Swallow – a Time and Phase Coherent Satellite


The Swallow is a fairly compact, 7 liters, 2-way satellite. It uses a very unusual driver, a GBS85N25PR03-04 midrange by Peerless. This driver is only 14.5mm deep. That’s 0.57 inches. If it works the way I intended, it would be a perfect candidate for a time and phase coherent design. I then would not need to slant the front baffle or use a step to align the acoustic centers of the woofer and the tweeter.

Fig 1 – GBS85 RAW response

It is with much anticipation when I measured the response of the GBS85. I expected to see a flat response as what is in their datasheet but what I got is quite a few waves from 1kHz~4kHz (Fig 1). That’s going to be a problem because it is exactly where my crossover will be.  

With a response like that, I would normally cross at 2.5kHz or less but that would mean having to use one of my more expensive tweeter. But it’s not simply the cost. The acoustic center of my Seas 27TBFC is farther back than this GBS85. I want to keep the design simple and cost as low as possible. There’s where my cheapo Audax TW010E1 came to the rescue. This is also a low profile tweeter. The acoustic center is almost the same as the GBS85. 

Fig 2 – GBS85 with Low Pass Filter | TW010E1 with High Pass Filter

The Red plot in Fig 2 is from the TW010E1. I just love this tweeter. All it took was one capacitor to get this beautiful response. The only weakness is a fairly high crossover frequency. This means that the GBS85 needs to cross at 4.5kHz. From the RAW response in Fig 1, that is very close to the cone breakup peak of 5kHz.

This is not an ideal crossover point but there’s not much of a choice if I want to use the TW010E1. After a few adjustments, I managed to tame the 5kHz peak so that the GBS85 (Blue plot) can cross with the TW010E1 without messing up the summing.

Fig 3 – Swallow Frequency Response

Fig 3 is the summation of the GBS85 with the TW010E1. It took me a while to get this response. Most of the time was spent on trying to control the GBS85. In the end, there’s a slight bulge at 5kHz. I doubt I can hear it on playback. I would be worried if it appears in the midrange though.

Fig 4 – Swallow Null

To check on the crossover, I flipped the tweeter wires around. I was quite relieved to see a notch at 4.5kHz. It is not as deep as I would like but it does show that the two drivers are crossing properly.

With the basic measurements out of the way, the next plot is the Step Response. This will reveal how close the acoustic centers of the two drivers are. 

Fig 5 – Swallow Step Response

I had a big smile when I saw the Step in Fig 5. This is what I’ve been aiming for. An extremely fast, clean transient with the apex almost at 0 msec. For all intents and purposes, this is time and phase coherent, just like the Thiel.

The beauty of the Swallow is I didn’t have to slant the front baffle or use a step for the woofer. The low profile design of the GBS85 automatically aligns this midrange with the acoustic center of the Audax TW010E1. Simple yet effective.

Fig 6 – Swallow Waterfall

The Swallow’s waterfall plot shows some artifacts in the treble. In particular are the two strong decays at 16kHz and 9kHz. They are not from the TW010E1 tweeter but the GBS85’s cone breakup.

Fig 7 – Swallow Toneburst Energy Storage

The Toneburst plot shows stored energy (light blue slices) from 5kHz~10kHz. These coincide with the cone breakup of the GBS85.

Fig 8 – Swallow Spectrogram

The Spectrogram offers a clearer picture of the stored energy when they are plotted against time. There is an obvious hot spot at 2.8kHz. That’s caused by diffraction at the baffle edge. Below 2.8kHz, there are a series of spikes but I doubt I can hear them. Their energy is dissipated by 6ms.

Sound Quality

This is my first time listening to the GBS85. Surprisingly, it sounds good. I was fearful that the sound quality would be compromised with the low profile but it was unwarranted. It is a well designed driver. With music playback, I couldn’t detect any flaws.

When coupled with the Audax TW010E1 tweeter, the sound quality improved tremendously. Gone is the brittleness in the treble. That’s the difference between a real tweeter and treble derived from cone breakup.

String instruments like acoustic guitars are perfect for the Swallow. I played Yoshio Kimura Audiophile Selections and it was a joy to listen to. The clarity and body of the notes are a cut above the norm.

The Swallow excels in voices too. Dean Martin’s Innamorata came out crystal clear. There’s no nasalness in his singing. For female vocals, I put on Nana Mouskouri. Her vibrato is exceptionally demanding on speakers. Any issues with the crossover and she’ll sound shrill. After listening to Amazing Grace and Only You, the Swallow passed with flying colors.

The next album is from The Mamas and the Papas. I want to see how well the Swallow fares with old analog recordings of the 60s. What I’m listening for is the quality of the vocal harmony. The Swallow did not disappoint. In California Dreamin and Dedicated to the One I Love, the vocal separation is superb.

Extending the Bandwidth

Now that I got the Swallow satellite sorted out, my next target is the bass. To keep things simple, I integrated my Toucan-SF to the Swallow. That extended the bass down to 40Hz. What a difference it made. The music is finally complete. 

In Summary

I am very excited with the success of this project. It is not easy to get quality sound with budget drivers. In the Swallow, it’s all possible because of the Peerless GBS85 midrange. I can foresee more Time and Phase Coherent projects in future based on this GBS85.

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.