David,
I'm not using the supply as drawn by John Broskie. I'm using an LM317/337 variant as laid out by Stuart Yaniger. No diodes.
Mosfets used have been IRFF220 and IRF820 or 840, Have to look up the #.
I've seen the filter inaccuracies from simulations. It was apparent that a low source impedance was needed to get the expected filter results. In fact, using Broksie's recommended filter scheme, which lacks any stage prior to the filter, adversely affects output of the preceding stage (in another piece of equipment, DAC, Phono stage,etc.) into the outputs of another filter, i.e., high pass & low pass. So, when I implemented my first filter, I did add a ~medium impedance gain stage (to be able to adjust for different amplifier gain levels) and used a high impedance filter. My rule was 1:10 minimum.
Since then, I found that a medium impedance filter sounds better (cleaner), i.e, 10k vs 100k, so now I need a 1/10 impedance gain stage.
But I'm trying to eliminate the line stage>filter by making it all-in-one, eliminating a series gain stage setup. I think a gain of 10 will suit any foreseeable amplifier combinations, so I was asking about an excellent sounding tube, with a mu of around 20, that I can use as simple grounded cathode, to drive a filter network with a ~10k impedance. Obviously. there will be variance, since the filters will be: -18dB/octave LP -6dB/octave HP & -6dB/octave LP -6dB octave HP
I understand that a follower here will have much lower impedance that driving off of a plate, but I'd really like to avoid adding six more followers and servos.
Stuart
|