Swift-IIIc (Ultimate-II)
(High Definition Series)

5 inch 2-way
13 Liters Bass Reflex Satellite

by Michael Chua

This design is free for DIY. Not for Commercial Use.
This article may not be published in part or full without the express permission of AmpsLab.

The Case for 2nd Order

Though the 1st Order Filter for the Vifa XT25TG30 solves the impedance issue, I believe the phasing could be improved.

Swift-IIIc addresses that issue. By adopting a 2nd Order Filter for the tweeter (Fig 25), the phases of the ZA14 and the XT25TG30 are much closer. The trade-off is the impedance now dips to 5 ohms at about 3.5kHz. This is still manageable. At 5 ohms, power amplifier's protection circuitry should not activate. We can check this out when distortion measurements are made.

Swift-IIIc Impedance

Fig 25 - Swift-IIIc with XT25TG30 2nd Order Filter

Fig 26 is the Distortion of Swift-IIIc. In the tweeter critical region (2kHz-5kHz), THD is 0.3%, a respectable 50dB below 90dB. The bulk of the distortion is 2nd harmonic (Red trace). More importantly, the dissonant odd harmonics are just below 0.1% at -60dB. There is no sign of distortion from the power amplifier at 3.5kHz. The majority of power amplifiers will have no problem driving Swift-IIIc.

Swift-IIIc Distortion

Fig 26 - Distortion Measurement of Swift-IIIc
(mic 20 ins On Axis with Tweeter | 5ms Impulse Window | 1/12 oct smoothing)

The acoustic slopes of Swift-IIIc drivers are shown in Fig 27. Disregard readings below 400Hz as room inteference takes over. For the lowest distortion, the 18dB/oct filter for the ZA14 has been retained. The XT25TG30, on the other hand, is now filtered with a 12dB/oct network (Red trace). The crossover summation is in fact, smoother than Swift-IIIb.

Swift-IIIc Frequency Response

Fig 27 - Swift-IIIc Frequency Response
(mic 1 meter Tweeter Axis | 5ms Impulse Window | 1/12 oct smoothing)
Disregard readings below 400Hz

The crossover network of Swift-IIIc is shown below (Fig 28). What is glaringly obvious is the omission of the LCR in the tweeter's network. After more measurements and auditioning, I found the LCR doesn't really contribute to better sound quality in this instance.

Swift-IIIc Crossover Network

Fig 28 - Crossover Network of Swift-IIIc

Auditioning Swift-IIIc (Ultimate-II)

Is the Swift-IIIc an improvement over the rest?

After listening to it for a few days, I would vote a resounding YES. It is not just a smoother crossover summation but rather one of coherency. Maybe I'm sensitive to phasing. The Swift-IIIc simply sounds "in-sync". The net effect is the music sounds more natural.

Aug 20, 2013


next > Swift | Swift-II Hybrid | Swift-II Passive | Swift-II Music
  Swift-III | Swift-IIIa | Swift-IIIb | Swift-IIIc | Swift Ultima | Swift Ultima-II


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