Phaedrus Phono Cartridges

PHLUX-II – second-generation active phono-cartridge

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.

Click here for a short video describing the PHLUX-II.

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’sStereo Lab needle-drop capture and equalisation software; a partnership which delivers reproduction of unparalleled accuracy.

As you can see in the specification below and at Pspatial Audio’s Stereo Lab PHLUX-II correction page, a combination of PHLUX-II and Stereo Lab delivers a frequency response accuracy better than 1dB overall variation over the whole audio band.

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 cartridges are 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

  • Generator: MM-active
  • Tracking force: 15 – 25mN; 20 mN (2.0 g) recommended
  • Stylus type: 0.4 x 0.7 mil elliptical diamond, alloy cantilever
  • Cartridge weight: 8.5g
  • Mount: Half-inch, M2.5 tapped
  • Tracking ability (300Hz): 68痠 lateral & vertical
  • Tracking ability (1kHz): 31.5 cm/s peak lateral
  • Tracking ability (2.5kHz): 47 cm/s peak lateral
  • Tracking ability (10kHz): 30 cm/s peak lateral
  • Tip mass resonance: >35kHz
  • Output voltage at 1000 Hz, 5cm/sec: 3.5 mV
  • Load impedance: 47k or greater, 200pF or less (see application information)
  • Frequency response: 30Hz – 20kHz: ±1.5dB† (±0.35dB with Stereo Lab software)
  • Channel separation: 28dB (1kHz)
  • Distortion (300 Hz 45痠): <0.3% lateral; 3% vertical*

† 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:

  • 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 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-II a hardware support 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.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

DisC – displacement-sensitive phono cartridge

Read a new article by Richard Brice about the DisC cartridge and how it works and be sure to visit our virtual listening room.

Or watch a video

DisC Phono Cartridge

In the last few years, there has been great interest in phono cartridges based on strain-gauge transducers in which the stylus is coupled directly to a transducing element made of silicon, similar to the base region of a transistor.

This type of transducer is pressure-sensitive so that the displacement of the stylus controls the resistance of a conducting channel. The movement of the stylus is thus translated directly into a signal voltage.

Rather similar to the old ceramic cartriges of the past, these cartridges are not electrodynamic generators and thus do not require movement of an armature to derive a signal. The transduced quantity is thereby not stylus velocity, as it is in a moving-coil or moving-magnet (or moving-iron) cartridge. Instead, the output voltage is a direct measure of the amplitude of the stylus movement within the groove. 

Another new cartridge type (although, in truth, as with the strain-gauge type, both modern variations on very old themes), is based on light-valves in which the movement of the stylus controls the amount of light falling on a photo-sensor. Once again, this type directly senses the displacement of the stylus as it moves in the groove1.

Away with RIAA!

Both these displacement-sensitive cartridge types, have been received to great acclaim by the audio press who attribute their particular quality to the minimal equalisation required by a phono cartridge of this type and for the concomitant simplification of the replay electronics.

Phædrus Audio DisC

Given the reputation of displacement-sensitive pickups as the primo inter pares in the world of high-end phonograph cartridges, we believed that we at Phædrus Audio ought to see if it was possible to develop a cartridge of this type.

It is….. and the result we have named DisC (for Displacement Cartridge).

A little history….

Displacement-sensitive phono cartridges aren’t new. In fact, there was a time that the vast majority of phono cartridges were of this type. Older ceramic cartridges sensed groove displacement and dispensed with electrical equalisation entirely. Generations of small record players used the signal directly from the “crystal” or ceramic pickup to drive a simple amplifier formed from a single valve (tube). The circuit diagram of one such unit, the Dansette Bermuda is illustrated below.

Go to the link for the reasons why a displacement-sensitive cartridge does not require equalisation. 

 
Circuit diagram of a record player by Dansette in which the ceramic cartridge directly drives the single valve amplifier (via the volume control). No equalisation is required.

The Phædrus Audio DisC brings forward this technology from the sixties to the present day with a phono cartridge which doesn’t need RIAA equalisation.

DisC technology

The Phædrus Audio DisC is neither a strain-gauge, nor an occluding optical-path cartridge. (Nor a ceramic type!) Instead, a moving-magnet cartridge element is operated as an element in a special very low-impedance circuit configuration such that the short-circuit current generated at the output of the cartridge is an accurate analogue of the groove displacement.

This arrangement is especially advantageous because it avoids the mechanical impedance limitations which beset the strain-gauge type – and the ceramic cartridge of yesterday2.

