The Objective Interim Amp Mk2

[aerius]

Finally found a way to get cheap output transformers for my amp after reading this page.

All resistors are 1/2W unless otherwise noted, coupling caps should be 250V or better. The power supply capacitors are all 240VAC motor run caps from Aerovox which I scrounged for $5 a pop from the surplus store, and the chokes are also cheap surplus store goodies. You don't have to go nuts on the PSU like I did, I just felt like building an amp with no electrolytic capacitors anywhere in it.

The output transformer is actually a 50W toroidal power transformer from Amveco (~$20 each from Digikey). If you have high impedance 'phones or simply listen loud then connect the secondaries in series instead of parallel, or get a 9V or 12V output model instead of the 7V one I use.

Good luck getting sockets for ECC40 tubes though. You can a 6CG7 or 12AU7 instead but you'll have to tweak the resistor values on the front end a bit.

Tube filament power is from a rack mount PSU I found in the surplus shop. It can run 6V, 12V, and 24V tubes so I can use a large variety of output tubes since I'm too cheap to shell out money for 6SN7's. Go go 12SN7's and 1633's.

Learn to scrounge and this amp can be built for $50-100.

[aerius]

Power supply updated to Mk2Mod2 status. Now using a bigger transformer from the surplus store along with a hybrid bridge rectifier. Uses UF4007 diodes and 12D4 vacuum tubes, all resistors are 5W. Quieter, with less hiss & hum. Dead silent with my Senn 580 and K340, just a bit of background hiss & hum with my Grado 225, but I suspect it would be a problem for Ety & Shure owners. Still have some grounding & wire routing issues to clean up which should kill any remaining noise, but I'm too damn lazy.

[aerius]

Made a couple more changes. Power supply transformer is now fixed at 240V, and the 820R cathode resistors on the output tubes are now switchable between 1k and 640R. 1k is for 6SN7 and 6BX7 tubes, and 640R is for 6BL7's. 6BL7's are happier and sound more open at this new operating point, and the other 2 sound the same as before as far as I can tell.

This amp will cleanly drive any dynamic headphone I can think of, even the K1000. Tyll said it sounds good, so build my amp damnit!