High Efficiency Speaker Asylum: RE: how to centralize a we555 compression driver by Jmmlc

Hello,

Here is my view on that question.

Inversely to the common idea that with Western Electric drivers you just have to screw the diaphragm and it will operate always at his best, this is not true.

Here is the result of my own exerience on the good way to set up WE555 or the Westrex or LMT compression drivers that use the 555 diaphragm and phase plug.

Measurements are the key for a proper optimization of the performance of the driver.

1) The driver being not mounted on a horn I remove the phase plug,

2) using LIMP in white noise mode, I acquire a first impedance curve.

If the main impedance peak is not in the range 350 to 450Hz, there is an urgent need to have your diaphragm perfectly centered.

3) if there is no main peak, probably you have iron dust inside the gap. First thing is to clean the gap (this is always needed with the Westrex and LMT versions of the 555 that possess an Alnico magnet). All the old drivers I could see, all of them have iron particles inside the gap. Sometime there is also a high Q peak around 100Hz a sign of problem!

4) To have the diaphragm centered, unscrew the 6 screws until you can move a little the diaphragm in every direction.

5) Still using LIMP (with the ARTA box) or using a white noise generator and a 50 ohms series resistor between the amp and the driver, move the diaphragm by small displacements until the white noise appears to contain more and more low frequency.

Re-do an impedance mesurement and note the main resonance frequency (it should be aroud 380 to 400Hz).

6) Set a sine generator to the previously mesured impedance frequency and still using a 50 ohms (or another high value, from 30 to 100ohms) series resistor. With very small moves of the diaphragm, try to obtain the largest audible level and the least distortion.

7) screw the 6 screws of the diaphragm

8) screw the phase plug.

If, when you do a sweep of frequency there is still audible distortion, it means that the distance between the diaphragm and the rear part of the magnetic circuit is too much small. (In old driver they often set the distance between diaphragm and phase plug too much large).

Then you have to unscrew the 2 little screws that maintain the different rings (on top metal ring, then generally 2 bakelite rings then the aluminum diaphgram then 2 others bakelite rings.

You can try to remove one of the 2 last bakelite rings and place it with the 2 upper rings under the metal ring. Then screw the 2 little screws to fix the diaphragm. Then redo steps 1 to 8...

Then your driver should deliver a low distortion sound inside a very wide frequency range.

Best regards from Paris,

Jean-Michel Le Cl�ac'h