The DisC cartridge thereby combines the advantages of the best of electro-dynamic cartridges with the advantages of the displacement-sensitive type. This includes:

  • Low-impedance operation (and therefore low-noise) – like a moving-coil;
  • Superior tracking; and
  • Low-equalisation requirement of the strain-gauge or optical type.

 

PHATHOM

Recovery of the current-mode signal from the DisC cartridge, suitably equalised, for sending to a standard preamplifier or integrated amplifier is performed by the PHATHOM base-station (the interior of which is illustrated above).

No modifications are required to turntable or cabling to support a DisC cartridge. The cartridge body is tapped, so that it is fixed to the headshell simply with 2 x 2.5mm austenitic stainless-steel bolts (which are supplied).

The DisC cartridge, the PHATHOM base-station (preamplifier) and its associated (RESOLVE) power supply are supplied as a package. Prices are here.

Specification Phædrus Audio DisC

  • Load: PHATHOM base station only
  • Stylus: 0.0003″ x 0.0007″ elliptical diamond
  • Tracking force range: 1.5 to 2.5g (15 to 25mN)
  • Tracking force recommended: 2g (20mN)
  • Mount: ½ inch; body tapped M2.5
  • Vertical tracking angle: 20°
  • Weight: 8g

Note that the PHLUX active cartridge is the subject of UK Patent application GB1705370.3

 

Why do displacement-sensitive phono cartridges not require equalisation?

 

The truth is that they do…. a bit! But the equalisation is not the well known RIAA type equalisation and it is more than an order of magnitude less intense.

 

The cutter head, which engraves the record groove is constructed very like a loudspeaker. It is mechanically coupled to a coil through which the signal currents flow whilst sitting within field of a powerful magnet. Just as with a loudspeaker, it is the bass frequencies which cause the cutter to move the most: the high frequencies cause it to move only very slightly. (If you’ve ever looked at a loudspeaker cone as it reproduces music you’ll recognise this.)

 

Thus, if the record cutter-head is fed a signal with has a constant amplitude with respect to frequency (a signal with a flat frequency response), the cutter will respond to this signal so that, as the frequency increases, and the slope of the signals steepen, the amplitude of the groove inscribed on the disc will fall. We say that the grooove has a constant-velocity characteristic.

 

Now, if this record is played with a cartridge which responds to the velocity of the stylus as it traces the groove (and all modern, electrodynamic cartridges do), it will reverse the effect of the cutter and the output signal will once again has a flat characteristic with respect to frequency. Put simply, the falling response of the cutter is cancelled out by the rising response of the cartridge.

 

This mirror-image nature of cartridge and cutter is why we can speak of complementary recording and playback equalisers. But in doing so, it’s easy to forget that the physical “wiggles” inscribed in the disc do not match the frequency characteristic of either the pre-equalised signal, or the post-equalised signal. They exist in a little-appreciated nether-world.

 

 
The frequency response of the groove inscribed on the surface of an LP record: mostly flat with a modest shelf at high-frequencies.

 

The signal derived from the displacement of the groove when recorded according to the RIAA standard, has a frequency characteristic which has two broad regions of flat-response at high and low frequencies, separated by a step, as illustrated above. The first region is due to the turnover filter in the recording equaliser. This cuts bass frequencies and thereby forces a rising response through the bass-middle frequencies. This complements the falling response of the cutter-head. The second region is due to pre-emphasis, where once again, the rising response complements the falling-response of the cutter. The shelf is caused by the flat part of the recording characteristic (in RIAA, between 500Hz and 2121Hz) where there is no compensation for the falling response of the cutter-head. In fact, the precise value of the offset of this shelf is derived from the ratio of the two upper time-constants of the RIAA standard thus,

 

75μs ⁄ 318μs = 0.23585 = -12.54dB

 

Now, if a cartridge is used which responds directly to the groove amplitude, nearly all that is required to correct a record recorded with the RIAA characteristic is an equaliser which restores the treble section to that of the bass: a signal manipulation which is just 4% of hundred-fold ratio enshrined in the RIAA characteristic. An appreciation of the modest requirement for equalisation may be appreciated by the fact that, in older ceramic type cartridges, this gentle equalisation was achieved with a gentle, mechanical resonance at high-frequencies.

 

Notes and references

 

1 Shedding Light – an Optical Phono Cartridge. Stereophile September 2015

2 The mechanical impedance of the transducing element in a strain-gauge cartridge is several hundred times greater than the permissible stylus tip impedance. It’s no easy matter designing a cartridge in which the stylus ultimately pushes and pulls upon a mass.