Woofer Impedance (Factory Rated 4 Ohm @ 25W)
|
Frequency
(Hz)
|
Voltage
(mV)
|
Current
(mA)
|
Impedance
(Z)
|
|
50
|
298
|
25.0
|
11.9
|
|
75
|
292
|
10.8
|
27.0
|
|
100
|
285
|
22.2
|
12.8
|
|
125
|
282
|
30.9
|
9.13
|
|
150
|
279
|
36.5
|
7.64
|
Even though significant
factors (especially near the free air resonance frequency) of inductive and
capacitive reactance are left out for simplicity, rest assured that this driver
is not anywhere near 4 Ohms. If I was to assign a meaningful impedance to
this port loaded driver, it would be in the 8 Ohm range.
The Satellites
Like the subwoofer, the
satellite specs are not specified by the manufacturer. Instead of round units,
Logitech chose oval ones so that tall, narrow enclosures would take up less
precious desktop space. Labeled as 3 Ohm, 8W units, I was able to make a few
measurements across the audio range.
Satellite
Impedance (Factory Rated 3 Ohm @ 8W)
|
Frequency
(Hz)
|
Voltage
(mV)
|
Current
(mA)
|
Impedance
(Z)
|
|
150
|
106
|
18.8
|
5.64
|
|
315
|
102
|
19.4
|
5.26
|
|
1000
|
100
|
20.8
|
4.81
|
|
3150
|
97.1
|
17.8
|
5.46
|
Affecting power measurements,
and thusly power ratings, the satellite impedance isn't anywhere near 3 Ohms--more
like 5 Ohms for the purposes of power calculations.
Rated R.M.S. Power
Output
At least Logitech didn't
fall for the ludicrous PMPO (peak music power output) specification. However,
I was shocked when I opened the woofer case to view it's mysterious guts.
Logitech has committed audio suicide by doing the unthinkable--using a switching
power supply instead of a linear one. Never intended for audio use and regarded
as taboo by all worthy audio engineers, the X2's were compromised before powering
them up. The main filter cap, the part that helps the amps remain stable during
periods of high output, was a paltry 2700uF. All this mess for a per unit
savings of maybe $2-4; what a shame.
The internal
amp is the tried and true Philips TDA 1554Q (q for quad). A quick trip to
the Philips 
semiconductor site produced the Adobe .pdf file, which shows a maximum power
output of 22W @ 10% THD for a BTL (bridged tied load) application, which the
subwoofer uses. The satellite single ended output was rated at 6W (each channel)
@ 10% THD. Both power specs are for 4 Ohms loads. Even if these numbers were
accurate, a THD of 10% is unlistenable for any length of time; a figure below
1% THD is more appropriate. If Logitech changed it's ratings for the SoundMan
to reflect that lower THD, the total output might drop from 40W to 25W--big
difference, especially for the marketing folks.
Frequency Response
The intentional bass hump
is caused from either the loaded port design and/or an intentional preamp
eq curve. These measurements were taken with the sub placed at the corner
of two walls, which gives a theoretical 12dB boost centering in the 100Hz
range. The 64-90Hz boost is not a reflection of the natural boost as free
air measurements confirmed the moderately wide hump.
The satellite driver produces
sound only in the low to upper midrange. Rolling off sharply after 8.5kHz,
no appreciable sound output was heard or measured in the 10-20kHz range.
Overall
Frequency Response
|
Bass
(rel. 100Hz)
|
Midrange
(rel. 1kHz)
|
Highs
|
|
90-64Hz
= +3 to +7dB
|
2.2-3.0kHz
= +1 to +3dB
|
10-20kHz
= Severe Rolloff
|
|
61Hz
= Flat (0dB)
|
8.5kHz
= 0dB
|
|
|
58Hz
= -3dB
|
|
|
|
55Hz
= -6dB
|
|
|
