Volume Potentiometer
One of my biggest surprises about the stock Darkvoice design is the volume pot. It's difficult to see, because it is mounted upside down on a small PCB, but I came to learn that it is using a linear pot, rather than a log (audio) pot. A linear pot!? Yes, a linear pot can be made to act somewhat similar to a log pot with a couple resistors, but there is nothing of the sort in the Darkvoice. That explains why the volume hit the roof when the knob was turned near the 9'o'clock position, even with high impedance headphones. I had two main options for the upgraded replacement: an Alps RK27, or a stepped attenuator. The stepped attenuator is the technically better upgrade, but there were two problems with implementing it in the Darkvoice: lack of space, and not knowing what the necessary attenuation range would be until I was completely done. So I went with the Alps RK27 "Blue Velvet" pot, and wired directly to the pins, foregoing the PCB for the original pot. Installing it was no problem, but there is only one millimeter of clearance between the back of the Alps and the front tube socket.
Between the other mods that have some attenuation of the signal, and the usage of a proper audio pot, I can now make the widest possible use of the volume knob when using the W5000s. Previously I had only about 15 degrees of movement from too quiet to too loud, I now have about 180 degrees of movement from quiet to loud.
Crossfeed
I've always been a fan of crossfeed since I first heard it, and had already incorporated a crossfeed module into the last amp I built, so adding crossfeed to the Darkvoice was an obvious mod. Not to mention I had an additional switch added to the front panel, for the original "Fitz mod", that needed a new purpose (this was before I was planning on making a new front panel completely). I decided to play it safe and use the same modified Linkwitz crossfeed by Chu Moy that I've used before and know I like, and set it up to have the switch toggle between off and high crossfeed. I didn't even consider using a premade PCB for it, because I knew I would have no room to securely mount it where it wouldn't short against anything. I decided to make the entire crossfeed module point-to-point, with the capacitors serving as the foundation for the whole assembly. The parts chosen were Solen tin foil capacitors and Vishay/Dale RN series resistors. I underestimated the size of the capacitors, but after trying out numerous designs on paper, came up with what you see in the Darkvoice now. This was quite possibly the most difficult mod in the entire amp, given the number in interlinked components in a tight space, with no easy way to assemble them in a linear fashion. The results are more than worth the extra hassle this caused, but I would definitely think twice before doing a point-to-point crossfeed in the future.
With the W5000, the crossfeed has proven that it deserves its place in the Darkvoice. While the W5000s do not sound as much like closed headphones as some others do, there is still room for improvement when an instrument is played heavily or entirely in one channel. When the crossfeed is turned on during one of these recordings, the entire sound becomes much more transparent and natural, and the feeling of pressure on your ear disappears. Quite often it is not a subtle difference at all, but a drastic change for the better.
Cathode Bypass (AKA The original "Fitz mod")
This is the mod that started it all. I originally came up with the idea for this while trying to reduce the hum I was getting from a very expensive tube I bought, and was going to be damned if I didn't make it quiet. I tried out several values of capacitors on the input tube's cathode until I found 100uF to be the sweet spot between reducing hum and increasing gain. This was originally on a switch to allow it to be taken out of the circuit (a bypassed bypass...), due to my uncertainty about its effects on sound quality using the cheap caps I had at the time. In the current version, I'm still using 100uF caps, but have upgraded from the cheap ones I had on hand to Nichicon KZ Muse caps.
This is single-handedly the cheapest and easiest cure for humming tubes on the Darkvoice. I have several tubes that used to have unbearable levels of hum, due to the AC heater supply used in the Darkvoice, but now I have to strain to notice anything even when the music is off.
). After that there will be a comparison to with the Little Dot 2+, and once I get my W5000s back in service, a comparison with the M³ as well as more detailed impressions of the new Darkvoice itself.
