Modifying my 24 year old B&W P4

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r3xj0hn570n
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Re: Modifying my 24 year old B&W P4

Unread post by r3xj0hn570n »

karatestu wrote: Fri Sep 17, 2021 9:01 pm Thou shalt not rejoice in point and squirt
tell me about this 'point and squirt'. Sounds messy.

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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

r3xj0hn570n wrote: Thu Sep 23, 2021 8:20 am
karatestu wrote: Fri Sep 17, 2021 9:01 pm Thou shalt not rejoice in point and squirt
tell me about this 'point and squirt'. Sounds messy.
It's basically speakers that have all the drivers pointing at you i.e nearly all of them :grin: A phrase that RD used a lot especially for Tannoys and similar. Omni's are not point and squirt.
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Re: Modifying my 24 year old B&W P4

Unread post by r3xj0hn570n »

Ah i see. I suppose the 'rejoice' part is a bit of a mitigation. My downstairs room is a case in point. Carefully controlled directivity is need there. Omnis would be terrible and dipoles won't fit.

Anyway, it will be interesting to see what you make of them. I suspect the woeful efficiency of the wideband driver will make it sound quite sleepy, but who knows.

Hmm, i have a pair of MG12s sitting in a drawer. One day...

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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

r3xj0hn570n wrote: Thu Sep 23, 2021 9:53 pm I suspect the woeful efficiency of the wideband driver will make it sound quite sleepy, but who knows.
Thanks for pissing on my bonfire.

The FR is roughly 84dB. That is the same as the cheap kevlar 5.25" I use and that is before they are heavily doped and pointed at the floor and ceiling. The B&W mid bass is 88dB. I am crossing over in the middle (-3dB) of the baffle step transition. In theory that will give me ~ 4dB of baffle step compensation.

It may not work out, we will see. Nobody in the FR room on diyaudio.com batted an eyelid at my proposal.

As an aside, I don't get people's obsession with high efficiency. Sure, it's nice to have and may sound "alive" but in a small living room at low levels even 80dB efficiency is enough .
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Re: Modifying my 24 year old B&W P4

Unread post by r3xj0hn570n »

karatestu wrote: Fri Sep 24, 2021 6:26 am Thanks for pissing on my bonfire.
Apologies, not everyone want or hears the same things. I didn't mean to discourage you.
karatestu wrote: Fri Sep 24, 2021 6:26 am As an aside, I don't get people's obsession with high efficiency. Sure, it's nice to have and may sound "alive" but in a small living room at low levels even 80dB efficiency is enough .
I'm not sure what it is about it. Once you've heard it it's hard to go back.

Did you figure out how to rebate the back of the baffle? If you have a 1/2" router you could safely extend out a bit and use the hole as a guide.

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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

r3xj0hn570n wrote: Fri Sep 24, 2021 7:37 am
karatestu wrote: Fri Sep 24, 2021 6:26 am Thanks for pissing on my bonfire.
Apologies, not everyone want or hears the same things. I didn't mean to discourage you.
karatestu wrote: Fri Sep 24, 2021 6:26 am As an aside, I don't get people's obsession with high efficiency. Sure, it's nice to have and may sound "alive" but in a small living room at low levels even 80dB efficiency is enough .
I'm not sure what it is about it. Once you've heard it it's hard to go back.

Did you figure out how to rebate the back of the baffle? If you have a 1/2" router you could safely extend out a bit and use the hole as a guide.
I was going to delete my bonfire comment but you were too fast. :grin: Grumpy just got up syndrome. Apologies to you.

I have never had anything over 90dB efficient. I don't want to hear any higher as if what you say is true then I will want it and this speaker building nonsense will never end :lol: I also need to keep the box size sensible and efficient bass usually requires a big box.

No I haven't worked out how to do the internal chamfer with a router yet. The inverted chamfer bits are non existent in this country. I don't quite understand your extending out idea :think:

baffle is 25mm mdf and the driver needs a 3mm rebate in the front. That leaves 22mm and the driver is only 35mm deep all told. It is going to have to be quite an aggressive chamfer
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Re: Modifying my 24 year old B&W P4

Unread post by r3xj0hn570n »

karatestu wrote: Fri Sep 24, 2021 7:57 am No I haven't worked out how to do the internal chamfer with a router yet. The inverted chamfer bits are non existent in this country. I don't quite understand your extending out idea :think:

baffle is 25mm mdf and the driver needs a 3mm rebate in the front. That leaves 22mm and the driver is only 35mm deep all told. It is going to have to be quite an aggressive chamfer
Route it from the front with a dovetail bit that isn't fully seated, using the shank against the already cut hole as a guide. Don't use a 1/4" router bit, it's not safe. 1/2" should be fine though, you can really crank those tight.

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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

Thanks for that, I hadn't thought of dovetail bits. Although looking at them, the biggest is 25mm wide. Take off the shaft width (12mm) and that leaves 13mm divided by two = maximum of 6.5mm taken off. I am not sure that is enough of an angle.
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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

As I will be using these as a stop gap until my spheres of doom are finished I may use a traditional speaker level xover with series inductor for low pass and series cap fir high pass. I was going to do it passively at line level by changing the amp low pass cap to ground.

But I won't be able swap speakers in and out without taking my amps to bits. My sphere creation won't need a low pass cos the mid bass will be driven wide open. That's the problem with passive line level filters using the amp's existing input filters - they are specific for a certain speaker, change speaker and you have to get the soldering iron out. It is without a doubt worth it though - 1st order filters done with no added components, only small film caps required because of the high resistances involved, no nsertion losses, no impedance problems, speaker drivers don't mess with the filter knee frequency as their impedance changes with frequency (no Lpads required).

Not that I will be swapping speakers in and out when the spheres are finished but until then ..........
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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

Today's foolery has been concerned with my B&W P4 speakers. I put them on their sides with 6.5" mid bass forward firing and put the little sphere with 2" FR above it. Did this so I could get the two drivers as close together as possible. Xover is at 3.5kHz using the original inductor for low pass. I had to put two caps in parallel for 11.5uf for the high pass.

Image

Image

It has been so long since I heard any part of these speakers (4 years). I can remember them being a bit harsh and the top end had lost it's sparkle a bit - ferro fluid drying out in the tweeters. As I said earlier the first order low pass at 3.5kHz is not enough to reduce the kevlar cone's nasties and that is still evident.

But bloody hell, there is bags of get up and go compared to the 6.5" doped cheapie. The TL bass is not as tight as sealed but it's not that bad. What I do know is that I moved away from point & squirt for a reason - the sound is delivered in your face and I don't like that anymore. The volume is louder than previously but I can't turn it down to a comparable level - no volume control :grin:

There is quite a bit to like about it. The 2" FR has no problem at all going down to a 1st order 3.5kHz high pass filter at all - unlike a dome tweeter, there is much reduced distortion yet things still feel alive and with a bit of sparkle.

Rex's comment up thread about the expected tone being bass heavy due to a spl mismatch between the two drivers has not occured at all. Yes, they are further from the wall than they would usually be but there is too much mid range if anything. I will put them closer to the wall and that will probably even out the response somewhat. Also my high pass filter may need tweaking.

More to come.......
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