Hi ronv,
I think you need not bother with the LM317. I have several of these:
The 505 series. The specs for mine are ±15VDC, at 100 mv, line and load regulation are ±0.02% and ripple + noise is 0.5mv rms. They are linear supplies. I can take one side or both sides as necessary. Real nice and easy to use. Are these good enough for a quiet pre? Could still do a bit more filtering on them if necessary I guess. I don't know how quiet a 317 will get. Thoughts?
Not to throw dog's bone in the soup, but I have continued research and found a specialty MIC manufacturer that makes almost exactly what I have in mind. They have published one of their prior commercial circuits that will fit inside an XLR plug!
Here is the schematic:
Here is where the schematic came from (good reading)
**broken link removed**
and here is the mic design (see the X-R series) and their little amp/buffer boxes too.
**broken link removed**
For my purposes I would put the cartridge in the end of a tube soldered into the RCA plug. In fact I already have one made with others in the wings. Just waiting for a finished amp design.
This fet design is really tiny but I do not know enough about LTS or electronic engineering to get it running properly in LTS. I am a builder at heart.
Apparently floating the capsule as he does allows for a greater dynamic range for the the mic than an OPA design and no surgery on the mic like Linkwitz. Nice. Also, no testing needed on the FET, apparently any GP FET will do! Nice. And I happen to have some MPF102 FETs on hand
Does it make sense to use the FET balanced cir for the pre and use an OPA for the buffer/amp to feed my unbalance line in on my sound card? That might actually be easier, smaller and less controversial too. Thoughts?
It appears that most of these ECM capsules put out around 4-6mv for an input of about 94dB SPL. That happens to be about where I test my speakers. I throw that in for a gauge to aid in gain selection. Now you know all I know and a whole lot more from your end.
Oh, BTW, I always intended to use the physically smallest caps possible to keep the design small enough to get into a mic wand and still be flat (not down 2-3 dB at the ends) from 20-20K. From my supplies that sometimes means larger value but physically smaller electrolytics rather than films.
Thanks