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- ... lectronicsI'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.