Just for the heck of it, I swapped the JAZZ16 for a VIBRA16 card.
Now, the JAZZ16 is a pretty decent card with good DOS support and SBPro mixer compatibility. The genuine Yamaha OPL3 synth is nice, but it's still a clone card and I had two major (and 1 minor) issues with it:
1. The MPU-401 emulation would not work. I tried lots of stuff, and even after setting the enable jumper, reserving IRQ 9, and configuring the driver, nothing would recognize it as present and working.
2. Occasionally digital sound would completely flake out and produce noise until restarting the application.
3. You can't disable the CD interface, so those resources are wasted unless you happen across a compatible LMSI drive.
I decided to try something else, the CT2940 VIBRA16. More than just a replacement for the JAZZ16, this card has some interesting features and obstacles:
1. It is a genuine Creative Labs chipset and features a CQM synthesis chip (the CT1978) instead of the OPL3.
2. It's a cost-reduced SoundBlaster 16 popular with OEMs and sold at retail as a value brand.
3. It's a Plug-and-Play card.
4. It's a Gateway card.
This last fact is important to this build on principle. You can tell it's from a Gateway system because of the Gateway part number sticker (SNDCRD005) and the date sticker (10/24/95) on the back.
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The fact that this is a Plug-and-Play card means there's no hardware jumper configuration. Instead, the card is meant to be configured dynamically by the operating system or ICU in a system with a Plug-and-Play BIOS. However, the secret to using this card is that none of that is actually necessary, and just like a hardware-configured SoundBlaster, the CT2940 is usable without any memory-resident drivers or management suites. All you need to do is "free" the card's IRQ in BIOS, execute a CTMU.EXE program to get the card going, and use the SB16 standard applications MIXERSET.EXE and DIAGNOSE.EXE to configure and soft-set the card. After that, it's ready to roll on port 220h and IRQ5 like any other SB16.
If instead you install the driver package as packaged, you end up with all kinds of extraneous TSRs in your config files, which slows down your boot and eats up your RAM. Ironically, the CT2940, a Plug-and-Play card, results in lower memory usage than the older JAZZ16.
The only problem I had swapping this card into the system was some CMOS configuration crud that produced benign errors at POST. After clearing NVRAM with the mainboard jumpers, this went away. The card works in DOS, Windows 3.1, and Windows 95 with virtually no hassle.
And with what should be an operational MPU-401 interface (albeit hampered by the infamous "hanging note" bug) I can move on to experiment with off board MIDI synths and emulators.
The output signal is lower but cleaner than the JAZZ, too. Even with the software mixer maxed out, I have to come off Unity gain on my hardware mixer to get a good output level.