From a emdded ebay card.
Replaced the original CT1978 CQM synthesis chip and its related DAC with a genuine Yamaha YMF289B OPL3 FM synthesis chip and companion YAC516-E DAC,
A number of modifications to the FM/AWE filtering, op-amps and capacitors used to improve overall sound quality.
Note that this modified card has somewhat heavier filtering on the FM and AWE output than usual for these cards. As the Yamaha YMF289B can generate some pretty gnarly treble,
Lowered the low-pass filter cut-off frequency to 16kHz to bring the FM output more in line with that of the SoundBlaster Pro 2. (This does not affect SoundBlaster DAC playback, only FM synthesis and AWE synthesis.)
Lowered the FM/AWE mixing gain somewhat to fix clipping problems with the original setup when loud songs were being played. (I took great care to not lower it too much, so that FM and PCM sound do not become excessively unbalanced.)
The FM and AWE outputs are both filtered by the same op-amp stage, so both outputs are filtered at 16kHz first-order, effectively.
The resistors that control the gain are R27 and R36, while the caps that control the filtering for this stage are C55 and C65.
The current gain resistors are 20K, the caps for filtering approximately 470pF. I personally prefer this more bass-heavy filtering as it makes for a nicer listening experience, but if you want to ‘open it up’ you can replace the caps at C55 and C65 with 180pF units for a 44.2kHz first-order filter instead.
The card originally had very poor quality MC3403 op-amps that were very noisy. Replaced them with higher quality units: I used an MC33079 for the FM/AWE mixing, and an MC34074, a TL974 and a TL074 elsewhere.
Replaced the larger-valued electrolytic caps, including two slightly oversized Nichicon 1000uF output caps for the TDA1517 outputs (which match its datasheet recommendations).