Thanks for all info, I just built the "Sadowsky"-preamp (based on Silent Fly's layout, with some modifications). It works great!
- 2.5n (in tonestack) => 2.2n
- 5u output cap => 4.7u (still electrolytic)
- 750R (tonestack to gnd) => 2k lin pot (mid control)
- Moved the 500k log Level control to between the tonestack and the second amp stage
- Skipped the 300p (second jfet gate to gnd) because I first had a 330n due to a mislabled bag, which resulted in a very noticeable hi cut. Took me a while to debug, as I first suspected the tonestack, but when I found the problem, I just removed it, works fine without.
- I run the preamp on 18V
- Another resource for a very similar preamp can be found here: http://www.diystompboxes.com/smfforum/index.php?topic=73173.0
- Built it all on vero
So far so good, it sounded great with my Vox headphone amp. But when I took it to rehersal, and plugged it into my PA Power Amp i use for head (Peavey IPR 1600, drove it with a Sansamp VT Bass before), there was lots of noise and very low output. Some investigation proved the output impedance of this preamp is much too high for my power amp (which takes max 7.5kohm).
So i found a rather simple solution. I built an AMZ super buffer (this one
, used OPA2134 for opamps) into the same casing, right before the output jack (so that the buffer is on even if the preamp is bypassed). This was a very good solution. The preamp is now completely noise free, and the output inpedance is so low that you can use this preamp as a headphone amp! The only thing that's missing is balanced DI-out, but I could never fit a DI tranny (or anything else for that matter) into this box.
My Sansamp VT Bass goes into the bottom drawer now
The preamp is the top left one. I didn't put in a footswitch, since it's always on, but there's a bypass switch on the back side, should I want to use it as buffer only: