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Needed:
-- 4 kg fresh sheep's wool, (the yearly harvest of one big animal).
Not cleaned: Don't use water to remove the shit and dirt.
-- Pack of moth-ball's
-- 4 parts of hard wooden beam, of 10-14 square cm diameter,
sawed at lengths of 30 cm in an angle of 12 degrees.
-- A set of 6 MKP-capacitors
of 250V: 2 * 68 MFD + 4 * 22 MFD
-- 2 Towels |
Modifications
Place the speaker on its
back:
Unscrew the 2 woofers. Place them aside carefully.
Mid-high-filter:
Unscrew the mid-high filter PCB behind the upper woofer.
Remove both the electrolytic capacitors 82 MFD and 22 MFD.
Replace the biggest (82u) by a MKP-capacitor of 90 MFD, this can be 68MFD and
22 MFD in parallel.
Replace the smallest (22u) elcap by a MKP-capacitor of 22 MFD.
Fix the loosy capacitors with glue or tie-wraps. Screw the filter-PCB in place.
Strengthening the box:
Knock on the side panels at different heights, investigating the weakest
parts with the lowest hollow sound.
These are behind the woofers, between the partitions, and just between front
and back panel.
Fix tightly the wooden beams horizontally between the side panels, at the weakest
places.
The low knocking response from the panels should disappear.
Damping the cabinet:
Fill all hollow rooms, behind the foam-rubber mats, around the midrange speaker-cylinder and at the
bottom with wool, 2 kg per box.
Place one moth-ball in each partition inside the wool,for killing all insects.
Check the absolute polarity of the 3 speakers. (In some kappa90's the connection
of the midrange-speaker is wrong).
Place the woofers back, closing the box. The wool should not touch the cone
of the woofers.
Plug one towel in the bass-reflex-pipe,
blocking the airflow.
The pipe can be used as cable channel for later modifications (active filters).
Test the speakers with a 40 Hz sine
at 50 Watt, listen to odd sounds.
Loosy cables can knock at the panels and should be fixed.
Leaky airflow in the pipe can be noisy. Plug in tighter.
Extending the low-frequency-range:
The closed box had a resonance-frequency
of 41 Hz with a Q-factor of 0.5, resulting in a 2nd order roll-off with 6dB
at 41 Hz.
This low Q is only possible
if the woofers are driven by 0 Ohms, from an amplifier. The passive filter increases
the Q-factor.
The roll-off was partly (between 50-70 Hz) compensated by the peaking of the
lowpass-filter, just above the speaker-resonance.
An exact compensation of the lowfrequency-response
is possible when the passive low-range-filter inside the kappa90 is replaced
by an active filter, followed by an separate amplifier.
Schematics and calculations
can be found at Linkwitz Lab.
We extended the low-frequency range
from 41 Hz to 20 Hz with an 12 dB/oct highpass equalization ("Linkwitz
Transform", Biquad).
A low-pass-filter should be 4rth order
wit a cut-off frequency (6dB) of 325 Hz, consisting of two 2nd order butterworth
filters wih a Q of 0.71 at 325 Hz.
The sluggy response of an open reflex-pipe is impossible to compensate, the
pipe should be closed. |
