Convert hybrid amp to pure tube output?

[Soldermizer]

I've heard this can be done (a DAC-01A headphone out to pure "tube.") I may want to do that, but I am doing something a little different. I have a Bravo Audio 12AU7 hybrid headphone amp. I use the headphone out to drive RCA ins to a power amp, so the load is pretty easy. I don't have a schematic. Would it be as simple as disconnect (I assume) the outputs from a power transistor to the headphone outs, and tap in to the outs of the tube (downstream of a cap of course!) ... never done this, don't want to electrocute myself (but this is a 24-V powered toy, so perhaps not lethal?)

 

[ballpencil]

I'm afraid it's not that simple. 12AU7 don't give enough current to drive a headphone directly, hence it needs the power transistor to give the punch. To have pure tube output, you need to replace the power transistor with a tube stage that can drive headphones directly. ECC99 as cathode follower might be good for the job, depending on your headphone impedance.

 

[Soldermizer]

Your point is well made; however, what I want to do is basically turn the headphone amp into a preamp. I am only using the "headphone" out because it produces the gain I need (which, of course could change dare I fiddle with the circuit...)

My tube out will only have to drive 47K ohm or whatever the similar input impedance of an amp's line in is.

 

[Merlinb]

Is this the schematic? If so, then yes, you could do what you are suggesting. Add coupling caps from the anodes, and pull-down resistors, and you have your line outputs. (You don't need to disconnect anything from the MOSFETs, just let them carry on regardless). Since this is a low voltage circuit there is very little that can go wrong; we're not talking precision audio here.

http://i30.photobucket.com/albums/c306/oohms/bravo12au7/shem.png

 

[kevinkr]

You might do as well or better just to use the mosfet source for the output and a good 1uF film cap to RCA jack with 1M pull down. (The large electrolytic is not an ideal capacitor for coupling to the input of your power amp.)

The plate current in the 12AU7 is about 300uA which means that tube is not going to be an effective driver of anything external, and the current source as rudimentary as it is is probably the only thing keeping it at all linear, and from an AC standpoint probably represents a load of several hundred K ohms - not likely that connecting a 47K load to the plate circuit is going to do anything good for gain or linearity..