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PTT6.5X08-NFA-01 6.5″ Ultra Low Distortion Long Stroke Driver 8 Ohm

The PURIFI PTT6.5X08-NFA-01 is a refined bass-mid woofer, incorporating state of the art technology that is based on extended research. The result is this ultra linear long-stroke woofer with unparalleled performance. 

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The PURIFI PTT6.5X08-NFA-01 a is a refined bass-mid-woofer, incorporating state of the art technology that is based on extended research. The result is a long-stroke woofer with unparalleled performance.



  • A whole new take on driver design
  • PURIFI’s Natural Surround
  • Extreme low distortion over full excursion range 
  • Engineered and manufactured in Denmark
  • Excellent midrange performance
  • High power handling
  • High linear excursion limit with constant force factor


Product Details

PURIFI PTT6.5X08-NFA-01 bass-mid-woofer 


PURIFI is a fresh company that designs and manufactures extremely high-performance transducers and amplifier modules. Don’t let the freshness fool you; Their leadership team consists out of some well-respected audio technology pioneers. PURIFI developed a deep understanding of many performances degrading aspects for both amplifiers and transducers. Thanks to this understanding the PTT6.5X08-NFA-01 is the most refined bass-mid woofer from our woofer category.


Besides this 8 ohm version, there is also a 4-ohm version and a 6,5" passive radiator available. Looking for similar performance, but something smaller? Take a look at the 4" PTT4.0W04.


The challenge 

Loudspeaker drivers are known to add a large, if not the largest, amount of distortion to an audio signal chain. Reducing this distortion is a challenge with the complication of fine-tuning to improve one part (e.g. harmonic distortion) often results in a degradation of another part (e.g. intermodulation distortion). These distortions consist out of several separate distortion mechanisms. By carefully mapping and completely understanding these mechanisms, PURIFI succeeded in improving the overall performance by a large degree.


A whole new take on driver design

PURIFI developed mathematical models for the motor system, suspension, cone, and surround. These models allowed for the optimization of each distortion mechanism separately. This systematic approach reduced all distortions mechanisms and keeps intermodulation distortion low. The result makes that the PTT6.5X08-NFA-01 is a compact 6.5” driver that parallels or even surpasses the bass performance of a low excursion, a large surface driver, while it also excels in midrange performance.


The mechanism


Low force factor modulation

Current through the voice coil is not distorted by cone movement. Gives undistorted midrange regardless of the excursion

Constant force factor over excursion - Bl(x)

Prevents amplitude devotions of midrange output during the large excursion (sometimes called ‘burbling’)

Low surround radiation distortion

Reduces harmonic and intermodulation distortion caused by surround

Low magnetic hysteria distortion

Reduces harmonic and intermodulation distortion caused by magnetic traces left by the previous cycle



Q: Should I go with this 4 ohm or with the 8-ohm version?

A: Both woofers have similar distortion performance and the T/S parameters are alike. The 4-ohm coils of the PTT6.5W04 allow more current to flow on a similar drive voltage. This gives higher sensitivity and is preferred in most applications. If you have an amplifier that is not comfortable with low impedance loads or has specific requirements, such as parallel woofers, this PTT6.5W08 might be more favorable.


Q: Why is this surround odd shaped?

A: This is the PURIFI Natural Surround. Conventional surrounds create distortion as they deform. The shape of the Natural Surround reduces cone edge resonances and distortion. It might be odd-looking, but form follows function here. A more detailed look shows that the surround actually represents a complex sound wave. We are told that the Natural Surround is also called ‘wormy’ by insiders.



8 Ohm
Driver type
86.9 dB @ 2.83V/1m
Frequency range
Resonance frequency (fs)
31 Hz
DC resistance (Re)
6 ohms
Mechanical Q (Qms)
Electrical Q at Fs (Qes)
Total Q factor (Qts)
Speaker diameter
Power handling
Voice coil inductance (L)
0.53 mH @ 1kHz
Effective mechanical mass incl air load (mms)
24.4 g
Effective piston radiating area (Sd)
132.7 cm²
Force factor (BL)
Equivalent volume of compliance (Vas)
26.4 liter
Overall diameter
176 mm
Mounting diameter
145 mm