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Thread: Suggestions for a Mega Amp 3.0 revision?

  1. #16
    Your current user title Nameless One donluca's Avatar
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    Quote Originally Posted by villahed94 View Post
    For audio stuff (at least CD quality) you only need around 2.5 V/us of slew rate.
    This page can give you some insight into the types of opamps you could use (bipolar or FET).
    http://nwavguy.blogspot.mx/2011/08/o...surements.html
    If you dislike the “sound” of 5532 that means you must also dislike most of the music available.
    This is the kind of guy I hate with a passion.

    He probably got a spectrum analyzer for Christmas and went crazy with various measuring.

    Praising an opamp on numbers is the same as saying that the Xbox1/PS4/whatever is better because it's better specced.

    One of the fundamental of audio is that our hearing is different from person to person and this is not accounting our personal tastes which is not measurable in any way.
    This is why everyone tells you to go and try some piece of audio gear with your own ears instead of blindly buying from Amazon.

    Good numbers are a *very* good start, don't get me wrong, but your ears must be the final judge.

    So, what are you aiming for?
    For example, if you're going for authenticity (making it sound as close as the original as possible, that is) then numbers are absolutely useless since the original opamp used was a low quality one, used for cheap guitar amps, and any of the modern ones are going to be more than sufficient. Although, reproducing the same sound might prove a challenge and the only thing you could possibly do is just test a bunch of them (go for the cheapest ones).

    If you're striving for the best sound possible, even if it will sound wildly different, then by any means try out some of the better ones, such as the NE5532 (which I strongly suggest you, to start with) and see what you like most and what you feel captures the spirit of the original sound best. Keep in mind that MD have a quite aggressive low pass filter, you might want to give it some thought since the composers made music with that in mind.

    P.S.: the car analogy is also terrible. Speed is something objective and measurable. The "pleasure" derived from listening to music can't be measured by scientific means.
    Sorry for the rant and OT.
    "Ask not what others can do for you, but what you can do for others."

    Founder of the 16-bit audiophile project
    http://16bap.theclassicgamer.net

  2. #17
    Outrunner Eep386's Avatar
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    I don't think the guy in the article was quite advocating praising op-amps by numbers, per se. (Indeed, he demonstrated how a cheaper NJM2068 could match the considerably more expensive '4562 in noise, and how the old fogey '4556 can still hold its own as a decent output op-amp.)
    I think the gist of sharing that article was to point out that 'high-end' op-amps like the LM4562, TLC084, etc. are probably not worth the expense in a circuit like this, considering that even with the 'unfiltered' option the Mega Amp will have a very heavy first-order low-pass filter, with a 15 kHz knee frequency at the highest. For ~15 kHz, you don't need anything even remotely LM4562-level. A super cheap quad-amp like the '2060 looks like a good way to integrate things a little, though again there's nothing stopping you from using two '5532s instead if you'd rather. (Heck, I'm still not entirely against the notion of adding unity-gain buffers all over the place, were it my own creation. The '5532 would make a mean unity-gain buffer, that wouldn't be disturbed much by loud sounds even on 5V.)

    For the record though, I'm still very much enamored to the NJM4556. That thing simply does not clip as a VA2/2.3 FM pre-amp. The TDA1308 seems to perform very similarly there too, and it's cheaper, though I could swear it is very slightly more prone to clipping than the '4556, but not enough to justify swapping it out again. (Bear in mind that the VA2/2.3 pre-amp gain is a whopping 8.18 stock; I've reduced it to 7.27 as part of a mod to improve the sound quality without excessively violating the 'stock' nature of the console.)

  3. #18
    Outrunner Eep386's Avatar
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    OK, at long last, I finally got off my duff and built a Mega Amp of my own, based on the 'mini' version of the V1.2 model, for a VA1.8 Model 2. I used an MC33078 instead of a TL072 along with a 2.5V VRef, and didn't bother with mono. I didn't have the exact resistors needed so I used some close values. Despite using some pretty crap quality carbon resistors it actually sounds pretty good (I am using 150K gain resistors with 82pF filter caps, so a 12 kHz cut-off) and I don't hear any clipping or crunching that isn't typical of 'average' 315-5684 based systems. (If anything, it clips and crunches less than 315-5684 systems.)

