Sparrow-II (SB13PFC25-08 with Peerless BC25SC08-04)

SB Acoustics SB13PFC25-08

This midwoofer has been bugging me for a long time. I’ve never encountered a 5″ that drops off at 1.5kHz (Fig 1) and just plateaus at -5dB till 4kHz. To make matters worse, there’s a violet burst as the cone breaks up. 

Fig 1 – SB Acoustics SB13PFC25-08 • Mic at 36 inches • Tweeter axis • No smoothing • 500Hz below in Nearfield

Ordinarily, I would cross at 1.5kHz but there are no dome tweeters that are comfortable at that low a frequency. I even tried using a compression driver with a horn but that presented a host of different problems. 

Fig 2 – Blue plot=SB13PFC25-08 with Low Pass

This time round, I went for broke and rolled it off at 2kHz with a 12dB/oct low pass filter (Fig 2). I avoided notching out the peak at 7kHz for I wanted to keep the crossover component count low.

Fig 3 – Sparrow-II Passband

Since I’m on a roll with the Peerless BC25SC08-04 tweeter, I decided to mate it with the SB13PFC25-08. After some tweaking, I managed to get a pretty decent summation. The crossover passband (Black plot in Fig 3) shows no cancellations on either side of the crossover frequency.  

Fig 4 – Sparrow-II Frequency Response

The final frequency response of the Sparrow-II is in Fig 4. Of all the Sparrows that I did, this version has the best crossover. It is virtually flat from 100Hz to 10kHz, except for a slight depression from 3.5kHz to 6kHz. 

Fig 5 – Sparrow-II Null Response

The Sparrow-II Null is not very symmetrical but it does indicate the two drivers are crossing at 2.8kHz, This tallies with the Low Pass and High Pass plots crossing at 2.8kHz in Fig 3.  

Fig 6 – Sparrow-II Step Response

The Sparrow-II Step response (Fig 6) is remarkably similar to the Soliloquy-III. There are no breaks in the leading edge of the SB13PFC25-08. From the step, we can see the tweeter and the midwoofer are both wired in absolute phase.

Fig 7 – Sparrow-II Waterfall

The Waterfall plot (Fig 7) shows minimal artifacts in the treble. This is attributed to the excellent performance of the Peerless BC25SC08-04 tweeter.

Fig 8 – Sparrow-II Toneburst Energy Storage

The Toneburst plot in Fig 8 has some light blue slices from 5kHz to 10kHz. They are quite benign. The ones that may cause smearing is lower down at 1kHz to 2kHz.  

Fig 9 – Sparrow-II Spectrogram

The Spectrogram in Fig 9 shows two hot spots at 1.1kHz and 1.3kHz. These are the light blue slices from 1kHz to 2kHz I mentioned earlier in Fig 8. They will not cause any smearing as they are fully dissipated by 6 msec.

Fig 10 – Sparrow-II Harmonic Distortion

The Harmonic Distortion plot in Fig 10 reveals the Sparrow-II is dominated by 3rd harmonics (Violet plot) from 700Hz to 4kHz. They originate from the SB13PFC25-08, not from the Peerless tweeter. I would not discard the SB13PFC simply for this because during auditioning, it didn’t irritate me.

Fig 11 – Sparrow-II Impedance

The Nominal Impedance of the Sparrow-II is 8Ω. However, in the treble region, it dips to 4Ω at 3kHz. At this point, the electrical phase dips to -45°. Overall, the Sparrow-II impedance is not too demanding on power amplifiers. Chip amp users should take note and design their amps for 4Ω to cater for the treble.

Summary

I am more than satisfied with the outcome of the Sparrow-II. The Peerless BC25SC08-04 tweeter is the solution to this annoying SB13PFC25-08. I could have used a tweeter that cost twice as much as the midwoofer and save myself a lot of stress but I dislike that approach. To me, it’s ridiculous to use a $50 tweeter for a $25 midwoofer.    

Fig 12 – Sparrow-II In-Room Response

One of the positives from the SB13PFC25-08 is I did not have to use any BSC to lower the midrange. My baffle width of 8-1/2″ seems to work quite well with 5″ midwoofers. 

The in-room response of the Sparrow-II in Full Space (4 pi) shows the bass is -5dB below the fundamental. The bass will be stronger when the Sparrow-II is nearer to the back wall (Half Space)  or in a room smaller than my lab.

The Sparrow-II and the Warbler-TA are great projects for beginners. Their drivers are very affordable. Crossover cost is as low as possible without compromising quality and finally a sound that exceeds the competition.

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