Headphone Correction Filter board

that can (and will) be corrected.

The Abyss AB1266 will be added.

I got the K701 version of the filter up and running. So far, so excellent. The filter overcomes some of the key failings I found with the K701. The bass is now deep and extended. I wouldn't say it's punchy or that it has slam, but it's clear and textured. And at the other end of the spectrum, the shrillness I used to hear with some horn and string passages is gone. (This latter problem is the reason I haven't listened to these headphones much lately.)

Overall, the K701 is much more enjoyable. I'll be listening to them more and, if I hear anything else worth commenting on, will report back here.

A couple of questions about the build. I used a DPDT switch so I could A/B the filter. I found that when I engage the filter circuit, the volume increases dramatically (and not just certain frequencies). Bypassing the circuit dramatically lowers the volume. Is this expected?

I found that turning the pots clockwise increases the filter activity i.e., more clockwise = less apparent 7 kH peak. Is this the expected behaviour? (I'd expected the reverse.)

I have a finicky connection on the left channel that isn't generally a problem, unless I open and close the enclosure. Then some judicious jiggling of the input cable clears up the problem.

Very nice work, Frans. Thank you.

Frans set me straight (I got clever and incorrectly wired the RCA jacks). When toggling between the filter circuit and bypass, there's now no change in volume.

However, turning the pot right (clockwise) definitely reduces the 7k peak. Turning it CCW increases the volume of the peak. (I tested this both by adjusting the pot as a ran a 7k tone through my system and by toggling between bypass and circuit when playing a 7k tone with the pot set to about 90% CW.)

Hi, Frans. I used a Bourns 3362R-103LF-ND pot.
www.digikey.ca/product-search/en?vendor=0&keywords=3362R-1-103LF

Thanks, Frans. That's good to know. FYI the manual is just about right, i.e., a setting of about 10-20% from full CW with these pots sounds pretty good to my ears (using log sweeps and some initial music listening). One observation: the shimmer and clarity that people hear with cymbals on the K701 is probably a consequence of the 7 kHz peak. Remove the peak and the shimmer ebbs. Fair trade off in my mind.

I put together the page 35 version of the T50RP filter and have been listening to it for several days. I have to say that, while the K701 filter tweaked the sound, this filter alters the sound more dramatically.

The treble from the stock build was too hot, resulting in a corresponding loss of apparent bottom end. I swapped the R103/R203 5.6 KOhm resistors for 8.2 KOhm resistors and the R106/R206 220 ohm resistors for 270 ohm resistors. The results are much more pleasing (but still may be a tad too bright for my taste). The sound is much more defined/detailed and even airy compared to the unfiltered sound. Dare I say that, in comparison, the unfiltered sound is somewhat dull?

I've been using two modded T50RP headphones, one with BMF's DBV#3 mod and the other with BMF's rock wool mod. I've used both an ODAC/O2 combo and a CDAC+/PSII combo (the CDAC+ is a nice variant of the O2).

EDIT: Just so that no one thinks that I'm damning with faint praise here, I should mention that I won't be going back to unfiltered sound anytime soon. I might be adjusting the sound, but it's already much better than what I was hearing unfiltered.