Kef LS50 Drive Unit


Up for test is the raw drive unit from Kef’s flagship LS50 monitor.

About a year ago I ran the gamut on some Kef drivers: the HTS3001SE concetric, Q100 5.25″ concentric, and the R-series concentric.  Knowing the lineage points to the LS50 drive unit as a mix of the Q-series concetric with it’s Z-flex surround and the R-series concetric with it’s ribbed cone for reducing breakup, I was curious to see the measured performance.  I finally was provided a drive unit to test, so here we go.

Photos:

Stock photo of the LS50 speakers captured from an image search via Google:

ls50lrg

My photos…

IMG_8503 IMG_8504 IMG_8505

 

 

 Woofer Testing:

Small Signal Measurements via Klippel:

Electrical Parameters
Re 3.01 Ohm electrical voice coil resistance at DC
Le 0.197 mH frequency independent part of voice coil inductance
L2 0.474 mH para-inductance of voice coil
R2 2.46 Ohm electrical resistance due to eddy current losses
Cmes 332 µF electrical capacitance representing moving mass
Lces 9.72 mH electrical inductance representing driver compliance
Res 27.62 Ohm resistance due to mechanical losses
fs 88.6 Hz driver resonance frequency
Mechanical Parameters
(using add. mass)
Mms 10.432 g mechanical mass of driver diaphragm assembly including air load and voice coil
Mmd (Sd) 9.706 g mechanical mass of voice coil and diaphragm without air load
Rms 1.137 kg/s mechanical resistance of  total-driver losses
Cms 0.309 mm/N mechanical compliance of driver suspension
Kms 3.23 N/mm mechanical stiffness of driver suspension
Bl 5.604 N/A force factor (Bl product)
Loss factors
Qtp 0.507 total Q-factor considering all losses
Qms 5.107 mechanical Q-factor of driver in free air considering Rms only
Qes 0.557 electrical Q-factor of driver in free air considering Re only
Qts 0.502 total Q-factor considering Re and Rms only
Other Parameters
Vas 2.4278 l equivalent air volume of suspension
n0 0.291 % reference efficiency (2 pi-radiation using Re)
Lm 86.84 dB characteristic sound pressure level (SPL at 1m for 1W @ Re)
Lnom 88.07 dB nominal sensitivity (SPL at 1m for 1W @ Zn)
Madd 9.79 g additional mass
Sd 74.46 cm² diaphragm area

ls50 woofer impedance 1

Large Signal Testing with Klippel’s LSI Module:

Displacement Limits thresholds can be changed in Processing property page
X Bl @ Bl min=82% 6.7 mm Displacement limit due to force factor variation
X C @ C min=75% 2.9 mm Displacement limit due to compliance variation
X L @ Z max=10 % >7.0 mm Displacement limit due to inductance variation
X d @ d2=10% 12.7 mm Displacement limit due to IM distortion (Doppler)
Asymmetry (IEC 62458)
Ak 7.59 % Stiffness asymmetry Ak(Xpeak)
Xsym 0.19 mm Symmetry point of Bl(x) at maximal excursion

Force factor Bl (X) Bl Symmetry Range Mechanical compliance Cms (X) Stiffness of suspension Kms (X) Kms Symmetry Range Electrical inductance L(X, I=0) Inductance over current L(X=0, I)

Frequency Response @ 2.83v/1m:

Note: Due to the rather tall surround and the fact I don’t have the trim ring used to flush mount the driver in a baffle, this driver was not flush mounted.  This will effect the high frequency response to some degree.

  • Black = 0 Deg
  • Orange = 30 Deg
  • Blue = 60 Deg

LS50 woofer 0 30 60

HD at 90dB/1m and 96dB/1m equivalent:

HD @ 90db

HD @ 96db

Tweeter Testing:

Small Signal Results:

Electrical Parameters
Re 3.14 Ohm electrical voice coil resistance at DC
Le 0.013 mH frequency independent part of voice coil inductance
L2 0.01 mH para-inductance of voice coil
R2 0.41 Ohm electrical resistance due to eddy current losses
Cmes 92 µF electrical capacitance representing moving mass
Lces 0.19 mH electrical inductance representing driver compliance
Res 1.02 Ohm resistance due to mechanical losses
fs 1205 Hz driver resonance frequency
Loss factors
Qtp 0.536 total Q-factor considering all losses
Qms 0.709 mechanical Q-factor of driver in free air considering Rms only
Qes 2.185 electrical Q-factor of driver in free air considering Re only
Qts 0.535 total Q-factor considering Re and Rms only

Frequency Response @ 2.83v/1m:

Note: Due to the rather tall surround and the fact I don’t have the trim ring used to flush mount the driver in a baffle, this driver was not flush mounted.  This will effect the high frequency response to some degree.

  • Black = 0 Deg
  • Orange = 30 Deg
  • Blue = 60 Deg

LS50 tweeter 0 30 60

 

HD at 90dB/1m and 96dB/1m equivalent:

HD @ 90db

HD @ 96db

3 thoughts on “Kef LS50 Drive Unit

  1. Can you give some details on the caps used in the crossover in the LS 50

    I am thinking of upgrading them.

    I had great success in upgrading the caps in a Maggie 1.6 to Hovlands

    Do you have any photos of the crossover PCB ?

Leave a Reply

Your email address will not be published. Required fields are marked *