PHLO-II

The PHLO-II is designed to facilitate the integration of the PHÆDRUS AUDIO PHLUX / PHLUX II active phono cartridges into a standard hi-fi system or computer audio-capture system.

This design is made available under the terms of the CERN OHL v.1.2 open-source hardware licence.


BACKGROUND

The PHLUX (I / II) phono cartridge differs from other phonograph pickups in that it has a small head-amplifier integrated with the moving-magnet generator assembly. This integration affords several benefits in terms of extending and levelling the frequency-response and reducing inter-channel crosstalk. Details are available at PHÆDRUS AUDIO's website:

http://www.phaedrus-audio.com/PHLUX.htm

The PHLUX head-amplifier is designed so that the necessary power is supplied via the turntable signal cables in much the same way that phantom power is supplied to a capacitor microphone. In this way, opting to use the PHLUX (I/II) cartridge requires no modification to the wiring of the turntable. However, power must clearly be provided to the cartridge at some point in the signal chain and, although it is quite possible to modify an existing phono stage preamplifier to provide this source of power, many hi-fi enthusiasts are wary and reluctant to modify expensive equipment with the attendant issues of voiding warranties.

To this end, the new PHLUX II cartridge is supplied with a small PCB where power may be injected from a PP3 type, 9V battery. This minimalist solution is excellent for debugging and may be arranged to offer good permanent performance. However, for the very best performance, it is better to combine the functional requirement to supply phantom-power with an active amplifier stage, so that the small PHLUX head-amplifier is not required to drive long connecting cables and the (often deliberate) capacitance of the phono input stage. The PHLO-II PCB is designed to fulfil this requirement.

BUFFER STAGE

In this build option, the PHLO-II is configured to act as a high-performance buffer as well as providing the necessary phantom-supply to the head-amplifier within the cartridge. This configuration provides the ideal matching from PHLUX (I/II) cartridge to a standard MM phono input.

NON-EQUALISING PREAMP

By varying the build of the PCB slightly, the PHLO-II may act as a high-performance, non-equalising phono preamplifier which may feed a computer sound card to capture needle-drop recordings made with the PHLUX (I/II) cartridge for subsequent equalisation in the digital domain. Not only has it been demonstrated that the frequency-response accuracy, dynamic range and distortion of such a conversion in the digital domain can outstrip any analogue implementation, the flexibility of software implementations ensures that accurate equalisation may be applied to records made before with widespread adoption of the RIAA recording characteristic in the 1960s. Furthermore, sophisticated noise and "click" reduction algorithms may be applied to the needle-drop recordings to render an audio experience superior to any straightforward analogue playback. See the link below for further details concerning the capture and subsequent equalisation and noise and artefact reduction using digital signal processing.

http://pspatialaudio.com/

CIRCUIT DESCRIPTION

The PHLO-II is a very simple circuit built around a high-performance, low-noise operational amplifier: the OP284 from Analog Devices. Other, less expensive, op-amps may be employed - for example, the ubiquitous NE5532 may be employed in buffer-mode. However, the OP284 is strongly recommended and is essential in the non-equalising preamplifier configuration where the rail-to-rail performance of the OP284 is required.

The operational amplifier type is dual-sited so that a conventional, DIP amplifier package or a surface-mount 8-pin SOIC package may be fitted. (The OP284 is normally considerably cheaper in the SOIC package.)

Power to the PHLUX (I/II) head-amplifier is provided by resistor chain R203L/R and R204L/R. C205L/R act to decouple the supply. The DC-sit of the PHLUX head-amplifier is approximately 3V and thus the signal may be DC coupled directly to the op-amp.

With R205L/R linked out (or a very low resistor value fitted) and R206L/R and C206L/R not fitted, the op-amp acts as a simple buffer, the signal being AC coupled to the output via C207L/R.

If R205L/R are fitted as 10k and R206L/R and C206L/R are fitted, the PCB acts as a non-equalising preamplifier and the signal level (with a gain of 33dB) is entirely sufficient to drive a computer sound-card line-level input for subsequent signal processing in software.

DC POWER SUPPLY

The supply current to the amplifier stage is almost entirely a function of the op-amp, the consumption of the head amplifier in the cartridge being only about 260uA (both channels). With the OP284 fitted, the quiescent current draw will less than 3.5mA on a single DC supply of between 9 and 15V. The power supply needs only to relatively stable and noise-free as the precision supply to the head-amplifier is heavily smoothed on the PCB itself. A simple battery may be employed. For example, a modern, lithium, PP3 type battery will offer many hundreds of hours of noise-free service. Alternatively, a plug-top DC power supply may be employed to provide power to the amplifier stage; although most modern units are switching supplies and may not offer the best possible noise-performance. The DC supply should be fed in at point "+12V IN" and "GND".

AC POWER

On the same PCB, but electrically separate from the amplifier stage, is a small, regulated power-supply which will accept a 12 - 18V AC input from the secondary of a small mains transformer. The transformer secondary should be connected to points: "+12 TO +18V AC" and "COMMON". Being a linear supply, this will guarantee very low noise and provides the convenience of allowing an entirely stand-alone, mains-powered unit to be constructed. When this PSU is employed, the output voltage from the regulated power supply "+12V OUT" should be connected to point "+12V IN" (the ground is connected via the PCB).

MOUNTING

The PHLO-II PCB has four fixing holes, all of which are isolated from signal ground.

FINAL THOUGHTS

The PHLO-II is intended to aid the integration of the PHÆDRUS AUDIO PHLUX / PHLUX II active phono cartridges into a standard hi-fi system or computer audio capture system. The amplifierís various power supply options, its small size and flexible mounting mean that the PHLO-II PCB will be useful in a variety of applications and environments. For example, it may be easily integrated within an existing preamplifier housing and fed from an internal, positive power-supply rail, or it may be mounted in a small, inconspicuous housing and fed from either a battery, a standard plug-top DC supply, or from the mains.

The PHLO-II open-source hardware design is licensed under the CERN OHL v.1.2 licence. See:

https://www.ohwr.org/documents/294

Right click on this link to download a zip file of all the hardware design files for the PHLO-II PCB.

Raw (un-stuffed) PCBs will be available from Phædrus Audio. See prices.

Contact sales@phaedrus-audio.com for more information.




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