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Thread: Fixing the sound of a VA2.3 Model II Genesis without a Mega Amp

  1. #16
    Creator of the Mega Amp Raging in the Streets Ace's Avatar
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    Do you not have a PC with a half-decent Realtek on-board audio chipset? You should just use that directly and invert the waveform (a lot of sound cards, ADCs and capture devices I've tested seem to invert the incoming waveform, the only exception right now being the SoundBlaster Audigy 2, whose Line In maintains the waveform exactly as it should be), that will get you to sample audio at 48KHz.
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  2. #17
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    Well, I cobbled together a really wonky but serviceable cable to record direct from the Genesis to my line-in jack on the onboard sound chip (Realtek ALC-somethingoranother) of my "lovely" HP computer. Perhaps this will give a better impression of how it sounds: https://www.dropbox.com/s/xmrizpnb38...rding.ogg?dl=0

    This is done from the RC4558-fitted VA2.3 with 120pF active bandwidth limiters/lowpass filters, 560pF pre-mix filters, 4700pF FM filters and 1500pF output filters (they were the closest I could get to 1600pF from Fry's... later when I order my 330pFs from Digi-Key I'll snag some 1600pF caps as well).

    And yes, you're right, my Realtek's line-in inverts the recording. But if you prefer I can simply invert the recording under Audacity and upload that.

  3. #18
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    Quote Originally Posted by Ace View Post
    Good to know. Shouldn't be too difficult to set up a voltage divider and filter capacitor to get a higher/lower VRef.
    Oh sh*t, not again... I was tracing the wrong pin. I re-examined the circuit (this time, using a multimeter to work around my wall-eyes) and realized that the FM pre-amp DOESN'T get its VRef from the ASIC, it gets it from pin 15 (REFOUT) on the 315-5684. According to the internal block diagram of the 5684, it's a '1/2 VCC' output pin (a voltage divider, I ASSume). Since this pin is already loaded by the high-pass feedback loop and pre-mix low-pass filter stage of the 5684, disconnecting the BA10358 FM pre-amp from it should not cause any problems, provided of course that you supply a new VRef for the FM pre-amp afterward.

    Vref Correction.jpg

  4. #19
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    Sorry to bump this, but I just wanted to say that I just tried an NJM4565 (that I pulled off an old AudioPCI card) in place of the BA10358 with the 120pF active filtering, and the 4565 sounds better than the RC4558! It's like the RC4558 but with less distortion and a slightly brighter, 'bolder' overall sound. I'll have to make a fresh set of recordings for this amplifier once 270pF caps arrive.

    It does draw more power though, so further testing with the 4565 is required before I recommend it for this purpose.
    Last edited by Eep386; 10-06-2017 at 06:58 PM.

  5. #20
    Creator of the Mega Amp Raging in the Streets Ace's Avatar
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    It's a higher-grade op-amp than the RC4558. Comparing the datasheets, the unity gain bandwidth is slightly higher on the NJM4565 (4MHz vs. 3MHz on the RC4558) and the slew rate is quite a bit higher (4V/uS vs. 1.7V/uS as specified in Texas Instruments' datasheet for the RC4558). Should do a much better job at handling high frequencies than the RC4558.

    Didn't manage to test with a TL072 yet?
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  6. #21
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    No, not yet. Had to make some use out of a crusty old obsolete PCI sound card, and just can't be arsed to replace the Vref atm to make it work. I just played through most of Garfield: Caught in the Act with the 4565, and it seems to be running without any issues. And yeah, the 4565 handles high frequencies far better than the 4558, BA3404 and (especially) any of the LM358 clones. The Sonic ring sound no longer crackles and the sound is very clear, nice and full. I do think the 7805 heatsink is a little bit warmer after the change though. (Not too surprised really, considering how the 4565 burns more power than the 4558.) I need to grab another VA2/VA2.3 and just leave it unmodded, so I can make proper comparison recordings. (And then do my experimentation on it after the comparisons are made!)

    I did replace the 315-5684's 1000pF pre-mix low-pass filters with 20pFs that I pulled from the same soundcard, and it sounds _delicious_ through my speakers. Not a hint of muffle anywhere beyond the 4700pF FM low-passes, and no nasal sound either. Guess that's the second-order low-pass you're talking about.

    Just to see what might happen, I did try removing those caps, and the muffle went away but the sound became quite harsh. (Naturally.) I still want to try 270pFs though, just in case my tinnitus-struck ears are hiding an oppressive amount of electronic noise.
    Last edited by Eep386; 10-06-2017 at 10:47 PM.

