On Fri, 18 May 2007 12:45:28 GMT, Eeyore
wrote:



The Phantom wrote:

The image Orig.png is the schematic of the originally published network.

The image T4_3_18.PNG shows the values when T4 is changed to 3.18 uS.

InvRIAA.png shows the formulas in pretty print.

I have a recollection that such a network cannot ever produce a true RIAA curve.

I dug out the Lipschitz paper from a box in the basement. The only place
he says *cannot* is in footnote 9 of the section where he discusses the
effect of inadequate open-loop gain in active equalizers. To quote:

"Of course, we assume that the shifts involved are not so large that the
roots of the equations (6), (18) and (20) become complex, for then the
configurations under consideration CANNOT be made to follow the RIAA curve,
and the amplifier's open-loop gain must be considered to be totally
inadequate."

Passive networks such as the ones under discussion lately can always
follow the RIAA curve if their topology gives the right pole-zero
configuration.


Lipschitz discussed this in depth in the AES journal about 30 years ago.

Lipschitz's paper, "On RIAA Equalization Networks", appeared in the JAES
in 1979.

I also found with Lipschitz's paper a copy of a 6 page reply by Peter
Baxandall which was published in the JAES in the Jan/Feb 1981 issue. He
showed an inverse RIAA network identical in topology to the one I've
analyzed in this thread. He used 27 ohms for R1 and also added another 27
ohm resistor in shunt at the input, solving the problem of ensuring
sufficiently low impedance at the input. He accounted for the effect of
the two 27 ohm resistors.


Graham