Bottlehead Crack

I arrived to the Bifrost in my searches but buying it from the dealer in Europe means about 510 for the 4490 and 730$ for the multibit.. on the second hand market they are very rare here.
That Audio GD DAC 19 looks very good tho, but a little bit over budget - i was hoping of getting a real improvement for a max of 600$ even if buying used - still i'll keep it as an option.

 

if it's 2 different sources causing noise, then i would be suspicious that the fault lies with the amp, assuming you're using the same wall outlet/power strip configuration and interconnects as before. possibly something/cold solder joint came loose in transit? do your ICs ohm out correctly? with the amp turned off and unplugged from the wall, ohm out all grounds in the amp, including RCAs and pot to the safety/chassis ground. if all else fails, you should have enough room to install a ground-loop breaker in the amp.

 

Thanks I will have to check this out. I also bought a Jensen isomax ground loop isolated (RCA to RCA) to see if indeed there is something going on there, but I will try out your recommendation of checking out the interconnects. I have been half thinking its something to do with the amp, but with the amp plugged in and turned on all readings look correct. Thanks or your help!

 

Without the SB upgrade, you've actually only met half the amp. I just finally got around to dropping in an E80CC and correcting R1 to get he plate voltage right. I have say the E80CC / 5988 combo is pretty sweet. I've been away from crack with the Mainline for months and its like coming back to an old friend.

 

http://www.hificollective.co.uk/sites/default/files/auricap_application_notes_0.pdf

 

@NightFlight - did you find correcting R1 for the E80CC voltage to have a noticeable impact on the sound? Or is it just something that should be done anyway for safe/correct operation of the amp when using that tube?

 

Thanks @fishski13, your advice helped me find the problem. I've read about following the ground, but didn't fully understand the the casing of the RCA jacks need to be grounded as well. These came loosing in shipping as your predicted.

Looking to install mundorf output caps next. Any one done this? I have some cable tie attachments and hoping that they will fit on the underside of the chassis. Are any other caps that should be replaced or small parallel caps added? If so, any significant sonic improvement?

 

glad to hear you solved the noise issue.

it's a tight squeeze inside for film caps. IIRC, Dayton and Mundorf have reasonably priced 100uF 250V caps that should fit. depending on heaphone impedance, you can go with a lower value like 68uF for the Senn HD6x0. i installed 68uF/400V Jantzen caps in Hands' Crack and they fit fine.

 

I own the mundorf's and will slap em in when I find some free time, but the fit looks tight!

 

Just finished Speedballing/Modding my stock Crack. I may try and squeeze in one more large film cap instead of the last electrolytic in the power-supply that I currently have bypassed with the small 2.2uF film. Need to see what I can get away with with physical size & capacitance.

I did listening tests and took notes throughout to write up soon - but it sounds alright you know.

 

Wow, beautiful build Jeb!

 

Jeb - Looking good! Did you do any sound tests before / after with the power supply bypass cap? I have not done this yet and am interested. Also any pics of the top side of your Crack? Looks like you have upgraded the pot and jack.

I recently put in the same output caps and thought they sounded great (pic below). Excuse the mess - my build isn't nearly as clean as yours. I noticed quite a bit more bass and cleaner on the low end. Overall more clarity too? Could be me wanting to hear it, but I am thoroughly enjoying.

 

Nice work @JoshMorr ! I also feel that the film output caps do improve matters but agree it can be difficult to be fully confident in the extent/nature of the change due to the time between fitting and listening.

As for bypassing the last electrolytic in the power supply, I only tried one cap in that position (a Mundorf supreme 600v 2.2uF) and based on that would not recommend it. I felt it was a step backwards producing flimsy unnatural weird sounding treble. I snipped it out in the end. I'm not sure if it was the particular bypass cap or the just the interaction between the two.

Anyway, that whole exercise convinced me of the influence of that final power supply cap on the sound. So, In the end, I replaced it entirely with another Mundorf 100uF / 250v and bypassed that instead. I really like the results. But you first need to fit the choke to be able to drop to that lower capacitance otherwise I think you'd need to try and squeeze in something closer to a straight swap (220uF/250V) film cap - which are enormous. Some people on the BH forum have done it though.

The choke is not expensive but slightly tricky to fit. Not terrible by any means though. I am a bit rushed today but should have time tomorrow to post full impressions and some pics of the layout and the top side.

 

Speedballing/Modding the Crack

General conclusions:

• The Speedball is the key upgrade. I reckon stock Crack + Speedball + half decent tubes = about 85 - 90% of the quality of sound with all the mods I did.
• That extra 10 - 15 % is quite nice though.
• Overall trend is towards a cleaner and brighter sound. I am now using tube combinations I considered to be slightly too warm before.
• Filling a budget amp with boutique/expensive components is not necessarily sensible or efficient use of cash. I now have about £150-£200 of parts upgrades in there now. With the kit + Speedball that brings the total cost of the whole package to a little over £400. There are other amp options to consider for that cash. Or build a Bottlehead kit with a better circuit altogether.
• You could also Mod for much less than this with cheaper components. Plus you can't put a price on some of the fun/experience of this kind of stuff.
• DIY electronics is fiddly but really satisfying. Doing things outside of the manual (but with the help of BH forum members) is challenging but a great learning process.

1) TKD 2CP 2511 POT

This was the first thing I did. This pot is £65. But I was surprised by the extent of improvement. I don't want to go overboard due to the time that elapsed between fitting and listening.

