1. Introduction
In the early 80's the CD-player has been developed with Philips. At those times also shape had been
given to a new phono MM-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! |
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2. Simulation of Cable Connection
In figure 1 is the simulation of the cartridge with a 200 pF cable connected to it. As expected the inductor in the cartridge resonates together with this capacitance. In spite of the rather large series resistance of the coil, the resonance peek is more than 35 dB at 15,4 kHz. This sounds horrible! 3. The RIAA-correctionThe RIAA-correction dictates poles at: |
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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 an OPA134. |
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4.1. An Update
To suppress the resonance of the cartridge induction Lc with the capacitance of the connected cable, it could be damped with a resistor Rp in parallel.
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4.2. Final Diagram
To conclude the diagram of the built-in phono pre amp is shown in |
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5. ListeningCompared 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 as worse tracking. Much later (in 2018) when I finished my loudspeaker system: '2 BMRs in een Baffle', the performance of the piano concertoes of Bartók on Deutsche Grammophon 2LP Stereo 2726005 sounded comparable to a CD on my highly upgraded CD player and SSA35 amplifier. |
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6. Measurements
The frequency characteristic is flat from 20 Hz up to 14 kHz within 1 dB. Over 14 kHz the amplitude increases gradually to +4 dB at 18 kHz, |
7. Conclusions
  •   An op amp (read: OPA134) is very well suited for implementing a MM phono pre amp. |
8. Some RecordingsTo get an idea of the quality of the cartridge and the pre amp, some recordings are presented here. Listen the 3D sound stage and the wet water in the sauna! |
9. Last Remarks
If you would simulate the cartridge loaded with a cable of, say, 200 pF and a parallel resistor of 5,6 kΩ, you will find nearly the same curves, but it sounds totally different!
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May 12 - 2013 june 27 - 2021 Herbert Rutgers. |