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Speakers clipping w/ TDA7492P

Discussion in 'Audio' started by Costantin, Jun 19, 2017.

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  1. Costantin

    Costantin

    5
    1
    Jun 19, 2017
    Hi guys, I'm new in this forum.

    I bought this board to make a portable boombox: https://it.aliexpress.com/item/TDA7...32651692103.html?spm=2114.13010308.0.0.LwDs8G. I have two pair of speakers, one is this: http://www.kenwood-electronics.co.uk/car/amp_spk/custom/KFC-E1762C/?view=details and the other one is this: http://uk.jvc.com/car-entertainment/16cm-speakers/CS-J620X/ (one pair of speakers per boombox). I'd like to power the system with a 18650 battery pack. The problem is, I tried multiple types of packs (3s1p, 3s2p, 4s1p, 5s1p) but with everyone of these, the speakers start to clip at the last two volume steps (I don't know how to call them sorry), even with both gain switches on the OFF position. any solution?
     
  2. davenn

    davenn Moderator

    13,496
    1,830
    Sep 5, 2009
    I assume you also have a pair of those amplifiers ?
    you will struggle running both off the same amp

    what is the battery pack output voltage ?

    neither of your 2 speaker sets are rated to 50W continuous, they may be going into distortion
    I seriously doubt that the amplifier can produce 50W+50W without significant heatsinking .... have you provided any ?

    ahhhhhhhhhh I looked at the datasheet for the TDA7492P .... it's only 25W+25W, would still need good heatsinking for anything more than around 10+ watts / channel out
    so the speakers you have should be OK with that

    there's a chance you are just plainly overdriving the amp
    also the battery pack may not be able to handle the current required at near full power settings
    Have you tried it on a decent power supply ?


    Dave
     
  3. Audioguru

    Audioguru

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    Sep 24, 2016
    A 3s 18650 battery is 12.6V when fully charged. The datasheet for the TDA7492 amplifier shows a graph of maximum output vs supply voltage. The lowest supply voltage shown is 15V when the power is 23W at horrible 10% distortion or 19W at 1% distortion that can still be heard into 4 ohms.

    A 5s battery is 21V when fully charged but 1P is only one set without cells paralleled so the 18650 battery cannot supply enough current and its voltage will quickly drop.

    The speakers will "bottom out" if they are not in a correctly designed enclosure.
     
  4. Costantin

    Costantin

    5
    1
    Jun 19, 2017
    Each boombox has 1 amp, 1 pair of speakers and 1 battery pack.

    3s1p/3s2p -> 12.6v fully charged
    4s1p -> 16.8v fully charged
    5s1p -> 21v fully charged

    No I didn't, I'll do it today.

    Thank you for the reply.

    I'll make a 5s2p pack and I'll try with that. I just tried to power the amp with a 19v 2A laptop charger, but at max volume it restarted itself. I touched the chip and it wasn't really hot, far from 80°c, when the protection triggers.
     
    davenn likes this.
  5. Audioguru

    Audioguru

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    614
    Sep 24, 2016
    How did they make that Chinese amplifier so cheap?
    1) The heatsink is too small.
    2) High current traces have copper too thin and narrow.
    3) Not enough filtering.

    You used a 19V/2A power supply and played both (?) 4 ohm speakers at maximum power when distortion was pretty high. The graph in the datasheet shows 10% distortion at 36W for each channel for a total output of 72W plus heating of about 7.2W for a total from the power supply of 79.2W. But the power supply can produce only 19V x 2A= 38W so of course it fell on its face.
     
  6. Costantin

    Costantin

    5
    1
    Jun 19, 2017
    I tried the 4s2p pack and the clipping was still there but it was less than before. I was thinking about changing the max volume of the bt chip with an spi-usb connector (I don't know if it is possible yet) so no one can turn the volume so high that the speakers start clipping, maybe it will work.
     
  7. BobK

    BobK

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    1,684
    Jan 5, 2010
    Are the speakers in a closed box when you are seeing this distortion? As AK said, the speakers themselves will hit their end of their range motion and distort if they are not properly loaded by an enclosure (without even producing that much sound).

    If the problem is simply that the input signal is too high and drives the amp into distortion, all you need is a voltage divider before each input.

    Bob
     
  8. Costantin

    Costantin

    5
    1
    Jun 19, 2017
    Yes, I put them in an enclosure as audioguru said but the distortion is still there. I dont actually know if the input signal is too high or not.

    Thanks for the reply.
     
  9. BobK

    BobK

    7,673
    1,684
    Jan 5, 2010
    If it is distorting only at very high loudness, then, by definition, the input is too high. Are you getting adequate loudness before it distorts? If so, 2 resisors for each channel will fix the problem.

    Bob
     
  10. Costantin

    Costantin

    5
    1
    Jun 19, 2017
    The problem is that the bluetooth chip (CSR8635) is soldered on the pcb, so I'm not able to solder the resistors on the output pins (that connect to the amp input).
     
    Last edited: Jun 20, 2017
  11. Audioguru

    Audioguru

    2,725
    614
    Sep 24, 2016
    Only a moron would turn up the volume too high.
    It is normal for an amplifier to have "too much gain" so that faint sounds can be made loud enough.

    You said, "so no one can turn the volume so high that the speakers start clipping" which is wrong because normal people will hear clipping and turn down the volume to eliminate the clipping.

    My amplifiers usually have the volume control at halfway and have never been turned up high enough to cause clipping. I do not let my grandson touch the volume controls because he is only 1.5 years old.
     
  12. Gadgetfan

    Gadgetfan

    1
    0
    Apr 29, 2018
    I have the exact same issue....the problem is that the clipping starts when the volume isn't insanely hard. I use two 8 ohms speakers off 50 watt. I use a 12v 1amp power supply. I am looking for a more powerful power supply and I think I need a 24v 5 amp DC power supply.
     
  13. Hopup

    Hopup

    227
    31
    Jul 5, 2015
    You have less than 12watts of real power then. I would expect for it to clip at low volumes...
     
  14. Audioguru

    Audioguru

    2,725
    614
    Sep 24, 2016
    Common sense shows that your weak little 12V/1A power supply has a max output of 12V x 1V= 12W.
    The datasheet shows a graph of output power with a little clipping (1% distortion) with the lowest power supply voltage that is 15V and the power in each 8 ohm speaker is 11W for a total of 22W plus a little more for heating.

    11W in an 8 ohm speaker is 9.4V RMS which is 9.4 x 2.828= 26.6V p-p but the amp is bridged so the actual output is half at 13.3V p-p. Then the voltage loss is 15V - 13.3V= 1.7V.

    Then your 12V supply will produce 12V - 1.7V= 10.3V p-p which is 3.64V RMS that produces 6.6W with a little clipping into ONE 8 ohm speaker. Your little power supply is overloaded when the amplifier is trying to produce 6.6W into two 8 ohm speakers.

    If you use a power supply that is 25V then the power into each 8 ohm speaker is 26W without any clipping. The current from the transformer will be about 2.2A.
     
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