For some time now we've been exploring development of a preamp buffer product as well as a headphone amp. As this has moved through a few initial stages these two products are morphing together such that they've more or less one and the same but for how the input/output is handled.
Developing anything with the word "tube" in it automatically implies "big, heavy, and expensive". For sure this isn't going to be a compact lightweight portable unit. A few pounds of transformers guarantees that just for starters. And since all audio gear (buffers/amps etc. ) are essentially all about the power supplies we aren't going to go light on that. But we are going to take a somewhat different path than most.
The hallmark of the theoretical "perfect" buffer is zero noise and no added coloration with both infinite input impedance and zero output impedance. Good luck with that. But to get close to this ideal we are currently taking the following approach.
Hybrid SS/Tube - Front end op amp gain stage (0 to ?) followed by 6SN7 triode buffer stage - 2 triodes (1 tube per channel)
DC Coupled Input & Output - No coupling caps, active offset trim adjustment with output relay safety disconnect. Part of the job of the SS front end is to remove any DC offset. We're going to test the hell out of the safety disconnect and if we aren't convinced it's foolproof we'll relent and put in an output coupling cap. But no coupling caps is the goal.
Split High Voltage Regulated Supply - Each channel has a pair of triodes working between a +/- high voltage supply rather than a single triode per channel and the usual single positive supply. Think of these split voltage triode pairs as being a rubber band held at both ends. You can pull on both ends of the rubber band while the center moves very little. Done right this provides optimal PSRR (low noise) while also making DC coupled output much more feasible. Yes, it does make the power supply design more challenging. But audio is power supply is audio. Regulating the split voltage supply further ensures the best audio quality.
Low Noise Split Voltage Op Amp Power Supply - While not exactly a true super-regulator, we are employing latest generation integrated low dropout regulators with noise down in the few uV/V range.
When fully developed, this hybrid buffer with front end SS gain stage can be packaged as a stand alone unity gain buffer, a buffer/LDR attenuator (i.e. an active LDR preamp), and as a hybrid LDR based headphone amp.
More to follow as this evolves.
