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    Chang's Combo Meal #4
    MKL's Avatar
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    Mar 2002
    Pordenone, Italy


    The audio issue of AES revision 3-4

    A few years ago, after buying a system from me, the customer (who considered himself an audiophile) complained that he could hear a buzzing noise coming from his TV speakers as well as from the headphone jack. He also mentioned that he had a low serial system that didn't have that issue at all.

    Since I had another (unmodded) system with the same PCB revision (3-4), I tested it immediately and much to my surprise it was having the same issues described by the guy. I couldn't believe I had never noticed it because once it was pointed out to me it was impossible not to hear it.

    To get a better grasp of the problem I ran tests with as basic a setup as possible: I ran the system without hooking up video, only with headphones plugged in and another system (revision 3-6) running next to it, also with only headphones in and only the 3-4 was having the buzzing issue. Both system were powered by the same power supply (without step-down transformer) so the PSU was ruled out as a possible cause.

    When the guy sent the system back to me along with his other "silent" system (that turned out to be the very first PCB type NEO-AES) the tests confirmed that 3-4 had the issue and that NEO-AES was buzz-free just like 3-6. The thing was all the more puzzling as NEO-AES and 3-4, despite being different revisions, have the same audio circuit (while on revisions 3-5 and 3-6 it's partly different).

    I was starting to put the blame on the different PCB layout of the 3-4 causing interference when I noticed the difference between the NEO-AES and 3-4 circuits: each audio IC (DAC, op-amps, headphone amplifier) on the NEO-AES has a decoupling (bypass) cap nearby (close to the VCC and GND leads) just like all the other ICs on the board which is what you would normally expect to see:

    The 3-4 system doesn't have them though. Not a single bypass cap in sight:

    After adding 0.1uF (104 marking) caps across the 5V and ground leads of each IC (on the bottom of the board) the issue was gone:

    The cap for the DAC (YM3016) seems to make the most difference.

    According to the datasheet, the bypass cap recommended for the headphone amplifier IC is a 100uF electrolytic cap.
    The value for this cap on MVS boards is 470uF/16V. I used 100uF/35V:

    Of course you will have noticed the 1000pF (marking 102K) caps soldered to pin 1 and 16 of the IC. Those are other components that SNK left out of the headphone circuit on AES boards but they're supposed to be there according to the datasheet (they are part of a low-pass filter on the inputs) and are not missing from MVS boards:

    A further example of the lack of attention in designing the audio circuit on this revision is the incorrect placement of two polyester caps in the L channel path:

    To rectify things desolder and swap them.

    How about the other revisions? Will 3-3 have the same issue as 3-4? I didn't have the chance to test one but judging by the lack of bypass caps for the audio ICs it's safe to assume it has the issue too:

    I mentioned above that revision 3-6 is buzz-free. Let's take a look at the audio circuit of this board:

    In this case the caps used for decoupling purposes are electrolytic instead of the usual multilayer ceramic ones. They're the 10uF caps numbered 1, 3 and 9 in the previous pics.

    Revision 3-5 is almost identical to 3-6 but guess what, the bypass cap for the DAC (number 1 on 3-6) is missing:

    I haven't tested a 3-5 system so I can't tell if the missing cap makes any difference.

    It may be worth mentioning here that the headphone out on revision 3-6 has the R and L channels reversed. I had noticed this years ago and wasn't the only one:

    Since then I have found why. It's another mistake of good ol' SNK designers. Apparently they realized it and made a correction on later boards by moving a couple of resistors:

    Last edited by MKL; 05-17-2010 at 04:56 AM.

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