Swift-X Zaph ZA14 with HiVi RT1C-A Planar Tweeter

The Best of Both Worlds

The Swift-X is based on two drivers, a Zaph ZA14W08 Aluminum Cone 5″ midwoofer and a HiVi RT1C-A planar tweeter. Both are outstanding in their own right but have their own shortcomings.

The RT1C delivers beautiful highs due to it’s planar nature. The only problem with this tweeter is it cannot be crossed low. By that, I mean it won’t do 2.5kHz. In fact, it needs to be crossed at 4kHz. Zaph’s ZA14, on the other hand, has great vocal reproduction. The issue with this midwoofer is the severe cone breakup at about 8kHz. If these two can work in harmony, I’ll have a speaker with fantastic vocal clarity and a treble that only planar tweeters are capable of.

HiVi RT1C-A Planar Tweeter

HiVi RT1C-A RAW frequency Response

Fig 1 – HiVi RT1C-A RAW frequency Response in 13 liter box

Fig 1 is the response of the RT1C mounted onto a 13 liter test box. The lowest frequency the RT1C can go is 3.5kHz. More worrying is the Baffle Edge Diffraction. With a baffle width of only 8.5″, the destructive effects of Baffle Edge Diffraction are clearly visible (the response on either side of the asymptote).

HiVi RT1C-A with 4kHz, 12dB/oct High Pass filter

Fig 2 – HiVi RT1C-A with 4kHz, 12dB/oct High Pass filter

The Red plot in Fig 2 is the response of the RT1C-A with a 12dB/oct filter adjusted to suppress the effects of baffle edge diffraction. Note how linear the roll-off slope is now. This is crucial in removing the harshness in the treble.

Zaph ZA14W08 Aluminum Cone 5″ Midwoofer

SWIFT-X ZAPH ZA14 Frequency Response. RAW and 18dB/oct Low Pass filter

Fig 3 – ZAPH ZA14 Frequency Response. RAW and 18dB/oct Low Pass filter

With the ZA14, I was concerned whether the cone breakup would be sufficiently suppressed with a 4kHz low pass filter. After working on different crossover orders, I opted for a 3rd order low pass. The peak at 8.5kHz is down 23dB. This is good enough as there’s not much music energy up there.

Swift-X Frequency Response

Summed response of ZA14 with RT1C-A wired in reversed phase

Fig 4 – Summed response of ZA14 with RT1C-A wired in reversed phase

Fig 4 shows the summed response of the ZA14 with the RT1C-A. Electrically, it’s 18dB low pass and 12dB high pass. Acoustically, the two drivers crossed at 4kHz, which is good. No cancellation is observed on either side of the crossover.

The Swift-X response is incredibly flat from 500Hz to 10kHz. There’s a +3dB bump at 900Hz and two smaller ones at about 2kHz. Nothing to worry about. Note that the plot is without any smoothing. If I had applied 1/6th of an octave smoothing, it’ll be virtually a flat line.

Swift-X Harmonic Distortion

ZA14 and RT1C-A 2nd and 3rd Harmonic Distortion

Fig 5 – ZA14 and RT1C-A 2nd and 3rd Harmonic Distortion

Disregard the notch at 150Hz in Fig 5. That’s an anomaly as a result of floor bounce. In real listening, there’s no suck out.

The Harmonic Distortion of Swift-X is generally -50dB below the fundamental which is pretty decent. Above 3kHz, the 2nd Harmonic (Red plot) rises slightly. They are no cause for concern because during playback, they were not audible.

Swift-X Impedance

ZA14 with RT1C-A Impedance plotFig 6 – ZA14 with RT1C-A Impedance plot

Swift-X is an easy load for most power amplifiers. In Fig 6, the impedance stays above 8 ohms throughout except for the crossover region where it dips to 6 ohms. The electrical phase is impressive. It is almost 0 phase from 400Hz~20kHz.

Swift-X in use

As I had hoped, the Swift-X exhibits amazing vocal clarity coupled with exceptionally detailed treble. The planar tweeter does a marvelous job with the highs. Not the slightest hint of harshness.

When placed in full space (4 pi steradians), the Swift-X sounds a bit lean on the bass. Fig 5 shows it’s about -6dB from 60Hz~125Hz. The bass will be louder when the speaker is up against a wall (half space), even more when placed in the corner of a room (quarter space).

Due to the softer bass, the Swift-X makes an excellent choice as Near Field Recording Monitors. Too much bass is not needed in this situation because near field monitors are mainly used for vocals. What is essential is a flat response. And this is where the Swift-X shines.

For listeners that place their speakers in full space, the Swift-X bass can be supplemented by a subwoofer. I tested it with a Dayton DA270 10″ Aluminum woofer, crossing at 125Hz with a 24dB/oct Low Pass filter. The result was disappointing. All I got was thud, thud. The mid-bass was totally absent. When I switched over to a Bandpass sub with a Dayton SD215A inside, the difference is shocking. The mid-bass is now clearly audible, with definition and punchy too. Moreover, I didn’t even need an electronic crossover because a bandpass already has a low pass filter built-in. Since mine is at 125Hz at 12dB/oct, it is just right for the Swift-X. You’ll still need an additional amplifier though. But it’s a simple and cheap solution to get great music.

One last note. The HiVi RT1C-A has to be wired in reversed phase. When wired in normal phase, I detected some minor phasing issues.

Crossover Values & Box Dimensions

If you’re interested to build this project, kindly send me a mail for the crossover and box dimensions. For non-commercial use only.