PHLUX-II - second-generation active phono-cartridge

Introduction

The Phædrus Audio PHLUX active phono cartridges represent a new development in phono cartridge technology in which a moving-magnet motor system is buffered by a miniature impedance-converter thereby securing the performance of a moving-coil cartridge with the superior tracking and general convenience of the moving-magnet type. PHLUX-II is the second generation product and replaces the original PHLUX.

The miniature impedance-converter which buffers the moving-magnet motor is supplied power via the signal wires, in much the same way a capacitor microphone is supplied with phantom power. By this means, the PHLUX-II may be fitted to any turntable and arm combination without modifications to the wiring.

The Phædrus Audio PHLUX-II active phono cartridge was developed for Pspatial Audio's Stereo Lab needle-drop capture and equalisation software. They explain in the software help-files what prompted the search for the best phono cartridge possible to ensure accurate needle-drop recordings.

A version of the PHLUX-II is available for enthusiasts of historic recordings. This version is called the PHLUX-II Libre and has a modified body which enables it to mount standard and aftermarket styli. A Shibata stylus, various hyper-elliptical types and a styli for early, mono or Quadraphonic LPs and 78 RPM records are all available, see pricelist for details of order codes.

A technical article on the benefits of active cartridge technology is given below as well as applications information for those wishing to use the PHLUX-II.

The PHLUX-II cartridge is supported by the Open Source Hardware project PHLO-II.

You can buy the PHLUX-II with or without an accompanying base-station directly from Phædrus Audio. Contact sales@phaedrus-audio.com

You can download the PHLUX-II manual here.


Specification Phædrus Audio PHLUX-II

Note that the PHLUX active cartridge is the subject of UK Patent application GB1517805.6


Technical description

The advantage of the greater output signal voltage from the moving magnet pickup is offset by the nature of the source impedance which rises with frequency. This creates a number of practical problems.

Firstly, the cartridge must be terminated with a resistance some 500 to 1000 times greater than the termination of the moving coil type or the high frequency response is unacceptably attenuated. As frequency increases, the increasing reactance of the cartridge fails to damp the electronic noise due to these high termination resistances, and this sets a lower bound on the noise performance of the moving magnet cartridge to a level where the electrical noise is not so far below that of the analogue medium that it may be said to be insignificant.

Secondly, the capacitance of the connecting cables interacts with the cartridge to produce a resonant network with a frequency response peak in the audio-band where the impedance of the pickup inductance and cable capacitance are equal and opposite.

Thirdly, the higher impedances of the moving magnet pickup circuit compromise stereo performance due to crosstalk between the right and left signals in the unscreened wires of the cartridge and tonearm.

Active circuitry

All the disadvantages of the moving magnet pickup may be eliminated by means of a lightweight, electronic amplifier close to, or inside the pickup; power to which is provided via the signal wires. This amplifier:

  • Isolates the moving magnet pickup from load effects which affect frequency response.
  • Isolates the inductive nature of the MM cartridge source-impedance and thereby secures a signal to noise ratio similar to the moving coil type.
  • Reduces crosstalk in the wiring of the tonearm.

In fact, it is possible to secure an electrical performance from the moving magnet type pickup which approaches or surpasses the moving coil type and combine these virtues with the lower cost, better tracking of the moving magnet pickup.

Test Results

The wideband, RMS noise-voltage, measured with an AC voltmeter post-RIAA equalisation with the active circuit is 8dB below the noise of the standard moving-magnet arrangement. The figures below illustrate the spectrum of the noise post RIAA equalisation. Listening tests confirm that, once the tonearm is removed from the record there is no perceptible noise from the loudspeakers; just as with a moving coil cartridge.

Residual noise (tonearm parked) standard moving magnet cartridge: 24 bit recording. With 0dBFS calibrated to 50cm/s, the 16 bit noise floor is indicated. This analysis is post RIAA equalisation.

Residual noise (tonearm parked) moving magnet cartridge with lightweight amplifier in circuit: 24 bit recording. With 0dBFS calibrated to 50cm/s, the 16 bit noise floor is indicated. This analysis is post RIAA equalisation.

Listening tests also reveal that gone too is any hint of the "splashy" top-end often associated with the moving magnet cartridge. Measurements confirm the frequency response dependence on capacitive loading is entirely eliminated when the lightweight amplifier is fitted. The familiar "hump" in the frequency response of a moving magnet cartridge is removed. Predictions from the SPICE model in the first figure below are confirmed by the measurements illustrated below that.

Theoretical prediction of response modification due to "activation" of the MM cartridge

Measured response modification (red traces - standard arrangement: black traces - with lightweight amplifier fitted)

And, because there is no LC "filter circuit" to degrade high-frequency group-delay, the result is transient handling which compares with a moving coil type.

Phase response of standard and active MM compared

The biggest surprise noted in the listening tests was the alteration of the stereo image which was radically improved with the impedance converter fitted. Measurements on the AT95E show that inter-channel crosstalk improves by >6dB over a sizeable portion of the audio band with the amplifier fitted.

