Support for Panasonic strain-gauge cartridges for CD-4

This feature is due for release in Version 3.n It is limited to registered users of Stereo Lab Audiophile version.

In a strain-gauge cartridge, the stylus is coupled directly to a transducing element made of silicon of a similar composition to the base region of a transistor. A constant current, supplied by special preamplifier, flows through the semiconductor and the movement of the stylus is translated into changes the resistance of the element so that the voltage developed across it is directly used as the audio signal.

Strain-gauge cartridges were popular for use with CD-4 records because the output impedance is low and non-inductive; unlike a moving-magnet cartridge. Without very careful selection of ancillary equipment, the inductance of the moving-magnet type cartridge can interact with cables and the preamplifier terminating impedance to destroy the high frequency performance upon which CD-4 performance crucially depends.

By contrast, the high frequency performance of the strain-gauge cartridge is excellent as the frequency-response curve for the Panasonic EPC-450C-II strain-gauge cartridge illustrates.

Another advantage of this type of tranducer for CD-4 is that the strain-gauge cartridge is pressure-sensitive so that the displacement of the stylus controls the resistance of a conducting channel. This enables the decoder to dispense with RIAA correction as explained here.

Stereo Lab now supports needle-drops made with strain-gauge cartridges like the Panasonic EPC-450C-II. The option to decode a CD-4 needle-drop made with this type of cartridge is engaed via a check-box on the CD-4 preferences tab.

Support for Panasonic strain-gauge cartridges (and indeed other displacement-sensitive cartridges) for non-CD-4 duties has been available in Stereo Lab for some years.

To invert, or not to invert...

It is a curiosity of the Panasonic strain-gauge cartridges that that the left/right signals presented to the preamplifier are out-of-phase. This anomaly must either be corrected in the hardware preamplifier or by inverting the phase of one of the channels in the software decoder.

Although phase-inversion in software is a trivial process, it makes more sense to perform the phase-inversion in the hardware, otherwise the preamplifier is only good for needle-drops into Stereo Lab. You can't listen to audio (seriously) without this phase inversion and even record-monitoring whilst making needle-drop recordings is uncomfortable.

This phase-inversion appears to complicate the requirement for a preamplifier for the Panasonic cartridges, but this is deceptive. Because of the requirement to provide a constant-current supply to the strain-gauge cartridge, a special preamplifier is required anyway.

One such preamplifier is made by our sister company. A version of the Groove Sleuth preamplifier (which was designed for use with Stereo Lab) is available which supports Panasonic strain-gauge cartridges.

Contact sales@phaedrus-audio.com for more information about the strain-gauge option for the Groove Sleuth preamplifier.

Links

For all support issues, go here.

For Pspatial Audio sales, email: sales@pspatialaudio.com