Tube DIY Asylum: Re: Dumb question # 1 by Kurt Strain Do It Yourself (DIY) paradise for tube and SET project builders.

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Posted by Kurt Strain on March 25, 2002 at 10:43:31

In Reply to: Dumb question # 1 posted by Ivan303 on March 25, 2002 at 09:24:34:

Nothing for this circuit because I never made one like it. It's totally a theoretical example, but I know it will work fine. B+ here should be about +250V.

Two phono stages I actually built that work well are shown in the following URLs:

http://www.sonic.net/~ktstrain/phono3schem.GIF
http://www.sonic.net/~ktstrain/phonoschem.jpg

The first one has a good accurate bass response by moving the grid grounding resistor of the second tube to the front of the network. Unfortunately I discovered that doing this leaves the RIAA caps "unbiased" (at DC), which adds a slight performance degradation in these caps.

The second circuit shown has fixed two things: the RIAA caps are now biased up to the plate voltage of V1 and the grid grounding resistor is so high it makes Rs much less than Rgrid which is desirable from two counts: insertion loss of the network is minimized and the bass response is more accurate.

Someday I would like to try the LC version of an RIAA, using inductors as series elements. That's supposed to be awesome if done with great components. The disadvantage of the RC RIAA is that there's that big Rs in there that tends to lose control of the second tube's grid and degrades the sound.

Kurt

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