12AU7/EL84 Headphone Amp

[sorenj07]

I'm trying to design a headphone amp intended for relatively higher impedance headphones, 200 ohms at least, so I don't think transformer coupling should be essential. The person this amp is for has a large collection of NOS etc. 12AU7's and EL84's so would prefer amps of this type.

For the output stage, I'm considering using trioded EL84's in a White cathode follower, with B+ 300V, at around 30mA of current. Ra would be 88 ohms (going off the EL84's transconductance of 11300 umhos), Rk 130 ohms. Bypass cap would probably be a Black Gate, and the output capacitor would in all likelihood be >50uF Solen.

The input stage would probably be a 12AU7 grounded-cathode, direct-coupled to the output stage. Probably no cathode bypass cap to reduce distortion a bit.

I'm a bit rusty on the principles of direct-coupling and how it affects the next stage's biasing. On paper at least, what needs to go on in order to make this work? Thanks!

I'd post a schematic but things are too up in the air right now. It's not essential to use both 12AU7's and EL84's, and they might not be the tubes I would choose, but that's what the guy has the most of...

[EWBrown]

What you want to do is go over to John Broskies "tube cad" site and look up the 411 on the "Aikido" amplifiers, these are a modified SRPP-meets-White CF-with a twist design, he covers headphone amps designs, which use capacitor output coupling, there are a lot of entries and he covers the operating theory as well as good and bad (to avoid) design approaches

/ed