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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.

 





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