My first Crack build

[Maxhawk]

I received the parts on Friday and by Saturday afternoon had it all assembled. I'm not happy with the way the stain came out because you can tell where the excess glue was due to insufficient sanding. I guess I should have just left it unstained/natural until I was in the mood to do the wood prep right.

I substituted the cheap stock stereo pot with an Alps RK27. Yes it's rotated just slightly to accommodate the new locating tab drilled a few degrees off to clear the original hole.

I also opted for some cheap poly 100uF/250V caps in place of the stock electrolytics. I opted not to go for expensive boutique caps since I figure even a cheap poly should be better than an electrolytic. I can't quantify whether it sounds better, but I know it will last longer.
Everyone recommends building the basic kit first and then adding the speedball upgrade later. But I don't have a lot of patience and really hate having to go redo anything I've built, so I decided to install the speedball kit at the beginning.
I powered it up initially using a variac while monitoring the main B+ supply voltage. A quick check of all the voltages looked OK so next was to fire it up with music. Thankfully everything worked first time and I didn't have to troubleshoot anything.

I've got 2 issues however:

• When connected to my PC, I had some background noise that was exasperated by CPU heavy applications running. The noise went away when I disconnected the amp's RCA inputs and I finally traced it to the ground from the coax SPDIF I had connected to the DAC. Strangely my solid state amp is dead quiet connected the same way. I changed to optical SPDIF and now the Crack is silent too except....
  
  
• I seem to have microphonics anytime I tap on the amp. It doesn't matter where I tap and it sounds kind of like a spring. I've swapped input tubes which made no difference, so maybe it's the output tube. I used a non-metallic probe to tap various parts on the bottom and nothing stood out except when I tapped the tube sockets. I double and triple checked all my solder connections and they all look good. Next step is to try a different output tube once I get my hands on one (have a Mullard 6080 on the way).

I'm using the amp with Sennheiser HD800. At first I found the Crack to be brigher than my homebrew, but after a while the brightness went away. I guess the tubes just needed some warm up time.

I'm actually disappointed that the 2 amps sound so similar. I only have the stock 6080 output tube but I have several input tubes on hand: stock unmarked 12AU7, RCA clear top, RCA black long plate, and CV4003. Now I will say that when switching tubes, I only let them warm up enough so that I don't get the glaring brightness when cold. But damned that I can't really tell much difference between the various tubes. I may have to tube roll by listening for several days at a time with a particular tube so I can get used to its sonic signature.

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[Paul Birkeland]

But damned that I can't really tell much difference between the various tubes.

This is another reason on the list of reasons to build the kit stock first. 

Generally a tube will take 20-30 minutes to warm up and reach consistency.  I think 6080's take a bit longer, as there's a lot more material in there to heat up in the first place. 

-PB

[Doc B.]

Based upon my own experience, critical listening is a skill that takes some time and dedication to master. Consistency in the comparison procedures, adequate warm up time, familiarity with the source material and other system components, and only changing one thing at a time are very important.

The last couple of critical listening sessions I had involved our new DAC. I took a well burned in prototype to Paul Stubblebine's mastering lab to compare to a Pacific Microsonics model 2. We connected everything up thru his mastering console and went away for an hour to let everythng warm up and stabilize. Only then did we listen.

We used a piece we are both very familiar with for evaluation -  a high res version of one we have put out ourselves in analog format. We also listened to one or two others. We listened back and forth three or four times to each track before offering impressions to each other, in order not to skew the other guy's observations.

At some point I realized that I had set the DAC to run in "battery on and charging mode", rather than "battery only" mode as I had intended, and so we redid the test after I switched to pure battery power.

We did indeed hear differences and though it is not really pertinent, the result was favorable. Paul and I have been doing this together for nearly 20 years, and he has taught me a lot about "listening deep". Like anything else it takes practice and consistency to be able to know what to listen for and, perhaps even more, it takes practice to be able to say "I can't hear a difference" and be OK with that.

The other listening is a work in progress, comparing the DAC running on different battery chemistries. This one we are doing at BHQ on my own system. That gives me a home court advantage as I know the system strengths and weaknesses intimately. Someone will ask - I have formed an initial opinion but I need to do a lot more listening before I am ready to share any results.

[Maxhawk]

Based upon my own experience, critical listening is a skill that takes some time and dedication to master. Consistency in the comparison procedures, adequate warm up time, familiarity with the source material and other system components, and only changing one thing at a time are very important.

We did indeed hear differences and though it is not really pertinent, the result was favorable. Paul and I have been doing this together for nearly 20 years, and he has taught me a lot about "listening deep". Like anything else it takes practice and consistency to be able to know what to listen for and, perhaps even more, it takes practice to be able to say "I can't hear a difference" and be OK with that.

I appreciate the insights into perceiving differences. I was hastily swapping tubes and expecting a big differences since folks always talk about sloppy vs tight bass, holographic vs narrow imaging, fatiguing highs vs silky smooth, etc. These comparisons don't seem subtle so I was expecting to hear immediate big differences.

As Paul suggested, I do need to give the amp some time to warm up and spend some time getting used to the sound signature. Now that I know to listen for more minute differences I won't go swapping tubes so quickly .

[Doc B.]

People will tend to credit things within their realm of understanding for creating what they hear. Many more people are aware of changing capacitors or rolling tubes because it can be done with little or no study. We here will tend to credit circuit design with much greater influence on the sound than those things because we have spent many years studying design. Which doesn't make us any better but rather implies that that is at the limit of our understanding. You can also see people who credit things like absolute polarity or cables as a cure all for any problem.

Which brings me to another great lesson I learned from Paul S. Whenever asked "What is the most important thing to consider in setting up a system?" his response is

"Everything"

[Maxhawk]

Today I was much more methodical in my tube rolling, spending more time listening to the same few tracks for about 20 minutes before changing input tubes. I'm using the stock 6080 tube (unmarked except for "6080" and "USA") with my HD800 cans. Here's what I found on bass and treble:

CV4003 -- This sounds most like my solid state OPA627/BUF634 setup. I'd go as far as to say it sounds most neutral tonally of tubes I have with bass that isn't quite as controlled as my SS amp. The highs sound the same as my SS amp.

RCA black long plate -- Lots of bass, bordering on sloppy. Sounds smooth because the treble is rolled off at the top end, but I get a sense of too much energy in the lower treble. But that might be a consequence of a rolled off top end rather than a peak in that region. To a solid state guy, this is what I would consider "tubey sound".

RCA clear top -- Bass sounds like the black long plate with a top end that's slightly brighter than my SS amp. Good for darker cans. I didn't like it with my HD800.

Stock Sylvania 12AU7 -- Sounds closest to the CV4003 with a little more bass, but much more controlled than the RCA tubes. The highs sound similar to the CV4003.

Longer listening sessions will allow me to get a better sense of imaging, spaciousness, and midrange quality. So far I'm liking the CV4003 best. Anyone have thoughts on whether a CBS 5814 Hytron would be a step up?

Edit: The RCA long plate is actually quite nice with music that isn't bass heavy. Will listen to this tube for a while...