    I guess I just wanted to say, that the older 1.2 revision is still viable so long as you don't use amplifiers that need to be perniciously babied like the TL07x. The MC33078 seems to work great, as does the RC4558. My board has a DIP-8 socket so I can now try out various op-amplifiers. Expect me to occasionally update this with my findings regarding various cheap jellybeans (might as well eliminate the ones that aren't so good for this kind of project).

    And yes, I'd imagine a TDA1308 or NJM4556 would be better still for the output side. For now though this is what I have on hand.
    Last edited by Eep386; 03-31-2018 at 09:31 PM.

  4. #19
    Outrunner Eep386's Avatar
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    Update: Aaaaand I (somehow) ESD'd my VA1.8 by trying to adapt a CXA1645 to it. It was working fine before I started muddling with the encoder. So further experiments with the Mega Amp will have to wait, at least on my end. (It was purchased as-is broken from eBay, and I had brought it to life.)

    Not (really) a biggie, I brought a battle-damaged Model II VA3 back to full life with a CXA1145M. I *could* try adapting the Mega Amp to it, but I already fixed up its onboard sound so not really a pressing need atm.

    I did do a few sound recordings of the Mega Amp 1.2 with the MC33078 though, while it was still working:

    Sonic the Hedgehog - Star Light Zone (apologies to Tiido for using MP3 - I had to use a format that a flippin' iPod would work with.)
    Sonic the Hedgehog - Scrap Brain Zone

    Aside from the noise, it sounds pretty nice I think, with a really wide audio response. The bass is also nice, warm and tight. The high level of noise is due to the fact that I played a little fast and loose on the layout, using cheap carbon film resistors instead of good 1% tolerance metal film resistors. My next attempt with a Mega Amp will use better resistors and more attention to detail, and I am considering ways to inexpensively add a dedicated 5V power plane using a low-profile 78L05. (I measured the power draw of a stock VA1.8 using a Kill-A-Watt and it was barely 6 watts, so I have full confidence that the aging power brick will easily handle another 100mA of load from an added 78L05.)

    By comparison, here's the VA3 Model II with my modifications to the onboard sound:

    Sonic the Hedgehog - Star Light Zone (Model II VA3 onboard sound)
    Sonic the Hedgehog - Scrap Brain Zone (Model II VA3 onboard sound)

    As you can tell, while it doesn't sound bad, the 315-5684's audio response is simply no match for the MC33078's. :P Some of it is due to the VA3 being configured with a 1000pF (5.8 kHz) output stage low-pass, and I removed the Sallen-Key filter stage and left just a 160pF filtering stage on the FM, with a 49.7 kHz cut-off. The Mega Amp I built uses a 12 kHz low-pass by comparison.
    Last edited by Eep386; 04-05-2018 at 01:52 PM.

  5. #20
    Wildside Expert fluxcore's Avatar
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    I would love a revision tailored to the nomad (including cart expansion audio circuit) There's lots of talk about how noisy the Nomad audio path is, but not how to best rectify it.

  6. #21
    Hedgehog-in-Training Hedgehog-in-TrainingNameless One pyroesp's Avatar
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    Quote Originally Posted by Eep386 View Post
    2. Consider reworking the FM mixing circuit to more closely match the Yamaha YM3438 datasheet's recommendations.

    This is probably a red herring/not worth it, but I wonder if we couldn't get better FM sound by following Yamaha's recommended circuit. Anyone who knows better than I do about this stuff (should be pretty much anybody else), I'd appreciate some insight as to this notion.
    Attachment 13913
    The one issue I might have with this, is the usage of pull-up resistors. I wonder if that would create DC offset, given my experience with the FM pre-amplifier used on the VA2/2.3?
    As an FYI, the capacitor after the first buffer amp removes all DC and only lets AC through. Because the opamps aren't powered with a negative rail, you have to add a DC offset or you'll just cut off the negative side of the AC waveform.


    If you guys make a Discord channel or whatever, I'd love to follow along.

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