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 »

Back to familiar ground. Semi omni with mid bass up firing.

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That has tamed the mid range a little and the presentation is not so in my face now :dance: I think I am so used to mid bass pointing up and doped that anything else is a bit of a shock and takes a bit of getting used to. So now I have semi omni again but no doping and a 1st order speaker level xover at 3.5kHz (ish).

There was still a bit too much mid range for my liking so I dropped the high pass capacitance to 10uf. that made a positive difference but it is still grating. I need to put them against the wall but it's hard when my cube speakers need moving and these TL's on their sides do not lend themselves very well to positioning.

This exercise has given me some courage to move away from doping but I don't think I will be doing it with a kevlar cone. The break up higher than xover is getting through and giving everything an edge that I don't like. You don't get this with a doped kevlar cone- it's liquid smooth but with some of the dynamics and detail taken away. Or maybe what I think is now detail is some frequency response problem. These 2" full range are still breaking in as well. At least with this lower xover they will get there faster.

I think these P4's will be passable once the xover frequency is decreased and done at line level passively. I might even have the courage to give these B&W kevlar 6.5" cones a spray of plastidip. It peels off because I removed some from Zebbo's old soft dome tweeters that he swapped out. The plastidip came off in one piece and looked like a dome.

I am aware of the need to match the directivity of the two speakers at xover, especially when both forward firing. This probably will be ok as the lowering of the xover frequency will make a better match between the 6.5" and 2" cones. I will have a 2" full range with 1st order high pass filter rather than what came with the speaker - a 25mm aluminum dome with ferro fluid and a third order ellectrical filter. In theory a 1st order xover is better for phase but there may be some other xover trickery going on in the original that I haven't got round to understanding yet. A first order low pass with a third order high pass. then there is any acoustical roll off to add in plus other things I don't know about.

Anyway, I am sure I can improve them even if they don't turn out perfect.
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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

Nearly cracked it with these speakers :dance: Close enough to know it's a goer, there are things to iron out

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I decided to put them the right way up, as they were intended to be used. Right back to the wall which is how I remember using them until I started faffing with home made crap in 2017. Even though they are a resistive quarter wave tapered duct design I never remember having any issues with the bass booming or lingering. That is indeed the case again now especially as are and will be used across a corner as in the picture above.

The bass timing is another issue, or not an issue depending on how you look at it. They are not as fast as closed design in the bass but I find transmission line to be the best of what I have heard out of the rest i.e reflex, passive radiator, my crap open baffle experiments. Never heard front or back horn loaded bass though. So it will do nicely I think. Even right back to the wall the bass is not over powering.

The 2" cone full range is proving to be a very adequate match for the 6.5" kevlar B&W driver. I have always found B&W drivers to be of good quality. It is the xovers that let them down imo. The P4 box is made of 25mm MDF front baffle and the rest is chipboard unfortunately. The wood used to create the tapered line inside the bottom two thirds of the enclosure does a good job of bracing that part. The top third has no bracing so I might add some in the future. Brace the driver magnet to the rear panel.

So back to the 2" full range. I can't really keep it in a sphere on the top unless I really lower the xover frequency to 300 - at that point they are still within a 1/4 wavelength from centre to centre, on the very limit though. Although I am kind of addicted to spheres and this pains me but i will probably just put the 2"FR in the vacated tweeter hole. It's a perfect fit but I need to rebate the front baffle and chamfer the back to let the 2 incher breathe.

Of course it can't share the enclosure of the 6.5 " mid bass but I have already cut a suitable pipe for both speakers that jams itself nicely between front and back panels. I will probably make a hole in the rear panel and extend this line down the back of the speaker towards the floor. With progressive stuffing (denser near the end of the line) I will have created a midrange transmission line that does not produce any of the sound from the vent, the stuffing eats it all up.

I was concerned about this pipe eating up valuable mid bass enclosure space but was assured on diyaudio that it was a minor thing in TL design. The main thing is the line stays the same length, cross section and taper to keep the tuning that B&W intended with the original drivers. Well I haven't changed any of that. The other important thing is to keep the distance from the centre of the mid bass to the end of the line (the top panel) the same. Running a circular pipe from front to back panels does not change this I am told.

So it should be a good result. I can't move the mid bass to up firing because it changes this vital distance just mentioned. I could build a new box for it though which would enable me to use the mid bass up firing and keep everything else the same but I am not sure I can be bothered.

