This is a chronicle of my errors and eventual success with making, upgrading, and modding my crack.
First about me:
I am the type of person who learns from mistakes, and the bigger the mistake the bigger the lesson. I also hate black boxes, and endeavor to understand the internal magic, usually through catastrophic disassembly, and release of smoke. That's not to say I am reckless to the point of danger, but curiosity combined with a sense of "I can do anything I set my mind to" tends to yield some emergent behaviors in any project I undertake. This is my nature, and try as I might to be otherwise, I find myself time and again with a bench full of parts, and a notebook full of lessons learned.
My company rewarded me with a pair of Sennheiser HD-650 headphones for 20 years of service. Much better than a gold pen or a watch. But I quickly discovered that expensive headphones didn't sound good. On my home stereo headphone jack they sounded fine, but elsewhere... So I set out to understand why. A quick refresher of Ohm's laws, and I was struck with a need for an amplifier that could overcome the ~300 Ohms of impedance the thin wire in these headphones created. I gambled on a "portable" amp/dac combo, and struck out. It sounded "OK" but nothing great. Commercial products were going to be expensive to evaluate, and there's just so much snake oil out there. Eventually I came across Bottlehead.
A kit was ordered, delivered and I began the construction of my Crack. I learned to solder from youtube. And initially it showed. First lesson learned: Go slow and get the hang of soldering. It's not about dropping a dollop of liquid metal between two contacts, its about creating an electrical bond between dissimilar metals. You can see the solder bond with wire and terminal strip. Halfway through my assembly I had to stop, and reflow my previous handiwork, then check every joint with a multi-meter for continuity. Second lesson: You can melt wire with your solder station. Those LEDs have very thin leads, and can be easy to break. Fortunately, Bottlehead shipped me some more, and I was able to complete the build, and do the resistance checks. Here's where things got scary. Everything checked out. Learning to solder while building an amplifier is a bit of in-flight aircraft repair. So I expected some iterations. But... everything checked out.
On to voltage checks. This part had me scared. My limited experience with house electric left me with fear and low confidence. Plugging something into the wall you just built made me worry my house would burn down. I actually had a fire extinguisher with me, just in case. I powered on the unit, and began probing a live circuit with a meter. I felt like I was cheating death. The voltages didn't quite check out. Turns out my LEDs were not installed in the right direction. After that realization, I turned it on and got all my voltages to check. I remember feeling like Victor Frankenstein: "It lives!"
Then came my first bit of adversity. Once I moved the amp to my source, I lost a channel. I had no idea why, and was very frustrated. Over the course of a week I figured out how to actually trace the problem. A skill that has proven immensely useful since. Turns out those RCA posts are difficult to solder properly. Once I figured that out I was in business and got about 3 solid weeks of enjoyment listening to music as if I'd heard it the first time. I had to brag about my creation of course, so I transported my amp to the office. That set off another round of solder, re-solder, cursing my lack of skills, and discovering that for me, a wiggle test of every connection is needed.
Then began my love affair with my music library. I decided to see if different tubes made a difference. Not to belittle you guys, but the idea of spending $$$ on a dutch made tube from the early sixties, which has sat in a warehouse under god knows what conditions for 4 decades, only to be offered up on the "audiophile" community at many multiples from another equally esoteric tube seemed a bit too far down the diminishing returns curve. I did however, notice a difference in the combinations of tubes. For example, a JJ gold pin 12au7 pairs quite nicely with a Sovtek winged "c". I got some ABX software, and started to train myself to listen. Went through the Golden Ears challenge, and started considering my next step.
Enter the Speedball (almost). I eagerly placed an order. Then had to wait, and wait. In the meantime I figured I would try some crazy bypass capacitors. The Ukraine is having a fire sale of Soviet electronics. Seems that manufacturing parts for the invading army limits your market, and they are selling surplus on Ebay for little money. I became a war profiteer and bought some teflon bypass caps. Thinking I would save the pain of new solder for a dubious upgrade, I decided to clip in the new parts with test leads. This was, in hindsight, terribly stupid. What I found was that capacitors act like resistors to DC. And that when shorted bypass capacitors act like wires. I pleasantly sent DC right into the drivers of my headphones. A great lesson was learned. BTW Sennheiser is a great company, and repaired the cans along with a chuckle filled phone call.
Next came the speedball for real. I bought a set of magnifying lighted glasses and stepped up my assembly game. Each solder joint is shinny, each wire cut to length and stripped nicely. I took a week of evenings to do the speedball just right. At the end, resistances were spot on, voltages equally within tolerances. I replaced the volume pot with a stepped attenuator soldered by someone with more time than I have, and then fired up the amp again. After verifying that NO DC voltage was being passed through the headphone jack I plugged in the new HD-650s and gave it whirl. The speedball is really a worthy upgrade.
Next I replaced the electrolytic output capacitors with some foil caps from Jentzen. This was rewarding but not nearly as much as the speedball. Finally, I added back those teflon caps, because I had them. I still cant hear a difference, but I am officially out of space so I am done.
Best lessons learned to date:
1) Magnifying glasses with lights makes your work better x10 (Thanks to Doc B for this tip).
2) Assembling slowly means you will likely get the enjoy your music sooner.
3) Shake test your connections.
4) Don't waste money on inconsequential or diminishing returns upgrades until you've got the speedball. It really is the biggest bang for the buck.
5) If you want to continue to hot rod your amp (and why wouldn't you!?) look at the new crack-a-two-a. Lots more space there, and a better starting point.
That's it. Thanks guys for the fantastic support via this forum and thanks to Doc B. and his team for creating a DIY kit that is stripped of the usual unverifiable enhancements and achieves an amazing electrical balancing act without feedback!
