PURIFI finally did a fully purified passive speaker design! The SPK 16 prototype is here - with a PTT tweeter

[Lars Risbo]

if going active you can use 3 active woofers and do cardioid etc

 

[Paul7777x]

if going active you can use 3 active woofers and do cardioid etc

Sir.

Just as I think I’m getting my head round the whole concept of my final, superb, speakers and eventually even getting them to be active … you go and throw in some obvious witchcraft.

 

[boXem]

The choice active / passive is normally done at the beginning of the design process, not at the end.

 

[Paul7777x]

The choice active / passive is normally done at the beginning of the design process, not at the end.

Indeed.

But with the impressive parameters of the Purifi drivers and the obvious care taken with the cabinet design I’d suggest there’s someone around here who could/will do an impressive job of designing an active x-over with what the SPK16 gives them to work with.

 

[Luvchampagne]

if going active you can use 3 active woofers and do cardioid etc

I'm one of the few where cardioid isn't beneficial for my use case as I have multiple mlps and sitting down for longer amounts of time is not medically possible.

 

[kyuu]

I'm one of the few where cardioid isn't beneficial for my use case as I have multiple mlps and sitting down for longer amounts of time is not medically possible.

I think you're more of the norm than people who sit still in a dedicated listening position for hours on end. Maybe not around here, where we have a lot of audiophile (minus the woo, mostly) folks, but certainly in general.

 

[Salt]

if going active you can use 3 active woofers and do cardioid etc

So, if I understood correclty, You would estimate,that an active approach with this enclosure could not only be realized but might give some advantage to former optimazation?

 

[bmc0]

[...] force of well over 100lb if it's hitting Xmech at 1Khz+...?

The voice coil would vaporize if you tried to hit Xmech at 1kHz.

 

[Roland68]

The choice active / passive is normally done at the beginning of the design process, not at the end.

In theory, yes; in reality, rather not .

 

[kemmler3D]

The voice coil would vaporize if you tried to hit Xmech at 1kHz.

I suspected this didn't actually make sense, but I lack the engineering knowledge to use the power rating (250w) to determine the maximum force we need to worry about for the housing.

 

[kemmler3D]

I just did a very rough quick test to see if a one-piece design with internal structure is an option.
It should work.

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Fun stuff, but I think the overhangs/bridges on the bracing will be too long at full size...?

 

[Luvchampagne]

The choice active / passive is normally done at the beginning of the design process, not at the end.

Are you sure?
I know some extremely prominent and popular speaker designers that design as an active speaker then once the design is locked in, including wave guide and other innumerable variables, then iterate on the passive cross over. This approach offers lots of benefits including but not limited to rapid prototyping.
Your view is very old school.

 

[bmc0]

I lack the engineering knowledge to use the power rating (250w) to determine the maximum force we need to worry about for the housing.

As a first approximation, you can compute the peak motor force. At 1kHz, the impedance is ~5Ω, so peak current at 250W is sqrt(2×250W/5Ω) = 10A. Bl is 7.7 N/A per the datasheet, so motor force at 10A is 77N or 17.3lbf (7.85kgf). The cabinet isn't infinitely stiff though; any force causes deflection, which means all the heavy bits of the driver need to move some amount in order to apply a force to the cabinet.
In practice, this is a non-issue. Sound quality would be severely affected way before you'd really need to worry about vibrational forces damaging the cabinet.

Edit: Forgot to add before that I'm not a mechanical engineer, so the above should be taken more as a layman's musings rather than authoritative.

 

[kemmler3D]

As a first approximation, you can compute the peak motor force. At 1kHz, the impedance is ~5Ω, so peak current at 250W is sqrt(2×250W/5Ω) = 10A. Bl is 7.7 N/A per the datasheet, so motor force at 10A is 77N or 17.3lbf (7.85kgf). The cabinet isn't infinitely stiff though; any force causes deflection, which means all the heavy bits of the driver need to move some amount in order to apply a force to the cabinet.
In practice, this is a non-issue. Sound quality would be severely affected way before you'd really need to worry about vibrational forces damaging the cabinet.

Thanks, super helpful. It seems like the force will be very much more in line with what I expect PLA or PETG to handle. This makes more sense than what I had posted previously. I think PETG is still probably preferable, the forces aren't trivial here, but at least I'm not worried the design needs major modification.

 

[kemmler3D]

A plate amp will necessarily remove some of the bracing and foam will it not?

I think the cabinet would need to have a compartment added for it, otherwise you're going to throw off the net FR.

A couple of power amps on the outside, or a four channel Purifi amp perhaps?

I’ve never seen the need for a plate amp really.

Saves desk space...

 

[Luvchampagne]

I think the cabinet would need to have a compartment added for it, otherwise you're going to throw off the net FR.

Saves desk space...

With all due respect, why would you prefer to save desk space vs. The inevitable replacing of a plate amp? What if you can't get the same plate amp or a plate amp with the same dimensions?
There's an enlightening thread on hypex capacitor failures. The eval boards use cheap caps that are sub 2k hour rated. Even the Nichicon caps are rated at 7k hours. I admit I'm an outlier since I listen to my system for 8+ hours per day. Work. Relaxing. Tinnitus therapy. But even in kit form the Purifi speaker kit plus cabinets are not cheap. For me this would be approximately 3 to 4 years.

You make many highly intelligent posts and I've learned lots from them. I don't agree with you on this one.

 

[bmc0]

Just for a bit of fun, I was curious how much power it would take in theory to drive the cone to Xmech (±14.5mm) at 1kHz assuming linear behavior (a big assumption in this case). Peak excursion is F/(ω²m), so F = 14.5mm×(2π×1000Hz)²×29.8g → 17059N (3835lbf). Current would then be I=F/Bl → 17059N/(7.7N/A) → 2215A, and apparent power would be P=I²×R → (2215A)²×5Ω → 24.54MW. Vaporize was the right word, I think .

p.s. The sound level at 1m would be approx. 160dBSPL. A little beyond what most people need.

 

[Paul7777x]

I think the cabinet would need to have a compartment added for it, otherwise you're going to throw off the net FR.

Saves desk space...

Aha... good point, but mine will be a home hifi. I’ll just make or buy a rack or stand appropriate to the number and size of any components.

 

[boXem]

Are you sure?
I know some extremely prominent and popular speaker designers that design as an active speaker then once the design is locked in, including wave guide and other innumerable variables, then iterate on the passive cross over. This approach offers lots of benefits including but not limited to rapid prototyping.
Your view is very old school.

They take the decision at the beginning to design a passive speaker and use active crossovers during development for the "lots of benefits" that you didn't care to list. Cool.
If the decision is taken to go active at the beginning, some basic choices are just not the same. Cabinet dimensions and target responses notably.

In theory, yes; in reality, rather not .

Theory has a tendency to forget bean counters, marketers and managers. Theory is the only safe space for engineers these days.

 

[Audionaut]

because the Purifi woofer itself is unbelievably performant (it's basically alien technology)