Jim,
I am considering building the Castinet strictly for use as a preamp, not as a headphone amp. My amps have a low input impedance of only 10K, but they have 30dB of gain, so the low gain of the Castinet is not a problem. However, looking at the specs of the output trannies shows they have a frequency response of 30Hz to 20kHz at full power. Does the fact that they are not flat down to 20Hz pose a problem when used as a preamp? Or, since they won't be being used at anything approaching full power (12 watts) will they be flat down to 20Hz when used at the low power required of them when running into the 10K ohm load of my amplifiers?
Thanks
Steve