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26th March 2012, 01:10 PM  #1 
Previously known as kingden
Join Date: Aug 2008
Location: Evanston, IL

RIAA Network Transfer Function
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. 
26th March 2012, 01:55 PM  #2 
diyAudio Member
Join Date: Nov 2009
Location: Cape Town

Where's the input and output?
You also need to take into account the source and load impedances. 
26th March 2012, 08:46 PM  #3 
Previously known as kingden
Join Date: Aug 2008
Location: Evanston, IL

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

26th March 2012, 09:18 PM  #4 
diyAudio Member
Join Date: Jul 2007
Location: Central Berlin, Germany

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 Last edited by KSTR; 26th March 2012 at 09:22 PM. 
26th March 2012, 09:28 PM  #5 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

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 ??
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26th March 2012, 10:24 PM  #6 
diyAudio Member
Join Date: Jul 2007
Location: Central Berlin, Germany

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 illdefined 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.

26th March 2012, 10:36 PM  #7 
diyAudio Moderator

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.
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27th March 2012, 01:35 AM  #8 
Previously known as kingden
Join Date: Aug 2008
Location: Evanston, IL

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? 
27th March 2012, 02:00 AM  #9 
diyAudio Moderator

You really do want to incorporate the buildout 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.
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"Pity, wrath, heroism, filled them, but the power of putting two and two together was annihilated." EM Forster 
27th March 2012, 02:17 AM  #10 
Previously known as kingden
Join Date: Aug 2008
Location: Evanston, IL

What are the buildout resistors? Where do they go?

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