Fig
1 - Vifa XT25TG30-04 Impedance
In Swift-III, the tweeter has been upgraded to a Vifa XT25TG30-04.
This tweeter is also a Dual Ring Radiator, similar to the
smaller XT25SC90 but with a lower Fs centering at 470Hz,
a more desired frequency. |
Summing
of Swift-III
Fig
2 - Frequency Response of Swift-III (ZA14 + XT25TG30)
1 meter - tweeter axis
Fig
2 shows the acoustic slopes of the ZA14 and the XT25TG30
crossing at 2,300Hz. What is really interesting is the highpass
of the XT25TG30. The slope is virtually a straight line,
resembling a textbook electrical filter.
In
the all critical crossover region, no summing anomalies
are seen. From 1kHz-4kHz, summing is smooth. No peaks or
cancellations. Judging from the plot, the XT25TG30-04 is
a better match for the ZA14 at 2,300Hz.
|
Frequency Response of Swift-III
Fig
3 - Frequency Response of SWIFT-III
1 meter - tweeter axis
Fig
3 above shows the summed response of the ZA14 and the XT25TG30
with a full passive crossover.
Disregard
the measurements below 200Hz as they are affected by the
room. The deep notch at 150Hz is attributed to a cancellation
node caused by a sound wave bouncing back from the floor
below the speaker.
|
Dispersion
Measurements
Fig
4 - Frequency Response of SWIFT-III at various angles
1 meter - tweeter axis
Swift-III
Horizontal Polar Response
Fig
5 - Horizontal Polar Response of SWIFT-III
Normalized Flat at 0 degrees curve
Swift-III
Harmonic Distortion
Fig 6 - Harmonic Distortion of SWIFT-III
Blue trace consists of combined 2nd, 3rd, 4th and 5th Harmonics
0.7% at 2.7kHz (blue trace)
1.4% at 95Hz (blue trace)
Swift-III Passive Network
Fig
4 - SWIFT-III Passive Network (18dB/oct)
The
crossover components are shown in Fig 4. Recommended types
are Polypropylene Capacitors, Air-Core Inductors (minimum
18 awg) and preferably Non-Inductive 10 Watts resistors.
To save cost, the 50uF and 170uF can be Non-Polar Electrolytic
Capacitors.
|
Swift-III
Passive Network Impedance
Fig 5 - SWIFT-III Impedance (18dB/oct)
Swift-III
Impedance
Swift-III's
impedance is an insane load. The drastic dip to 2 ohms at
about 2.7kHz is sure to stress most solid state power amplifiers.
At such a low impedance, there is a strong probability that
power amplifiers' over current protection circuits will
be triggered.
After
listening to Swift-III for a couple of days, it's apparent
that there's distortion somewhere. My guess is my power
amplifier's protection circuit was activated during certain
parts of the music. To test my theory, a slight modification
was made to the crossover. The next version shall be Swift-IIIa.
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