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PostPosted: 18 Jan 2013, 21:41 
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After receiving a prototype pre-assembled K272B cct. brd, featuring the JAN6418 pentode, I was anxious to mount it in my usual ABS plastic case and take a listen. I had just finished building two OPA2134 CMoy style HP amps, which I enjoyed thoroughly, and now keen to hear the difference. Both CMoy(ers) sold so I have none on hand for an A/B test but do remember the sound of both. In the K272B, sent to me, low grade components are used, carbon film resistors, green polyester caps etc. Normally when assembling these kits I swap out the low grade bits for better parts. I left the PT2308 HP driver chip in place. Usually I replace this with an OPA2134 but I wanted to listen to the kit "as is".

Take a look at the images below and you wee see the new "B" is a much narrower brd. I left in place most of the wiring which came with the pre-assembled and only replaced that which was required. Normally I would not use multi-strand hook-up wire but I wanted this kit up and going ASAP. I employed my "safe-on" technique of using switched HP jacks and a push-button on\off switch. With "safe-on" no power is supplied to the cct. brd. unless both plugs have jacks inserted and the power switch is on. Also in my last two builds of these portable HP amps I have a battery test point at the back near the power switch. Putting the probes of a voltmeter here (positive in the center and negative outside) and having the unit powered-up will allow you to check battery health without dissembling the case. I use two 9V batteries in parallel.

This K272B rang like hell when I had the top off and was testing it. Lid on and sitting beside me on the couched there is no ringing at all. So how does it sound? If you like a crisp, dry, clinical wholly accurate sound build a CMoy clone with the OPA2134 solely. BUT if you want a rich, full, earthy, gripping auditory headphone experience, wait for the K272B to come out in kit form. It sounds weighty and harmonically textured. From the first few seconds of FM radio from my Cowon S9 I knew this not only sounded different but was going to be a winner. Love it. While on the bench I use a test CD to get the balance between channels set. The left was well down on the right. I used central voice, a 1Khz test tone and pink noise to tweak the balance. If you build the kit upgrade all cheap parts with better parts. Mine sounds good as is but there is probably more juice in the lemon.
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K272_New.jpg
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K272AandNew.jpg
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K272B.jpg
Attachment:
K272BRear.jpg


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PostPosted: 22 Jan 2013, 02:17 
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I had the brass name plate engraved today. You know me and my silly names well this HP amp is called "6418-B". Here are some shots. I made up a special player to HP amp cable from wire wrap wire. Two earths are heavily twist with left and right channel together. One earth only is terminated at the 6.5mm (HP amp) end. Heat shrink tube over the twist with additional heat shrink tube at the end for durability with solder and gold connection. Looks pimped!
Attachment:
6418-B_I.jpg
Attachment:
6418-B_II.jpg
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6418-B_III.jpg
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SkinnyLeads_I.jpg


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PostPosted: 22 Jan 2013, 03:34 
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I really like the look of those leads.


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PostPosted: 22 Jan 2013, 07:29 
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Based on a phone call with Oatley's chief engineer the K272B will be available as a kit from Oatley in the next two weeks.

I expressed to Oatley that the seperate filament voltages (2X1.5VAAA) and an 18V buffer stage would be prefered. Oatley is happy to look at this as possibly the K272C.

Further more Oatley has developed an electret microphone which employes the JAN6418 as the preamplifier stage. The mic sounds extremely exciting. Looking forward to this one. Also a MC\MM stage to come.

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PostPosted: 22 Jan 2013, 09:52 
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I've just had a look at the website and I think I'll wait for the B revision as it's due out so soon. I need to source the parts for a mains powered headphone amp but I'd best finish my Poddwatt first.


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PostPosted: 22 Jan 2013, 10:01 
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Joined: 05 Nov 2010, 21:07
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Location: South East US - Tennessee
:idea: Instead of a kit that is "set" to be either singularly powered or dual powered, how about making so the builder can choose how he wants to power the kit - whether to use a single source power supply or a dual source power supply. I, for one, prefer the single source power supply like the original 272 has over the dual source found on the 272A, and since I assisted another builder in finding a way to convert the "A" model to a single source model, I would seem not to be alone in this.

:beerchug:

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The key to a successful build is to keep the smoke IN the circuit.
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We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
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PostPosted: 22 Jan 2013, 20:23 
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Hi Les, I couldn't agree more - like an "experimenters headamp / preamp". Just supply the PCB, tubes and small parts. Keep the packaging small and let the buyer add the bulky parts on his end. This will keep costs and shipping to a minimum. Then have flexibility with different power supply options and gain. If a dual balanced supply option was available you could eliminate the output capacitor and you open the door to a bunch of opamp / buffer chips. That would be a great low cost kit to get folks hooked on DIY and Tubes! :lildevil:

Cheers

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PostPosted: 22 Jan 2013, 20:38 
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I think until we start building our own from scratch we aren't going to get the best out of this arrangement. As I have stated: JAN6418 front end with a unity gain CMoy back-end.

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PostPosted: 04 Feb 2013, 05:36 
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There is s chance the K272B may never be released at least not in it's current form. After more discussions with Oatley a 18V version using the OPA2134 chip may result. As I have suggested in an earlier post, a CMoy with 6418 tube front end.

I suggested two 9V batteries be used to form +\- 9V to run the chip with a differential supply, similar to the way the CMoy is set up. Oatley for some reason want to use 1.5V batteries to make 6v then employ a small step up cct. develop 18V. I think two 9V batteries is a better idea. They don't. Any thoughts?

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PostPosted: 04 Feb 2013, 12:23 
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mwhouston wrote:
.....want to use 1.5V batteries to make 6v then employ a small step up cct. develop 18V


Two thoughts on this:
First, it is conceivable this method would yield more run time. A 6v AA pack would carry about a 2000mAH (2AH) capability where as a 9v is only about 300mAH. So I can sort of see the positives of the method. However, and secondly, I don't like SMPS in audio circuits. To step the 6VDC up to 18VDC an SMPS would be required. SMPS produce HF that, even with really good filtration, is introduced in to the audio signal. At minimum, it colors the audio signal. IMHO

I'm going to side with you on this Mark.

:beerchug:

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The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
_________________________________
LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board


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