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After reading the umpteenth post about how bad DC filaments sound I just have to post. I'm one of the few that has ACTUALLY tried an apples to apples AC/DC filament evaluation. Years ago, on my path to optimizing my Welborne Ultimate Laurels, I set up 2 filament supplies on each monoblock (one AC one DC). I could switch back and forth between the supplies with double pole double throw switches.
My findings were that the stock DC setup, using a diode bridge, cheap electrolytic caps (10,000uf), with a resistor between sections (CRC supply) did sound inferior to AC. I changed the DC supply to use a Schottky bridge, Elna Cerafine caps (3,300uf), with very nice vintage potted Chicago filament chokes between cap sections (CLC supply). That was another story, DC was better than AC. This was not only my opinion, but that of everyone who heard the demonstration.
So as usual, blanket statements about what is best are useless. Topology, implementation, and system synergy are important factors that don't lend themselves to simple generalizations.
BTW reputable sources say that choke input is even better, but I didn't have the funds to experiment with a LCLC supply, and what I had sounded PDG. My simple DC supply: 6.3V filament winding > Schottky bridge > 3,300uf Cerafine > 75mh .3 ohm choke > 3,300uf Cerafine. Voltage was a little higher than I wanted but close enough (5.2V).
I'd think that by using an LCLC supply, and adding a cheap common mode choke at the end to get an LCLCL supply, you could further improve it by being able to significantly reduce the value of the C, and still get no hum. It's my SPECULATION that the very large amount of electrolytic capacitance, in the typical DC supply, significantly contributes to DC's bad reputation.
Anybody else experiment along these lines? I know about the regulated current supplies, and they might sound perfectly fine, but I have a hard time dealing with the idea of "sand" anywhere near the signal path. I know I'm being somewhat of a hypocrite about not trying the "sand" approach, but just want to stay in a more "purist" vacuum state. So...anyone out there experiment with relatively high L, low C, low DCR filament supplies? Remember, I said relatively Jeff!;^)
twystd
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