Modifying my 24 year old B&W P4

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

Unread post by karatestu »

Bit more progress.

90 degree upvc bend 68mm

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I chopped a bit off so the down pipe will be closer to the back of the speaker and a bit more hidden from view.

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Attached the bend over the protruding yellow pipe temporarily with blutack just to test ergonomically and visually.

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I installed a rough bit of pipe into it just to see what it would look like

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View from in front of the speaker. You can't really see the bend until you get quite close or you are vertically challenged.

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View from above

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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

I could have gone for something fancier than black upvc but the bends were not the right size or shape.

The 68mm upvc bend is just slightly too big (yellow tapering pipe is 60mm across at its end and this bend is for 68mm pipe. It covers the yellow pipe nicely though and keeps the pipe close to the speaker's back panel.

The rounded bend is good (rather than a sharp 90 degree change in direction) as it is seen less from the front. It also helps the sound waves go round the bend :lol: and through more fibreglass - could be bullshit that bit :think:

The pipe through the speaker tapers from 86mm OD where the driver is mounted to 60mm OD where it terminates at the rear of the enclosure. The bend is for 68mm pipe. If I want this pipe on the outside of the speaker to continue tapering then I have to use another of these yellow pipes and cut it to 68mm diameter to fit on the bend. Otherwise I could just use the straight 68mm black upvc pipe. Not decided on this yet.

In transmission lines the tapering allows for a shorter line length. This is when you are trying to use the lines output to augment the driver. In this case I don't want any output from the end of the pipe. Not sure how critical the tapering of this back pipe is in this situation :think:
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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

What the hell is going on ? I have assembled these speakers with the original mid bass which has dust cap removed, voice coil vent stuffed and cone doped with a bit of wickes black gutter sealant. Never goes hard and can be removed quite easily unlike that evil bison kit adhesive.

Tweeter is a 60mm wide range driver in a tapering tube stuffed with fibreglass.

I fell out with point and squirt when I first tried a semi omni. I am starting to think that maybe I should not be so against it. Class D amps anybody ?

There is much to like about these speakers but they do sound different to my sealed enclosure spheres. Wish I could combine the best bits of the two. Everything is a compromise and subject to our individual taste. I can happily live with transmission line bass even though I can hear what is different about it compared to sealed enclosure. I am using exactly the same drivers in both .

Starting to think some of the directions I have chased off in are not necessarily better. :angry-steamingears:
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Latteman (Fri Jan 27, 2023 8:51 am)
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Re: Modifying my 24 year old B&W P4

Unread post by Latteman »

When I returned to a pair of goodmans mezzo SL from my diy semi omni I to was amazed that I actually preferred them- I have subsequently progressed to an open baffle design and couldn’t be happier
It’s interesting how u go in certain directions and where u eventually end up
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karatestu (Fri Jan 27, 2023 9:28 am)
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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

I'm hearing more of the mid bass' output now that they are pointing at me instead of at the ceiling.Before the levels between the two drivers was perfect for my taste. The level has changed so something has to be done.

Options

1) change feedback resistor on tweeter amp boards to bring their level up.

2)change fb resistor on mid bass amp to bring their level down.

3) put padding resistor in series with mid bass

4) Something else

5)lower the high pass filter -3dB frequency slightly

I refuse to put a padding resistor in. Full stop. Especially on the bass where it will increase qts and required box size. Of the other options bringing the tweeter level up by reducing feedback looks the best (as long as its safe). It doesn't need much increase.

I decided to do something else. Removed the fibreglass stuffing out of the tweeter back tube. That has livened things up a bit :dance: Not sure I need to do anymore, we will see after more listening.

I do wonder about removing the 1" thick open cell foam that lines the upper chamber walls behind the mid bass driver. It is a classic part of the doc mods after all. Won't be messing with the fill in the first half of the transmission line though - that is very important to correct operation.

I'm gonna do it, remove the bit of foam.
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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

Forgot to say , I did close the end of the tweeter pipe off with an inch deep plug of fibreglass.