Response and crosstalk standard MM cartridge (pink noise tracks spectrally "whitened" prior to analysis)

Response and crosstalk in MM cartridge fitted with lightweight amplifier (pink noise tracks spectrally "whitened" prior to analysis)

The illustrations above also show that the measured frequency response when only one channel is driven is also greatly improved with the lightweight amplifier present and remains essentially flat, rather than rolling off by about 12dB at 20kHz which the AT cartridge demonstrates under standard conditions. In other words, the frequency spectrum of an element in the mix remains constant irrespective of its position in the stereo image. It is probably this which accounts for the remarkable transformation of the stereo image with the lightweight amplifier fitted.

Technical conclusions

In short, the sound quality of a standard moving magnet cartridge is transformed when the lightweight amplifier is fitted. All the virtues associated with a moving coil type are secured whilst retaining the significant advantages of the moving magnet type: lower cost; replaceable stylus; and often better tracking due to the very lightweight motor mechanism. Listening tests made with other, much more expensive, moving-coil cartridges were not preferred to those made with the PHLUX-II cartridge.


Applications information

A "one-stop-shop" solution to use the PHLUX-II with any pre-existing hi-fi system is given below.

Otherwise, supporting the PHLUX-II is relatively simple and virtually all phono preamplifiers may be modified easily to supply adequate power for the cartridge which only consumes a minute 0.6mW. A typical circuit is illustrated below.

Power to the PHLUX-II is supplied from rail Vp via resistor Rp. Vp must be +9V or greater and the value of Rp is calculated with the equation,

Rp = ( Vp - 3.5 ) / 0.08

Where Vp is in volts and the result is directly in kilohoms. Thus, for example, if Vp is +9V, Rp will be 68kΩ.

Note that the standard values of 47k and approximately 220p for Rt and Ct respectively are not ideal. It is better if Rt is as high as possible (commensurate with providing adequate bias to the amplifier). The presence of Ct in this circuit is not ideal as it simply serves to reduce the stability margin of the impedance-converter circuit and it is better removed.

Also note that the signal sits on a bias of approximately +3.5V which may affect the choice, and possibly the polarity, of C1.

A practical design is given below in which the phono preamplifer may be selected to be in standard moving-magnet mode or PHLUX-II mode by moving a link.

PHLUX-II and PHLO

To avoid modifying a preamplifier to support the PHLUX-II, we have made the PHLO base-station board available. This tiny (40mm x 10mm) surface-mount PCB may be fitted inside the preamplifier or any small housing. It only requires a single-rail nominal +9V supply. A 9V (PP3 / PP9) battery is very suitable and certainly a good place to start. The current consumption for the PHLUX-II and PHLO together is only 9mA, so a PP9 will give 1000 hours service.

The PHLO contains the pull-up resistors to provide power to the PHLUX-II active cartridge and a wide-bandwidth, very low distortion buffer amplifier. The output is AC coupled. The PHLO may thus drive any moving magnet preamplifier input. A wiring diagram is available here.

Important note
The PHLO is supplied fully built and tested. Nevertheless, it made available to competent enthusiasts and professionals. Using this component requires the ability to solder and to interpret correctly the signal and power-supply requirements. Phaedrus Audio will only supply limited debugging support for the PHLO.



Groove Sleuth MINI

The first commercial phono preamplifier to support the Phædrus PHLUX-II is the Groove Sleuth MINI from Pspatial Audio. The Groove Sleuth MINI is a cost-effective non-equalising preamplifier for audiophiles. It is non-equalising because it is designed for equalising needle-drops in Pspatial Audio's Stereo Lab software.

However, the Groove Sleuth MINI provides iLOOP (a very low-noise loop-through) and it may thereby be used as a base-station for the PHLUX-II, with the iLOOP signal going to your existing phono preamplifier. Like this....

Whilst the Groove Sleuth MINI may be operated with the Pspatial Audio RESOLVE top-of-the-range power-supply, it incorporates a rechargeable battery and a front panel switch which permits total disconnection of the PSU. By this means, the unit runs solely on low-noise, low-impedance battery power and the mains power-supply cannot influence the audio quality.

Groove Sleuth MINI may be thus used with a low-cost plug-top power supply without any compromise in audio excellence.

The unit has about 24 hours operational charge, but Pspatial Audio recommend battery play be limited to approximately 10 hours. You can buy the Groove Sleuth MINI directly from Phædrus Audio. Contact sales@phaedrus-audio.com

The manual for the Groove Sleuth preamplifier range is here.


PHLUX-II Libre

The PHLUX-II Libre has a modified body which enables it to mount standard and aftermarket styli. A Shibata stylus, various hyper-elliptical types (line contact) and a styli for early, mono or Quadraphonic LPs and 78 RPM records are all available, see pricelist for details of order code for PHLUX-II Libre.



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