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PostPosted: 14 Feb 2010, 12:32 
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I got a chance to play with the new Oatley Electronics K272A Tube Heaphone Amp / Preamp Kit. I don't see the new version on their web store, but you can buy it from Oatley through eBay: http://stores.shop.ebay.com.au/Brankos- ... lectronics
I'm sure it will be on their web store shortly.

The new kit is essentially the same as the 6418 Valve (Tube) Preamplifier / Headphone Amp Kit reviewed by Mark with some changes around the heater circuit and the addition of a large grounding plane to the PCB. Now two AA batteries are used for the heaters (instead of running off of the 9V battery). This will greatly increase playing time and save some money on battery costs.
Attachment:
K272A-6418-Tube-Headphone-Amp.jpg

Using an inexpensive portable mp3 player and without any changes to the circuit or the supplied parts, I tried out the headphone amp kit with a pair of inexpensive in-ear canal buds (~17ohm)
Attachment:
6418-Tube-Headphone-Amp-Kit-Ear-Canal-Headphones.jpg

and a pair of Sennheiser HD595 open back headphones (~54ohms).
Attachment:
6418-Tube-Headphone-Amp-Kit-Sennheiser-HD595-Headphones.jpg

Consistent with all other reports on this kit, the 6418 valves are extremely microphonic, so using grommet tube dampers and paying attention to damping the enclosure will be a must. Resting the enclosure on a rubber pad or mouse pad should also help.

The circuit or my input leads seem sensitive to picking up stray EMF / EMI - using an enclosure to provide shielded is likely going to be necessary as well.

With the ~17ohm ear buds there is audible background noise. The amp can drive the buds with authority and there is plenty of available output when driven by a portable mp3 player. Without the background noise this would make an excellent amp for these buds. I will check again after I install the grommet tube dampers, apply shielding and some sort of damping.

Moving to the Sennheiser HD595 (~54ohms) the background noise is almost inaudible but the output level may not be sufficient for some. The amp does a good job with these headphones, a huge improvement over the portable player and a combo I could live with but it did leave me wanting a little more as I know what these excellent headphones are capable of with the right amp.

I'm thinking that 32 ohms may be a very nice load for this headphone amp. I have a pair of Grado SR80 that I will test out. I will also try some different opamps and more voltage on the B+, so more notes to follow.


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PostPosted: 14 Feb 2010, 15:44 
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Gio - as you know I have used my K272 as a portable triode headphone amp a lot on the train. I find if there is a mobile phone nearby I get the radio interference from the phone. My case is a heavy plastic though it is lined with bitumanised AL foil. So a metal case maybe required.

I have no noise issues with both my 30 ohm phones. The triple grommet trick on the tubes will kill the microphonics, it is a must.

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PostPosted: 16 Feb 2010, 14:37 
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Gio!
What about use 2 x 6418 like lampizator for CD and DVD players. Inside there is +12,+15v?

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PostPosted: 16 Feb 2010, 21:49 
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Hi Nikola, the 6418 could be used there, but given how microphonic the tube is, the spinning disc could be a problem. I did also try the headamp connected to regulated 12V and 15V supplies and with the higher voltage there is an improvement.

So I got a chance to properly dress the little 6418 tube headamp kit. I installed two grommets per tubes (had to use a bit of Vaseline to get them on). Huge improvement - forget A/B comparisons, just install the grommets / tube dampers right away. The board was mounted using grommets as spacers rather than nylon spacers. Also, for listening the amp was separated from the table using a thick bed of paper towels. These and a shielded enclosure helped reduce the noise floor quite a bit. Again I used an inexpensive mp3 player as the source.

With ATA in-ear headphones (~17ohm, ~$10) there is still audible background noise which may bother some during quiet passages. The amp can drive the buds loud and with authority. The cheap in-ear headphones are much more enjoyable with the amp over just the player.

Grado SR80 open-back headphones (~32ohm, ~$100) very slight background noise which is only audible if there is no music. The amp does a very good job driving the headphone and the output level is reasonably loud and should be more than sufficient for most. This made for a very satisfying combination.

Sennheiser HD595 open-back headphones (~54ohms, ~$300) virtually noise free, but the output level will likely not be sufficient for some. The amp does a good job with these headphones but these excellent headphones are capable of much more with the right amp.

Overall the sound from the little headamp kits is quite comparable to that from a CMoy running off a single 9V cell. I would describe the 6418 tube kit as warmer and more full than a CMoy headamp. The $30 kit is decent using only the stock parts and sure to be an improvement over a portable player.

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PostPosted: 17 Feb 2010, 16:11 
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Location: Netherlands
I've read that higher value resistors are inherently more noisy than lower values. The opamp in a stock build is run at unity gain using two 1 Meg resistors per channel. Would changing both the resistors to the same but a (much) lower value have an effect on the noise? Any side effects when that's done?


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PostPosted: 18 Feb 2010, 11:34 
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Location: Menifee, California U.S.A.
I am new to the forum and would like to purchase the K272A kit. It would be a nice addition to my Ipod. I am located in the U.S.A. Sun City, California. I have done some searching on the internet however I could not find a supplier in the U.S.A. I do not wish to bid on it on the ebay site.
Is there a site that I can order this from ?

After finding a Magnavox FM-45, 40 watt stereo at a home sale, buying it and taking it home and cleaning it up. I found that it works ! I was hooked on tubes. My next idea was a simple tube-amplifier for my Ipod, I don't know what is out on the market for that.

For Christmas I received a tube-Amplifier from Acoustic Research.

I am thinking a portable tube-Amp would me fun to build and right up my alley to start off with for my second tube project.
Rob


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PostPosted: 18 Feb 2010, 13:50 
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Robbie7 wrote:
I do not wish to bid on it on the ebay site.
Is there a site that I can order this from ?

Rob, just use the "Buy it now" button here. No bidding needed. You pay AU $30.00 for a very nice piece of kit. Not expensive IMO.


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PostPosted: 18 Feb 2010, 15:57 
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Ive built a number of them and the portable triode headphone amp is the way to go. I use mine with Phiaton MS400 HPs and Cowon S9.

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PostPosted: 20 Feb 2010, 17:43 
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For those wondering about the cathode of VR2 which is not tied to ground, here is Oatley Electronics response:
Quote:
If VR2 were connected directly to ground, the grid of V2 would effectively have a bias voltage of at least -1.2V with respect to pin 3 of the cathode, thus possibly cutting the tube off. The way it is connected in the kit, the Grid to Pin 3 of the Cathode voltage is 0V, same as the first stage. The only difference between the two stages is that effectively the second stage has a lower supply voltage and thus produces a slightly lower gain. The gains can be balanced by adjusting VR1 and VR2.

If the filaments were connected in parallel, the supply current would almost double. The two stages would now be identical in appearance, but their gains would be different anyway, as no two tubes are the same.

So for this reason a stereo potentiometer will be off balance.

I'm waiting for a more recent update on the life of the 6418 tubes. There is a two tube test underway and after 480 hours the maximum change in anode current of the tubes was about 1%. So a short tube life is not expected to be an issue.

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PostPosted: 21 Feb 2010, 02:07 
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Gio - I have no further updates from Branko of Oatley Electronics. But I feel (and I believe he feels) that the tube life should be excellent with the sub-minis.

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