Swallow-TPC (GBS85N Time & Phase Coherent)

Swallow-TPC
Peerless GBS-85N25PR03-04 with BC25SC08-04
7 Liters Sealed Box

In this Swallow-TPC, I replaced the Audiopur ribbon with a Peerless BC25SC08-04 tweeter. I could have used another tweeter that is easier to work with but this BC25 meets my price point and she has a short horn. Hopefully, the acoustic centers of the two drivers will be a better match.

Fig 1 – Peerless GBS85N and BC25SC08-04 RAW Frequency Response • Baffle Width=8″

The plots in Fig 1 are the relative loudness of the GBS-85N25PR03-04 midrange and the BC25SC tweeter. They are in a 7 liters sealed box with a baffle width of 8″. The tweeter is flush mounted but she can be used surface mounted. There’s hardly any difference as baffle diffraction doesn’t affect the response much because of her horn.

From these plots, it’s obvious that I will have a handful with the crossover. The early roll-off of the GBS85N is bad enough but there are these violent cone breakup peaks after that, the worse one at 5kHz. I will likely need to use a notch filter to tame this peak because I am planning to cross at 3kHz.

The BC25SC08-04 tweeter has her own set of issues. Her resonance frequency (Fs) is a high 1.8kHz. If I want to cross her at 2kHz, I will have to use a conjugate network (LCR) to flatten her impedance peak. That will add to the cost. But it’s not only that. I’m not certain whether the BC25 will be comfortable at such a low frequency even with an LCR in place. To be on the safe side, I will cross her at 3kHz. This will give me some margins for the tweeter to breathe better and I can do away with the LCR.

Fig 2 – Peerless GBS85N with BC25SC08-04 (Flush Mounted)

After going around in circles, I finally managed to tame the two drivers (Fig 2). They are still not as perfect as I would like but they’re much better than my previous attempts. There’s a bump at 2kHz in the tweeter roll-off which will cause some issues during summation. I can install an LCR to remove the bump but I don’t think it will be so bad to warrant the additional cost.

However, with the GBS85N, there is not much of a choice. I had to install a notch filter to remove the 5kHz peak. Without it, the treble will certainly be mangled.

Fig 3 – Swallow-TPC Passband

The Black plot in Fig 3 shows good summation in the crossover. There is a light bump at 2kHz caused by the bump in the BC25 roll-off.

Higher up, the GBS85N peak at 8kHz caused a dip in the treble. I can install another notch filter but it’s not worth the extra cost. I seriously doubt anyone can make out a dip at 8kHz.

Fig 4 – Swallow-TPC Frequency Response

Fig 4 is the final frequency response of the Swallow-TPC. Frankly, I’m quite proud with the outcome considering that these two drivers are really difficult to work with. During playback, I didn’t detect any excessive presence from the bump at 2kHz.

Fig 5 – Swallow-TPC Null

The Violet plot in Fig 5 is the null response when I flipped the tweeter wires around. There are some jaggedness in the slopes which are caused by the lack of smoothness in the roll-off but they are not serious. What is important is the null is deep and centered at 3kHz. This confirms the two drivers are crossing right at 3kHz. 

Fig 6 – Swallow-TPC Step Response

This is where all my hard work pays off. The Swallow-TPC step response (Fig 6) is like the time & phase coherent Thiel. The difference is there’s no tilting of the baffle or using steps like in the Dunlavy. But it’s more than that. I did this without having to resort to 1st order networks.

Fig 7 – Swallow-TPC Waterfall

The Swallow-TPC shows very few artifacts, particularly in the treble (Fig 7). There seems to be some decays from 1kHz to 2.5kHz.

Fig 8 – Swallow-TPC Toneburst Energy Storage

The Toneburst plot in Fig 6 shows some excess energy at 2.5kHz. Those from 3kHz onwards probably are inaudible. 

Fig 9 – Swallow-TPC Spectrogram

As I expected, the treble from 1.5kHz to 20kHz is clean (Fig 9). However, there a a few delayed spots below 1.5kHz. Fortunately, they are fully dissipated by 8 msec.

Fig 10 – Swallow-TPC Harmonic Distortion

The Swallow-TPC Harmonic Distortion is quite low (Fig 10). The 2nd (Red plot) and 3rd (Violet plot) harmonics are generally at -50dB below the fundamental. The sum of the 2nd to the 5th harmonics (Blue plot) is only at -45dB. This is a far better performance than the Swallow-AP where I used the Audiopur ribbon.

Fig 11 – Swallow-TPC Impedance

The Swallow-TPC is generally not a difficult load for power amplifiers (Fig 11). Her nominal impedance is 4Ω but she does dip briefly to 3Ω at 2.5kHz. Her electrical phase is well mannered. The max is +20° at 4kHz. Other than that, she’s mostly at 0°.

Summary of Swallow-TPC

The Swallow-TPC was a challenge to work on. It took a bit of work to get the two drivers to behave the way I wanted. I generally do not like to fight with drivers but the Swallow-TPC is an exception. In the end, it is worth the effort. I now have a mid/high section that is as good as time and phase coherent. The Swallow-TPC is not expensive. The GBS85N cost only $20. Add $22 for the BC25SC08 and they total up to a measly $42.

I am listening to the Swallow-TPC with my Palila bandpass sub as I’m writing and they sound really nice. For those who are not fans of bandpass, a conventional direct radiating subwoofer can be used instead. Actually, using a subwoofer with a plate amplifier and active crossover build-in is an easy solution. For those who are really into diy, use your favourite woofer and build a 3-way.  The Swallow-TPC is very flexible in that you can cross the bass woofer to the GBS85N anywhere from 125Hz to 500Hz.

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.