Split passive RIAA Network
for Tube Phono Amplifier

The split RIAA method involves having two separate RC (passive) filters, each is driven by its own active stage.

The network which is usually place first is that of R1, R2 and C1.
The time constants for first stage are 3180µS (50,05Hz) for (R1 + R2)C1 and 318µS (500,5Hz) for R2C1.

  • Typical standard values used in tube circuits are: C1=33nF, C2=1nF
The second stage defines a low-pass filter with a -3dB point of 75µS (2122Hz) and its time constant is 75µS (R3C2).


Calculate R1, R2 and R3


Capacitor values for Network

C1 = nF
C2 = nF



Tube data (optional)

Ra = Ω (resistor between plate and power supply)
Ri (internal tube plate resistance) =  Ω (from tube datasheet)




Ri + Ra = Ω
R1 = Ω (incl. Ra and Ri)   Ω (theoretically)
R2 = Ω
R3 = Ω (incl. Ra and Ri)   Ω (theoretically)


R1 = Ω parallel Ω = Ω, Error = %
R2 = Ω parallel Ω = Ω, Error = %
R3 = Ω parallel Ω = Ω, Error = %




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 R1, the error becomes more noticeable and moves in the opposite direction, giving a positive boost at 10Hz (+2.74dB).

without changing C1 value and

without changing C2, 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, nothing has to change...





RIAA Playback Curve
Attenuation in dB relative to 1kHz


> Frequency

> Attenuation in dB

 

> Frequency

> Attenuation in dB

10

19.74

 

1000

0

20

19.27

 

1200

-0.61

30

18.59

 

1500

-1.4

40

17.79

 

1800

-2.12

50

16.95

 

2000

-2.59

60

16.1

 

3000

-4.74

80

14.51

 

4000

-6.61

100

13.09

 

5000

-8.21

120

11.85

 

6000

-9.6

150

10.27

 

8000

-11.89

180

8.97

 

10000

-13.73

200

8.22

 

12000

-15.26

300

5.48

 

15000

-17.16

400

3.78

 

18000

-18.72

500

2.65

 

20000

-19.62

600

1.84

 

30000

-23.12

800

0.75

 

40000

-25.6





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