6V6 line preamp

[buzzforb]

OK thanks.

[Salas]

Originally Posted by SGregory

AC voltage on the heaters for the 6v6 was inho the way to go. I tried various DC set-ups including a SSLVR and all of them sounded dead to me. So I am back to a simple tranny and a resistor to keep the voltage down to 6.1V. It is a matter of opinion though.
No Hum.

Did you also try the 317 chip current source heater like in the original schematic by the way?

[SGregory]

Yes, I did try it. If I recall correctly, it was a similar perception. Kind of dull, or missing the life. Hard to describe exactly. In truth none sounded really bad, just prefered the AC set-up better.

[Salas]

Is there there a bit different harmonic content in THD measurement when with AC heating?

[SGregory]

This is what I have. I don't recall why I stopped testing it as I don't really like the trash that I see with the 60Hz harmonics. I know I had a significant reduction in 60,120,and 180Hz with DC.

Regarding the 1kHz signal and harmonics, virtually identical or at least no difference I could see. +/- 1 or 2dB

I recorded this almost a year ago, 30Dec11, so I am a little sketchy on all the details, but I know I had better traces for the AC filament

[Salas]

So red is some time not your best AC heating, blue is DC heating. 1kHz harmonics have a nice fall scale.

[buzzforb]

Salas. Did you try lifted AC heater.

[SGregory]

Yes red is a not so good AC spectrum. If you recall I was having a bit of an issue back then with ground loops on my test bench. I know I have a clean one somewhere in my files, but that was a computer ago. The 60,120, and 180 Hz peaks were about the same magnitude however.

[AJT]

i heard reports by some local diy'ers that using a CMC on the ac filament yielded improvement, the choke is a ferite type sized to handle the current required...

[Salas]

Originally Posted by buzzforb

Salas. Did you try lifted AC heater.

Not in this one. DC voltage source and DC current source. I preferred the later.