• Made me realise the slightly rough, grainy nature of the stock pot.
• TKD pot is extra clean and quiet. Perfect balance. Crisp, precise, less smeared sound.
• Seems to tighten & extend the bass.

Mechanically, in combination with a nice new metal knob, this pot is damn smooth to use. Padding the pot with 33k and 75k resistors (described in the FAQ on the BH forum) to reduce the gain was also a massive improvement for the user experience, allowing precise setting of the volume. Comfortable listening is now about 12 O'Clock on the dial (previously 8-9 O'Clock with very little room to adjust).

Fitted neutrik locking jack and some nicer RCA jacks for reasons of vanity. Used a step-drill to make the hole for the locking jack.




2) SPEEDBALL

Good learning curve from doing the stock build & the manual is great again. Never done PCB work before and found this quite challenging until I got my technique down. Lifted a pad due to noobiness and had to scratch off some of the board and solder directly to the trace. Argh.

Relatively speaking, a big upgrade on the stock sound:

• Overall sense of previous blurriness coming into tighter focus throughout the range. Tonal shift towards neutrality but still a warm amp.
• Less blended, less overdone decay. Slightly less mid-centric. Vocals take a small step back
• Staging width and depth improvement.
• Treble: better focus & zing
• Bass: nice improvement to articulation and depth. Makes the stock Crack bass seem pretty damn sloppy. Even so, Speedball bass will still be considered quite soft/fuzzy by some.

A More complete improvement than even the best tube rolls.



3) FILM OUTPUT CAPS

I went for Mundorf 100uF/250v. These are among the smaller of the 100uF Caps available. They fit "comfortably". Attached with 2 x 24mm cable tie mounts on each side. I like the tie-mounts with two holes on the outside rather than one hole in the middle.

• Wow these sound nice and smooth. Upper mids/treble in particular are smoother and LESS GRIT
• If Speedball was a tonal shift toward a brighter sound, these seem to go back slightly the other way while retaining the same if not better levels of resolution. Fleshy and convincing.
• Midrange clarity. Holographic.
• Could be a bit laid-back for some perhaps. Slight bass thumpiness.
• Maintains better integrity at higher volumes than stock caps

4) TRIAD CHOKE

Fitting this is slightly tricky due to needing to drill so close to the transformer and squeeze in a standoff next to the ground screw. I used M3 x 25mm nylon standoffs with female thread one end and a stud on the other.

• Pretty subtle (more so than Cap change) but worthwhile effect, again towards a slightly cleaner, brighter, more resolving sound.
• Adds a welcome bit of bite and snap to high end
• Seems to improve swing, impact and dynamics.

5) BYPASSING THE LAST CAP IN THE POWER SUPPY

A ratio of 1:100 was suggested so I went for Mundorf Supreme 2.2uF / 600V. (£12)

• Oh No. Not really feeling this. Sound is flatter and more narrow. Treble is uneven, unnatural and brittle.
• ABORT! ABORT! ABORT!

6) REPLACE LAST CAP IN POWER SUPPLY WITH A FILM CAP


This cap could be considered as important as the output caps. Initially uncertain if I could squeeze in another large film cap but read that the choke would potentially allow a lower capacitance to be used. Fitted in another Mundorf 100uF / 250V on 2 x 25mm standoffs + cable tie mount, in 'Star Trek' formation (see pic below). The screws are between the power tube socket and transformer.

• very pleased with this. Further cleans things up. Extra dose of the smooth Mundorf sound.
• Helps further reduce treble brittleness/scratchiness
• Seems to produce a more coherent sound overall.

7) BYPASSING THE ABOVE FILM CAP

Experimented with this (carefully) with clip leads to try and detect differences. Predominant difference seemed to be in very subtle changes to treble presentation.

NOS Russian Teflon 0.056uF 600v : extra snappy/extended up top with impressive decay. A bit shrill and piercing. May annoy over time. £2 !

Rike S-Cap PIO 600V 1uF: an improvement - articulate and precise but perhaps a bit more forward and aggressive sounding. Pretty nice.

Mundorf Supreme 2.2uF 600v: hated it on the electrolytic, not bad here though. Little boost to treble focus. Still slightly flimsy sounding

Jupiter 1uF 600V HT Beeswax Paper : Immediately liked this. Extended but also smooth & natural. Never tiring. Never too sloppy. Slightly extra-euphonic. Only issue is the heat, since this is rated at max 70 degrees C, rather than the stock caps which are 85. Will keep an eye on it as it does get fairly warm under there. Using an expensive cap like this in this position is quite silly.



Initial impressions are that the E80CC tube is excellent.



Cheers!

 

Wow, great write up Jeb! This will be a great resource for anyone interested in Crack modding going forwards.

 

Amazing post! You should send a link to the bottlehead guys.

 

I randomly bought a pre-made Crack + Speedball (this is already exceptionally rare in the Netherlands I assure you) which is coming in on Tuesday and I really wanted to dig in deeper, hell I already had the Power choke, Mundorf M'Caps and Valab stepped attenuator ready for order. I have no experience with amp building (I know how to solder and what to do just no experience with it) so this is all somewhat exciting and at the same time overwhelming.

Honestly after reading that level of in depth response and your pictures, you just saved me a ton of extra research @Jeb, thanks! I think I speak for anyone that wanted a bit of an extra that this extremely helpful for us. Absolutely kudos there!

 

Thanks guys - I appreciate it!