance by ohm-meter, and what impedance is written on their nameplate? And what power rating?
    This amp should have enough power to smoke little computer speakers. Has that happened?

    Let's do simple arithmetic:
    To get the 30 watts this amplifier can easily deliver into 4 ohms requires how much current?

    watts = amps^2 X ohms
    so amps = sqrt(watts/ohms)

    amps = sqrt(30/4) = ~2.74
    and that's for each amplifier.
    If you plan to use four amplifiers get a ten or twelve amp [STRIKE]transformer[/STRIKE] power supply.

    And don't neglect to use a LARGE heatsink. The amp will shut itself down if it gets hot.

    That looks to be a very nice amp by the way.



    Here's a thread where a beginner built an audio amp, albeit one more modest than yours:

    http://www.discovercircuits.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=7783&page=1

    follow it far enough to see the nice speakers he built for just a ten watt amp !

    good luck, and good listening !
    Put your effort into the speakers,
    and don't scrimp on the power supplies. I like toroid linears not switchers.
     
  12. Aug 6, 2013 #11

    Baluncore

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    What confusion.
    We do not know if a single or split power supply is being used.
    We do not know the voltage(s) of the supply/supplies.
    We do not know where or how the current is being measured; nor if it is average, RMS or peak.

    Consider a pair of amplifier modules, being used together in a bridge configuration to drive a single 4 ohm speaker. The power supply to both amplifiers can be a single 0V(ground) and +18VDC rail.
    Current flows from one rail, through one amplifier, on through the speaker then through the other amplifier to the other supply rail.

    A full amplitude square wave will result in a supply current of 18 / 4 = 4.5A
    Now two channels will need 9A total from the single 18VDC supply.

    A bridge configuration is used when AC coupling is not possible or there is insufficient supply voltage available. So why use two amplifiers in a bridge configuration on 18VDC when you can use one amplifier on 36VDC (= split +/-18V) for the same power?

    If you do not require full power you should be able to increase efficiency and reduce heat generation by lowering the power supply voltage or increasing speaker impedance.
     
  13. Aug 6, 2013 #12
    It IS a split supply, from an 18-0-18 transformerI am running it on the split supply circuit I showed up above. I heard distortion as I cranked the input audio up to max(which is to be expected) not while changing the 18v supply. So I put everything into this amp and the MAX I could read with a multimeter that I was drawing even without a regulator was as I preveously mentioned 50ma. And when I was reading 50ma the speaker was VERY loud as it should be.

    (In bridge I am only going to use 8ohms. but lets just focus on the single amp for now)

    So it just really is weird that you guys are saying it's supposed to drive a couple amps for each amplifier but I am running this one off of a wimpy 400ma transformer and it works great!

    I am hooking up the amperage meter on the positive line. am I supposed to do something different with a split supply?
     
  14. Aug 6, 2013 #13

    Baluncore

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    With a split supply, all current to the speaker passes through the ground connection. But that current is AC. Half the time that current comes from the positive rail, the other half of the time it goes to the negative rail. You should be measuring the current in both the +Vs and -Vs. It is possible that there is a DC offset on the amplifier so the currents may not be the same. Is that the case?

    An “18 – 0 – 18” volt transformer secondary would be rectified with a 4 diode bridge and the charge stored in two capacitors, one for each rail.
    18VAC has a peak voltage of 18VAC*1.4142 = 25.45VDC
    When rectified it will fall by one Vpn, 25.45 – 1.0 = 24.45VDC peak.
    So your rails should have peak voltages of –24V and +24V.
    Have you measured –Vs and +Vs ?
    What are those measured voltages ?
    What is the capacitance in uF of each of the two storage capacitors?
     
    Last edited: Aug 6, 2013
  15. Aug 6, 2013 #14

    sophiecentaur

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    The fact is that a few hundred mW of music into a cheap speaker will produce a lot of sound. High quality monitor speakers tend to be much less efficient and may need driving quite hard to produce a 'loud' sound. But you can't get away from the fact that you won't get any more audio power out than the real DC power you are putting in. 'Intuition' can lead you to the wrong conclusions! Loud may not mean high power - even if your parents tell you to turn it down.
     
  16. Aug 6, 2013 #15

    meBigGuy

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    Let's get this straight. If you put 18V across a 4 ohm speaker you will draw 4.5 amps. period. End of story. Assuming the supply will handle it, the current will peak at 4.5 amps as it clips.

    If you use a 400ma supply the supply will go out of regulation and the voltage will reduce. You are distorting.
    If you drive a sine wave you will hear the distortion easily. Also, the peak to average ratio for a sine wave is fixed (I'll make you work that out).

    So, drive a 200Hz or 1KHz sine wave into the amp (make a wav file with cool edit or something) and listen carfully to determine when the distortion starts.

    The RMS current you measure with a sine wave is meaningful. Current measurements made during music are not.
     
  17. Aug 7, 2013 #16

    davenn

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    if you put 18V DC directly across a speaker, its likely to have its ( the speakers) lifespan measured in milliseconds

    Dave
     
  18. Aug 7, 2013 #17

    jim hardy

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    Maybe the point is this

    That amplifier IC is capable of almost fifty watts, which in my days of vacuum tube hifi was a real sledgehammer.
    A sledgehammer needs a substantial grip.
    Driving even a single TDA2050 with a measly 400 milliamp supply is trying to swing a sledgehammer head with a toothpick handle.

    On the bright side - probably that's why OP's speakers are not burnt up.
     
  19. Aug 7, 2013 #18
    I meant to say 9-0-9, sorry. I also tried it with a 2amp transformer into a 4 ohm speaker. And it's pumpin just like before, little difference in current draw, and even if I am sopposed to measure negative AND posotive current, it's still coming out as only 100ma.

    I am not calling you guys wrong or anything, I understand that this should be drawing a LOT more, it's just a little freaky that it doesn't seem to be! Although It's pushing my coaxial high quality speaker to it's near MAX excursion when I turn the volume up! And the chip is getting hot when I do turn the volume up high so It seems like it's drawing a significant amount of current but it is easily running off of such a tiny supply.

    Also when I meant distortion I meant simply having your audio input too high for the amp, it sounds loud and fine untill I get around max input, this happens with all audio ics that don't have a resistor or voltage divider or somthing to cut down the input. I was just saying that I was running this chip at MAX, and still barely any current draw.

    but I will check my multimeter and mess around, do you guys want pics or anything?
     
  20. Aug 7, 2013 #19

    sophiecentaur

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    I don't think anything is amiss. It's just that you aren't measuring what you think you are measuring. Do you have an oscilloscope? What is the peak to peak excursion of your output, measured to a pure resistive load? Even without a scope, you may be able to work this out with your DVM on peak hold, measuring from each rail in turn.
     
  21. Aug 7, 2013 #20
    I just put may amp meter in seriece with ground and the speaker and was reading about .3A,

    by excursion I meant on the speaker voice coil.
    No I don't have a oscilloscope, I do this at home on a small table in what used to be my living room.
    And I'm really tired of working on this, can anyone point me to an 18v transformer of switching power supply module or SOMETHING that will run all this well?

    If i have 2 bridge channels with 4 tda2050, what current will the power supply need to have?
     
    Last edited: Aug 7, 2013
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