I have a new public-domain DIY preamp designed by Hugh Dean of Aspen Amplifier that was inspired by the Lender topology. It is capable of 40vpp drive into 7kohm load with low distortion. The distortion it has is all SE Class A in character (mostly H2, a little H3 and nothing else.) It has 0.002%THD driving 20v peak-to-peak into a 7kohm load (more than enough to drive any power amplifier). The dynamic range and headroom are enormous. Gain is adjustable from 12dB to 22dB with change of feedback resistor,
More info here:
http://www.diyaudio.com/forums/analog-line-level/314563-aksas-lender-preamp-40vpp-output.html
There is a group buy for some nice PCBs designed by a pro (JPS64). A second round GB may be possible if there are more interested parties:
http://www.diyaudio.com/forums/group-buys/315521-aksas-lender-preamp-40vpp-ouput-gb.html
Briefly, here is the very elegant circuit by Hugh Dean, using only 5 actives (1 is a CCS):
Here was my SMT prototype (there are nice SMT versions of all the TH components in the above schematic, and they actually work even better):
And here is measured FFT for 20vpp into 7kohm load with 0.002% THD, note the absolutely clean flat silent noise floor at -130dB:
Here is the proposed finished PCB with two core amp module daughter boards and input selector, cap multiplier based PSU and onboard CLC and CRCRC filter:
Amp core module daughterboards are available in SMT, TH, or metal core SMT:
This preamp was designed to be able to drive power amps with unity gain output stages (Pass F4 for example) or a simple single MOSFET common collector output type SE Class A amp. But I have found that it makes any amplifer sound better by giving it the proper driving headroom and clean low noise floor that it needs. PSU will be external SMPS walwart from 12v to as high as 35v. A 19v laptop SMPS is perfect.
Oh, and if you have no need to drive 20vpp into an amp, here is what it can do driving typical Line-Level inputs at 4vpp - 0.0011% THD, and this is at 22dB gain. If you only need 12dB gain, the distortion will be much lower, about 0.0007%THD 0 that's 7ppm!:
Finally, what does this have to do with headphones? Well, I use it to drive my Silicon Harmony SE Class A and the sound is glorious. I have also used it to drive a unity gain SE Class A, the MoFo amp as a headphone amp. Finally, with a simple swap out of 3 resistors to the above circuit (to increase bias current from 12mA to 90mA), this preamp becomes a bonafide low distortion SE Class A headphone amp!
Here is 1vpp into 50ohms at 90mA bias current and achieves 0.0056% THD: This low level of THD is hardly ever seen in a SE Class A headphone amp:
Sounds very very nice.