This is the second of the is style of headphone amp I have built. The first got sold, should never have done it. Here just SS rectification and three stage filtering. A small 5H Hammond choke starts the smoothing process. A Hammond power tranni which should be in spec but runs awful hot. I built this for someone else but because of the tranni issue I'm keeping it and building the client another with a more robust power tranni and tube rectification.
The 6.3V winding is rated 2.5A but also the 12AU7 filament is being drawn from here adding an extra 300mA. Could this be the heat problem? The tranni is rated 65 or 75mA depending whether you believe the Hammond web site or the sticker on the tranni. Total draw is 60mA. As best as I could measure the tranni gets to 40C or 100F. Too hot? I can hold my hand on it for about 5 seconds.
Power tube is a RCA 6AS7 which I tested is 105% emissions and the 12AU7 is an Ozzi Miniwatt testing 95%. Good combo of tubes. Sockets are ceramic and most of the wiring is solid core. Alps Blue Velvet pot with solid brass knob. All WBT silver soldered (45).
With a 12AX7 there is a lot more gain but you can just detect a very faint hum. With the 12AU7, less gain (not a problem) and no hum. The amp drives both my 32ohm and 250ohm HPs without any problem. Lots of bass. The schematic is similar to the standard Bottlehead design. I'm using only 100uf 100V NP output caps. On the schematic they are recommending 680uf. I can't see how that is required I'm finding bass plentiful. All power caps are snubbed and the output cap is snubbed with a 220nf poly. I like to snub any large audio signal cap. I think is makes them quieter and I believe it allows better flow of HFs.
Base once more by my amp buildimg buddy Don and is Australian Spotted Gum. Top plate is thick ABS. The sound is absolutely fabulous.
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