Viewing 11 posts - 1 through 11 (of 11 total)
  • Electronics engineers/DIY HiFi enthusiasts, help please! Why do my amps not work
  • captaindanger
    Full Member

    I have a couple of these amplifier boards:

    https://www.ebay.co.uk/p/Xh-m253-Tda8954th-Core-BTL-Mode-HIFI-Class-420w-High-Power-Mono-Digital-Amp-Z4w4/21020333806

    Based on this:
    https://www.nxp.com/docs/en/data-sheet/TDA8954.pdf

    I am trying to test them for a project and am using a raspberry pi headphone out as the input. I am using a 19VDC laptop charger instead of the 24V AC printed on the board (I am pretty sure the ebay ad is wrong), as there is a bridge rectifier straight after the power terminals, so this should give around 18V to the chip (I think). There is a blue power LED which is lit fine.

    The headphone out works fine with headphones, and I can get the speaker working running the headphone cable straight to it, so the problem is definitely the amp.

    The resistance across the input terminals of the amp is measuring infinite, would this cause a problem as headphones would obviously have a finite resistance.

    Not getting anything from the speakers, and not measuring an AC voltage across the input terminals (not sure if I should be able to with a standard multimeter?)

    What have I missed? Any help appreciated!

    Thanks

    captaindanger
    Full Member

    Page 6 of the data sheet is about the mute and standby, do I need to close both of these for operation? If so that may be the problem, I did try shorting each but only one at a time

    captaindanger
    Full Member

    Or is it that I have not provided a negative supply voltage?

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    tillydog
    Free Member

    I am using a 19VDC laptop charger instead of the 24V AC printed on the board (I am pretty sure the ebay ad is wrong), as there is a bridge rectifier straight after the power terminals, so this should give around 18V to the chip (I think).

    I guess that the board needs 24V AC so that positive and negative supplies can be generated on board (something like +/- 15V).

    Chip protection circuits will shut it down if total supply voltage is <20-25V or if there is >30% imbalance between +ve and -ve supply voltages.

    Buy a transfomer… 🙂

    funkynick
    Full Member

    A bridge rectifier is used for converting from AC to DC, so the board needs an AC input, and 24V sounds about right from looking at the supply voltage specs for the device.

    As above, get a transformer… you should be able to find an plug in supply for 24V AC, but make sure it’s rated enough for the amp… it’s gonna need to supply about 20A at full load!

    Also, amplifier inputs are almost always high impedance, so I wouldn’t expect to see any meaningful resistance there.

    Finally, a multimeter won’t be able to measure audio, it’s just not fast enough…

    ajaj
    Free Member

    The eBay listing shows a centre tap 12-0-12 transformer, which would give roughly +16v and -16v DC rails in line with the chip spec of +/-12-42v. Or you could use a transformer with independent secondaries wired in series to create a centre tap.

    I presume there’s some voltage regulation going on somewhere.

    You’d probably want to ground it somewhere sensible too.

    leffeboy
    Full Member

    As above, it looks like there are 3 wires to supply power and not just 2.   So 0v and +-16v AC.

    And, you need to check the current rating as well of course. That thing looks like it eats power. You might even need a fan

    Cougar
    Full Member

    A bridge rectifier is used for converting from AC to DC, so the board needs an AC input

    Or to protect against backwards polarity in the supply.

    captaindanger
    Full Member

    Thanks chaps. I can’t find a power supply for less than £40 so think The amps may be destined for the shelf for a bit longer, and I’ll get a ready made pi amp

    ajaj
    Free Member

    Something like this would get you going for £15. Add in £5 for a box, slow-blow fuse and plug.

    Wire the secondaries as shown here

    tillydog
    Free Member

    At the recommended supply voltage for that board, the max output will be ~ 2 x 20W as single ended amps, or ~30-40W bridged (which is how I assume it’s wired, but haven’t dug into the Chenglish details).

    Something like a 50 or 60VA transformer would be big enough.

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