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Thread: Take your Genesis audio to the next level - The Mega Amp 2.0 is here!

  1. #211
    Hedgehog-in-Training Hedgehog-in-Training Ice Man's Avatar
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    If anyone is interested, I got 5 Triple Bypass boards fully assembled that I have no use for. For more info PM me.

  2. #212
    Outrunner Eep386's Avatar
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    Yes, TR1/TR2 are the pass-through amp transistors for the Sega CD.
    They pass the YM3438+PSG+32X through to the Sega CD.
    What this means is, if you remove these transistors only Sega CD PCM and CD-DA will be passed to the RCA jacks on the Sega/Mega CD when hooked into the Genesis/Mega Drive in question. They will NOT affect the Sega CD audio going back into the console though.

    The transistor circuits were half-assed by Sega CS, probably why they're picking up noise.
    Last edited by Eep386; 12-04-2019 at 05:10 PM.

  3. #213
    Outrunner Eep386's Avatar
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    Also, be mindful to terminate unused amps after installing the Mega Amp.
    Leaving unused op-amps floating without terminating them is a terrible idea. They will (not 'can', will) inject evil amounts of trash into Vcc and GND, and burn loads of power doing so too. (They will switch extremely fast between rails as the inputs are essentially left in an undefined state, and will be 'toggled' by noise currents.)

    Allow me to share a slightly humorous example provided by Texas Instruments:
    TERMINATED.jpg

    Creative Labs left unused portions of quad amps completely floating on various models of the Soundblaster 16 card, probably partly why they have such a reputation for noise. (Other causes for SB16 noise are noisy DACs and mixer chips and logic, the infamous TEA2025 headphone amp which is ludicrously noisy compared to modern amps, etc.) Their approach would therefore be the one aptly labeled 'BRAIN-DEAD' by TI here. :P Not terminating the amps on the VA0~1.8 after installing the Mega Amp would be somewhere between that and 'JUST-AS-BAD' as there's still an input that's essentially left floating. The fact there's an input resistor dangling on nothingness won't prevent noise from making the amp slam that input into the rails randomly as floating wires are essentially just antennas in all but name, regardless of any 'jewelry' they may wear.

    When Mega Amping a VA0~1.8 you can simply remove IC9 (that's the '324 quad amp on the right with the cartridge ports facing you) as frankly, it's a minor PITA to terminate that amp as it has a 50-50 mixture of inverting and non-inverting setups, and there's no point in keeping a totally unused quad on the board anyway. But you can't remove IC10 as one of its four amps is used as a comparator for /SRES, unless you make a custom Mega Amp that has an LMV321 just for the /SRES comparator. So you'll have to terminate its three unused op-amps.

    To terminate IC10, you remove R42, R43, R44, R45, R46, R76, R77, R78, R79, R80, R81, C56 and C58, and put 0-ohm jumpers on R48, R78 and R81.
    What you're doing is converting these circuits into unity-gain non-inverting followers with VRef feeding the positive inputs, keeping the amps rock-solidly centered on VRef so noise can't make them switch between rails. VRef as supplied on these systems is 2V, a little low as it ideally should be 1/2 Vcc, but it shouldn't really make much of a difference in this case.
    Last edited by Eep386; 12-07-2019 at 03:11 PM. Reason: derrherher

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