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Original Message

RE: Note to folks performing my PAS RIAA mod

Posted by sbalfour on March 23, 2011 at 20:19:13:
Joe,

My comment was a bit of extrapolation. The R component of the 3180us
pole is off by a factor of 2.5X, and that's the pole that's at issue.

I saw the graph in your white paper. The other (parallel) components added
were 3% and 5% of the original component values. So ALL the components
on the phono board (nearly, anyway) better be 5% (or 1%) components and
in spec, otherwise the changes are indistinguishable from the values of
the original components. If the components are original, a large number
can be expected to be out of spec; a practical mod would have to re-stuff
the whole board first. Then we get to impedance match the triodes...
and I don't see that mentioned in the mod.

If someone is solder challenged, the mod isn't trivial.
(That's why you had to include in one of the postings of the mod a warning not to remove the 47K feedback resistors - why would anyone
do that?... maybe they can't read the schematic, don't know the resistor
codes, or don't know how to trace the board layout :-( ). But I think
most modders can locate a pair of 4.7Mohm resistors reliably.

Stock response on the graph, with the exception of the rise below 100hz, looks like it fits within a 1db range. If the RIAA is within +/- .5%,
that's darn good (and I think it will be, if the 4.7M ohm -> 2.0Mohm),
because I doubt the rest of the circuit is that good. The other issue is can you HEAR differences as little as .5db at typical listening
volumes of 70-90db? The speakers with the flattest frequency response
I've auditioned are something like +/-2.5db, about as good as it gets.
They have non-linearities of 2-3db, and sound quite flat.

I can't take the debate any further without measurements, which I don't
have at this point. I'd like to mod-the-mod to preserve Decca/78RPM
equalization, so I'm looking for an 80/20 solution that'll minimize the
ripple effects of this mod (like requiring 100K ohm volume pots, which
means forgoing the loudness function, losing the other equalizations,
re-stuffing the board...) but substantially reduce or eliminate the
non-linearity around the 50hz pole. How that 4.7Mohm resistor got there
I think had something to do with offbeat 78RPM equalizations. You don't
think David Hafler made a "mistake" like that!

Stuart