Yet another Purifi SPK5 speaker

[Rick Sykora]

After shipping damage did in the first SPK5 speaker @sgoldwin sent to Amir to test, another was sent with the crossover mounted internally. The second had an unknown issue, but was sent to me to fix. I later identified the issue as an incorrect polarity marking on the crossover board. Not sure where it happened, but the brace had broken free too and I had no simple way to fix. Rather than have me fix, @sgoldwin decided to send me another. This was around the holidays and it never made it to me ...

After some more iterations that need not detail here, we figured out 2 things. First, despite appearances, the Purifi crossover was NOT meant to be mounted on the brace. Second, the back baffle was leaking and needed to be sealed. So this set of posts documents the design mods done to redress.

Let’s start with the before pic...

Note the crossover mounted on the brace and the nice foam tape used to seal the back baffle. This pic also lets you see the twisty port design of the SPK5. Not as obvious is the wear spot where it is rubbing against the woofer coil (notice the light area on the edge at about eight o’clock).

 

[Rick Sykora]

...In case you do not know where to look, the wear spot is here...

This was keeping the back from being able to close completely and allowing air to leak. So removed the crossover and split it into the woofer and tweeter sections. That looks like this...

Note the small terminal strip mounted on the upper right corner of the small board. This was my solution to provide strain relief to the fine wire of the small inductor just below it. This inductor requires a ground connection, so knew it could be damaged when external wiring gets pulled.

Next these boards were mounted into the cabinet on the right land left sides. Like so...

This allowed the back baffle to seat properly and still had room for the port.

 

[Rick Sykora]

Next will tackle the SPK5 brace. In at least one build, the brace came loose in shipping. I surmise this was due to the additional weight of the crossover and some rough handling in shipping. After considering the other builds, went back to original tested by Amir. While it still had a plywood brace, decided to replace it with an MDF one and secure with some small cleats and wood screws. See pic in previous post.

Suspect glue would suffice normally but since this SPK5 needed to be worthy of world travel, the extra bolstering was a precaution. After these changes, unfortunately the cabinet was still leaking as shown by the impedance curve...

As with Amir’s original test, the peaks and valley are not at the frequencies from the Purifi design spec and the magnitude of the right peak is way too low. For the less experienced, the left peak is the port resonant frequency, the right one is the woofer resonant frequency and the valley between them is the box resonant frequency. In this case, the box tuning is around 70 Hz and should be around 30 Hz.

 

[Rick Sykora]

One way to diagnose cabinet leaks is with a stethoscope. They are relatively inexpensive and are a must for larger boxes or multiple driver speakers (like a line array). In this case, went the quick route as already suspected the back baffle was still leaking. So, used wide masking tape to temporarily seal the baffle joints and saw immediate improvement as shown here...

Now the impedance magnitudes are better defined, but more key is the box tuning and other resonances are at the specified frequencies. Since the masking tape looks pretty ugly, my normal solution would be to glue on the back baffle. This was deemed unacceptable, so pondered other approaches. @sgoldwin had already used 3/8 inch gasketing tape along the back bevel of rev 2, but rev 1 did not have. I happened to have 3/4 inch weatherstripping tape but seemed a bit thick at 3/16 inch. The tape was wider than the bevel, but was handy. I removed the masking tape and applied the weatherstripping tape. I was concerned the screws might catch on the tape, so before screwing on the baffle, I used a countersink to prepunch screw holes in the tape. To my surprise, after torquing evenly, the baffle was now airtight!

 

[Rick Sykora]

So, what's next? Should mention that getting the SPK5 tuning close was also a matter of getting the damping right. The specs call for a cotton-based material and plenty of it. I did not use near a much as the spec required as am pretty sure the port airflow would have been affected.

As you might surmise, I shipped the speaker back to Amir. Expect you will hear more from him about it soon. I ended up with parts for SPK5s as comp for my time. Since I had it around, already tried the SPK5 with the SB 5x8 passive radiator and so will be sharing those results. Also plan to use with a minidsp and try as active speaker. Hoping to try with lower crossover point for the AMT. Am open to other tinkering (another tweeter maybe?), so let me know if you have ideas to try.

 

[Dj7675]

So, what's next? Should mention that getting the SPK5 tuning close was also a matter of getting the damping right. The specs call for a cotton-based material and plenty of it. I did not use near a much as the spec required as am pretty sure the port airflow would have been affected.

As you might surmise, I shipped the speaker back to Amir. Expect you will hear more from him about it soon. I ended up with parts for SPK5s as comp for my time. Since I had it around, already tried the SPK5 with the SB 5x8 passive radiator and so will be sharing those results. Also plan to use with a minidsp and try as active speaker. Hoping to try with lower crossover point for the AMT. Am open to other tinkering (another tweeter maybe?), so let me know if you have ideas to try.

Look forward to the re-review.