Woofer Impedance (Cont.')
Significant
factors (especially near the free air resonance frequency) of inductive and
capacitive reactance are left out for simplicity. I would rate the woofer
as a 9 Ohm unit for testing purposes. It should be any easy load to drive
for the TDA8563Q BTL amp setup.
The Satellites
Like the subwoofer, the
satellite specs are not specified by the manufacturer. The satellite cases
were snapped (not screwed) together, so I left them intact for fear of cracking
a case or two.
Satellite
Impedance
|
Frequency
(Hz)
|
Voltage
(mV)
|
Current
(mA)
|
Impedance
(Z)
|
|
315
|
104
|
13.9
|
7.48
|
|
1000
|
100
|
17.0
|
5.88
|
|
3150
|
94.4
|
15.7
|
6.04
|
Affecting power measurements,
and thusly power ratings, I will assign a nominal 6 Ohm rating for this review.
Similar to the easy going nature of the subwoofer, the sat drivers will be
an easy load to drive for it's quad amp IC, a TDA8561Q.
Frequency Response
The unintentional bass
hump is moderately narrow in frequency range, but very pronounced. Remember,
a 10dB boost essentially means a relative doubling of loudness for sounds
produced in that affected frequency area. The low frequency cutoff in room
measured an average-for-size 42Hz, with only a small rolloff beginning at
51Hz.
The midrange response
was a disappointment. In two critical ranges, the curve shelved upward in
the two areas noted below. This had a dramatic effect on the perceived sound
quality.
Overall
In-Room (2 Walls) Frequency Response
|
Bass
(rel. 100Hz)
|
Midrange
(rel. 1kHz)
|
Highs
|
|
63-84Hz
= +6 to +11dB
|
350-422Hz
= +1 to +3dB
|
7.8KHz
= -3dB
|
|
51Hz
= -3dB
|
2.1-2.9KHz
= +1 to +3dB
|
9.3KHz
= -6dB
|
|
42Hz
= -6dB
|
|
|
After noting the huge
low end boost I pondered if someone had measured the 7500's in an anechoic
chamber and not a more typical 2-wall setup of most computer users. So I re-measured
the FR with the sub in nearfield manner and violá! A much flatter curve
resulted.
Nearfield Response
|
Bass
(rel. 100Hz)
|
Midrange
(rel. 1kHz)
|
Highs
|
|
53-150Hz
= -3 to +2dB
|
280Hz-7.8KHz
= -3 to +3dB
|
7.8KHz
= -3dB
|
Composite
Nearfield Response
53Hz-7.8KHz ±3dB
When
the two nearfiled curves were spliced the smoothest response ever measured
was the result. BUT, this curve can only be achieved with the sub placed approximately
3-4 feet out into a room, away from any walls. Not really a computer friendly
arrangement is it? Also, note the poor high frequency response from the small,
nearly full range satellite driver. Again, the perceived sound quality correlated
to the measurements. No appreciable highs were heard or measured.
Rated
R.M.S. Power Output
Since BA
chose to hide any power estimates, I assumed the task of uncovering as much
as possible. The amplifier chips were the TDA8561Q 4-channel opamp and the
TDA8563Q opamp on a BTL setup. The power supply configuration was of very
poor quality despite the massive Jamicon 22,000uF 25V filter capacitor. At
this price level, nothing less than a toroidal transformer and split power
rails (ie. Vcc & Vee) is acceptable. BA chose to go the cheapest route
possible, with an unshielded non-audio quality external wart and a single
16.5 VDC power rail.
In order
to test the woofer amp, the box had to be sealed so I did the best I could.
An estimate of 11 watts for the sub is my best guess. The satellites
are a different story, being directly measured at 4.39 Watts per channel (1KHz),
four channels total. When the dust settled, I'd estimate that the 7500's could
output approximately 29 Watts RMS.
As I had
great difficulty reaching maximum output due to low amp gain (sensitivity),
peak values may be higher depending on the individual soundcard used. See
the next section for those measurements.
