Primus inter pares - displacement-sensitive phono cartridges

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 of a similar composition 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. A constant current, supplied by a base-station, 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.

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 force applied to 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 groove.

Both these cartridge types, apart from arousing curiosity, and despite certain technical limitations, have been received to great acclaim by the audio press who appear to attribute their particular quality to the relatively modest degree of equalisation required by a cartridge of this type and for the concomitant simplification of the replay electronics.

Given this reputation as the primo inter pares in the world of high-end phonograph cartridges, we believed that equalisation for these cartridge types should be included within Stereo Lab.

This equalisation parsimony isn't new. Older displacement-sensitive ceramic cartridges dispensed with electrical equalisation entirely and generations of small record players used the signal directly from the 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.

Circuit diagram of a record player by Dansette in which the ceramic cartridge directly drives the single valve amplifier (via the volume control).

Why does a displacement-sensitive cartridge not require equalisation?

The truth is that it does! But the equalisation is not the well known RIAA type equalisation and it is 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 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,

75μs318μs = 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.

This reduction in the required degree of equalisation liberates the cartridge designer to opt to use mechanical engineering to affect the necessary tone-control. If the designer chooses to take this approach, the pickup (as it was with the ceramic cartridges of the past) is contrived to use the dimensions and materials of the drive, back clamp and damping so as to give a mechanical transfer characteristic that is the inverse of the recorded amplitude characteristic1. In other words, mechanical equalisation may be employed, instead of electrical equalisation. In broad terms we can say that the cartridge designer arranges the various mechanical resonances involved with driving arrangement of the stylus pushing and pulling on the transducer to compensate for the treble-shelf.

It is this use of mechanical resonance to effect the tone-modification which had led some people to believe that no equalisation is required when a displacement-sensitive cartridge is employed. This is incorrect. True, electrical equalisation may not be present but there is a real need for equalisation of some sort. The degree to which this form of equalisation is successful is demonstrated by the measured response of the Panasonic strain-gauge cartridge (EPC-450C-II) from the 1970s (and designed to play Quadraphonic recordings).

In the case of the photo-sensitive, light-valve type, electrical equalisation may be required as it, unlike the strain-gauge or the ceramic type, does not have to act physically upon the transducer and thus a mechanical filter may be inconvenient or impossible to contrive.

The bass shelf

There exists a slight complication, indicated by the rising response in the graph above below 50Hz (time constant 3180μs). This is known as the bass-shelf which is the defined attenuation limit of the turnover filter. Below this frequency, in the lowest octave of the audio range, the signal inscribed on the disc returns to constant-velocity and the signal derived from a displacement-sensitive cartridge rises. To compensate for this, the playback equaliser must incorporate a high-pass filter. The implementation of this filter has been criticised in the hardware equalisers which accompany displacement-sensitive cartridges2. This problem is evident from the frequency response of the Panasonic EPC-450C-II shown above in which the rise in response below 50Hz is very clear.

Stereo Lab

The designer's option to employ mechanical equalisation (or not) forces a certain complication of how to deal with needle-drops recorded using displacement-sensitive cartridges.

In software, of course, it is easy to implement the bass-shelf filter with great precision so this is included as part of the equalisation of displacement-sensitive cartridges in Stereo Lab; especially since this seems to be an issue with all types.

But do we include the high-frequency shelf filter or not? Clearly that depends on the make of cartridge and whether it is possible to record the unequalised signal (if electrical EQ has been employed).

In the end, we took the opinion that, because modern pickups are required to have vertical tracking weights of the order of 1.5 grammes or less (to ensure minimum record wear), the use of parallel mechanical resonances to control or influence the frequency response must be questioned since they reduce the driving point impedances and work against proper tracking at this restricted downforce.

We hope therefore that, by including precision equalisation for displacement-sensitive cartridges in Stereo Lab, we will encourage designers not to opt for mechanical equalisation.

There are two versions of EQ-P (as this EQ is named) in Stereo Lab.

  • EQ-P1 - bass-shelf filter and full treble EQ (+12.5dB). This assumes a displacement-cartridge with flat response with no mechanical equalisation.

  • EQ-P2 - for use with the DisC, Phædrus Audio's, innovative displacement-sensitive cartridge design.

Notes and references

1 At the same time taking into account the fact that the mechanical impedance of the transducing element 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.

2 Shedding Light - an Optical Phono Cartridge. Stereophile September 2015


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