Wow! Thank you BH team.

[CrackMeUp]

I've just finished my first BH kit - the Crack - and had to jump on here immediately to say thanks for making this possible guys. I got the kit for Christmas and I've taken my time building it, not wanting the fun to end really. And it really has been so enjoyable. Great manual to follow, too. I'm not entirely new to electronics, but it's been a while, and getting into the audiophile hobby has rekindled my passion.

It all went together well and tested fine. When I plugged my HD650s in, which I bought to pair with the Crack, and set Corrine Bailey Ray playing on my old iPod, I had goosebumps within seconds. Such a great sound! I had lots of upgrades in mind, but even though nothing is burned in yet (headphones included) it's got me wondering why I should bother, although I know I will. That's what the audiophile journey is about after all.

In case you're not yet tired of seeing Crack build photos, here are some of mine. The only variations I made were the wire colour choices from the IEC to the mains transformer primary, as being in the UK red is a hot/live colour and never used for Neutral. And I twisted the ground wires from the pot along with the signal wires to continue the common mode rejection. I have to say that I was amazed at how quiet this amp is, with zero audible hum or hiss even at max volume.

Another thing I was particularly impressed with was how close the HLMP LEDs matched one another in terms of forward voltage drop. Do you match them, or did I get lucky? They measured exactly the same.

On my resistance and voltage readings I did get a channel discrepancy.
Resistance checks:
Right Channel RCA centre pin - 97.6
Left Channel RCA centre pin - 92.9
Voltage checks:
1 - 76.9V (Right CH.)
5 - 80.7V. (Left CH.)

The voltage readings correlate with the higher resistance on the right channel, which tells me I should expect some level of channel imbalance in my headphones. Could the resistance difference between channels be due to the wiper on the inexpensive the stock pot? Thoughts on that and how best to get the readings closer would be appreciated. Or does this somehow not affect channel amplification and therefore not matter? Or perhaps it's too small a difference to hear in a headphone system.

Anyway... Thanks again for developing these kits and supporting them as you do. I've really enjoyed the build and now have a great headphone amp too.

-- Steve

[Paul Birkeland]

Hello Steve,

The differences in overall resistance in the potentiometer don't necessarily mean much for channel imbalance and these values don't have any influence on the OA/OB voltages you're measuring.  Those will move with whatever 12AU7 you use.  In the most general terms, the plate voltage on a super fresh 12AU7 will be around 75V, and this voltage will drift up over time until you get to around 95-100V with a bit of a dead 12AU7.  We also find that these OA/OB voltages on the small board can start somewhat high with a tube that has been sitting around and not used for decades and will settle down eventually over time.

The HLMP-6000 LEDs do indeed have very, very tight voltage matching. 

-PB

[CrackMeUp]

Thanks Paul, that's good to know. I did wonder whether the input tube was influencing the difference I'm seeing. I can have some fun tube rolling a few 12AU7s and measuring their plate voltages. Amp sounds great to me though, and I'm sure my years aren't perfectly balanced anyway!

[Paul Birkeland]

The pots we provide do sound pretty good but admittedly don't have perfect channel balance at low levels.  In the Crack FAQ sticky there's a note about adding a few resistors to help with that if you end up with that issue.

[CrackMeUp]

Thanks again Paul. I'll take a look.


--Steve

[bucketofguts]

Lovely specimen of a Bottlehead Crack.! Thanks for posting.

[eharent]

Hello there,
how did you manage to have the wires so straight ant tight. it looks amazing!
would have loved to do it this way
Best,
Emmanuel

[CrackMeUp]

Hello there,
how did you manage to have the wires so straight ant tight. it looks amazing!
would have loved to do it this way
Best,
Emmanuel

Hi Emmanuel, you have to go your own way with the wire lengths to position them like this. I attach one end before I cut, run the wire how I'd like it, including the bends etc, to the other end, then cut it 7mm past the termination to allow for that 7mm bit to be stripped. Tight bends on twists and braid are easy enough if you take your time. They're not long runs. One thing to watch with the supplied PVC wire is that the PVC can melt away pretty easily where you have a bend in the wire close to the solder point. I would usually try to solder those ends first where possible, keeping the wire straight until after it's soldered. Not always possible though if you're closely following the construction order in the manual. No problem with Teflon/PTFE dielectric of course if you use that.

--Steve

[oguinn]

Do you ever get close to running short of wire? I know Bottlehead packs a little extra in.

[CrackMeUp]

Do you ever get close to running short of wire? I know Bottlehead packs a little extra in.

I had 10-12 inches of each colour left over, so yes there's plenty provided in the kit to get a little creative if you want to. Less of the green wire, but still 5 or 6 inches more than I needed.

--Steve