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I've been working on a project for connecting a Yamaha YM2612 to an arduino and triggering sounds to be played from the chip. I'm having trouble on the audio part of the circuit. I've worked out a lot of the noise in my original circuit but I'm having trouble getting the level of amplification I want (not anything huge, just something line level you can use for headphones). A while back, I modded my Sega Genesis with a custom mod called the Mega Amp (I'll link schematic below). That works 100% and has no problems with making the signal clear and loud. I figured I could just rework that circuit for this project, but I'm not getting the volume or clarity I expected. I'm not sure if this is due to there being something else in the actual Sega console but I don't think it is. I've looked into op-amps and think I have an ok enough understanding of how they work, but I might not be seeing something important. Here is the schematic of what I have on my breadboard as well as the mega amp mod that I used. I broke the mod down into just the parts I think I needed. For the time being, I've only been testing this by hooking up one of the audio outputs on the Yamaha chip and running this through my computer's sound card. Any help and insight for this would be immensely appreciated! I'm still learning and would like to know more! :D

Breadboard circuit: https://gyazo.com/b69beac00f659da8f619841d61877566

Mega Amp mod circuit: https://i.imgur.com/DDulM2r.png

TDA1308 datasheet: https://www.nxp.com/docs/en/data-sheet/TDA1308.pdf

YM2612 Pinout: http://partnumber.datasheet.netdna-cdn.com/pdfhtml/0870/865751/page1.png

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The polarity of the audio coupling capacitor C4 is backwards which causes a DC unbalance and the amplifier amplifies it. Two of the many coupling capacitors in the Mega Amp mod circuit are also backwards.

The audio gain is 240k/6.2k= 39 times which is crazy since the audio input to C4 is probably at least almost 2.8V peak to peak.

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  • \$\begingroup\$ So should I switch the cap around or use something non-polarized like a ceramic? The audio of the yamaha chip is quite low in general. The signal has a dc offset of around 3.5V with a peak to peak value of about 1V. I probed this directly off the pin before the pull down resistor. I tried rotating the electrolytic cap and it didn't seem to alter the volume at all. Here's the picture of the sample sound I have the chip playing for testing. This is AC coupled and .5v per division. imgur.com/bxLVh7l \$\endgroup\$
    – TokyoScarab
    Sep 10 '18 at 3:33
  • \$\begingroup\$ I'm also not sure if that cap is backwards. I've run this on some circuit simulator programs and each time I flip the cap, after a short period of time, the cap breaks in the simulation \$\endgroup\$
    – TokyoScarab
    Sep 10 '18 at 17:39
  • \$\begingroup\$ I'm also not sure if that capacitor is backwards. I think it's correct as it is. I think in that circuit there's a chance for to negatively biased for a portion of the conduction range but on average I think it is correctly biased. if there's concern that this is really the issue you could replace it with a ceramic or film capacitor. Note ceramic capacitors are not great for audio as their capacitance shifts with voltage. \$\endgroup\$
    – MadHatter
    Jan 26 '19 at 16:05

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