Cost of DIY Kumisa-III build starting to spiral!

[Thomasr]

My mind tends to jump from pet project to pet project, but every now and then I'll start obsessing over an idea and start researching the hell out of it. That is what's currently happening with a planned DIY build of the CK2III.

I already own the SMSL HO200 and want to explore different sound signatures in an amp. I found an Ali store with a bunch of bare PCBs and kits for some common DIY amps, including the Cavalli-Kan Kumisa III. I want to prioritize rich, euphonic, holographic sound (and yes, I know that a tube amp is the obvious choice if I'm chasing that sound, but I'm obsessing over the CKK3 right now - plenty of time to obsess over a DIY tube amp later) so I started looking into upgraded components, as the ones included in the kit are mostly decent, but also mostly what you'd expect from Aliexpress.

Well, as I start tallying up things like Mundorf Evo Oil coupling caps, Burson V7 Classic op-amps, upgraded power smoothing and buffering caps (especially considering how much more power the discrete Burson wants compared to the stock TL081), beefed up rectifier diodes, better voltage gain transistors, better regulators, a hefty toroidal transformer to power everything, etc. ... well, I could easily see this amp build run up to $1000.

I guess my question to the group is: worth it? Will a heavily upgraded CKK3 sound like a $1000 amp? This started out as a "learning build" but it's morphing into something very different.

 

[dunring]

If your learning what you want then it's worth it. There's easier and cheaper ways to do it. A class A like the Singxer SA-1 V2 I just had with an SMSL DAC with filter and color settings can get a really smooth sound in combination.

 

[Thomasr]

Another thing I'm thinking is that the CKK3, even with a bunch of upgrades, has relatively modest current output. I'm wondering if my planars (Sash Tres 45, Monolith M1570) will reach their full potential.

 

[funch]

Have you seen this? It's the original, not a ChiFi knock off. No, it's not for planars. https://www.amb.org/audio/ck2/

 

[Thomasr]

Have you seen this? It's the original, not a ChiFi knock off. No, it's not for planars. https://www.amb.org/audio/ck2/

Yeah I saw that, but when at the very top I saw "News: January 2007" I wasn't sure how active that site is. More importantly, it looks like Canada is now in a trade war with the US, so I'm forced to prioritize global suppliers elsewhere.

Re: planars, I figure this CKK3 should be able to push at least 100mA per channel continuous, and if I beef up the buffering caps it should cleanly go higher during peaks. I wouldn't expect to run the HE6 off this amp, but I'm hoping my SASH and Monolith (plus an Arya Organic or HEK Stealth I'm considering in the future) would have enough headroom.

 

[funch]

AMB's site is very active. Go to the forums site and ask about planars. I'm sure you will get good info there.

 

[funch]

Just remembered that I sold one to a member here recently. He said it drove his M1070 'without stress', so it probably depends on which planers.

 

[ericj]

My CKK3 developed a buzz a few years ago, before i bought my large hifiman planars, so i haven't used it in a while, but it never lacked any oomph driving any of the planar magnetic headphones. I haven't updated my gear list in a while but go take a look.

I find that it's a very musical design.

I would say, either use AMB's board, BOM, and maybe the discrete regulators, or get the aliexpress kit, build it as-is (unless some components seem extra sketchy), and upgrade a few things at a time.

The audible difference between a Mundorf and a Panasonic can be damn near nothing.

 

[Thomasr]

The audible difference between a Mundorf and a Panasonic can be damn near nothing

Yeah that's the part I'm wondering about. If I've learned anything in this hobby over the last couple years, it's that I don't have Golden Ears™. It's really easy to start looking at tantalum resistors and Evo Oil Supreme film caps and next thing you know you've turned a $50 Aliexpress kit into a $500 shopping cart.

For now I'm in waiting mode. Some stuff has arrived and other stuff is in transit. One good thing is that a bare Kumisa PCB has arrived, so I have the ability to test continuity between pads and get a better idea what each component does. So at least if I do decide to chase upgrades I'll know what's what. For example I figured out there are 4x 3k3 resistors that carry the op-amp output to the driver stage, so those could be places to experiment with carbon resistors.

