Dayton Audio D250P Polyimide Compression Driver Review

Dayton Audio D250P With more than ten compression drivers in my collection, I have more than enough to play with. But the majority of them are with Titanium diaphragms with a few exceptions made of aluminum. Since this D250P is with a Polyimide diaphragm, it would be a first for me. I’ve never heard this driver before, so I’m approaching it with an open mind. Before I proceed further, as in all my other posts, this is not a paid review.
Manufacturer’s Frequency Response Plot On paper, this Dayton looks promising. Frequency response is 1kHz~20kHz. Minimum crossover recommended at 1.6kHz/12dB slope. And looking at the response plot, it is pretty flat with a Dayton H812 horn.	(Fig 1)
Measured Frequency Response I did a sweep with the H812 horn attached. Bass was removed to protect the driver. Fig 2 shows a chunk of frequencies missing from 2kHz to 3kHz. This is clearly not in Fig 1, which is from the manufacturer. My first thought was perhaps it’s a manufacturing defect. If that be the case, there’s really nothing I can do. Nonetheless, I persevered. Next is to measure distortion.	(Fig 2)
(Fig 3)
I was absolutely horrified by the results. I’ve never seen such high distortion before. What concerns me most is the 3rd harmonic, which is the Violet trace. This is another indication that something is not right with the compression driver. Undeterred, I proceeded to disassemble the D250P. First to come off is the end cap. It looks a bit flimsy with the thin plastic but I doubt that is the cause. Next is the polyimide diaphragm. To remove it, simply lift it by the speaker terminals. The voice coil looks fine. No deformity on the diaphragm. After that, I checked the voice coil gap.To my surprise, there were debris inside. On careful examination with a magnifying glass, the largest piece was about 1mm in diameter, shaped like a doughnut. To compound matters, it’s made of iron which makes it difficult to remove because the magnet made it stick to the side wall. It took numerous attempts but half an hour later, I managed to clean up the gap. That offending doughnut was removed, along with all the smaller ones. It looked to me like they were remnants of a milling process. I then reassembled the driver and anxiously did another distortion sweep.
(Fig 4)
Fig 4 is the new distortion sweep with the gap cleaned. I’m relieved to see distortion is now much lower, in particular, the worrisome 3rd harmonic. Eventhough it may seem high at 2kHz, it’s about 40dB below the fundamental. This works out to approximately 0.5%. Bear in mind that no crossovers are used in this sweep. So what we’re seeing is worse case scenario.
Encouraged by the lower distortion, I eagerly await the frequency response sweep.The Blue trace in Fig 5 is with the clean gap. Compared with the Black trace, the one with the debris, the new plot shows a smoother response. That huge chunk of frequencies from 2kHz to 3kHz that were missing have been restored.It’s safe to assume that this is what the D250P should be. There’s nothing else I can do to improve on it. Next is to get a speaker working so that the D250P can be auditioned.	(Fig 5)

Auditioning the Dayton D250P For convenience, the D250P/H812 combo is placed on top of the Dayton RS180S (Fig 6). The R180S is loaded onto a compact 19 Liters bass reflex box tuned for an F3 of 45Hz.Even though Dayton recommends the minimum crossover to be at 1.6kHz/12dB, I decided to cross at 950Hz with a 24dB slope. If the D250P doesn’t complain, then it would be an ideal frequency to mate with my 12″ Dayton PA310 (future project).	(Fig 6)
(Fig 7)
I ran more distortion sweeps on the D250P/H812 combo, this time through an electronic crossover. High Pass was set at 950Hz, 24dB/oct Linkwitz-Riley. I was hoping to see lower distortion with a 24dB slope. My wishes came true. In Fig 7, the 2nd harmonic is reduced considerably. More importantly, the 3rd harmonic up to about 1.5kHz, is 60dB below the fundamental. This is a massive improvement. It would have been fantastic if it maintained at this level throughout but considering the low price of this compression driver, one can’t expect perfection.
Now that distortion is acceptable, the 2-way can finally be crossed. In Fig 8, the Black trace is the Dayton RS180S and the Blue trace is the D250P/H812. Both crossed at 950Hz (24dB/oct). Disregard the measurements below 500Hz. My room is interfering with the measurement as it is no longer gated.	(Fig 8)
The Red trace in Fig 9 is the summed response of the RS180S and the D250P/H812. Due to the offset of the drivers’ acoustic centers, a notch resulted at about 1.2kHz. What is also visible is there’s summing on the left of the notch and cancellation on the right. Simply reversing the polarity of the D250P will not resolve this. The only way is to Time-Align the two drivers.	(Fig 9)
In order to do this, the D250P is re-wired for reversed phase. A delay is then applied to the RS180S, which is the driver closer to the microphone. When the delay time is correct, the sweep will show a notch centering at 950Hz. In Fig 10, the Red trace recorded a beautiful high-Q, 35dB deep notch. This indicates correct time alignment of the two drivers.	(Fig 10)
In Fig 11, the Red trace is the summed response with the D250P re-wired back to normal phase. Notice that there are no cancellations on either side of 950Hz. And with a 24dB/oct, Linkwitz-Riley, it sums flat. Now, sounds from the RS180S and the D250P arrive at the microphone at the same time.	(Fig 11)
Sound Quality After a few days of listening, this polyimide diaphragm indeed sounds different. Most apparent is the absence of brightness that’s associated with titanium diaphragms. By the same token, it may be too “mellow” for some. The highs are there but they lack the sparkle in the notes. A good comparison would be the sound of a nylon string versus that of steel in an acoustic guitar. I don’t know whether this polyimide diaphragm can be classified as more hi-fi sounding. What I do know is that it is not for me. It’s simply too dull. The music is lifeless and sounds compressed. My Selenium D220Ti is clearly more lifelike. Music is more dynamic and there’s depth. This in no way implies that the D250P is a bad driver. Far from it. It’s just a question of preference. For those that can’t stand titanium diaphragms, give this D250P a listen. It may just be what you’re looking for. All measurements were made with the microphone at 1 meter, on tweeter axis. Gating is at 5 msec and No Smoothing applied.