I have been building DACs from kits and have been reading a lot about tweaking CD players, but I did not have the courage to butcher my CD players.
Early this year, I was able to buy a very decent, excellent sounding Denon CD player, it is so inexpensive that I felt so encourage to tweak it.
First thing that I did was replaced the PSU capacitors to Aerovox electrolytics, noticeable improvement on sound. Encouraged, I replaced the electrolytic output capacitor with a very inexpensive Wima MKP (polypropylene), and wow, better dynamics, bass improved. Next was the removal of muting transistors, with trembling hands, I removed the 4 muting capacitors (on hindsight, I should just have wired the output caps directly to the RCA sockets, simpler and safer!)
Encouraged again, I figure I will replace the opamp output stage with a tubed output stage. So I took note of the DAC of my vintage CD player, it is the Burr Brown PCM61P, 18-bit DA converter, 8X oversampling. This particular DAC (lucky for me) has both Io (current output) and Vo (voltage output), which means the opamp is built in into the DAC. Every DIYers on the net says that the best is to tap from the Io (current output) of the DAC, build an I/V (current to voltage converter) and feed the signal to the grid of the tube stage.
After a few weeks of research, I decided to use my favorite 9-pin tube, 6n1p (beside from being CHEAP, it sounds great).
According to the data sheet of the PCM61P, the Io is capable of 1V output, so I really don't need a lot of gain.
After wiring and soldering the tube stage, power up the CDP, it sounded so awful, no mids and no bass, just shrill noise. Back to the drawing board, after a couple of weeks of banging my head (this is my first SS into a tube stage project), I decided to isolate the current output of the DAC totally from the circuit and tied all the opamp input of the DAC to ground (reduce noise), voila, music! A lot better! After a week or so, I decided to tweak the I/V converter, and wow! Sound worth all the effort and time I put into it.
Here are some pictures:

Now that only tweak I need to do is have the scratched chassis painted at my work (Nippon Paint), flat black, or maybe some nice Honda color.