  7. #22
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    OK, I finally tried a TL074 in my VA2 - it worked as-is! I didn't even have to change the Vref feeding it either - it's running on 2.5v. Apparently the problems I was having earlier when I tried something similar with TL082s, was because I had the gain factor totally out of whack (stock 27K gain resistors plus 3.9K pull-downs on the FM), but after changing the gain resistors to 24K with stock 2.2K pulldowns the TL072 seems to work well.

    It sounds very similar to the NJM4565, I can hardly tell any difference between them actually. There's just an infinitescimal amount of crackle on some very loud sounds just like on the 4565, which might be due to the 4.7K pull-up resistors Sega added to the outputs, presumably to re-bias the 358's output stage into Class-A to get rid of the crossover distortion. I tried removing those on the BA3404 and BA10358 but got even cracklier sound, but seeing as the 4565 and TL07x don't suffer from crossover distortion I might try removing those and seeing how it fares without them.

    Edit: PERFECTION...? Removing those 4.7k pull-ups seems to have eliminated the extremely minor crackling that was remaining. Let me test this some more and I'll update this post after I subject this to a battery of brutal sonic tests. And yes, now that I listen to it more closely, I do hear some minor differences between the TL072 and NJM4565. They're pretty close, but the TL072's high range sounds just slightly more precise, and the 'ringing' on the cymbals used in Yuzo Koshiro music sounds more prominent. Both op-amps are miles ahead of the BA10358 / LM358A / BA3404 and RC4558, and seem to work well for this application.

    One amusing artifact of removing the 4.7K pull-ups, is that the power-on 'pop!' noise is much less pronounced. This is true for both the TL074 and NJM4565. Lowered DC offset?
    Last edited by Eep386; 10-07-2017 at 03:58 PM.

  8. #23

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    I have a VA2.3 that has yet to have a MegaAmp installed, so I'm considering this route to see if I find it satisfactory, as it's easier than finding a place to mount an extra board for me, but I do have some questions;

    * I know that Ace biases TL07x's closer to 3v than 2.5v to avoid clipping issues: can you (or Ace if available) provide any insight as to why that doesn't seem to be an issue here?
    * I only have 1500pf caps available (instead 1600pf) for C21 and C23: What would the effect of that be, slightly higher corner frequency on the output mix filter?
    * I'm also curious if there are any issues with 32X and/or SegaCD inputs that need to be addressed or if they're already fine stock once the FM and PSG adjustments have been applied.

  9. #24
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    Hello!

    Not sure why the TL072 seems to be happy at 2.5v VRef, at least in my case - possibly because we're reducing the FM gain with this mod? I also should mention that the 4.7K resistors at R12 and R13 should be removed, as they're only used for the stock BA10358 to bias its output stage into Class A to get rid of crossover distortion. The 4558, 4565, TL072 and BA15218 don't suffer from crossover distortion however, so if you're using these amps, R12 and R13 should be removed otherwise the output will clip.

    Yeah, the 1500pF output caps will be OK, it'll just have a slightly higher corner frequency on the output mix. I tried some 1500pFs on mine and they seem to sound alright.

    I currently don't have a 32X or SegaCD so I don't know how they'll react to this mod, but I don't see them malfunctioning with it. The 32X and Sega CD's filtering is covered mainly by the pre-mix second order low-pass filter (which with 270pF caps will have its corner frequency set to approx. 58 kHz to avoid muffling things) and the output mix filters.

  10. #25

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    Thanks. I do have a spare TDA1308 around from previous work with the newer Mega Amp so I may use that just to be on the safe side, unless there's a reason not to.

    As for 32X/SegaCD I didn't mean malfunctions so much as volume level balance.

  11. #26
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    The TDA1308 is a headphone driver, meaning its output is WAY stronger than the TL072's - too strong in this case. I wouldn't use the 1308 for this.
    I had good results with an RC4558 after I removed R12 and R13. (The crackling I was getting earlier, was because I didn't remove those resistors.) 4558's can be found on old ISA modems, or from Digi-Key/Mouser/Farnell at rock-bottom prices. Digi-Key part #296-9650-5-ND, $0.35 USD per chip.

  12. #27
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    Update: I rewrote the guide heavily, omitting the crappy 'passive' filter approach in favor of a purely active filter, and now including options for keeping the full third-order filtering intact, with new filtering values that try to retain the original Q factor, overshoot and damping rates as closely as possible while offering a higher corner frequency. I also found out the '4556 op-amp seems to work the best of all the op-amps I tried. Designed as a high-current output amp, it's got a relatively beefy output section and can drive capacitive loads with relatively little fuss, meaning loud sounds are less likely to poop out at the high gain levels it's set to. It's also only nominally more power hungry than the '4558 yet offers 2x the slew rate (3V/uS) and 2.5x the gain bandwidth (8MHz), fairly good performance.
    Last edited by Eep386; 01-02-2018 at 01:54 PM.

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