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Just wanted to share one simple mod I've done recently to my Denon PMA500. The device has Loudness function, which boosts both sub-bass and high treble. The thing is my Roland RH-A30 are already very good at treble, and boosting them more makes it really harsh. On the other hand they would nicely benefit from boost in frequency range below 40Hz. It's like listening with and w/o subwoofer:D So I came up with idea, that I will disassemble this amp, and I'll find where is this loudness located, and possibly remove part of a circuit that boosts high treble. At the beginning I was afraid that they used middle pass filter, and they put what comes out to inverting input of opamp, but fortunately they didn't, i.e. there is separate section for bass and separate for treble, and both are simple RC circuits. And because of that it was sufficient to remove resistor in treble boost section.

This is how this amp looks inside:

Here you can see two resistors removed:

And basically this is it. Now when I push Loudness button I get sub-woofer feel from headphones. I tested this new feature with all my headphones, and here are my observations:
  • Momentum - sound the same with and without boost, so I think reasonable to say would be that they don't have sub-bass at all
  • HD570 - they already do bass very well, so boost caused them to sound not even veiled but even worse, i.e. like behind 2nd curtain
  • HD215 - basically sub-woofer on your head, i.e. they were shaking and my whole head vibrated, wow! But treble not good...
  • RH-A30 - maybe not head shaking, but solid sub-woofer experience and finally treble were pleasant

Why would one boost sub-bass? Well, my answer to that is to not only hear the music, but also to feel it on your skin:D