Downunder55,
I'm really happy that you got it working, and that you like it. That bodes well for my own version. You would need the volume pot in front of the preamp to make sure you have some attenuation. If not, the full signal will be fed into the preamp all the time. That could lead to clipping of the signal, and it most certainly did in your case since it sounded terrible.
You could, if you want, try to lower the gain of the circuit by increasing R3 and R8 (the 560 ohm resistors from jfet source to ground) a bit. I am using 715 ohm, but you should get the same effect by using anything from 700-820 ohms. Do this only if the sound degrades at high audio volumes of course. That stopped my preamp from clipping even with full input from my HifiBerry DAC.
The PSU filter I'm using is exactly the one from Beavis, it seems to be fine with my switch mode PSU. I'd try that, and see what happens. It looks perfectly fine on the scope. Do not skip the resistor, as it is part of the filter. You're essentially creating a low-pass filter that removes high frequencies. The resistor can be higher, up to 500 ohms or so is no problem, but you don't want it to be too high as it brings down the voltage to the circuit.
Remember also to add the smaller capacitor, as it is more effective at filtering high frequencies. It would preferably be a ceramic, I'm using a polyester film cap. If it seems like the sound quality is worse, add one or two more 220uF electrolytic capacitors in parallel. Switch mode power supplies can be sensitive to capacitive loads, so be conservative when adding filter capacitors.
The circuit should work with anything from 12-30 volts, the PSU that's powering your amp would be fine. The output capacitors don't have to be polarized, non-polarized would probably be better and if you can find a film capacitor at the same value or higher (such as 4.7uF) it would certainly be better. Electrolytic capacitors are not linear with different frequencies, but I doubt that you would notice much difference at all.
All in all, this is a very resilient circuit in so many ways. The only thing I wouldn't change is the bias resistor from V+ to jfet drain. At least not without proper measurements. Even then, as long as you're within a reasonable similar value you'd be fine.
Finally, thanks for sharing the stripboard layout. I'd keep the original picture, it's in context and mistakes are what makes us improve.
BTW, isn't it übercool and satisfying to know that you have built this yourself?
by Downunder55 » 23 Sep 2014, 01:02 