I am using a speaker level 1st order passive xover at present. Coil from the old Marantz imperial 7 and a 22uf film cap for the high pass on the 2" mid tweeter. I am not exactly sure what the xover frequency is at present :lol: apart from to say that the coil gives a figure of -3dB at 800 hertz if I have measured it correctly with my DMM. When I used a 47uf for the high pass which works out roughly the same -3dB figure I didn't like the result - too much mid. Since I backed the cap off to 22uf it is much more to me liking. I might even go a little further.

So does this mean that I like a dip in the mid range ? I wouldn't be surprised considering my crap ears. Nothing sounds as smooth as a well doped mid bass to me so far. Anyway, I can't play with the low pass coil because I don't have any of the right value to lower the xover frequency. I don't want to be buying a load of them when I might not be using them. Good ones aren't cheap. If I go passive ine level with this (which was the intention) then I need to install the low pass cap on some pcb pins so I can change the value easily without having to rip the nva amp boards out every time I want to alter the cap value. The high pass capacitor is easy to change out as it is off the pcb anyway.

Starting to feel good about this project now. There is scope for it to improve through tweaking and the 2" going in thetweeter hole very close to the mid bass. Dropping the xover frequency even more will probably help as long as the 2" mid tweeter doesn't start to suffer from distortion. There is also the PLLXO which will improve things over crappy inductors and big film caps at speaker level. I hope it doesn't surpass my other project otherwise these modified P4's will be staying in the main system. :shock:
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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

I couldn't leave these alone for very long. I am going to have these as a second pair of speakers that I can swap in and out when the mood takes me. A change is as good a rest. At this rate I am going to end up with a spare room full of speakers :lol: A different pair for every day. Not a bad idea really, weekday speakers and weekend speakers :think: If I set the passive line level filters the same for every speaker then they can go in and out as required. That entails designing each for the same xover frequencies which may or may not work out.

I took the little spheres off the top of the cabs and put them on 60cm lengths of 110mm drain pipe on the inside of the main cabs. This way I can get the two drivers as close as possible. Xover is speaker level passive at the moment with the original B&W inductor and 11.5uf cap for xover at 3.5kHz. Not ideal but it will do for now. I stopped using the marantz imperial 7 inductor cos they are crap and my dmm didn't measure them right or I am being a numpty - the -3dB point is nowhere near where i thought it was :roll:

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There was something still not quite right which I assumed was to do with the 6.5" kevlar cone's break up coming through as the xover is still at 3.5kHz like the original and I know the 2" is sweet. Plus I think the 6.5" is too sensitive compared to the smaller driver. I know I said I wouldn't be doping this B&W driver and I haven't but I have done the time old trick of adding a worm of blu tack at the join of the cone and the dust cap. Less invasive than contact adhesive and easy to remove if it all goes wrong.

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Well what can I say. The driver is transformed :dance: When you read about doping and adding weight MOST say it is a bad thing as regards transient response, detail, sensitivity. Well in this case I can afford the loss in sensitivity however much that is exactly. Detail and transients are not noticeably affected imo. I could happily live with this at it is now.

A further observation about this blu tack addition. I think it has had as much effect as pouring dope on in the respect of calming break up resonances and attenuating the top end a bit. It has obviously dropped the mid volume down but I don't think the bass end has been affected as much.

I also note that this blu tack has much less contact area with the cone and dust cap (reduced cone area in contact). The dust cap in particular is only in contact with the blu tack at it's edges and not totally covered with contact adhesive. It is of my opinion that this gives a much superior result than doping the way I have been doing it. Plus it is so easy to do and removeable - you can adjust the amount applied very easily. No more doping for me bring out the blue tack (other colours are available :grin: )

It seems to let the cone and dust cap sing in a way doping does not. Very important. I may not need this blue tack damping when I lower the xover frequency and do it at line level, we shall see. I think it looks better than bison kit and plastidip as well - must get some in black so it blends in with the dust cap.

I think I have decided on a 500Hz xover for the final thing. Not as low as I hoped but I am concerned about the distortion when trying to put too much low frequency through this 2" cone especially with only a 1st order 6dB/octave high pass. I only listen at low volume in a small room (unlikely to ever change) so maybe I could get away with the lower xover. Will try both but I suspect the higher one will be better even though it is further into the telecom frequency range than I would like. I could combine the electrical filter with the drivers acoustic roll off but that makes it 3rd orderand not symmetrical with the 1st order low pass on the MB.

The directivity of the two drivers should still match reasonably well at 500Hz rather than 300Hz. 500Hz is also close to the middle of the baffle step transition (-3dB point) for this speaker enclosure - 20cm width. The directivity will certainly match much better than the original speaker with xover at 3.5kHz and using a 1" dome tweeter.

