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Thread: Genesis 1 & Mega Drive 1 stereo mod no headphones volume control

  1. #271
    Road Rasher ironnick23's Avatar
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    This looks fun, room on that board to switch between resistors so I can have va3 bass with va2 treble? Lol

    The teradrive came with a 3438, wondering if itís because they sound better on soundblaster speakers.

    I run my headphone out with a nice aux cable to my SCD, though it would be nice to switch to volume controls out the back because why not? Do you have a comparison showing the headphone amp distortion? When I keep my volume at a 5-7 instead of full blast it seems to do the trick with any high frequency distortion

  2. #272
    The legendary gargoyle Road Rasher Firebrandx's Avatar
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    Quote Originally Posted by ironnick23 View Post
    This looks fun, room on that board to switch between resistors so I can have va3 bass with va2 treble? Lol

    The teradrive came with a 3438, wondering if itís because they sound better on soundblaster speakers.

    I run my headphone out with a nice aux cable to my SCD, though it would be nice to switch to volume controls out the back because why not? Do you have a comparison showing the headphone amp distortion? When I keep my volume at a 5-7 instead of full blast it seems to do the trick with any high frequency distortion
    On my VA3, the headphone amp doesn't really 'distort' the sound so much as severely filter high frequencies. Since it wasn't meant to drive anything other than headphones, recording it into line-level equipment makes it sound worse. Here's a quick demo of my mod work to date versus stock on the M1VA3:

    https://filetrip.net/dl?y8rNoY7chR

  3. #273
    Road Rasher CrossBow's Avatar
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    That looks amazing, but I'm trying to figure out how it mounts now LOL! Would you have to run wires manually to the spot where the 2612 was or...?

  4. #274
    Road Rasher ironnick23's Avatar
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    https://shmups.system11.org/viewtopi...=61168&start=0

    Changing the 5600pf smd caps for 4700pf lowers, but doesnít eliminate, high frequency distortion. It matches the fidelity of the va2, without its bass distortion.

  5. #275
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    Quote Originally Posted by ironnick23 View Post
    https://shmups.system11.org/viewtopi...=61168&start=0

    Changing the 5600pf smd caps for 4700pf lowers, but doesnít eliminate, high frequency distortion. It matches the fidelity of the va2, without its bass distortion.
    This. Did it on my VA4.

  6. #276
    The legendary gargoyle Road Rasher Firebrandx's Avatar
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    Quote Originally Posted by Xaranar View Post
    This. Did it on my VA4.
    That was a mod I had done based on Ace's suggestion. However, with the line amp board, it isn't needed. So I've put the original 5600's back on.

    Quote Originally Posted by CrossBow View Post
    That looks amazing, but I'm trying to figure out how it mounts now LOL! Would you have to run wires manually to the spot where the 2612 was or...?
    It's going to use socket extensions from the upper left interface straight down into the Genesis. This will raise it above the cap line, with still plenty of room before it hits the top metal shroud.

    Edit:

    Another minor update: Decided to lower the line amp volume resistors to 150K. 200K was just a touch too much for line-level output. I've also contacted Ste in regards to the attenuation method going on for the YM3438. I'm skeptical about the series cap and series resistor being the proper method. Everything I've read about volume attenuation suggests a series resistor and a pull-down resistor. But I'm not an expert, so I'm going to wait for his advice.
    Last edited by Firebrandx; 06-05-2018 at 09:51 PM.

  7. #277
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    Quote Originally Posted by Firebrandx View Post
    Another minor update: Decided to lower the line amp volume resistors to 150K. 200K was just a touch too much for line-level output. I've also contacted Ste in regards to the attenuation method going on for the YM3438. I'm skeptical about the series cap and series resistor being the proper method. Everything I've read about volume attenuation suggests a series resistor and a pull-down resistor. But I'm not an expert, so I'm going to wait for his advice.
    That sounds about right, as a resistor comfiguration like that creates a voltage divide based on the ratio between the two resistors, where Vout = Vin x (R2 / (R1 + R2)), and R2 is the pull down resistor.

  8. #278
    Road Rasher CrossBow's Avatar
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    Which might explain why I've noticed that the 2612/3438 combo seems to generate some heat? But in the current setup and scheme the resistors are only used on the output lines and not the input to the chip itself? The cap is just there to filter out additional DC of course.

    In any event, it sounds like different resistor values might be required depending on which model 1 variant you are using?

