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K272C Oatleys JAN6418 tube based headphone amp\preamp
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Author:  mwhouston [ 12 Apr 2013, 02:25 ]
Post subject:  K272C Oatleys JAN6418 tube based headphone amp\preamp

Because of my continual bantering of Oatley to produce a K272X which had filament batteries and powered by 18V the K272C has been developed. The K272B will probably never see the light of day being a big improvement over the K272A but not being an 18V amp and not having separate filament batteries. I liked the "B" having seperate filament resistors and thought it sounded very good in a headphone amp I built up. I think Oatley had the K272B PCBs produced but just not packaged as kits at this point. My suggestion to them was to sell the PCB for the "B" only at a reduced price and let the constructor supply his own parts. I have no answer back on this at the moment.

Blow the trumpets and roll the drums the K272C is here. I believe it will go into production. I have been sent what I think is a prototype or first run compete unit. Looks good. Big filament batteries and 18V ready. It may be a week or so before I have it as a complete portable HP unit but I am very excited to try it. I still have the K272B and as I said love it.

Oatley have included in the kit the OPA2134 high performance Fet front-ended OPAMP chip. Two leds monitor battery health. One led will indicate when the 18V is fallen to 14V by first growing dull then finally going out. Another led, monitoring the filament voltage, goes out when the filament batteries fall to 1.4V (from just over 3).

The biggest improvement is the 18V supply. Some K7272s hissed and I think it was because the driver chip (OPA2134 or PT2308) was running on too lower voltage. Also with the higher voltage across the driver chip a greater dynamic headroom should be available. I would have liked the tubes to also have a 18V HT but it appears from the cct. that there is only 9V across the tubes. Oh well.
Attachment:
K272C.jpg

Author:  atanguay [ 12 Apr 2013, 10:17 ]
Post subject:  Re: K272C Oatleys JAN6418 tube based headphone amp\preamp

Cool! I'll be first in line to get one of these guys.

Author:  mwhouston [ 12 Apr 2013, 17:47 ]
Post subject:  Re: K272C Oatleys JAN6418 tube based headphone amp\preamp

atanguay wrote:
Cool! I'll be first in line to get one of these guys.

I have asked Oatley for a release date and price. As soon as they get back to me I will post the result. Also I have asked if the tubes, with some mods to some resistor values, can also be run at 18V. This should be possible with the change of three resistors.

Author:  mwhouston [ 16 Apr 2013, 16:56 ]
Post subject:  Re: K272C Oatleys JAN6418 tube based headphone amp\preamp

I have just been informed by Oatley that the K272C will be available for purchase in 14 days - 1st of May 2013. I am still trying to get an answer if the tubes are now on 18V. The storage cap just prior to the tubes is marked 10V??

Author:  mwhouston [ 20 Apr 2013, 03:09 ]
Post subject:  Re: K272C Oatleys JAN6418 tube based headphone amp\preamp

My wife through a steel can at me and said "build an amp in this". The steel can sat around for a few weeks then I got the K272C prototype cct. brd. I started to think "can I do this". I will save you a few dozen thought processes and move you to the present.

I'm going to build what I call a nood (Nu*de) build and it is a headphone preamp based on the K272C. What it is is that though the headphone amp will be contained in the steel can you will have to pop the top to connect to the amp. The cct. brd. (quite pretty) will sit on a varnished and polished ply base with the headphone jacks and switches exposed. The batteries will be slung underneath. Hard to explain and future photos will fill in the gaps.
Attachment:
CanAnd.jpg
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CanAndII.jpg

Author:  atanguay [ 20 Apr 2013, 10:19 ]
Post subject:  Re: K272C Oatleys JAN6418 tube based headphone amp\preamp

That's going to look really cool. I like the idea.

Author:  mwhouston [ 21 Apr 2013, 08:27 ]
Post subject:  Re: K272C Oatleys JAN6418 tube based headphone amp\preamp

The cct. brd. was pre-assembled so my job was to fit it to some sort of case. In this build the case would only ever house the nood cct. brd. and by popping the top you could connect what ever device you wanted. The PCB sits on a twice sanded and varnish light wooden plinth. I originally wanted the cct. brd. to sit on sticky feet but due to the fact I mounted the phono jacks too wide, thus raising the cct. brd. to high to fit in the tin, I had to swap to slit rubber grommets. This may have been a blessing as the final result is far superior to the original idea.

Two 9V batteries (18V) hang under the plinth in battery clips and phono jacks are double-sided-taped to the bottom of the plinth. I have once more employed my "safe on" technique which ensures no chance of leaving the device on and packed away. Have the PCB being able to be removed easily from the can means you can check the two battery level leds and therefore know when to replace batteries. I'm pleased with the whole build. Studs will be added to the bottom of the tin so the can will sit still and upright.
Attachment:
FinisI.jpg
Attachment:
FinisII.jpg
Attachment:
InCan_I.jpg
Attachment:
InCan_II.jpg


On a very brief listen I was impressed with total warmth and bass weight. I need a lot longer listening time for a better assessment but this one (K272C) feels and sounds like a winner.

Author:  Gio [ 21 Apr 2013, 22:54 ]
Post subject:  Re: K272C Oatleys JAN6418 tube based headphone amp\preamp

Nice. I was wondering how you would fit the headamp in it the metal can.

Cheers

Author:  mwhouston [ 22 Apr 2013, 00:49 ]
Post subject:  Re: K272C Oatleys JAN6418 tube based headphone amp\preamp

The earlier images really did show how this whole little project hangs together. The lid is under the preamp doubling as a stand.
Attachment:
K272C.jpg

Author:  atanguay [ 22 Apr 2013, 11:25 ]
Post subject:  Re: K272C Oatleys JAN6418 tube based headphone amp\preamp

What's the plan for the end? How about laser cut black acrylic? 8-)

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