Drew ][ TDX - Apple IIGS Stereo Card
Unfortunately these are currently sold out.
This started out as my first real electronics project, and after a lot of work (and fun learning) the Drew ][ TDX was born. Making money was never on the list of objectives, and was really about doing something more with the IIGS and giving something back to the community. The current version of the card is version 1.1 and is shown above. I was considering adding input, but there isn’t really demand for it and isn’t something that I am particular interested in, so unfortunately any future cards will be output only. In terms of a future version I am looking at moving to surface mount components to make a slot-less version. I don’t have a time frame for this, but probably some time next year...though i have already started ordering parts of prototyping :)
The initial Drew ][ TDX card was based on information gathered from the Apple ][ FAQ provided by Jeff Hurlburt (Actually named “TDX Stereo Board”). With a modification to use a quad op-amp (used by Polymorph). I wanted it to be as complete as possible and decided to include a backplate as part of the version 1.0 card. The card is designed to be connected to line-in on an amp or to a set of powered speakers.
The Apple IIGS as standard outputs all audio channels produced by the onboard Ensoniq via the onboard jack in mono. When you connect headphones or speakers to the onboard jack you will get sound out of both speakers, but this isn’t true stereo sound. Though the GS can support more than 2 channels the majority of software only supports 2 channels aka Stereo over Channel 0 (right) and 1 (left).
Though Apple did not provide the necessary circuitry to support stereo output (I am not sure why they didn’t considering the S stood for Sound or at least included it on the ROM03) they provided a 7 pin molex connector (J25) that has a number of pins to cover supporting multiple channels as well as a single input channel (via add on cards i.e like the HyperStudio Input card).
As standard the GS will output all channels, with the right components you can take this audio and split it into the separate channels.
The TDX card does this via an analog demultiplexer (74HCT4052N or similar). The sound enters the IC and the Ch0 and Ch1 is switched to the 2 outputs (aka left and right audio). The sound is then cleaned up and some gain applied via a TL084 quad op amp. This can then be connected to a set of powered speakers or amp. This works in the same way as many other stereo cards that were around in the late 80’s like the AE SonicBlaster, AE Audio Animator etc. Unlike those cards the current Drew ][ TDX only provides output and does not provide any input capability.