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Tweaking a CK²III

post #1 of 13
Thread Starter 
For my first DIY build I made a standard BOM CK²III - it worked perfectly first time. Since getting a pair of AH-D5000's it has once again become my main amp so I'd like to tweak it as I made two newbie mistakes and no doubt it can be made better with better parts. My searches could find no mention of these changes, probably due to the multiple spellings of the name. Not that I have any idea what site I was reading - it was nearly two years ago!

1 - I built it with a gain of 4.1 and an expensive 10K stepped attenuator. I listen at step 3 or 4 and while this isn't a problem with volume it is generally agreed that amps perform better when not near their quietest, so it could sound better. Should I reduce the gain, buy a 50K pot/attenuator or both?

2 - I remember reading at the time upgrading one part (in the power supply?) made a big improvement - would anyone happen to know this magical part?

I also intend to install the ground loop breaker as it was omitted for testing and I never got round to adding it

A link to any thread discussing mods would be great, if you have one in your bookmarks/favourites
post #2 of 13
maybe you could post a link to your bom, for comparison.
And I'd also like to know if it's possible to lower gain below 4.1 without problems.
post #3 of 13
Thread Starter 
I don't exactly have a BOM and don't even have excel, but this was the kit order form from as sent to Jeff at Glass Jar Audio. The only change was line 22 was changed to 22pF caps, to lower the gain to 4.1 as per the guidance on AMB.org.
From a maths point of view dropping 1.82 to .82 is the same drop as 9 to 4.1, so lowering R12, R13, R36, R37 more should lower the gain, but assumedly the caps would need to be upgraded further if it would work at all. I'm not confident saying it would!

Here is my pseudo BOM

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post #4 of 13
upgrade the pot and consider adding a o-22 PS, bypassing the on-board PS.
post #5 of 13
Output transistors should have the cheapest and largest affect on sound at this point. Which ones did you use in the default BOM?

The bottom of this page has some comparison's of output transistor pairs: discrete diamond buffer - parts list. I believe it is a little old and not updated, but new useful for audio pairings don't come out that often :P

Old threads used to bring up the 2sc2238/2sa968 combination. I believe these parts are hard to find now and often faked. I think Toshiba no longer produces them and replaced them with 2SC4793 / 2SA1837.

Other than the reverse pin-out anyone know if you can use 2SC4793 / 2SA1837 and if it will sound like 2sc2238/2sa968?
2sc2238/2sa968 seemed like the top recommendation for the CK2III.
post #6 of 13
Quote:
Originally Posted by suicidal_orange View Post
1 - I built it with a gain of 4.1 and an expensive 10K stepped attenuator.
Quote:
Originally Posted by fishski13 View Post
upgrade the pot
Considering it already has a stepped attenuator, I don't see the point.

Quote:
I listen at step 3 or 4 and while this isn't a problem with volume it is generally agreed that amps perform better when not near their quietest, so it could sound better. Should I reduce the gain, buy a 50K pot/attenuator or both?
It has nothing to do with amp performance, but rather attenuator performance and usable range. If you already have a good attenuator in there you probably can't improve that, but if you drop the gain to 2, you will give yourself more usable range. But someone better than I will have to confirm this can be done, and the amp will still be stable.

Switching to a 50k pot will be a backwards step.

Quote:
2 - I remember reading at the time upgrading one part (in the power supply?) made a big improvement - would anyone happen to know this magical part?
Output transistors would be the key improvement. Pop heatsinks on them as well to increase the quiescent current.

Quote:
consider adding a o-22 PS, bypassing the on-board PS.
Seems like using a sledgehammer to crack a nut. If you are going to that extent, I would lean towards building a new amp.
post #7 of 13
What'd you want a Sigma22 for anyways? Wouldn't a Sigma11 be entirely sufficient?

I've got a question though: How hard would it be to implement a gain switch to switch between, say, gains of 2, 4, and 8? I figure you'd need some breadboard, a lot of air wiring, and some extra bits to work out the resistance/capacitance to the right values.
post #8 of 13
Quote:
Originally Posted by Beefy View Post
Considering it already has a stepped attenuator, I don't see the point.
i misread.
post #9 of 13
Quote:
Originally Posted by Ntropic View Post
What'd you want a Sigma22 for anyways? Wouldn't a Sigma11 be entirely sufficient?
the CK2III requires a dual rail 15+/15- PS.
post #10 of 13
The main CKKIII threads are over at Headwize HeadWize: DIY Workshop > New Cavalli Kumisa III plus Boards & HeadWize: DIY Workshop > CK²III: Cavalli-Kan Kumisa III headphone amp (part 2)

Apart from changes already suggested re output transistors and increasing the quiescent current the other change that will make a difference IIRC is changing C19, C22, C23, C24 from 10µF bipolar electrolytic capacitors to 1µF 63V polyester film capacitors, you will also have to change a few resistor values if changing these caps, check AMB's site under parts recommendations & options for details.
post #11 of 13
I agree with the above, swapping out the output transistors is a good first step. I use the Toshiba 2SC2238/2SA968's and they rock. If you aren't already, try using Panasonic FM's or Nichicon HE's (preferable the FM's) for your electrolytics (polar) as this has an effect on the sound as well.

Also, try putting the transformer in a different case from the rest of the amplifier as this helps as well.

And yes a sigma-22 is overkill much for this amp. If you don't like the sound of this amp, might step up to another amp. While you're trying to find what kind of sound you like, the CTH rocks, because the amp itself is very neutral, all that's left is the coloring added by the individual tubes. It's more complex than the CKKIII but not as expensive/complex as the B22 or EHHA.

You don't want to throw too much into the CKKIII, its pretty dang good as it is but if you're really going to go all out for an amp, you're better off pouring your time and effort into a "better" amp that has more customization options (B22, Millet MAX, etc)

And I guess it needs to be said, if you like the sound of your amp as it exists, why change i?
post #12 of 13
As far as output transistors, my understanding is the MJE243/253 have a higher f-sub-t (alpha frequency). If you have volume problems at a gain of 4.1 then what source are you using? The Carat Peridot listed in your sig has a volume control.......duh!
post #13 of 13
Thread Starter 
Thanks, I'll have a read of those (huge!) threads. Then again, as suggested no point throwing silly money into this build but if a few small changes can improve things why not? I have to open it up to make the ground safe and while it's open would be the time to do them. Maybe just the reg's and move the transformer (should have mentioned in the OP, it was planned).

The Style Audio does have a volume knob but that only affects it's internal amp, which I'm not using. I'm far from a DIY expert but not a technophobe

I actually have a CTH fully populated, but it doesn't work and I'm not good enough to debug it. See the "tweaks" thread on it if you have experience and would like to help, I was planning on one final bump before selling it but it's effectively an unused Christmas present which is never a good thing to sell on
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