The foam in the mid bass chamber is out and so far I prefer it. Opened things up more, a bit more illumination just where I wanted it.

I also have a bit of a bass tone control in the fact I can insert a 1" thick cut out of foam to fit in the TL output vent. Tried it just now and it did knock the bass back a touch on a known problem track.

I can see these speakers going home and being used in the living room whilst I continue refining the semi omni sphere speakers. Going to need more mid bass drivers cos I have them back in the speakers they came from.
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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

My mind really has to stop racing ahead. This little devil in my head keeps whispering in my head - why not try it as a three way ? It goes against simplicity and as i do my filters passively af line level it requires two more channels of power amp.

But apart from that it could be done by employing a tweeter in a tapering pipe which is situated on top of the speaker, much like what B&W do on some models. Same SB acoustics 2" wideband driver for mid and tweeter. Bass driver used up to -3dB point of the baffle step transition (575Hz) - that requires the amp's low pass filter capacitor changing. Mid amp needs it's low pass cap changing and needs a cap for high pass at 575Hz..Tweeter stays as usual with high pass filter at around 7KHz.

What I need to weigh up here is......is it worth having extra filters in the area of 575Hz with the theoretical result of having a driver which produces mid frequencies but doesn't have to do much low bass. Is it more hifi rather than music ?
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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

Things moved on a little. One of my inner tube speaker isolators got a puncture :cry: after I moved the speaker slightly and it sat on a small length if copped wire. Having said that, these speakers don't seem to benefit as much as the up firing sphere speakers :think: I can only guess that it is because they are TL and not pressurised like the closed boxes. Being point and squirt might have a bearing as well.

Down to a bit theory now. Best graphical :grin: results are said to come from matching the drivers for directivity at xover and crossing over before they start to beam. Run a 6,5" mid bass fully open and it starts to beam at about 2.7 KHz ( frequency = speed of sound / effective diameter of cone). This driver will be beaming alot before it runs out of puff at 7-8 KHz. The 2" driver will have a much wider dispersion at xover. Off axis frequency response will be much different to on axis.

Also crossing over so high, you can't get the centre of each driver to be close enough ( within 1/4 wavelength of thd xover frequency) to act as a point source. Lobing and changes in the sound as you move your head about are the theoretical consequences.

I must be bloody cloth eared or my ears completely shagged as I can't detect any of these problems and believe me I have spent time trying. Also, the sound does not change that much when moving around the room (like semi omni) all be it at a distance of at least two metres away. Maybe it is the distance that I am listening at that is the reason I don't hear it, or maybe it is the case that the theory does not translate to real life ?

Beaming drivers must remove some of the problems with reflections and more so as you go up in fewquency. Seems strange saying this when i have been playing with semi omni for the last six years..
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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

Seeing as though I value the curvy form so much :shock: I thought I better make the effort with these speakers. The outer shape of a speaker cabinet is very important and my steel spheres have taught me this. There is a sense of calm and order, imaging, soundstage and detail retrieval all improve quite a bit in my opinion.

These B&W P4 have a slight bevelled edge on the two sides of the front baffle. It is supposed to help (like a round over) but what's the point with piddling about with 20mm roundovers which have little effect.

Behold the beauty of my latest creaction. Bendy card taped on to make the sides and back a circle shape. :lol:

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Baffle width is now 30cm instead of 20cm and it makes them slightly deeper as well. Had to cut holes for the speaker cables .

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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

It has improved the sound quite a bit. Bet my wife (and antonio) will love this latest development :grin: . It makes the speakers half as wide again but a curved shape is less imposing. There is obviously some dead space between the original enclosure and the outer one.

If I were to go ahead and do this quite substantial modification properly then only the front baffle will be left of the original still visible. I would have to add another top panel on top of what is already there. I would have to use something bendy to construct the outer curve. Bendy plywood or a bendy plastic sheet.

A lot of dead space but could be used to my advantage by filling with vibration eating materials. Sand might be a bit heavy. Cat litter or polystyrene could be an option.

I haven't fully decided if I am going to attempt this properly. The music is very addictive though :guiness;
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