  9. #279
    The legendary gargoyle Road Rasher Firebrandx's Avatar
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    Quote Originally Posted by CrossBow View Post
    Which might explain why I've noticed that the 2612/3438 combo seems to generate some heat? But in the current setup and scheme the resistors are only used on the output lines and not the input to the chip itself? The cap is just there to filter out additional DC of course.

    In any event, it sounds like different resistor values might be required depending on which model 1 variant you are using?
    Well the original guy that came up with 2.2k admitted he didn't thoroughly vet this as the proper mixing volume. Like me, he found certain sounds were still overloading the circuits.

    As for the heat, I've noticed this too, and I'm expecting the mod board will help dissipate that since the chips will be separated instead of stacked. It might even be a thing where I separate the power supply to each chip and rig that with a toggle, which means using the NAND gate mod to set pins A0 and A1 instead of using the 2612's timing to prevent crashes.

  10. #280
    The legendary gargoyle Road Rasher Firebrandx's Avatar
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    Sent off the finalized 'mount anywhere' compact line amp board for fabrication:



    Once that gets back and I do a few more tests on the resistor values on the input side, it will be ready for public modding!

    On a down note: The 4-layer board for the YM3438 mod is ULTRA expensive. The charge was going to be over $60 for 3 prototypes, so I'm going make sure everything works with breadboards first before I splurge that kind of money on the prototypes. I don't want to get the boards in and then find out there's a fundamental problem, so it's best I get this covered first with breadboards.

  11. #281
    Road Rasher ironnick23's Avatar
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    Nice work! One day, my Segaís internals are going to need a second tier for mounting all these mod boards

  12. #282
    The legendary gargoyle Road Rasher Firebrandx's Avatar
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    Current status: Did some more testing with the YM3438 + line amp board and determined the 5.1K input resistance was causing slight distortion in the deepest bass notes (stage 6 music in Streets of Rage). So I set the volume resistance back to 200K and tested several input resistors from 5.1K up to the original 10K, and found 7.5K was the sweet spot. So the line amp configuration has been change to 7.5K + 200K on the input side.

    Also heard from Ste on YM3438 volume attenuation and he confirmed the series caps + series resistance is fine for attenuating the volume. With that in mind, I rigged up an external testing device that let me change the resistance on those lines live, and was able to confirm that 4.7K is absolutely required for proper attenuation. Anything lower than that starts to distort the "SEGA!!" chime at the beginning of Sonic the Hedgehog. So let it be known hence forth the YM3438 mod for the model 1 Genesis MUST use 4.7K, not 2.2K.

    Still waiting on OSH Park to finish fabricating the finalized "mount anywhere" design. I did find a more suitable mounting adhesive instead of using looped electrical tape, and in this case, I'm going to start recommending "Scotch Mounting Pads". They come in 1-inch squares, so you just cut to fit the bottom of the board, peel and press it where you want inside the console. I used one in my RGB bypass mod I did in my Super Grafx with Voultar's board, and it worked beautifully! Here's a pic of that work in case anyone was curious:


  13. #283
    Road Rasher CrossBow's Avatar
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    That does look really good! I need to get me some 4.7k resistors then...oh..wait!!! I actually just ordered up a bunch for replacing ones removed from Atari systems so I actually have some really small 1/4 watt ones ready to go now that I think about it!

    I use 3M velcro on my PCB mounting. This way there is some flexibility to move the board around if needed and it can be removed easily without just flat out coming off the board. As long as you clean both surfaces with alcohol, the velcro isn't coming off anytime soon. But those scotch pads look to be much slimmer and offer a more flush looking fit of course.

  14. #284
    The legendary gargoyle Road Rasher Firebrandx's Avatar
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    Another note on the 4.7K resistors: Use NO higher than 1% tolerance. This is important because the higher the tolerance, the more imbalanced the stereo sound might be. I took out a box of 1% resistors and manually tested all the 4.7K ones I had with my Fluke 115 until I found two that were identical in rating. This ensured the left and right channels were perfectly balanced on volume level.

    Also just got notified from OSH Park they shipped the finalized boards! I can hardly wait to try them out and show some pics of one mounted in my VA3.

  15. #285
    The legendary gargoyle Road Rasher Firebrandx's Avatar
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    Got the new boards in and installed one in my M1VA3 as a permanent mod:



    And here are the new recordings with the 4.7K resistors in place. You'll notice the stereo separation is more subdued like it is on the YM2612, but this has to be the case for proper attenuation:

    https://filetrip.net/dl?i1miIjK3Cu

    So the board is finished. However, I have to save up some money to produce a batch for sale. I'll need about 3 weeks to get them ready to go on my web store.

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