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Saturday, December 15, 2018
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    Calculate passive RIAA EQ Network

Find resistor or capacitor value How to measure the Phono Amplifier

The passive RIAA Phono Equalizer is an appealing circuit from an audiophile perspective for 2 main reasons.
  • The high frequency reproduction of the curve is extremely accurate
  • The two amplifiers operate with a constant feedback gain. High Frequency overload is lower than an active feedback design but this seems never to create problems in real world applications.

The only concerns are to be sure that the driving tube V1 has a low enough output impedance relative to R1 and that the following tube V2 has an input impedance of at least 50 times R1 to prevent loading.

You can enter data for R1 and for the 3 frequency settings.
Those frequencies represent corner frequencies of the 3 stages of the RIAA curve, the low end, the mid and the treble.
If you have a different curve you would like to equalize, input the frequencies of interest and the calculator will give you the component values.

Passive RIAA EQ Network
R1= R2=   C2= nF
Hz  uS C1= nF
Hz  uS  
Hz  uS  
Choose a value for R1 and compute. Adjust that value until the values for C1 and C2 come as close to standard values as possible.

Please keep in mind that the output impedance of the first stage V1 throws the network time constants off.

For example, a 12AX7 used in a grounded-cathode configuration with a 150k plate-load resistor results in an output impedance of roughly 44k (62k in parallel with 150k). When this value is added to equalization network 75k, the error becomes more noticeable and moves in the opposite direction, giving a positive boost at 10Hz (+2.74dB).

If we reduce resistor R1's value to 37k, the output returns to close to flat, but with a slight bump (+0.05dB) in low bass.

In the case of the op-amps, the output impedance is usually well under 100 ohms.

more info about RIAA Networks (very technical and very complete)

RIAA Playback Curve

Freq. (Hz) dBFreq. (Hz). dBFreq. (Hz)dB
2019.36240 7.042400-3.39
2219.242706.25 2700-4.04
2519.043005.57 3000-4.65
2818.833404.80 3400-5.43
3118.613804.16 3800-6.17
3518.294303.49 4300-7.02
3917.964802.93 4800-7.82
4417.545402.38 5400-8.70
4917.126101.86 6100-9.64
5516.616801.43 6800-10.50
6216.027601.02 7600-11.39
7015.378500.63 8500-12.30
7914.679500.26 9500-13.22
8913.931100-0.23 11000-14.44
10013.181200-0.52 12000-15.17
11012.541300-0.79 13000-15.85
12011.941500-1.31 15000-17.07
13011.381700-1.80 17000-18.14
15010.361900-2.27 19000-19.09
1709.462100-2.73 21000-19.95
Reference: 1000 Hz0.00 dB

  This calculator finds the closest standard resistor or capacitor value.

Desired Value:

IEA Series Closest Value Error Comments
E6 most capacitors, some inductors
E12 some capacitors, 10% resistors
E24 5% resistors
E48 2% resistors
E96 1% resistors
E192 0.5%, 0.25%, 0.1% resistors