RIAA Equalizer Amplifier Design

RIAA Equalizer Amplifier Design

This article explores the intracacies of phono amplifier design,  culminating in a few practical designs. Special emphasis is placed on overload margins (critical if you want good sonics from your EQ amp), driving the EQ network adequately and noise.

RIAA Equalization Amplifiers V2.0

Below is the RIAA Calculator Excel spread sheet.  Please read the article above, before attempting to use the tool below.  Once you have calculated your RIAA values,  you are strongly encouraged to check your design out using a spice model – I use LTSpice,  but any  Spice simulator will work.

Here are some guidelines to help you use the spreadsheet tool:-

1. In his paper, Lipshitz discussed how all the time constants interact in the classic all-active RIAA (which is what I focus on in my article above and I recommend for best performance).  It is the most difficult to design, but you get superior overload and noise performance using this approach i.e. no trade off’s to make as in the case of passive, active-passive or passive-active approaches.
2. Lipshitz  provides a methodology with which to accurately calculate the component values despite the fact that they are all interacting with each other – so the process in the spread sheet is a bit iterative to converge on the correct values.  Remember, you cannot simply calculate the RC values from T=RC – if you do, your RIAA break points will be all over the place
4. Follow the steps below precisely to use the tool correctly
A.  Set L18 to 1 kHz (I actually should have fixed this in the spread sheet at 1 kHz but did not – learning point for a future version )
B. Select R0 in J6.  This is the lower arm resister in the feedback network that goes from the inverting input to ground, or to the DC blocking capacitor.  A value of between 100 and 500 Ohms works best.  If you try to go outside of these values, you can run into difficulties with getting the RIAA values to converge in the spread sheet, or you will get crazy values.
C. Adjust T1 (cell E7) for the correct gain in L22 (magnitude) and L24 (dB).  I usually go for a gain magnitude of 50x to 60x which =~50-53 dB at 1kHz.
D. Now comes the hard part: Adjust T6 (cell E12) for EXACTLY 1.000 in cell J14. You should find the value in J14 to lie typically between about 300k and 1.5 million.  Its important that you iterate on this step until you get 1.000 in J14.  This sets the resistor and capacitor values in the feedback network very accurately to the RIAA time constants.
E. The next step is to calculate the secondary post filter value by inserting a capacitor value into L11.  A value of 10nF is a good starting point.  Do not have values of R31 less than 50 Ohms as you don’t want to capacitively load the opamp at HF (it might go unstable).  R31 should typically lie between 50 Ohms and 330 Ohms.
F. I highly recommend you then put the values into an LTspice model and run  an AC analysis to check the conformity.  I typically get 0.2 dB 20 Hz to 20 kHz, and by tweaking the values slightly in the simulator,  easily get to 0.05 dB 20 Hz to 20 kHz.  However, in practical terms, anything better than 0.5 dB is very good.