EDIT: This design has gone through a bit of re-working. Jump to the major revisions here.
It was suggested to me on a number of occasions that a balanced JISBOS with a little gain might be a good accompaniment to my E-Mu 1212m. I have some other switching/source needs, so one thing led to another, and....
(Higher res version here).
Caveats - This is just a "work-in-progress" basic block diagram--and has a lot yet to be desired. A really well thought-out ground path, for example, needs to be worked out. Power supply needs to be added in (I'm planning on an LCBPS). In an effort to make this as "idiot-proof" as possible, there's perhaps more switches in the signal path than I would like (a total of 5 between input and line out if you count the normalled TRS jacks), but I really didn't want a situation where you might be sending out-of phase signals or a gained signal to a line output.
A brief overview of each section:
Obviously, I wanted balanced input capability from my 1212m. I also needed the ability to patch in an unbalanced line-in from a tuner, and sometimes an input from a DAP. I threw in the DAC just as a placeholder for "future expansion." OPUS or y2, anyone?
For signal selection, I'm thinking of a 4 pole, 12 load rotary switch like the NKK MRA403. Break-before-make, looks solidly built, and not too expensive. There are also some options I found on E-bay but it's a bit of a crap-shoot.
BALANCED or Dual-Stereo MODE
The "Mode Switch" reverses L+ and R- so you end up with the positive and negative phases for each channel paired. Doing this in conjunction with Alps stereo faders enables manipulation of a single balanced stereo output ganging the two faders, or two unbalanced stereo outputs with their own volume controls. An additional 4 poles on this switch would send the appropriate buffered signals to their respective balanced or unbalanced line outputs on the back of the preamp. I suppose you could add a phase inverter before Line Output 2 if you wanted to drive, say, a stereo amp and send a second signal to a subwoofer.
For now I'm settling on dual 1/4" TRS jacks for the headphone output. Not strictly kosher, I know, but having the switching between headphones and speakers integrated into the jack is a bit more elegant than yet one more rotary switch, I think. In "unbalanced" mode, it also has the virtue of being able to handle two standard headphone plugs with no additional modding or adaptors.
The Gain Switch would tap between the appropriate spots on the JISBOS buffer signal and divert the signal through resistor dividers and back to the boards. The specifics for adding gain to the JISBOS are in this thread, and there's quite some discussion about it on Headwize as well.
I'm not sure if this was necessary, but I added a "gain defeat" feature that prevents the signal from the JISBOS boards reaching the rear line outputs when gain is switched on. Technically the gain would only be 4.
I have a number of questions about the design:
1.) Will it work?
2.) Is it necessary to defeat gained output going to the line outs, or am I being overly cautious?
3.) Is there any way I can simplify this thing?
4.) Is there a better way I can do all the switching--and still have it be cost-effective?
5.)Any suggestions as I work out the ground paths? I do want to try and minimize unnecessary crosstalk and other grounding issues as much as possible.
6.)Looking towards the DAC, would either the OPUS or the y2 lend themselves to this application? Would they be reasonable upgrades from my 1212m?
I know it seems silly to name it since it's only amalgamation of existing circuits, but I've toyed with naming it the "Odyssey," since I sold my beloved old game console to help finance the project...
...or perhaps because that's what the signal has to go on to find an output somewhere.
I'd appreciate any other quips, comments or suggestions as well. Thanks for looking!
Edited by Zaubertuba - 9/20/10 at 11:53pm