|It has for long been my intention
to model a 2½-way speaker with the following attributes:
1. Relative low cost.
2. Good sensitivity, 87-88 dB.
3. Easy load on amplifier.
4. Modest size, 30-35 litres.
5. "Easy" drivers that will allow a relatively
simple crossover. "Easy" drivers mean drivers
with smooth frequency response profiles.
So, one of my friends brought these speakers and asked me
to take a look at the crossover as he wasn't happy about
the sound. The system was bought as a kit and read below
what was going on.
HDS164 (850438) + Vifa XT25TG-30-04
Cabinets made from 19 mm MDF and added a 4 mm black
acrylic faceplate. Cool!
drivers. The HDS164 (850438) has been discontinued, but
there may be a
few of these around and the shown crossover may be
interesting for others.
The HDS164 (850438 and -39 = different
chassis colour) has been replaced by the 830874 (pp
membrane) or the 830875 (Nomex membrane), apparently
having almost identical frequency response and impedance
profiles, but whether these new drivers will fit in with
this crossover is a very good question. If you would like
to make a construction with the new drivers, lend me two
or four drivers and I'll fine-tune the crossover to work
response of original loudspeaker.
I shan't reveal the original crossover,
but from the schematics it was supposed to have a 15 mH
coil to the lower bass. On the coils were written 14 mH
and the CLIO said 11.3 mH. Hmm...
Next the 0.8 mH to the upper bass would really mean that
this driver should have quite some help from the lower
bass in the upper bass/lower midrange, but basically we
have a speaker with a weak bass and an upper midrange +
treble some 4 dB above the basic sensitivity of the
speakers. No wonder it sounded much too forward and
aggressive on most recordings. I guess that if the
cabinets were placed in two corners, the overall response
might have been good, but with 1 meter to the rear wall
and free of side walls the speakers just had way too much
midrange. The midrange and treble was well balanced and
the FR from 600 Hz to 20 kHz very flat. The crossover was
a 6/12/12 dB set-up and the phase-tracking between upper
bass and treble was quite poor despite making a smooth
is seen a simulation of the original crossover and the FR
is pretty close to the actual measured response.
The phase-tracking between upper bass and treble isn't too good.
Impedance measurements. Green: system imp.
Red: One bass driver. Blue = tweeter. One of the things
that made a week bass was the port tuning at 30 Hz (green
graph). These drivers need a much higher vent tuning and
the 50 x 150 mm vent was cut off to 70 mm length.
the frequency response of the HDS164 and XT25TG drivers
Disregard what's below 350 Hz.
New crossover simulation
new crossover is basically a 6/12/18 dB set-up. Above the
LspCAD predicted response.
The lower bass has been added an impedance correcting RC
circuit of 6.8 ohm + 10 uF.
All drivers have positive polarity. See actual response
of old and new system. Red(imp)/blue(phase)= old system.
Green and yellow = new system.
Generally this is an easy load on any amplifier. 4 ohms
minimum impedance and the phase profile is good too.
new crossovers ready for wiring.
frequency response has been tilted to produce the BBC
dip. Blue = minimum phase.
I'm not sure of the reduced level of the
XT25TG above 5.5 kHz. I haven't seen this before, but I
guess the symmetrical placement of the tweeter is
responsible for this. Also there are some wrinkles around
2.7, 3.7 and 4 kHz not usually seen from this tweeter. If
you want more treble, bypass the 1.5 ohms resistor in the
The cabinet is 20 x 28 x 96 cm (WxDxH), 35
litres, and made from 19 mm MDF with three bracings
between drivers. The vent is 50 x 70 mm (DxL).
Important!: Driver placement is 75, 230
and 430 mm from top and all drivers are centered on the
I would ad a base plate to the construction, lifting the
speaker some 30-40 mm. The optimum listening height is a
little low - for me. Allow the base plate to make the
bottom panel of the speaker (see SP38) allowing the
crossover to be attached to the front panel.
Take a look at the SP38, 2.5 clone, PL18 and other
constructions to render details of cabinet construction.
No more will be given here.
This construction comes very close to what I had in mind
for a 2½-way with the intended features. These speakers
will play anything with a wide soundstage, some really
good bass and with excellent midrange transparency.
Getting the XT25TG tweeter level right is - as always -
very important. I have found the 1R5 resistor to the
tweeter to give the best results. Usually I start with a
too high tweeter level and slowly increase attenuation
until a level where it just seems to
"disappear", i.e. it gels into the overall
sound without making an act of itself. It's a pity
Peerless has discontinued these sandwich diaphragms. I
hope the new Nomex and PPB cones will work as good as