True Headphone AMP?

If I were you I would still connect the headphone amp onto the tape(record) outs if you have such. I would not want my headphone amp depend on the preamp volume settings. If you don't want the signal go to the main speakers just use MUTE (once again if you have it). It will not mute the record out.
In DIY area to build a descent headphone amp is a matter of within $100 that would match your headphones AND your preamp perfectly.
In consumer market area not all of them may please you for the budget. The ultimate champ in budget-no-option is Grace M903 which is also an excellent preamp and 24/192kHz USB DAC. I've also heard lots of goodies about Violectric.

I've done this in the past to check for sound characteristics at low volumes. This was to see if what I was hearing was caused by the amp, or speakers. I would not want all of these amplification stages altering the sound in my headphones though, when a headphone amp doesn't need or use them. But for safety reasons, I would use a 1/4 to 1/2 amp fuse in each channel.

Others with more electronic knowledge might give a more appropriate value. This would be in case something happens to go wrong for example, an output transistor shorting and passing 50 volts or more to the headphones. Besides the loud volume, the phones can catch on fire (if not protected), with all of that power there.

If people reading this are considering doing this on a tube power amp, make sure you keep a reasonable load on the speaker outputs on amp at all times. I would guess a 8 ohm non- inductive capable of handling the output power from the amp.

I think most preamps have enough gain to drive headphones but have to high of an output impedance to work. Is there a non active way to drop the impedance down enough to work with headphones?

I think atmasphere preamps can drive headphones from their main outputs. I believe that is because of their low output impedance.

Please correct me if i'm wrong.

Sarcher30 raises some interesting points. A preamp having very low output impedance, and the ability to drive low impedance loads without significant frequency response issues or increases in distortion, should work well into many headphones (especially those that combine high impedance with good voltage sensitivity). But those are a lot of constraints, and if the preamp and phones were not chosen very carefully it would be easy to end up with poor sonic results.

Is there a non active way to drop the impedance down enough to work with headphones?

A suitably chosen step-down transformer would lower output impedance, while also reducing gain. I'm not aware of any transformers that are specifically designed to interface between preamps and headphones, though, and again I would imagine it would require careful selection of all of the components to achieve a good match. It might also cost more than many dedicated headphone amps.

Regards,
-- Al

Thanks Al.

I figured there was some good reasons that such a device does not exist. I thought there might be some issues but do not have the technical knowledge to know what they would be.

Might make an interesting project. But sounds like it would not work for but one combonation of pre and phones.

Sean