Cartridge tests

Frequency response of a modern Moving Magnet pickup cartridge, an Ortofon 2M Red.

FREQUENCY RESPONSE

WHAT IT TELLS US

Pickup cartridge frequency response is a good guide to tonal balance and accurately tells us what to expect, since correlation between measurement and this aspect of sound quality is good. Although a transducer, like a loudspeaker, a pickup cartridge is far simpler. An electrical generator feeding a load, it offers a perfectly flat frequency response up to 2kHz or so.

Typical MM pickup cartridge generator and, below, its frequency response. Simulation by LTSpice.

Old (1970s) Moving Magnet cartridges, like the Shure M97xE still available today, had falling high frequency output into the standard 47k load, producing a warm tonal balance and an easy, amenable sound.

Shure M97xE has falling treble and a warm sound.

Modern cartridges like the Ortofon 2M Red shown above have rising midband output and a more forward and detailed sound as a result. Frequency response measurement reveals such differences.

What is called ‘tip mass resonance’, a mechanical resonance between stylus tip effective mass and vinyl compliance, occurs around 18kHz and peaks up treble sharply. In old cartridges this restored upper treble. In modern cartridges, notably Moving Coils, it makes for a bright sound overall and this can emphasise distortion from poorly recorded / cut or damaged LPs, and on inner grooves.

Audio Technica AT-F3 Moving Coil cartridge with treble peak caused by tip mass resonance.

HOW WE MEASURE IT

We measure frequency response using the JVC TRS1007 test disc. Cut at half speed, it is known for its accuracy and is commonly used by manufacturers, so our results compare well. Most other test discs give significantly less accurate results, often wildly so, and results are neither correct nor comparable. TRS1007 is played through a custom designed preamplifier and equalisation network, as specified for the disc (test discs are rarely cut to RIAA curve). The preamplifier also provides a range of capacitive loads from MM cartridges and resistive loads for MCs so we can assess load sensitivity.

JVC TRS-1007 test disc, cut at half speed and very accurate.

Our published curve, recorded by the one-sixth octave analyser of a Clio measurement system, shows lateral frequency response close to outer grooves as a greet trace, and the same plot on inner grooves is shown in red. Tracing loss caused by the stylus being unable to fully read the narrow mechanical wavelength of inner grooves causes output to fall away above 10kHz. However, with a good modern stylus inner groove loss can be very small, just a few dB.