Another Phono Amp for MD-cartridge

1. Introduction

In the 80's the CD-player has been developed with Philips. At those times also shape had been given to a new phono MD-cartridge as the successor of the GP412MKII. It should have become the GP620, but the marketing was interrupted by the advent of the CD-player!
I was the lucky chap who could put his hand on a proto type of the 'GP620'.

When I fixed the cartridge on my old Thorens TD124 with a heavily modified tonearm, I was very disappointed. Reading the documen-
tation much later, I saw that the inductance of this cartridge is 500 mH with a resistance of 800 Ω! My measurements: 535 mH with 816 Ω. It is not so nice to connect this to the phono input at the power amp via a 1.5 meter long screened cable!

2. Simulation of Cable Connection

In figure 1 is the simulation of the cartridge with a 200 pF cable con-nected to it. As expected the inductor in the cartridge resonates to-gether with the capacitance. In spite of the rather large series resis-tance of the coil, the resonance peek is more than 35 dB at 15,4 kHz. This sounds horrible!
Even with a 1 meter shorter cable (1 pF/cm) the resonance is still present in the sound. At 18 kHz the level is enhanced with 10 dB. The resonance frequency is 21,8 kHz.

The alternative is to put the phono pre amp in the player close to the pivot of the tonearm. The capacitance in case is less than 25 pF.

3. The RIAA-correction

The RIAA-correction dictates poles at:
3180 μs (50 Hz),
318 μs (500 Hz) and
75 μs (2120 Hz).
To avoid distortion because of auditory feed back, tonearm reso-nance and rumble, it is wise to add a pole at 20 Hz. This offers an amplitude-frequency characteristic near 6 dB/oct so that a decent op amp (which frequency characteristic falls with 6 dB/oct) could be used very well.

4. The Pre Amp

At 1 kHz, the amp should amplify about 40 dB to match the sensitivity of my power amp. For more than one reason (see other LF-designs on my web site) I choose the implementation with the op amp: OPA134.
At 1 kHz its open loop gain is 80 dB so that the overall feed back is near 40 dB if the RIAA-correction is built in the feed back! Figure 3 shows the diagram and figure 4 the simulated frequency charac-teristic.
The output impedance of the OPA134 (open loop 10 Ω) is much smaller than R3, and R1 = 0,01.R3, so that the curve will meet the RIAA-correction within 0,1 dB.

4.1. An Update

To prevent any resonance of the cartridge induction L_c, it could be damped with a resistor R_p in parallel. If we choose R_p = 2πf.L_c for
f = 2120 Hz, and remove C3 we get the same RIAA-correction! With L_c = 525 mH and R_p = 6,3 kΩ, f = 2120 Hz.

Moreover this load of the cartridge very well could realize some electro-mechanical damping of the stylus.

Figure 5 shows the effect of R_p. Even the rest capacitance of the cable inside the tonearm (25 pF) does not influence the frequency characteristic.

4.2. Final Diagram

To conclude the diagram of the built-in phono pre amp is shown in figure 7. The frequency characteristic is shown in figure 8. Compared to the original RIAA-curve in figure 4, it is only 1 dB lower over the total frequency range without any linear distortion.

5. Listening

Compared to expensive moving coil cartridges, the GP620-with-the-new-pre-amp sounds about the same. The compliance is larger and the sound is so clear and transparent that I do not want to change with any moving coil cartridge with their typical idiosyncrasies.

6. Conclusions

  •   An op amp (read: OPA134) is very well suited for implementing a phono pre amp.

  •   The large inductance of an MD-cartridge does not digest a capacitive load of more than some 30 pF.

  •   To avoid capacitive load of more than 30 pF, put the phono pre amp underneath the turntable close to the pivot of the tonearm.

  •   To avoid any resonance, the cartridge could best be loaded with a parallel resistor. If this resistor has the right value, it will replace the
      75 µs time constant.

  •   It could be, if the magnetic coupling between the stylus-magnet and the coil is strong enough, that an extra progressive damping of the
      stylus movements takes place.

  •   Compared to the 'old situation', the sound of the system has been enhanced tremendously.

7. Last Remarks

If you should simulate the cartridge loaded with a cable of, say, 200 pF and a parallel resistor of 6,3 kΩ, you will find nearly the same curves, but it sounds totally different!
Don't ask me why.
Simulations can bring you to splendid ideas but the proof of the pudding is the eating. Always listen to what you are doing!

This pre amp could very well be followed by a BUF634 if the load is too heavy due to a long cable and/or low input impedance of the following system. For details see 'A Solid State Headphone Amplifier' on this web site.

This pre amp could be built for every MD-cartridge! The value of R_p depends on the induction L_c of the cartridge. As long as R_p = 2.π.2120.L_c and the original 75 μs time constant has been removed, it will work!