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PFM Special - a simpler and more affordable DIY loudspeaker design

Next step: Upper Baffles (front and back layers):

WIP-TopBaffle.jpg


I haven't drilled the holes for the speaker screws yet. I'm planning to use T-nuts, but I'll sandwich the nut bases between the two layers, so they can't work loose. You'll also not see the screw holes from the back. Here are the steps:
  1. Use a center punch to mark the holes in the front. (BTW, I changed my mind twice on the positioning of the midrange screws, which is why you see all the noise on the left.)
  2. Align the front and back layers, then drill a tiny pilot hole through the top (front) layer, and part way into the bottom (rear) layer.
  3. From the inside (back) of the front layer, use a Forstner bit to create 1mm pocket for the T-nut base.
  4. Use a regular (metal) bit to continue that hole from the back, to create a snug pocket for the T-nut barrel.
  5. Use a another regular bit large enough to just accommodate the M4/M5 screws. This will be drilled from the front face of the front layer, to keep things neat and tidy.
  6. Use that same bit to extend the pilot hole part way into the rear board, in case the screws extend beyond the end of the T-nut.
I think my screws may be short enough that they won't extend into the rear layer, but I may do this regardless, so it acts as as a "backer board" to keep the holes from blowing out.

I'll be doing all this with a drill press, which is rather important given the proximity of those holes to the edge. (It's too late now, but I could have routered those tweeter holes a bit smaller and still accommodated the tweeter body.)

More later the baffles later this week.

In the meantime, I need to do some diagnostics on an NCC300 amp that's started blowing fuses. (I suspect one of the transformers. The bolt wasn't tight enough, so it slid around during transit and I suspect it's been damaged.)
 
BTW, I've been doing some work deciding the layout for the passive crossover components. Recall that I decided there was too much stuff (especially with the larger inductors and capacitors) to fit into the cubby. Consequently, I'm building a separate box that will likely hang on the back of the speakers. I'm considering the following component positions for the midrange (first pic), tweeter and woofer (second pic). Hopefully there's enough space between the various inductors for them not to interfere with each other.

WIP - Crossover Idea - M.jpg

https://avzine.com/images/PFM Special/WIP - Crossover Idea - M.jpg

WIP - Crossover Idea - WT.jpg

https://avzine.com/images/PFM Special/WIP - Crossover Idea - WT.jpg
 
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I've no idea why the second image isn't appearing for me in the prior message. :confused: I've tested a few things, and it should be fine. If it doesn't appear for you, click the hotlink below the "image missing" placeholder to see the picture.
 
Mike Hanson said:
I haven't drilled the holes for the speaker screws yet. I'm planning to use T-nuts, but I'll sandwich the nut bases between the two layers, so they can't work loose. You'll also not see the screw holes from the back. Here are the steps:
  1. Use a center punch to mark the holes in the front. (BTW, I changed my mind twice on the positioning of the midrange screws, which is why you see all the noise on the left.)
  2. Align the front and back layers, then drill a tiny pilot hole through the top (front) layer, and part way into the bottom (rear) layer.
  3. From the inside (back) of the front layer, use a Forstner bit to create 1mm pocket for the T-nut base.
  4. Use a regular (metal) bit to continue that hole from the back, to create a snug pocket for the T-nut barrel.
  5. Use a another regular bit large enough to just accommodate the M4/M5 screws. This will be drilled from the front face of the front layer, to keep things neat and tidy.
  6. Use that same bit to extend the pilot hole part way into the rear board, in case the screws extend beyond the end of the T-nut.
I think my screws may be short enough that they won't extend into the rear layer, but I may do this regardless, so it acts as as a "backer board" to keep the holes from blowing out.

I'll be doing all this with a drill press, which is rather important given the proximity of those holes to the edge. (It's too late now, but I could have routered those tweeter holes a bit smaller and still accommodated the tweeter body.)

More later the baffles later this week.
Click to expand...
You overthink like I tend to :D

Concealed T-nuts may be a neat solution until you suffer a cross-threading mistake and you can't re-seat or re-align them. Since you are using ply, a much neater solution is to use wood screws. There is plenty of purchase and I'd certainly not recommend using T-nuts for the tweeter.

Just need to drill perpendicular pilot holes and you're done.
 
James said:
You overthink like I tend to :D

Concealed T-nuts may be a neat solution until you suffer a cross-threading mistake and you can't re-seat or re-align them. Since you are using ply, a much neater solution is to use wood screws. There is plenty of purchase and I'd certainly not recommend using T-nuts for the tweeter.

Just need to drill perpendicular pilot holes and you're done.
Click to expand...
You may be right. I'll put further thought into it. ;)
 
James said:
You overthink like I tend to :D

Concealed T-nuts may be a neat solution until you suffer a cross-threading mistake and you can't re-seat or re-align them. Since you are using ply, a much neater solution is to use wood screws. There is plenty of purchase and I'd certainly not recommend using T-nuts for the tweeter.

