The Jantzen Audio JA-8008 HMQ™ is a very special 8” paper cone midwoofer, designed by Danish loudspeaker designer Troels Gravesen. Manufactured exclusively for Jantzen Audio by SEAS in Norway. A very popular design used by both OEM manufacturers and as part of the bestselling DIY speaker kits, designed by Troels Gravesen. This re-invented “vintage” woofer has received praises from audio professionals and DIY aficionados alike. Improvements compared to the previous model (JA-8008 HES):
New voice coil former
Improved surround
New phase plug
Increase magnet size
Driver features, as described by Mr. Troels Gravesen:
Voice coil former:
The voice coil available at that time was wound on an aluminum voice coil former. Aluminum voice coil formers introduce damping of cone movement by shorting eddy current around the magnet gap. The result was low mechanical Q, actually 1.7, which is not too bad, but one thing that can characterize vintage drivers is low mechanical loss and a more vivid and dynamic presentation. I write can characterize, because I've had vintage drivers that did not have high Qm despite paper voice coil formers and "low loss" suspensions. Low loss suspensions were sometimes added some viscous resin to strengthen the suspension and ensure long term stability, but also had the impact of introducing a lot of damping to cone movement. The aluminum voice coil former is now replaced by a fiber glass voice coil former with similar properties of e.g. Kapton and titanium. The result is a huge increase in Qm, actually I measure Qm = 10 from the driver samples fitted with a center phase plug.
Surround:
Foam surround is one of the best surround materials available when it comes to low loss suspensions. This primarily due to low mass and excellent elastic properties, and when applied a suitable coating it will also have a reasonable life span. What was not available back then (stock item) was foamed rubber surrounds as an alternative to coated foam. Foamed rubber provides lower mass compared to the standard rubber used for bass drivers and in many ways properties similar to foam surround. The new JA-8008 HMQ is fitted with a narrow (low mass) foamed rubber surround added a damping ring to reduce the all too common ~800 Hz resonance.
Phase plug:
Phase plugs were actually available at the time the previous JA-8008 HES model was put together, but discarded as it did not offer any sonic advantages to the paper dust cap. With the current HMQ driver the phase plug offers the best frequency response and the highest mechanical Q, thus phase plug it must be. The phase plug is held in place by a solid copper ring above pole piece.
Magnet size:
The magnet size has been increased on the new driver to compensate for slightly increased mass of suspension and loss of radiating area of cone (no dust cap) and we maintain an efficiency of 94 dB/1W/1m making 95 dB/2.8V/1m sensitivity
Specifications
resonance frequency fs = 46 Hz
impedance R = 6.07 Ohm
sound pressure level SPL = 94.3 dB (2,83V; 1m)
DC resistance Re = 8 Ohm
force factor BL = 11.1 Tm
effective piston radiating area Sd = 219 cm2
effective mechanical mass incl. air load mms = 18 g
equivalent volume of compliance Vas = 44.3 l
total Q factor Qts = 0,25 (Qms=10.0, Qes=0,26)
voice coil diameter = 12 mm
maximum peak linear excursion vibration xlin = +/- 3 mm
