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This is an amazing project. So many electronics instructibles are noobs-(mis)leading-the-noobs so it's great to see an exception to all that.Looks like you're using only one of the primary windings on the transformer. Probably not an issue since it's got a lot more VA than you're drawing from it.I'm really tempted to build your meter shield even though I've already got a big stack of incomplete projects I ought to build first..... :/
Yeah, there's a DC offset on the output which will toast the speaker. The easiest way to get around this with single-supply operation is to use a full-bridge circuit. Yes, more parts, but you get better fidelity too. See:http://www.infineon.com/dgdl/an-1071.pdf?fileId=55...[International Rectifier AN-1071: Class D Audio Amplifier Basics]Adding the input buffer/preamp is an inherently good idea, though I don't see why you'd use a small power amp for the job. No harm in doing it this way, though.The 74HC04 would be even better if you used it to buffer both phases. Fast clean edges are best. And ground the unused inputs on the IC!
Why did you remove the components for the other outputs?
How to Build an 8-Bit Computer