Help needed with amp design...

Discussion in 'Electronics & Electrics' started by TERRA Operative, Oct 8, 2006.

  1. TERRA Operative

    TERRA Operative Member

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    I am currently redesigning the plate amp in my HT sub, as the volume/xover display thingo doesnt work and I want to get rid of all equalisation and let the 5.1 reciever do it all.

    I am going to reuse the chip amp (part no. TDA7296) and associated componentry as much as possible, but have hit a slight snag, being the poor quality of PCB layout diagram in the datasheets.

    I have cleaned up the layout as much as possible, ready for a CAD layout to be drawn, but am uncertain of a few points (circled in green), namely the connection to pin 2 on the chip amp itself, and the ground plane connection under C6. Anyone able to clarify this for me? I have included my drawing, the drawing supplied with the data sheet and the schematic.

    Have fun! :)

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    Last edited: Oct 8, 2006
  2. OP
    OP
    TERRA Operative

    TERRA Operative Member

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    Just updated OP with better layout picture. I think I've got it pretty close, I'm still just not 100% sure on the green bits..
     
  3. mAJORD

    mAJORD Member

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    so did you take the reference board and redraw it in eagle or something?

    It looks fine to me.

    -ed if it wasn't just being used for a sub I'd comment on the aparent messy and unoptimised layout of the reference board, but it will work fine :p
     
    Last edited: Oct 8, 2006
  4. OP
    OP
    TERRA Operative

    TERRA Operative Member

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    Heh, the top pic is a trace of the reference design (second pic) done in Photoshop. I am currently redrawing the diagram in Rhinoceros 3D...

    What's the go with the ground plane doubling back on itself? Can I get rid of the loops or are they needed? If I could get rid of them, it would just neaten the PCB up a little.

    I'll be using Nichicon Fine Gold caps and 2x 4700uF of capacitence on the supply rails if that's any consolation... :)
     
  5. OP
    OP
    TERRA Operative

    TERRA Operative Member

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    Ok. Here is the revised layout. I've fixed up the ground plane a bit and thickened both the supply rails.
    I think it's about done. Now to print and etch...

    [​IMG]
    Click to view full-sized image!
     
    Last edited: Oct 8, 2006
  6. mAJORD

    mAJORD Member

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    looking better.

    I see what you mean about the 'looping back' , I'm not sure why they did it that way tbh. Ideally the sgnd would have it's own trace back to the thick ground plane down the bottom, I'm not sure how that standby setup works, (pin 4) , but as you've wired it now should be fine in comparison to the original at least, If anything it will lessen the chance of nose on the sgnd line.
     
  7. Dr feelgood

    Dr feelgood Member

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    just curious, how much difference to overall SQ would there be between this layout and something better? and what sort of changes would make it better?
     
  8. mAJORD

    mAJORD Member

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    I don't know what measurable differences drfeelgood, I've seen comparisons of optimised vs unoptomised class D chip amp circuirts and the differences are there, especially in noise.. In a class AB chip amp like this, the biggest thing is stopping oscillations. But I guess noisefloor would also be affected.

    Re. Changes, A proper star earthing layout would be the first.. In honesty it's not too bad, but if it was a high power design you could get some high currents between the speaker ground return and the filter caps, so there could be voltage differences at that point where S-gnd joins the power ground plane.

    C9 and 7 could be kept closer to the IC itself so there is no resistance and inductance between them and the IC.

    Feedback resistor R3, feedback path should be kept small, and close to the IC as posisble, It's connected directly to the output terminal which is good, but the path back to the IC is like a maze,

    best way to achive all this is a double sided PCB.. I'm not expert in PCB design so I'm not baging that one out, I'ts hardly easy to achive on a single sided board when everything has to make it's way back to one IC. I'm struggling enough trying to design my own DOUBLE sided board ATM..

    National semiconductor are prettymuch the king of IC amps, and have some good literature on there site, I'll find it if you're interested. There's certainly a different set of considerations than with a discrete AB amp, mainly becuase of the higher bandwidth, they're more prone to instability from poor layouts.
     
  9. OP
    OP
    TERRA Operative

    TERRA Operative Member

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    Well the chip-amp has an RMS rating of 30W and a 'music' rating of 60W, so it's not too extreme.
    It's funny you mention double sided PCB... I had to use a double sided blank, as that's all I had. I didn't even think of using the other side. :rolleyes: Ah well, we'll see how this one goes......

    I'll post a worklog in the electronics section once I get a little more done. :)
     
  10. Dr feelgood

    Dr feelgood Member

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    ahhh!, that makes a bit of sense, my first impression was it may have had something to do with capacitance, etc along the paths and between layers (if double sided). I know in one of rod elliots designs he stipulates that the PCB layout is of extreme importance but he does not go into detail as to why.

    thanks.

    TERRA, looks good, hope all goes well. be sure to post pics of the end result for all to admire. :)
     

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