Marvey posted:
"I am curious what amps were used by the folks who said the mod made the bass more woolly or increased bass volume as this was not my experience. I can see removal of the dampening increasing the impedance peak ~100Hz. Maybe certain amps that are more voltage limited are having a more difficult time with this."
That was me. I have been doing more thinking and work on this so I thought I'd respond.
My source (dual mono Buffalo II's into a Lundahl LL1684 transformer) has a Zout of approx. 277 ohms. I also experimented with an Apogee Groove (a current source, as you know). I have always compensated for the increased voltage into the headphones because of the low-frequency impedance peak with EQ to restore the frequency response. I think this can be calculated and done with very good accuracy.
Based on your comment, I wonder if the impedance peak is being altered in magnitude by removing the rear damping and thereby threw my compensation off, thus leading to my perception that the frequency response change was too great. However, I also felt the bass was a bit uncontrolled with a voltage source as well (ifi micro).
I do think that that the HD650 benefits from some boosting in the bass. I have previously done this in my set-up by not fully-correcting the frequency response secondary to the elevated Zout of my source(s). Lately, though, I have been correcting the impedance-related frequency response change fully (the impedance changes effect farther up the frequency range and perhaps don't extend low enough) and instead generating a curve to have the HD650 match the Harman curve in the bass (like Tyll).
My thoughts:
1. I suspect the rear damping removal mod is doing acoustically what I have been doing electrically regarding damping the low frequency diaphragm resonance
2. I stopped before doing the "coin mod" to the front foam as it wasn't reversible; this may have restored the balance/improved my perceptions, as:
3. Detailed work on a Harman curve leads to a bit of a "smiley" EQ, the upper range of which I speculate may be somewhat similar to the "coin mod."
4. I don't have the ability to take acoustic measurements and therefore all this is simply speculation; I would love to see frequency response and impedance magnitude measurements of un-modded and fully modded HD650s.
5. While I can't fully eliminate all variables and therefore can't describe causation, there is something certainly appealing subjectively to current drive to me.
6. Implementing a curve to fully achieve a Harman response is very, very interesting.
Best wishes,
Bill