Just need to drill perpendicular pilot holes and you're done.
Click to expand...
One issue with wood screws (especially for the tweeter) is that the holes are quite near the edge. I'm concerned that the pressure of the screw will easily blow out the edge of the ledge.

Given your concern with embedded T-nuts going south, perhaps I should just mount them on the back of the baffle, even though it doesn't look nearly as nice.
 
I like the cloverleaf PCB from soundimports.eu for the crossover PCB. They are located in The Netherlands and shipping costs was very reasonable to New Zealand.

https://www.soundimports.eu/en/crossover-components/printed-circuit-boards-pcb/

I do not like using T-nuts: there is a flexible gasket between driver and cabinet that can result is slight movement of the driver. KEF in the KEF 101 mounted the B110 on some rubber gasket and used rubber washers between the fastening bolts and B110 chassis to prevent any audible issues. I've seen pictures of some KEF unique drivers mounted in the same way.

I would not worry about using woodscrews - ProAc uses MDF and woodscrews and MDF is not as strong as plywood. Pre-drill properly is the key here. (As long as you do not plan on removing the drivers umpteen times.)
 
Mike Hanson said:
One issue with wood screws (especially for the tweeter) is that the holes are quite near the edge. I'm concerned that the pressure of the screw will easily blow out the edge of the ledge.
Click to expand...
If you think there isn't enough of a ledge for wood-screws, you definitely won't have enough for T-nuts or threaded inserts (with are better, by the way). Of you pre-drill for the diameter of the screw shank, the substrate won't blow out.
 
Rather than T nuts I used threaded inserts, embedded in the back panel of the top baffle. And yes they were very close to the edge of the tweeter baffle, took some careful drilling that did :)

With an insert you simply have a little brass well showing at the back, not as "rough" as the wings of a T nut.

In fact I used threaded inserts for all the speaker mounting hardware.
 
My boxes are particle board with a much thinner mdf skin a la Peter Comeau and as I don’t know how to use a router , by cutting a larger circle in the mdf than the one in the particle board I get inset drivers easily. I have taken out my drivers umpteen times and if ever I note that a wood screw holding them begins not to bite as confidently as before I insert a wooden matchstick, break it off and then my old allen headed screws bite into new wood.
 
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You can drill the affected hole out and used fluted wooden dowels, glued in place. These repair a damaged hole nicely. Leave slightly proud and trim back with a sharp chisel. Then pre-drill with say a 2.5mm drill for #8 wood screws. Used this on a damaged screw hole on a kitchen cabinet.
 
chiily said:
Rather than T nuts I used threaded inserts, embedded in the back panel of the top baffle. And yes they were very close to the edge of the tweeter baffle, took some careful drilling that did :)

With an insert you simply have a little brass well showing at the back, not as "rough" as the wings of a T nut.

In fact I used threaded inserts for all the speaker mounting hardware.
Click to expand...
I think I was following your lead too closely this time. :) I noticed the hole on the back panel was a bit large, and should have made the hole a bit smaller for the front panel.

And I just realized I could have simply measured the plastic that perfectly cradles the tweeters in their packaging. A slight bit bigger than that would have been perfect.

I do have a few off-cuts. If I don't need them for anything else, a may recreate the front panels with smaller tweeter holes.
 
Mike Hanson said:
I think I was following your lead too closely this time. :) I noticed the hole on the back panel was a bit large, and should have made the hole a bit smaller for the front panel.

And I just realized I could have simply measured the plastic that perfectly cradles the tweeters in their packaging. A slight bit bigger than that would have been perfect.
Click to expand...
One of the things I learnt early in my loudspeaker adventures is to take measurements directly from the drivers to ascertain the size of the cuts I need to make, so they fit snugly and flush. Making test cuts on scraps and making small adjustments is also a good idea.
 
James said:
One of the things I learnt early in my loudspeaker adventures is to take measurements directly from the drivers to ascertain the size of the cuts I need to make, so they fit snugly and flush. Making test cuts on scraps and making small adjustments is also a good idea.
Click to expand...
Oh, I learnt that lesson too.

And, always double check the tightness of the router bit in the router before cutting a rebate. Having the bit drop, spin and still be cutting makes a real mess.
 
I've verified that I've got a couple of nice off-cuts of the BB to redo the front panels of the upper baffles. I'm going to salvage portions of the incorrectly routered baffle boards to form the back panels. Then I'm going to sandwich them together before routing, and edit the routing vectors to do it correctly this time (with the two layers attached).

I understand the risk of mounting the T-nuts between the panels (especially inside the sealed bass cabinet), so I've given up on that. I'll probably use threaded inserts, as Garf suggested.
 
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Just with the inserts, drill the hole slightly smaller than the OD of the insert, then tap in to place from the back. I used a touch of Araldite to make sure it wasn't going anywhere. Finally, I used a threaded bolt to hold the insert square in the hole.

2007_08010267.resized by Garf Arf, on Flickr
 
Well, I think we'd all agree an important milestone anniversary and vacations are much MUCH more important than making loudspeakers. You've got your priorities right.
 
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