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.

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

Graham

If by "true RIAA curve", you mean that the curve has only the 3 time
constants:

T1 = 3180 uS
T2 = 318 uS
T3 = 75 uS

then I suppose it would be technically correct to say that the network
cannot ever produce the curve "exactly", since this network does in fact
have a 4th time constant, which must be placed at some frequency.

If the theoretical component values are used, and the network performance
is compared to the "official" 3 time constant RIAA curve, the error
smoothly increases as we approach 20 kHz, reaching a maximum of .0004 dB at
20 kHz. A physical network made of real components will never be this good
anyway. It seems to me that this is good enough for practical work.

And, as I said in my response to Jack, if you add the 4th time constant
(which for this network with the originally published values, is at about 2
MHz) to the "official" RIAA curve, this network with the theoretical
component values has an error of substantially less than 1 nano dB. Even
this error is an artifact of the limited precision arithmetic. The network
can match the 4 time constant RIAA curve exactly.