I am now starting to develop a dislike of tweeters :hand:
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Re: Modifying my 24 year old B&W P4

Unread post by SteveTheShadow »

Years ago, I used a worm of black tack at the junction of the cone and dustcap on the Fostex FF225WK wideband drivers, fitted to the 5ft tall Metronome speakers I used to have. It’s a very effective way of controlling breakup modes.
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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

SteveTheShadow wrote: Fri Oct 29, 2021 10:44 am Years ago, I used a worm of black tack at the junction of the cone and dustcap on the Fostex FF225WK wideband drivers, fitted to the 5ft tall Metronome speakers I used to have. It’s a very effective way of controlling breakup modes.
Yes, I did not realise how effective it was going to be. I wish somebody had told me years ago and I wouldn't have bothered to dope with Bison kit. I am flabbergasted at how it does just the right thing - controls the headache inducing break up but still lets the driver do it's thing i.e be open and dynamic. IMO doping with contact adhesive just goes too far.

I was worried that lowering the Fs of the driver too much would ruin the workings of the TL as the line length is tuned to the Fs of the driver. I am sure it has had an effect but I can't hear it :dance:

I will try running the driver without low pass inductor and see what happens but I will have to revert to the 7kHz line level high pass for the 2" that I have in the amps I am not using at the moment.
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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

It always pays not to throw stuff out :grin: I found some Ikea kids table legs which might be very useful for tapered mid and tweeter transmission line enclosures. It's like they were made for the job

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Re: Modifying my 24 year old B&W P4

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Does Long John Silver want his leg back?

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karatestu (Fri Oct 29, 2021 2:42 pm) • Latteman (Sat Oct 30, 2021 9:25 am)

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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

Fretless wrote: Fri Oct 29, 2021 2:19 pm Does Long John Silver want his leg back?

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He's not havin' em :grin:

Nearest thing I could find for what I was thinking of is below. Could stick a sphere on the front and have the table leg as a tail behind it :lol: Progressive stuffing all the way to the end of the tube and leave the end open. No sound should get out the end or reflected through the cone.

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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

Had another mess about with these P4's (or what's left of em :grin: ).

This forum seems to attract mostly people who prefer sealed box bass alignments. I was a bit wary of posting all this up here cos they are quarter wave tapered transmission line. Impressed the hell out of me as a serious hifi newb back in 1997. They do go low but I progressed in my audio career enough to know that it is a bit lagging as the bass enforcement from the vent is slightly behind that produced by the cone directly. It has to travel a metre or so down a tapering pipe and then pops back out again although at least it in phase.

Since adding the blu tack to the cone my thoughts wandered back to trying this driver in a sealed enclosure again. I tried it in my cubes a while back and the 17 litres was too much volume - little bass. I poured four litres of loose sand in the bottom of the enclosure and listened again. 13 litres was still too big although the bass was stronger. I concluded that the drivers qts was probably too low for sealed which needs to be 0.4 - 0.5 approx. I abandoned the idea.

Well, I decided to try again. Rather than take the doped drivers out of the cube I decided to block the hole to the transmission line where it exits the top chamber. Where the 135 measurement is in the diagram below. I used some bits of pine plank. It gives me an enclosure volume of 9 litres which is still a bit low for the average 6.5" driver.

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This has been a very interesting experiment there is no doubt. Listening to the speaker in TL mode and 9 litre sealed. I was just hoping for the best that 9 litres was going to be about right. Turns out it is about right :dance: The blu tack has raised the qtc through added mass and made the driver more suitable for sealed use.

Listening to the sealed version right now. :epopc:
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Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

A few things learned from this ;

Sealed enclosure bass is the tightest and for me is superior
Blu tack is the way forward
Point & squirt is not so bad
Tweeters suck big time
The cheap Chinese drivers are crap - I was in denial but they have served a purpose
A xover is not the devil - if done well and a simple 1st order with drivers that allow it

Although the bass does not go as low now it's timing is superior and doesn't wallow at all when the enclosure is the right size for the driver of course. I am missing the lower bass a bit but the cleanness of what is there more than makes up for it. I think I will play with the enclosure volume a bit more and add a litre of wood.

I did say up thread that I didn't mind TL bass. Well I suppose I don't but this latest experiment proves where my heart is - sealed.

I am not sure of the future of this speaker anymore. I might end up filling the TL with sand or something entirely different. The mid bass driver is a cracker when the blu tack is added. I still think the qts is on the edge of what is acceptable in a sealed enclosure but at least the box volume required isn't stupidly large. I don't really want to add any more blu tack to increase qts any more.

I now think about transferring these B&W drivers to my spherical omni project :think: I could get some more but they are £60+ on evilbay. Who said low qts drivers couldn't be used sealed.
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