Posted by PakProtector [ 152.163.97.27 ] on September 23, 2005 at 20:45:10:
Alright, got some bits on the way for testing and experimenting. In the raise my eyebrows, a 1:1 output TX for the linestage. -1dB high frequency point is in the low RF according to the manufacturer( Dave Slagle of Intact Audio ).
The plan is to make a low input capacitance, low output Z circuit for the active buffer. For pentodes, cathode Z is 1/gm. So either take a pentode, or build one. I am building one with a 5687 triode. With a gm number at the OP of 10 mA/V that beastie will be 100R output Z. Add the TX and we're looking at a few times that number... Still pretty good if you ask me.
I drew up the schematic whilst waiting on a scheduling meeting today. Too bad audio does not pay so well as my day job! But take from that, the schematic is simple, and few in parts. I'll find some way to get the schematic up on the Projects file if I have to send somebody a hand drawn version.
WOT, the gain will be nearly 1, and even with a 12B4 for my current linestage, I am very near to not needing any additional gain. Low output Z is quite another story, as the amp sounds best with a low value grid leak resistor in the input/PP driver stage.
Aside from the OPTx, the build is quite comparable to Guinevere, maybe simpler by 15%.
Anybody care to measure the effective gain of the pre- at listening levels? My bet is for less than 1 by an easy to compute margin.
cheers,
Douglas
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