When I bought an Apple //e in the mid 80s (replacing a very hacked Apple ][+) it came with a small card with a speaker on it called “SAM – Software Automatic Mouth” which allowed the Apple to talk.
The SAM card was an 8 bit DAC, implemented with a latch, DAC chip and amplifier for the speaker. I’d connected it to my sound system and discovered I could get pretty good crunchy 8 bit sound playback (less noisy than the Mac+ I also had at the time, no dither noise).
Around the time I’d written a drum sequencer that triggered a real 70’s era analogue drum box through the 4 game port annunciator outputs. One of the Mac music programs had some drum samples and I tried playing them using the SAM card. It worked great and in 1987/88 I ended up writing a 16 channel / 4 simultaneous voice mouse driven drum machine for the Apple II+ and //e.
Unfortunately the SAM card got left behind in a move and its a rarity on eBay. Recently after researching a few DACs and even an R-2R ladder option, I found an interesting chip that had promise, the AD558. This is pretty much a one chip solution. Internally calibrated, running on a single 5V supply, direct CPU bus compatibility and with a buffered output, it can be configured for a 0 to 2.8V output, perfect for a line level audio out.
From the AD558 datasheet and Apple II reference manual, I came up with this circuit:
The only additional passive components are:
- 0.1uF power decoupling capacitor – unpolarised greencap, 25V rating would have been enough, the one I had was double that.
- 6.8uF electrolytic capacitor. This blocks the DC offset in the 0-2.8V output, giving a ±1.4V output, which should be plenty as its going into my mixer.
- 20kΩ resistor, this is there to let current charge the blocking capacitor if the output isn’t connected (avoiding a transient on connection), I’d originally thought 10kΩ but settled on it after finding it first, its value is not critical.
I used an Apple II prototyping board which are still available here, an Apple II slot card which includes solder pads yet breaks out all the port pins to a wire wrap header. I initially planned to wire wrap it all, but I hadn’t ordered a wire wrap socket (and the pins might not have easily fit through the plated-through holes anyway), so I used one of the spare 20 pin sockets I’d recently bought to fix my mouse card, carefully ignoring the extra holes as the AD558 has 16 pins!
With the 8 data inputs on the AD558 arranged neatly in a sequence that corresponds to the data bus on the Apple slot connector, I directly soldered them to the connector using stripped wirewrap wire which, being silver plated, is a breeze to solder. For me it involved a lot of bright lights, looking through a number of magnifying glasses and some patient assistance from my wife, Oak.
Keeping the solder from running up the length of the slot connector was not easy, I eventually discovered that gravity can be your friend. For each of the 8 data bus wires I attached the slot end first then stretched the fine wire to its pin on the socket, (which I’d soldered in first) so it was just a case of heating a pin up so its corresponding wire could be slipped next to it, then the remaining wire cut off.
I took these photos as part of inspection, you can see the 8 fiddly data wires at the bottom right. Ignore the solder along the top edge of the board, its just strain reliefs for the line out cable.
On the row opposite the data pins on the AD558 (pins 9-16), a few adjacent pins need to be bridged. I ran a wire across them, trying to avoid touching the board itself as all the slot pin have traces criss-crossing to the wire wrap header (on the right on the photo below). If the thin insulation on the board was breached by solder, things would get unpleasant. I took particular care to keep away from the 12V line (pin 50 on the slot connector) as really bad things happen if that gets into TTL chips. I know from childhood experience.
Looks real classy doesn’t it.
The proto board’s half solder/half wire wrap design is really not ideal for building much on the board itself. I think it was intended to have a daughter board (eg: Arduino) hang off it. I would have preferred either bigger holes just for wirewrapping or a board without the wirewrap header and associated traces across most of its area, other proto boards have little pads for soldering to at the slot connector.
I did use wire wrap and the wire wrap header to connect to the device select, read/write and power lines. I’d bought the tool and wire after all, and wanted to stay away from the slot connector after my experience with the data lines.
Mounting the capacitors and resistor was uneventful and I ran the audio to the shielded audio cable at the top of the board with little consideration for what its crossing along the way, fully expecting I’d deal with noise later, once I’d tested the board.
With the card in slot 5, writing alternating 1, 0, 2, 0, 4, 0, etc to $C0D0 generated progressively louder clicks. Both the SAM software and my drum machine worked perfectly first time.
The audio output is surprisingly quiet given the unshielded wires and cross-crossing bus on the board. Plus I’m running without any special power supply regulation for the DAC, the AD558 has its own internal reference. What a great chip, glad I bought a few as I’ll have to do something for the Arduino with one or two.