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Seas 27TBCD/GB-DXT with Scanspeak PL18WO-06-08 The Goldfinch-DXT is an evaluation of the Seas 27TBCD/GB-DXT tweeter. I paired it with a Scanspeak PL18WO-06-08 out of convenience. If I like the sound of the DXT, I will test it with the other midwoofers I have at a later date.
This Seas DXT tweeter has been around for maybe 10 years. When I came across it, I shied away because I didn’t want to grapple with a response like that (Fig 1). I bought the 27TDFC and the 27TBFC instead. Years passed and one day, I said to myself, “Why not” and here we are.
First, I wanted to see the response of the DXT with it surface mounted (Blue plot in Fig 2). Due to the thickness of the face plate and the very short horn design, it resulted in two notches, one at 9kHz and the other at 18kHz. These are caused by diffraction between the edge of the DXT faceplate and the front baffle. When I flush mounted the DXT (Black plot), the roll-off is perfectly linear. Notice that at 2.5kHz, there is another notch, not as severe as the one at 9kHz but nonetheless, it’s there. Flush mounting the DXT, helped to lessen it but the depression is still there. This notch comes from diffraction at the edge of the front baffle. To completely eliminate it, I will have to either chamfer the edge or round it off. I don’t have time for that now. I will continue with it flush mounted as is. I can deal with the edge diffraction later if I decide to make a proper box.
The Red plot in Fig 3 is the DXT with a 3rd order filter set at 1.5kHz. This is an insanely low crossover frequency but I wanted to see how well the DXT cope with it.
Now that the Seas DXT is settled, let’s have a look at the Scanspeak PL18WO-06-08. Fig 4 is the RAW response in a 15 liters bass reflex. We can see the midrange rising response due to the baffle step. At 1.5kHz to 4kHz, the PL18 is incredibly flat. There’s vitually no cone breakup, save for a minor peak at 6kHz. Note how smooth the roll-off is. This midwoofer is a dream to cross with a 1st order filter(Goldfinch). Unfortunately, for this project, it is not so simple.
After numerous attempts, finally tuning by ear, I found the right response. The difficulty is not so much the slope of the crossover but the balance between the bass and the midrange. Too much bass and it will completely overwhelm the midrange. The Blue plot in Fig 5 is the final response made with a 2nd order network.
Fig 6 shows the two drivers with their respective filters. As I had hoped, they are crossing at 1.5kHz. The PL18 is rolling-off at 15dB/oct even though the electrical filter is 2nd order. The DXT is at 24dB/oct with an electrical 3rd order network. By the way, this is not your textbook crossovers. I tweaked them to get the slopes as such. As some might have guessed, I’m using a asymmetrical crossover.
The Black plot in Fig 7 is the summation of the PL18-06-08 with the Seas DXT. No cancellations are seen in the crossover passband, indicating good summing. Note the slight depression at 3kHz. That is from the DXT. There’s nothing I can do about it until I build a proper box.
The final frequency response of the Goldfinch-DXT is in Fig 8. Measurements below 500Hz are in nearfield. The Goldfinch is flat from 500Hz to 8kHz (+/- 2.5dB). After that, it rolls off gently at about 6dB/oct. It would be great if I could get the super high frequencies (10kHz to 20kHz) to be at the same level as the rest but I don’t want to mess with the crossover now. It is complicated enough as it is.
I’m relieved to see a well defined null at 1.5kHz. It is not -40dB deep but it’s good enough. It shows the phase of the two drivers are quite close.
The Step Response shows a very clean transient in the Scanspeak PL18. There are absolutely no breaks in the attack. It is not as fast as some of my other midwoofers of the same size but looking at the overall response, including the smooth decay, I would say it’s pretty impressive.
The Waterfall shows very few artifact from 2kHz upwards. Looks like the Seas DXT tweeter is quite clean.
Fig 12 – Goldfinch-DXT Toneburst Energy Storage The Toneburst plot recorded very light excess energy. This is actually very impressive. I’ve seen much worse.
As expected, the Spectrogram showsonly one streak at 1kHz. There are a couple of hot spots at 2kHz. Neither one of them is audible because they are dissipated by 6 msec. About the Seas 27TBCD/GB-DXTWhat started out as an evaluation of the Seas 27TBCD/GB-DXT ended up as another project. Before I touch on the Goldfinch-DXT, I shall address my findings of the DXT first. To be frank, I was flabbergasted when I heard the DXT. I wasted my money buying the 27TDFC and the 27TBFC. The DXT is in a class of her own. What sets her apart is the details in the treble. I could distinctly pick out the layering in the cymbals. It’s as though the DXT is able to produce the harmonics that most tweeters fail to. As for crossing her at 1.5kHz, I didn’t hear her complain. But then again, I wasn’t playing at insane volume. That’s not my way of enjoying music. The downside of the DXT is she’s not a friendly tweeter to work with. Actually, it’s not just the DXT. My ER18RNX and U18RNX share the same difficulties. It appears that Seas manufacture their products strictly to meet their engineering objectives. If their frequency responses ends up looking like whatever, well it’s up to the loudspeaker designers to work it out. the Goldfinch-DXTI must say I am more than pleased to be able to get this sound quality out of the Goldfinch-DXT. It took quite a bit of effort and a keen ear to make her sing like this. In the end, the crossover is rather complex but it’s well worth the cost. Now, this Goldfinch-DXT is at the level of the Starling-V and the Myna. Unless otherwise stated, all measurements were made with the mic at 36 ins, tweeter axis. Impulse Window=5ms. No smoothing applied. |
December 9, 2020HIFI DRIVERS, Projects
Fig 2 – Seas 27TBCD/GB-DXT RAW Response • Baffle Width=9.0″
Fig 3 – Seas 27TBCD/GB-DXT RAW and High Pass Filter
Fig 4 – Scanspeak PL18WO-06-06 RAW Response
Fig 5 – Scanspeak PL18WO-06-08 with Low Pass Filter
Fig 6 – Scanspeak PL18WO-06-08 Low Pass • Seas 27TBCD/GB-DXT High Pass
Fig 7 – Crossover Passband
Fig 8 – Goldfinch-DXT Frequency Response
Fig 9 – Goldfinch-DXT Null
Fig 10 – Goldfinch-DXT Step Response
Fig 11 – Goldfinch-DXT Waterfall
Fig 13 – Goldfinch-DXT Spectrogram