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 the electrodynamic motor system is buffered by a miniature impedance-converter thereby securing excellent electrical performance with excellent tracking. 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.

Unparalleled accuracy

The Phædrus Audio PHLUX-II active phono cartridge is excellent in any application. However, it was specifically developed for Pspatial Audio's Stereo Lab needle-drop capture and equalisation software; a combination which delivers reproduction of unparalleled accuracy for all records, at all speeds and at all recorded diameters (see specification below).

Historic value

A version of the PHLUX-II is available for enthusiasts of historic recordings. This version is called the PHLUX-II Libre. Styli for mono (0.001") and 78 RPM records (0.003") are available.

Technical and partnering equipment

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 phono cartridge is supported by the Phædrus Audio GROOVE SLEUTH non-equalising preamplifier..

The PHLUX-II is supplied with an adaptor board manufactured by Phædrus Audio.

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

You can download the PHLUX-II manual here.


Specification Phædrus Audio PHLUX-II

† JVC TRS-1007 lateral; includes variation due to wavelength loss on inner grooves.
* CBS STR-112
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:

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 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

The PHLUX-II phono cartridge is supported by both the Phædrus Audio PHONO preamplifier and the GROOVE SLEUTH non-equalising preamplifier..

A very simple and low-cost "one-stop-shop" solution to use the PHLUX-II with any pre-existing hi-fi system is the PHLUX-II ADAPTOR.

Phædrus Audio has also created the PHLO-IIa hardware supprt and buffer design for PHLUX-II made available under the terms of the CERN OHL v.1.2 open-source hardware licence.

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 ADAPTOR

The PHLUX-II ADAPTOR is available as a low-cost solution to get you "up and running" quickly with a PHLUX-II cartridge. A complete PCB product (built and tested) it provides the necessary power to the active cartridge and the on-going signal to any, standard phono preamplfier. Gold-plated phono connections are available to the turntable and preamplifier.

The low-noise power for the cartridge is provided by a PP3, 9V battery (not supplied). The card has a simple, slide on-switch and the current drain is so minimal that a single battery should provide many months (even years) of service.

Once the cable connections are made, the circuit board operates at a low-impedance point in the circuits and is thus quite tolerant of placement. For the best noise and hum performance, the board is provided with fixing holes (both on the board and on the phono connector mountings) and so may easily be fitted in a small, screened enclosure as a permanent fixture.

The PHLUX-II ADAPTOR is available at minimal cost, see pricelist.


PHLUX-II Libre

The PHLUX-II Libre is the version of the PHLUX-II for historic recordings.

Mono (0.001") or 78RPM (0.003") styli can be fitted.

See pricelist for details of order code for PHLUX-II Libre.



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