Theory of operation
Sound in ATARI XL/XE series computer is generated by Pokey chip.
Designers of this chip provided a way for adding more than one chip to the system, by equipping it in more than one CS (chip select) line.
All one have to do when adding another chip is to disconnect jumper from CS1 line, and connecting there negation of A4 from system bus. Now, if A4 is low, first chip is selected...
As old way approach (using 5 ttl chips to accomplish the task) seems a bit odd to me, and i had some TLC7528 from TI sample program i"ve put them to use - quick glance on datasheet showed the way to interface them to 6502 bus
But i would have to use 2 of them to get four channels - not good, if board space is limited.
I needed something else
Browsing through on-line catalog of one of Polish electronic suppliers i found similar device - TLC7226 - 4 channel 8bit DAC, the only problem was it needed a bit more juice to power up - i"ve decided to test it with 5V supply anyway - results were prommising...
This is very basic summing inverting amplifier application on LM324 chip, it provides reference voltage for virtual ground (analog circuit) and signal separation and leveling from Pokey chips, DAC"s outputs and GTIA beep. There is also mono output provided, so users using only monitor or tv out would hear both channels.
TLC7226 was designed for use with Z80 style bus interface in mind, so there is no RW and Phi2 signal - WR signal has to be generated as boolean multiplication of Phi2 and negated RW signal, on schematics You will find another signal EN_S and EN_C responsible for turning on/off parts of extension