Have you checked the patent number?
http://www.google.com/patents/US4812847

Though I can't say if they really used the stated component values, but it gives some insight about the design.

I only had a self-made version when I was a kid. It did not even use R-2R network but 8 resistors that were approximately powers of two.

Sheet 4 and 5 seem to indicate fancier designs but were they ever implemented or did they just use the simple resistor only one?

Perhaps they did implement the more complex design at first (the silver model), then simplified it (the white model). I think the white model is the later one because the label on the silver model only has a TM for Trademark and just U.S.A. The white model is an R in a circle, which is a Registered Trademark and emphasizes the product was "Made in U.S.A." The more complex design in Fig 4 has many more passive components, and Fig 5 has a buffer chip. Both Fig 4 and 5 are designed so that they could be used for a printer port that uses an open collector data port. IBM PC compatible printer ports use a two-state data port latch, so I think that Covox eventually realized that the extra components probably were not worth it. I have also looked at several IBM PC and Tandy 1000 compatible printer schematics and all have pull up resistors on the data lines, so I am fairly convinced that Covox was probably using the design in Fig 3 for its white model.

It's amazing how you can't seem to find any images of actual covox pcb's of someone just opening their converter. Still I can't imagine them going for a more expensive design if they could get away with their cheaper solution sounding good enough. You'd have to wonder if they picked the resistor with decent enough precision or just went with whatever cheapies they could find.

There's a video on YouTube about the Covox Speech Thing (https://www.youtube.com/watch?v=spOenlrSSOE) that shows a genuine Covox Speech Thing being used, it actually sounds not all that bad. At 2:28 it briefly shows the innards of what I would presume is a Covox, but it could be a clone, hard to say. Also, the Covox Speech Thing seen in that video is a different style than the two shown above, it doesn't have parallel port passthrough.

That would refer to this image

But i'm fairly sure that's a disney sound source with that ics1453 fifo/dac chip.

torindkflt wrote:

There's a video on YouTube about the Covox Speech Thing (https://www.youtube.com/watch?v=spOenlrSSOE) that shows a genuine Covox Speech Thing being used, it actually sounds not all that bad. At 2:28 it briefly shows the innards of what I would presume is a Covox, but it could be a clone, hard to say. Also, the Covox Speech Thing seen in that video is a different style than the two shown above, it doesn't have parallel port passthrough.

Actually, its not a genuine Covox-made Speech Thing. LGR so much as states that. He acquired his genuine Speech Things after he made that video.

Harekiet wrote:

But i'm fairly sure that's a disney sound source with that ics1453 fifo/dac chip.

Ah, I would believe so too now that I see the product code starting with "DSS" in the lower left of that PCB.