 

[ericj]

Yeah that's the part I'm wondering about. If I've learned anything in this hobby over the last couple years, it's that I don't have Golden Ears™. It's really easy to start looking at tantalum resistors and Evo Oil Supreme film caps and next thing you know you've turned a $50 Aliexpress kit into a $500 shopping cart.

For now I'm in waiting mode. Some stuff has arrived and other stuff is in transit. One good thing is that a bare Kumisa PCB has arrived, so I have the ability to test continuity between pads and get a better idea what each component does. So at least if I do decide to chase upgrades I'll know what's what. For example I figured out there are 4x 3k3 resistors that carry the op-amp output to the driver stage, so those could be places to experiment with carbon resistors.

The opamps in the CKK3 are just doing DC servo duty. Just zeroing the offset so that your headphones don't cook. The resistors shouldn't change the sound at all there.

 

[Thomasr]

The opamps in the CKK3 are just doing DC servo duty. Just zeroing the offset so that your headphones don't cook. The resistors shouldn't change the sound at all there.

Interesting! That would explain the kitmaker's decision to use one of the crappiest op-amps (TL081) in the history of op-amps. It's just there as offset servo. So I guess the 10uF caps associated with the op amp aren't coupling caps in the conventional sense, and juicy film caps would be wasted there.

 

[ericj]

Interesting! That would explain the kitmaker's decision to use one of the crappiest op-amps (TL081) in the history of op-amps. It's just there as offset servo. So I guess the 10uF caps associated with the op amp aren't coupling caps in the conventional sense, and juicy film caps would be wasted there.

When i built mine years ago, Cavalli and Kan specified opa2134 which was and still is kind of an expensive chip, even though there are now far superior chips from Ti that are cheaper. I tried using an 072 and it didn't work right, was told that the circuit was designed for the properties of the 2134. I didn't have the chops to re-engineer it to use a cheaper chip, but I'm not surprised that someone did.

I've ordered the bare board out of curiosity. I may have nearly all of the required components already. Not sure if there's a standard enclosure it's supposed to fit into.

Edit because of stupid forum change:

Oh yeah, AMB sells small boards that are effectively high-end linear regulators that can be plugged in to replace the 7815 and 7915. I haven't used them, but the impressions i've seen were positive, and yeah people use them on the original CK2III board.

I have to admit that a big part of the reason i pulled the trigger on this project is because when the original board came out i complained about some shortcomings of the board and made suggestions for a next revision, and AMB assured me that there would be no next revision.

I think i recall that at some point someone suggested a kumisa 3 power amp? don't remember for sure.

AMB chided me a few times for using very nice Toshiba transistors for the output stage instead of the recommended bd140/139 or mje243/253 or whatever it was so it's interesting to me that this kit maker is using transistors that are also now near unobtanium. I seem to have put my box of power transistors somewhere stupid so i am not sure what i am going to use at the output stage. I think i have examples of benny jorgensen's original pick somewhere.

Edit Again: No sign of my 2sd1763/2sb1186 pairs but i found two pair of sanyo 2sa1011/2sc2344 so i will probably use those.

whoops, BC560C is obsolete. They are no longer available from 1st tier vendors. So if you don't buy the kit, you're gonna have to get ebay parts. Good luck. I still have plenty i bought years ago.

this no double posting configuration is dumb.

I had all the small resistors in 1% metal film except for 300k, the noninductive sandstone resistors are not available in that formfactor from US vendors, at least not in small quantities, so I'm waiting for some to arrive from china. The other small transistors are still available.

Though 16mm diameter power reserve caps are specified, 18mm fit fine. Which is good because i have a bunch of 2200uf 35v in 18mm.

The upc1237 is a very common and popular and old chip that you can't get from a 1st tier vendor. Which is why i bought a baggie of them on aliexpress years ago.

Edit: Geeze this is annoying.

Was looking through docs today and realized that one of the differences with this design is that it doesn't have the jfet input that the ckk3 has. It's probably possible to hack it back in. Matched sets of low-noise fets are like $30 + ship.