It was 2 yrs ago that I tested these drivers in an active bi-amped system (Sparrow-19ST). What really bugged me then was the SB13PFC25-08 frequency response. It is anything but flat. At 1.5kHz, it starts to slope downwards. To compound matters, it then exhibits a violent cone breakup resulting with a peak at 7kHz. With a response like this, the crossover would be complicated, meaning many components. The challenge is I want to avoid a situation where the crossover cost more than the drivers. And I want to do it without compromising the sound quality.
In a flash of inspiration, I figured out the solution and I’m happy to say it worked out perfectly. In the end, the Sparrow-TA is a classic 2nd order crossover. Only two capacitors, two inductors and a few resistors.
Fig 1 – SB13PFC-08 RAW Response. Baffle Width=9″
The Black plot in Fig 1 shows how challenging the SB13PFC can be. For a 5-1/4″ midwoofer, I expect the response to be flat to at least 4kHz but that is not the case with the SB13PFC. To get a flat response after integrating in a tweeter, I would need to have the SB13PFC low passed at 1.5kHz ~ 2kHz. That’s awfully low especially for a budget design. Furthermore, where on earth am I going to find a budget tweeter that can cross at 2kHz, let alone 1.5kHz.
Fig 2 – SB13PFC with Low Pass Filer (LPF). SB19ST with High Pass Filter (HPF)
The Blue plot in Fig 2 is the SB13PFC with a 2nd order network. I adjusted the crossover to attenuate the 7kHz peak down by -10dB. It is still there but is now more manageable. The Red plot is the SB19ST tweeter. The attenuation of the tweeter is critical. Not enough and the treble will be too bright. If over attenuated, the music becomes dull. Some people describe it as “Dark”.
Fig 3 – Crossover Passband
The Black plot in Fig 3 shows the summing of the two drivers. No cancellations are observed in the passband, indicating the two drivers are crossing properly.
Fig 4 – Sparrow-TA Frequency Response. 500Hz and below in Nearfield.
For clarity, I removed the plots of the SB13PFC and the SB19ST, leaving only the Black plot (Fig 4). This is the final frequency response of the Sparrow-TA.
The response is not as flat as I would like it to be. This is something I’m prepared to accept for a budget design. What is more important is the Sparrow-TA sounds right. There’s no point having a flat response but sounds terrible.
Fig 5 – Null Response
To check on the phase alignment, I reversed the wiring of the tweeter. It resulted in a deep null (Fig 5 – Violet plot). This is exactly what I’m after. The Sparrow-TA is time-aligned.
Fig 6 – Bass Response
To have an idea of the bass, I included the in-room response below 500Hz. This measurement is no longer quasi-anechoic. It includes reflections of my room. What I want to see is the bass response in real live situations. Disregard the huge notch at 150Hz. That’s caused by a floor bounce in my setup.
We can see the sensitivity of the bass is quite low when compared to the midrange. This is typical of small 2-ways measured in full space (4 pi steradian). In a smaller room or when the speaker is placed nearer to a back wall, the bass will be louder.
Fig 7 – Step Response
This is something I didn’t expect. The Sparrow-TA step response in Fig 7 shows the SB13PFC with a very fast attack. The transient is a straight line, almost vertical. The apex is just after 100 microsec. Impressive for a budget midwoofer.
Fig 8 – Waterfall
Fig 9 – Toneburst Energy Storage
The Sparrow-TA Toneburst plot (Fig 9) shows some excess energy at 2kHz and 5kHz. What I find perplexing is the one at 2kHz. In the frequency response plots, there is no sign of any cone breakup at 2kHz. The 5kHz excess energy is probably from the cone breakup of the SB13PFC.
Fig 10 – Spectrogram
The Spectrogram gives a better idea of what is happening at 2kHz. There is a bit of a hot spot at 2kHz at 4.5 msec. This looks like a delayed energy. It doesn’t last very long though. It’s fully dissipated by 6 msec. I doubt it’s audible.
Fig 11 – Sparrow-TA Impedance
The Sparrow-TA is not a difficult speaker for power amplifiers. The lowest is 6Ω at 300Hz. In the higher treble region, it dips to about 3.5Ω at 10kHz. This is typical of 4Ω tweeters.
Sound of Sparrow-TA
The Sparrow-TA is easy on the ears. No listener fatigue even after long hours. The treble is not harsh nor does it draw attention to itself when it comes on. Vocals are clear enough. Male voices don’t sound nasal. No shrillness in female vocals.
For budget drivers, the sound quality is quite remarkable. Add to that a crossover than doesn’t cost an arm and a leg, the Sparrow-TA is a diyers’ dream speaker.
Unless otherwise stated, all measurements were made with the mic at 36 ins, tweeter axis. Impulse Window=5ms. No smoothing applied.
September 3, 2020HIFI DRIVERS, Projects