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Thread: DM500 Audio Level Problem

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    Member nickbo's Avatar
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    Default DM500 Audio Level Problem

    I'm buggered if I can find it.

    I remember seeing it somewhere...

    the mod to fix the audio output problem on the DM500 clones.

    Before Z80 was dispatched to the sin-bin, I think he may have detailed what and where to mod the ouput op-amp(s). But i can't find it.

    There's a couple of posts referring to cutting tracks and inserting resistors to bring the levels up from 250mV to 500mV. It will save me a bit of mucking about if someone's done it and can tell me which tracks to cut (as in pin numbers) and the resistors required. I haven't opened it up yet so don't know what's in it but i suspect they aint gonna be a couple of LM741s marked "Left & Right"

    Cheers



Look Here ->
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    Here ye be

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    Edit, a little too late.

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    Gentlemen, you are both legends. Thanks

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    Quote Originally Posted by nickbo View Post

    Before Z80 was dispatched to the sin-bin

    Where what why?
    It is a hobby so mileage varies
    Loving IPTV with XBMC and android M8
    Join in the XBMCHUB community and visit me at http://tvaddons.ag/

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    Where
    When you do things right, people won't be sure that you have done anything at all

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    Jeez i hate SMD !

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    Quote Originally Posted by nickbo View Post
    Jeez i hate SMD !
    Yup!
    Broke pin 3 on a card reader chip yesterday Had to use a Dremel to grind back the epoxy to expose the copper enough to get a reliable contact.

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    Any luck Nickbo?

    You know, your DM is the guinea pig. LOL

    The low level sound is starting to get up my nose too.

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    Member nickbo's Avatar
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    Nup.

    So far I have lifted the pins but am buggered if I can see how I am gonna get a reasonable mechanical attachment to the wee little pad that's under it. It looks like a 3 or 4 layer PCB with the pad about 0.7 mm. Basically, if I attach a soldering iron to a bee's pecker, and train it to solder SMDs, it may be a start. That or a bloody electron microscope with the same bee's dick heated up.

    That aside, you wonder how anyone could reverse engineer this beasty. Someone must have pinched the design stuff from dream.

    To be honest, I realy think a little op-amp set up is gonna be easier and safer. Drive it straight off the psu input and tap in between the rca solder tag and the female pin receptor.

    Just interested if anyone's done that using say, an LM1458 (dual op-amp)?

    If not, I will try and give it a go. Plan B is to come off the back of 'U15" - but I can't seem to locate that on the PCB.

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    Quote Originally Posted by best4less View Post
    Where
    a sad day for this section of austech ...
    It is a hobby so mileage varies
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    Quote Originally Posted by jonc View Post
    a sad day for this section of austech ...
    Zed will be back soon, not long to go, busy with other things ATM he says.

    I dont think he tried the audio level mod, it was just a recommendation from what i remember.

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    Well,

    DONT do the mod. I've now lost Audio after trying to reinstate the thing.

    Anyone seen any bees about ? Preferbaly one with an smd appendage ?

    nickbo

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    Changing the gain, by adding two resistors on the OP-amplifiers output, cannot be done without first changing some values of the filter capacitors, otherwise you get gain on the frequencies that you try to reject.

    The circuit is a 3-pole sallen-key active filter, and they are just using one OP-amp, that's why the modificated feedback amplifiers aren't helping.
    If the Sallen-key filter used two OP-amps, then it would have 1 single pole stage on one OP and one 2 pole stage, and adding feedback-resistors should get the gain you wanted.
    If gain is to be changed, I would recommend feedback with 4.7k and 10k to ground, and also double the feedback capacitors, marked C134 and C131 (1nF in the schematic).
    I have simulated it and I will try it on my dreambox too. Hope to return with results soon.

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    would it be safer to "attach" a ampli-Op?
    The gain of the IC on dreambox schematics is at its best and if you try to change the resistors and capacitor values you'll end up with some strange sounds coming out of the box.

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    Maybe safer, but I'm not sure if the values are chosen with care of the amplitude. Stange noise comes out because of the change of the filters, giving gain to the 48kHz sample, instead of suppressing it. That's why you have to change more components, not only adding some gain.

    The other thing is that this is NOT easy. To add gain-resistors, you have to first lift up the OP-amp chip TLC2272, then cut the lines below the component and then add very tiny resistors (0402) in the output. This work cannot be done with a home-soldering kit, but demands som microscope and the correct components, some professional equipment.
    I'm just doing this for experimental purpose, see if it can be done.

  • #17
    Member nickbo's Avatar
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    Guys,

    I buggered mine trying to do it.

    Plan B is to stick a couple of op-amps straight in line with the RCA jacks. mechanically its much easier to fit.

    Nickbo

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    First tests done !
    Just changing the gain with two resistors gave unwanted gain to the 48kHz, the frequency you want to reject. Adding resistors requires some filter changing too. My goal was to try changing as few components as possible, but you cannot !
    Later I tried to remove the resistors, short circuit the output and the negative input to get unity gain and the sound got back to normal.

    Now I will continue with the next test, add gain and change the filter to adjust the filter characteristics with the new load.
    I'll be back !
    Last edited by viairdeto; 07-04-09 at 06:06 AM.

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