Yes sometimes a digital inverter can be used for analog signals too.

Good news is that I was able to figure out the connections of the digital parts (bus address decoding, buffering, clock divider, etc) and the connections match my expectations. Chip select decoded from address bus with the two 138s go directly to OPL2 -CS and 245 -OE. Doubly inverted (buffered) -IOWR goes directly to OPL2 -WR and doubly inverted (buffered) -IORD goes to both OPL2 -RD and 245 DIR. Address pin A0 goes directly to OPL2 A0.

Bad news is that it is extremely hard to see how the analog parts are connected together, as I can't see the PCB wires under the chips and other components. The surroundings of the operational amplifier (U8) is most unclear, as well as some of the output amplifier connections (U9), including the volume control knob and the output mute parts (Q1).

If I had a card on my hands, I would measure what is connected to where with a multimeter by using the continuity test beep mode. So anyone with a Adlib, multimeter and basic electronics skills can help me fill out the blanks.

robertmo wrote:

but this one must be old:
http://www.dearhoney.idv.tw/Museum/soundcard-13.php
😉

That may be a clone. No marks from Adlib are anywhere to be seen on the board, and the board falsely states that it is in stereo.

not maybe, it is a clone 100000% 🤣

That clone is similar to one that was reverse-engineered by someone else (Grischenko Alexander, Latvija).

The reverse-engineered schematic (digital parts only) had Soviet clone markings, those with K555xx something, instead of 74LSxx.

Otherwise it does exactly the same thing, but the 14.318MHz bus clock is divided by 4 for OPL with 74LS74 chip instead of 74LS109. Just as same, as long as it is divided by 4.

HunterZ wrote:

Interesting analysis Jepael.

The Nintendo NES has 2-3 74LS04 hex inverter chips, and uses part of one of them as some kind of weird preamplifier stage for the audio (which itself comes from the CPU chip on 2 pins that get mixed together). That was one of the many weird things about the NES audio circuitry that made me abort trying to do a better stereo mod than the crude ones I've seen floating around, since I'm very much an amateur when it comes to electronics (especially analog circuits).

The NES has one 74LS04 chip, and only one of the gates is used for audio, the rest have to do with other aspects of the system (lockout chip mainly.) The 74HCT368 can do the same thing and does do the same preamplication on the Famicom.