Designed and manufactured in Denmark, the PTT6.5X04-NAA-08 and PTT6.5X08-NAA-08 are the new aluminum-cone members of the famous PTT6.5X extended-stroke product family. As the company details, Purifi’s research has identified the parameters that have so far prevented extended stroke drivers from breaking through high performance audio parameters. "The distortion of the acoustic output of a driver is a combination of several separate distortion mechanisms in the motor, cone and suspension. When testing a complete driver with sine waves, a situation often arises where a small tweak seemingly improves harmonic distortion (HD) by letting two mechanisms counteract each other. This always leads to a clear worsening of intermodulation distortion (IMD) that becomes obvious when testing the unit with a more complex signal. This is why traditional HD tests fail to predict subjective sound quality."
Purifi takes care to optimize the various distortion mechanisms separately, thus guaranteeing that any measured improvement truly reflects a real audible improvement no matter what the signal is. Accurate mathematical models are developed for the motor, suspension and the vibroacoustics of the dome and surround. These models clearly explain several distortion mechanisms in each domain, which provides fundamental insights into better ways of constructing the motor, cone and suspension. Finally, the same models are used to numerically fine-tune the geometry. This way of working reliably reduces multiple distortions mechanisms which secures low IMD for complex signals.
The Purifi engineering team targets Low Force Factor Modulation to prevent voice coil current from modulating Force Factor (Bl). The Purifi motor completely avoids this classical Achilles’ heel of long stroke drivers. This translates into low intermodulation (IMD) for clean, undistorted midrange even in the presence of a heavy bass, and equates to low impedance modulation, meaning that drive current is not distorted by cone motion.
Another area of attention is to achieve Very Constant Force Factor over Excursion, to prevent voice coil position from modulating the force factor. This is the classical cause of “burbling” i.e. amplitude modulation of the midrange by large low-frequency cone excursions. And because the surround also contributes to sound output and produce distortion as they deform, Purifi's Neutral Surround geometry reduces harmonic distortion as well as intermodulation distortion, without constraining motion.
As detailed in the company's research, another major goal for Purifi is to achieve Low Magnetic Hysteresis Distortion. "Hysteresis means that magnetic domains in iron retain traces of previous magnetization / demagnetization cycles which cause distortion when magnetized or demagnetized again. This distortion masquerades as benign harmonic distortion when tested with a sine wave but takes on a crackling or noisy character with more complex signals. Removing hysteresis distortion translates into a very “fluid” presentation with excellent front/back separation in the stereo image and a perfectly black background between instruments".
The combination of the above characteristics results in a compact long-stroke driver that delivers complex sound with a clarity and lack of effort that was previously the exclusive province of large-cone short-stroke drivers. Additionally, thanks to the small cone size, the driver has excellent midrange reproduction, and is highly suited for two-way systems.
The Purifi PTT6.5X04-NAA-08 (4 ohm) and PTT6.5X08-NAA-08 (8 ohm) woofers are rated for 250W IEC power handling, featuring a pleasing black anodized aluminum cone, all designed and manufactured in Denmark.
www.purifi-audio.com
