Another Crackatwoa build

[Larpy]

I ordered one of the restocked Crackatwoa kits and finished the build yesterday. As a repeat Bottlehead offender, I've built my fair share of kits over the years (3 Foreplays, a Moreplay, Eros, Mainline, S3X, and a Kaiju), but I never got around to buying a Crackatwoa kit before it was retired a couple of years ago. It turns out the Mainline was not my cup of tea, and so I'm grateful I got a second chance at the Crackatwoa. My first taste of tube audio was buying one of Pete Millett's Wheatfield HA-2 headphone amps back in 1999, and I've long suspected that BH's Crack (and by extension the Crackatwoa) was influenced by the Wheatfield's OTL topology. I never built a Crack (I heard one and decided I was happy with my Wheatfield), but the Crackatwoa intrigued me as a possible Wheatfield upgrade.

And it (mostly) is. I built my Crackatwoa stock except for a lighted power switch and a Goldpoint attenuator instead of the volume and balance pots (and velcro instead of plastic ties to secure the output caps). To my ears, the Crackatwo finds a near-perfect balance between resolution and tubey sweetness and warmth. It's not as lush as the Crack or the Wheatfield, but it still sounds plenty tubey, and it's noticeably more detailed/resolving than either.

The Wheatfield still has a certain magic, perhaps due to its using a 6SN7 rather than a 12AU7 as the driver tube and running both it and the 6AS7 cathode follower mercilessly (plate voltages are twice as high, and the 6AS7's cathodes measure an astonishing 190 volts each), but I think I prefer the Crackatwoa's overall presentation: a little less lush but more resolution. More relaxed and assured.

And this impression is with the stock electrolytic caps in the Crackatwoa's output. I'll replace them with film caps later this week. I also have a 12AU7>6SN7 adaptor on the way from Garage1217, so I'll try a few 6SN7s in place of the 12AU7.



A few questions:
1) I have 47uF film caps in my parts collection but no 100uFs. If I use only high impedance headphones (Sennheisier 800s), I'm assuming 47uF output caps will go low enough in the bass. Is this assumption correct or do is there more to the 100uF specification than bass extension?

2) If it turns out I prefer the 6SN7 as a driver tube, are there any modifications to the C4S board I should make to optimize it for the 6SN7's parameters? (different resistor values for example?)

In my view, the Crackatwo is an unheralded gem. For whatever reason, it seems it didn't sell well when it was first offered. There are precious few reviews or even mentions of it online. The Crack was such a smash hit that it seems to have hogged all the attention. But the Crackatwoa sounds wonderful. True, it's not cheap (though I spent almost the same amount of money on a new Wheatfield HA-2 back in 1999), but I for one find it more engaging to listen to than either the S3X or the Mainline, let alone the Crack. I'm something of a headphone amp obsessive (I've built 6 or 7 of them), and the Crackatwoa is up there with the best I've heard.

Looks like there's stilla Crackatwoa in stock for anyone who, like me, was on the fence was it was first discontinued.

 

[Paul Birkeland]

You are correct about the Wheatfield being influential. Back in the day, I really wanted one, but I couldn't justify throwing down that kind of cash, so instead I spent a bunch of time trying to figure out how to build OTL amps, and eventually the Crack ended up falling out of those efforts.

Pete runs a lot of cathode voltage on the 6080 so that he can get the resistance of the cathode bias resistors up nice. The Speedball/C4S loads free us from that constraint, so we don't have to flog things quite so hard.

For the output caps, 100uF DC link caps are so cheap now that I would just purchase a pair.
https://www.mouser.com/ProductDetai...EpiMZZMsh%2B1woXyUXj1PVPNxBwqheELxn03s%2B3yg=
Two of those + some black RTV from the autoparts store to glue them down and you're good to go!

The C2A specifically was what I envisioned as an "upgraded" Crack. Typically builders would put in a choke and big film output caps, but I thought that shunt regulating the whole amp was more worthwhile, and ultimately a lot more effective than simply adding a choke. Even when quantifying that the shunt regulator is like adding a 950H choke to each channel, this just doesn't have the A+B=C of "stock Crack + expensive parts=better Crack".

 

[Larpy]

Thank you for the cap recommendation, Paul. I just placed an order for a couple of them. I'd already swapped out the stock 100uF electrolytics for a couple of 47uF Obbilgato Gold caps, but as you point out the 100uF DC Links are cheap enough to be a no brainer.

The Obbligato caps do sound better than the electrolytics. Bass sounds more solid and perhaps deeper, and there was a scrim of graininess to the overall sound with the electrolytics that I now notice in its absence, but it's not a night and day difference. Of course, I'm in my early 60s and my hearing is nowhere as good as it was when I was younger, but when I first started along the DIY path 25 years ago the mantra was that the cheapest film cap sounded better than the best electrolytic, so I've avoided them for coupling purposes all these years. But when I first listened to the Crackatwoa a few days ago with the stock caps, I didn't immediately think "I've gotta change out those caps!" So either my wizened hearing now masks the size of the sonic difference or electrolytics have gotten a lot better than they were.

Today the 12AU7>6SN7 adaptor I ordered arrived and I plugged in a vintage gray glass RCA 6SN7 in the place of the stock Bell and Howell branded OS 12AU7 (which given its black plates and O getter I'm guessing is from the 1960s). Sounded fuller and bigger than the 12AU7, though the 6SN7 has a slightly higher mu, so part of what I hear could be a slight increase in volume. Years ago I remember reconfiguring one of my Foreplay IIIs to run 6SN7s and thinking the difference was a revelation, but with the Cackatwoa the difference seems less stark. Compared to the RCA 6SN7, the 12AU7 sounds thin and maybe a bit strained, but before I made the switch I didn't think the 12AU7's midrange sounded wonky at all. But that's the fun in experimenting.

WIth either tube doing driver duties, I'm still really loving the Crackatwoa.