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Thread: Question about potential mod

  1. #16
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    Okay after discussing things over with Eep386 via PM I've decided on the route I want to go with this, I've decided to replace the resistors for the onboard sound mixer at R40 & R41 from 10K to 12K with a 1% tolerance (for more even channel matching), and the filtering capacitors at C45 & C48 from 5600pF to 3900pF which will give me a 3.4kHz filtering just like the VA2 but without the original design flaws the VA2 had (I also believe that 3.4kHz filtering is what SEGA originally intended for the Genesis/Mega Drive from the start) as opposed to a 3.38kHz filtering I'd get if I left the original 10K resistors alone and simply replaced the 5600pF caps with 4700pF ones (I had originally considered using 11.5K resistors and 4000pF caps instead but decided against it after doing a bit more math and figuring out that I'd get a 3.46kHz filtering which seems a bit excessive and not as well rounded when compared to 3.4kHz). As for the mono mixer I've decided on replacing the 33K resistor with a 24K one which will cut the mono mixer down to 2.4kHz which should work nicely with the rest of this mod, so in essence I pretty much have almost all the info I need however, there's just one more thing I need help with, could anybody post a pic highlighting where R41 and R74 are located on a VA6 motherboard? I already know where C45/C48 and R40 are located so I don't have to worry about those 3, and I think I have an idea about where R41 and R74 are located but I want to be absolutely sure so I won't screw my system up by mistake when I go to do this mod.

  2. #17
    Outrunner Eep386's Avatar
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    BTW changing the mono mixer resistor will NOT affect the filter knee for that output as mono out is derived from the preamp output coming from the CXA1034P, which in turn gets its input from the preamp caps and resistors I mentioned. So changing the preamp gain resistor to 24K will still give 3.4 kHz with the resistors and caps you want to use, just at a lower gain figure.

  3. #18
    The legendary gargoyle Road Rasher Firebrandx's Avatar
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    Quote Originally Posted by Superrayman3 View Post
    Okay after discussing things over with Eep386 via PM I've decided on the route I want to go with this, I've decided to replace the resistors for the onboard sound mixer at R40 & R41 from 10K to 12K with a 1% tolerance (for more even channel matching)
    You might as well go to 0.1%. I always do when it comes to stereo pre-amp and output resistors whenever possible. Reduces balance fluctuation chance even more.

  4. #19
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    Meh

    Quote Originally Posted by Firebrandx View Post
    You might as well go to 0.1%. I always do when it comes to stereo pre-amp and output resistors whenever possible. Reduces balance fluctuation chance even more.
    Thanks for pointing out that 0.1% would be a better tolerance to use Firebrandx (Eep386 mentioned that he prefers a 1% tolerance for the resistors he uses so that's what I decided to go with but now that I know that a tolerance of 0.1 would reduce the chance of balance fluctuation even more I'd probably be better off going with that) although now it makes me wonder if I could (or should) go even lower than that, would using a tolerance less than 0.1% be a good idea or would that be complete overkill (when it comes to the stereo audio modding using native hardware I want there to be as little chance of balance fluctuation as I can possibly get, hence why I ask this). Also would I want to use a specific tolerance for the 3900pF caps based on resistor tolerance I go with (Eep386 mentioned that the lower the resistor tolerance the more even the channel balance is, would a similar principle also apply to the caps as well or does it really matter what the tolerance is with those?).

  5. #20
    WCPO Agent CrossBow's Avatar
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    I use 1% without any issues. But then I also measure the resistors before installing them and will go through as many as 10+ before I find two of them are reading the exact same. That is ultimately what you want to look for. You could even get 5% resistors and as long as you measure them and they are near the value you need them to be and if you get a pair that are exact to .001 of each other, you would be fine.

  6. #21
    The legendary gargoyle Road Rasher Firebrandx's Avatar
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    Quote Originally Posted by CrossBow View Post
    I use 1% without any issues. But then I also measure the resistors before installing them and will go through as many as 10+ before I find two of them are reading the exact same. That is ultimately what you want to look for. You could even get 5% resistors and as long as you measure them and they are near the value you need them to be and if you get a pair that are exact to .001 of each other, you would be fine.
    When ordering surface mounts, I don't want to have to probe each one for accuracy, so paying an extra 20 cents for 10 times more accuracy doesn't bother me. ;-)

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