OK, so I have been doing some thinking about this project. Its scope could be from slim to broad. Does it make sense to design the enclosure such that it can accommodate all options as stated below?
1) Basic: Unity Gain, (1)pair RCA in, (1) or (2) pair RCA out.
2) Transformer coupled inputs allow for 0, 6, 12dB gain. This would not work with a passive, unless Transformer based VC, because of impedance matching. There is a sonic magic to transformer coupling. There is full galvanic decoupling, and a quality to the sound that comes from inductive coupling which is exceptional! This also achieves optional gain in a way that I prefer!
3) Volume Control: Volume control would be switched between input transformers and buffered output stage.
4) Multiple Inputs: Say 1-3 inputs, 3-way toggle. No need for expensive rotary selector or relay switching and PIC or microprocessor controls which unnecessarily complicate the design.
5) Balanced outputs: This could be accomplished with another pair of output transformers (expensive cost add). Or, with input transformer coupling, the use of four tubes could also accomplish the same goal. Best sound however may be achieved with transformer output coupling.
Summarizing options below, please let me know which configuration(s) would be of greatest interest to you.
Simple RCA-RCA Buffer.
Volume Control, switchable in or out.
+6dB, +12dB transformer based gain.
XLR fully balanced outputs.
This begins to look more and more like a preamp, when you add in all options. I suppose it is a matter of semantics, but my goal is to produce a unit that will be the 'Analog Bridge' between your digital and analog source(s) and amplifier.
I would like to keep the size of the product fairly compact. It could be 17"W, but not necessarily. If a simple bridge, it could easily be 1/2 width.
What sets this apart from a preamp?
A preamp typically has remote control. This could be done, but then we are looking at adding a PIC and digital controls with clocks and other high frequency artifacts that would not be present in a simpler design.
A preamp typically has active gain and ideally a buffered or low output impedance.
A preamp may offer balance control, Home Theater Bypass, tape loop, etc. I don't want that level of complexity.
A preamp is a very active device. This would not be a passive device, but using transformer coupling for gain and unity gain active stage means there is VERY little room for noise to be added. Our power supply design and layout will rule noise out.
Simplicity and sonic purity are the goals of this project. I also want it to be affordable. It need not be dripping in billet AL and weigh 50lb. If offered factory direct to our mod customers, it could be reasonably priced. The more feature laden option would be more expensive of course, but still reasonable.
This could suit a main 'big rig' 2CH system.
It could work for a desktop system with powered speakers.
It could work with a headphone system.
It could be scaled to meet your needs!
Please post your thoughts and desires per my above comments.
This product WILL come to market, but I see an opportunity to make a very versatile product here, that would be unique and suit many peoples needs well.
President, ModWright Instruments Inc.