Using the '5200 and '51XX, you don't have to level-shift either, do you?
I pretty copied the schematics of the TI99 speech adapter for my TMS5200 protoboard. (you can hook yourself TTL to that one).
As far as the TMS51XX is concerned, I tried a few things, but the best was to hack the negative voltage the way the Stern Bagman board does it (voltage dropping diodes). But its flaky, it would have benefited from proper leveling that one.