[dvw]

By accident I found out my D-33 has a single power supply rail and used capactor to block DC.

The problem with this scheme is with low impedance headphones, there is a very severe bass roll off. I was comparing a XD200 to a V6. Not only there is a lack of bass detail with the XD200, it is also brighter than the V6.

There was a solder joint problem at the power supply that I fixed. I also changed the decoupling capacitor to 330 uF. The XD200 now has very fast bass, the high end mellowed out and there are lots of bass detail. (I used Gladiator's track 3 for test).

Did some research. Most headphone chip designed for low voltage portable devices uses single supply with decoupling capacitor. So they more or less have some bass roll off with low impedance phones.

Here's my suggestion:
1. Use higher impedance phone. 64 ohm is a good compromise (V6, Koss). 120 ohm if you have a strong headphone out.
2. Use an in-line volume control to add to the impedance. But this is not efficient.
3. Replace the decoupling capacitor with a higher value cpacitor. (you'll need some soldering skill) This is the best solution. Make sure the capcitor you pick is the same physical size or the board will not fit back into the case.

 

[dvw]

The D-NE300 series of CD players use output capacitor less amp. So increased impedance does not help the sound. For this player, a 16 ohm headphone will be optimal.

The 707 series of NetMD do use output capacitor. But it has a very limited voltage swing, increased impedance will reduce power. The amp will not perform regardless. The best you'll get out of this series of MD player is with a 32 ohm headphone.