RIAA Network Transfer Function

[kingneb]

Does anyone know how to find the component values for the attached RIAA network?

I am building a tube differential phono stage and will verify it spice.

[godfrey]

Where's the input and output?
You also need to take into account the source and load impedances.

[kingneb]

More specifically, the circuit is like this. The network is between the two differential outputs.

[KSTR]

Look here, this seems to be the equivalent topology to what you have in mind :
phonostage

General information :
Cut and Thrust: RIAA LP Equalization | Stereophile.com

And in LTspice, a reference can be simply constructed with an AC voltage source of magnitude 10 connected to an "e"-source (VCVS) with the transfer function given in direct Laplace notation, using the known time constants :
laplace (1 + 318u*s) / (1 + 3180u*s) / (1+ 75u*s)
Comparison with the Stereophile article shows 100% precise accuracy (compare the detailed numbers given in the text) as expected right away, but always better double check

[AndrewT]

I think it is Harm that gives a method to determine the component values.
There is also a method given by one of our audio celebs, Bax or JLH or ??

[KSTR]

Calculation, for a starting point, is quite straightforward. RIAA EQ is a one decade wide bass shelf (from 50Hz to 500Hz) with a pole on top (at 2.1kHz). Problem is the somewhat ill-defined plate impedance of the tubes for the actual circuit which influences all three time constants, being part of the source impedance the network works upon.

[SY]

I've attached the topology you want. There's an RIAA calculator here: KAB Electro Acoustics http://www.kabusa.com which you can use as follows.

Connect the two resistors marked R! in my drawing to the 12AX7 plates. For the R1 value in the calculator, use 2 times R1 in the drawing plus two times the source impedance of the driving stage (the formula for the source resistance of a diff amp may be found in Morgan Jones's "Valve Amplifiers" as well as any other standard tube text. The other components can be directly plugged into the KAB calculator.

[kingneb]

Tried it in simulation. I receive what resembles an RIAA curve, but the voltage differential is supposed to be reasonably flat, as I am running inverse RIAA networks on the inputs, along with the sine wave generator to simulate a cartridge.

The plate impedance for a 12AX7 is around 70k, in parallel with 220k, and double those quantities is about 560k. I put the values in the calculator and came up with the values in the schematic below. I wondered if I calculated it properly?

[SY]

You really do want to incorporate the build-out resistors- that will lower distortion, especially at high frequencies, and reduce the dependence of the frequency response on tube aging and drift. There is something of a noise penalty there, unfortunately. There are also ways around that, but with added complication.

Let's pick 100k as the buildout resistor. The equivalent series resistance is then 200k + 2x R_L||R_p. The second term is about 53k, so the total equivalent resistance is 306k. Plugging that into the KAB kalculator, R2 is 44k5, C1 = 7n15, C2 = 2n45.

[kingneb]

What are the build-out resistors? Where do they go?