Magpie-XT (SB17NRX2C35-4 with Peerless XT25TG30-04)

Magpie-XT
13 Liters Bass Reflex

Before the Magpie-XT, I loaded the SB17NRX in a 27 liters, followed by 18 liters bass reflex and still couldn’t get the sound I wanted. It is in this 13 liters box that everything came together.

Fig 1 – SB17NRX2C35-4 RAW Frequency Response • Baffle Width=8-1/2″

What I really like about the SB17NRX is the midrange (Fig 1). Vocals sound natural, clear and effortless. My design approach with this 13 liters bass reflex is to retain the midrange quality while at the same time get some decent bass.

Fig 2 – SB17NRX2C35-4 with Low Pass network

The Blue plot in Fig 2 is that of the SB17NRX. It was only after several attempts that I finally got the midrange to match the bass. The nasty cone breakup peak at 7kHz was proving troublesome and it necessitated a notch filter.

Fig 3 – SB17NRX2C35-4 with Peerless XT25TG30-04 (Flush Mounted)

The Red plot in Fig 3 is the Peerless XT25TG30-04 Ring Radiator tweeter. The crossover frequency is at 2.5kHz. When I tried to cross at 2kHz or lower, I lost the midrange. It seems that this is the sweet spot for the SB17NRX. Please note that the tweeter is flush mounted otherwise baffle diffraction will mangle the response.

Fig 4 – Magpie-XT Passband

The Magpie-XT crossover passband is the Black plot in Fig 4. There is a mild cancellation at 7.5kHz that is caused by the cone breakup of the SB17NRX.

Fig 5 – Magpie-XT Frequency Response

Fig 5 is the final frequency response of the Magpie-XT. She is not as flat as some of my other designs but she’s sounds great. That is more important to me than a ruler flat response.

Fig 6 – Magpie-XT Null

The Violet plot in Fig 6 is with the tweeter wires connected in reversed phase. It resulted in a symmetrical null centered at 2.4kHz. The depth of the null indicates the alignment of the SB17NRX and the XT25TG30 is quite close.

Fig 7 – Magpie-XT Nearfield

The Blue plot in Fig 7 is the nearfield response. This measurement was made with the mic about 1/4″ away from the cone surface. Because of the close proximity, it doesn’t record any room reflections. Essentially, a nearfield measurement is an approximation of what the Magpie-XT response will be when in an anechoic chamber. Note the huge suck-out at 150Hz is not in the nearfield measurement. This confirms that it’s an anomaly. It’s actually a floor bounce that my mic recorded.

Fig 8 – Magpie-XT Port

The Brown plot (Fig 8) is the response of the port. Measurement is made with the mic inserted about 1/4″ into the mouth. I am surprised that the port output is not a sharper peak. It looks more like a bandpass. At the higher end, the pipe resonance at 1.2kHz is sharp. Fortunately, it’s -15dB below the fundamental so it doesn’t interfere with the midrange.

Fig 9 – Magpie-XT Step Response

The Magpie-XT is fast (Fig 9). The minute the SB17NRX takes off, she shot straight up, hitting 200 microsec at 85%. She then slows down to hit the peak at 350 microsec. All the while maintaining a smooth transient. Not a single discontinuity is recorded throughout. That’s what makes her sound great. The Magpie-XT sound is “lively”, not slow or sluggish. That’s why I said she sounds “natural”.

Fig 10 – Magpie-XT Waterfall

The Waterfall (Fig 10) recorded a strong decay at 5.5kHz~ 6.5kHz. We should not be too alarmed by this because this is a highly magnified view (1.5 msec window).

Fig 11 – Magpie-XT Toneburst Energy Storage

The Toneburst plot in Fig 11 shows the decay in cycles. These unwanted energy is actually ringing caused by the cone breakup of the SB17NRX. Even though it is -10dB below the fundamental, the effects are seen. 

Fig 12 – Magpie-XT Spectrogram

When reference to time, the Spectrogram in Fig 12 shows no ill effects of the ringing at 5kHz~6kHz. however, it recorded some delay spots from 2kHz~2.5kHz but they very quickly lost their energy. By 6 msec, they are almost gone. What is interesting is how clean the SB17NRX is at 1kHz. There is hardly any streaking.

Fig 13 – Magpie-XT Excess Group Delay

The Excess Group Delay (Fig 13) shows the Magpie-XT at -2.05 msec at 47Hz. For a bass reflex, this is quite good. I expected worse.

Fig 14 – Magpie-XT Box Modeling

This is where it made all the difference. It is with a smaller 13 liters box tuned with a +3dB peak that sounds best (Fig 14). In doing this, I sacrificed the quality of the bass for loudness. 

Fig 15 – Magpie-XT Impedance

The impedance of the Magpie-XT is slightly demanding on power amplifiers (Fig 15). This would be rated at nominal 4Ω but in reality, she dips to 3Ω at 5kHz. Her average impedance is 3.5Ω as seen in her Re. Her electrical phase, on the other hand, is not demanding. She only dips -35° at the crossover at 2.4kHz. Apart from that, she doesn’t stray far from 0°. The port tuning is found in the saddle on the left. Magpie-XT bass reflex is tuned to 55Hz. 

Sound of Magpie-XT

I am very pleased with how this Magpie-XT turned out. She doesn’t have the bass impact of the Peerless 835025 but her redeeming qualities are in her tonal balance and super midrange.

Vocals and harmony are best for the Magpie-XT. Music like Oak Ridge Boys, Diana Krall and Boyz II Men passed with flying colors. 

about the SB17NRX2C35-4

Even though I managed to get the SB17NRX to sound pleasant, I would not recommend this woofer purely because the Thiele & Small parameters are way off. SB specs say Qts=0.32. I measured Qts=0.53. So, from a nice tight bass to a mediocre bass. I do not know whether it’s due to a lack in quality control but if SB Acoustics wants to be taken seriously, they should buckle up. There are cheaper woofers with Qts=0.53.

Crossover is available on request. Free for DIY. Not for Commercial use.
Unless otherwise stated, all measurements were made in Full Space (4 pi) with the mic at 36 ins, tweeter axis. Impulse Window=5ms. No smoothing applied.