Modifying my 24 year old B&W P4

A place for DIY project discussions.
User avatar
karatestu
Posts: 4790
Joined: Sun Jan 08, 2017 4:40 pm
Location: North Yorkshire
Has thanked: 1048 times
Been thanked: 616 times
Great Britain

Modifying my 24 year old B&W P4

Unread post by karatestu »

This little project is not to replace my other speaker journey, just a little side project that will get used until my forever changing diy jobbies are finally finished. I'm sick of not having a hifi in the living room :roll:

B&W P4 speakers I bought new in 1997 with my long suffering (and highly modified) Naim CD3.5 and a Nait3. Nait 3 is long gone. I auditioned half a dozen speakers in the £700 (1997) region and the B&W's won me over.

Image

These are a two way with 6.5" kevlar mid bass and 1" metal dome tweeter. They are a quarter wave tapered line (kind of transmission line). Due to the distance the 6'5" is from the end of the line (Zd is a key parameter of the design) the mid bass can't be moved so this will have to stay a point and squirt but looking on the bright side it stops me messing about too much and taking forever to complete :grin:

These were my main speakers for 20 years until I found this forum and started dabbling in the dark side of doping and drivers pointing in unnatural directions. They served me well and I was satisfied with them for 15 years. But as my sources and amps got better they started to annoy me a little.

I don't have a problem with TL bass like some do so it makes sense to keep that part of these speakers. TL design is not easy and they were designed with that particular mid bass in mind so the only option is to keep the 6.5 MB as well. With it only being a two way then certain compromises had to be made. The crossover is at 3.5 kHz as the 1" dome has to be crossed over that high to avoid distortion.

The MB only has a single inductor in series (1st order) so only attenuates at 6dB per octave above the xover frequency (3.5 kHz). Being kevlar doesn't help with it's nasty break up. This is the main problem with this speaker - it lets too much of the MB's nasties through.

The tweeter has an electrical 3rd order high pass filter which is required to roll it off enough before it's resonant frequency is reached. I would like to avoid anything over first order filters in this speaker.

So the tweeter has to go. :shock: I don't want to just replace it with a better tweeter that can go lower. I can't increase the MB's xover frequency as that will introduce more nasties and the difference in directivity between the two drivers at xover will just get more pronounced. Want to try and keep the off axis response as good as possible.

Doping of the MB is not going to happen with this. It has a smaller than needed magnet and a much too relaxed suspension and spider. Plus I don't believe doping and no filter works for a point and squirt as the MB beams too much at higher frequencies.

So what am I going to do then ? Well the MB driver will have it's first order low pass filter knee brought right down from 3.5 kHz to 500 Hz which is the -3dB point in the baffle step loss. This should keep out more of the kevlar cones break up resonances higher up. It also lets me use different sensitivities for the drivers and so do some baffle step compensation for free (as it were). I hope that is low enough not to mess too much with phase in the all important mid frequencies where the ear is much more sensitive to such things. If I want to lower the xover frequency below 500 Hz then I will have to make the baffle wider and I don't really want to get into that.

But what about above 500 Hz ? The traditional thing to do would be to go to a three way design with mid and tweeter. I have considered that but there are other alternatives. A compression driver - no thanks. That leaves the option of using a single full range driver. That would make what is known as a FAST or WAW speaker.

I think I like the sound of that the most out of the three options and it gives me flexibility to add a tweeter later on if I can't get on with the FR's top end. It's not like adding tweeters though as a FR needs an enclosure and is not closed back. Plus although enclosure volume for a TL design is not that important (within reason) the FR will be taking away the MB's enclosure volume. So that led me to look at very small FR drivers, the smaller the better in fact.

I came across on my travels a small 2.5" FR which is very well thought of and doesn't break the bank. It has a very flat frequency response and won't mind a 1st order roll off at 500Hz. It also fits right in the tweeter cut out without any modification :dance: (I couldn't quite believe my luck) except having to chamfer off the rear of the cut out to give some airflow. It will need rebating also into the front panel. It has a sensitivity 4dB lower than the mid bass so will be ideal to enable the baffle step compensation with no added components :dance:

I have cut a section of pipe to fit between the front baffle and back panel which will comprise the mid tweeters enclosure. This takes away a little of the TL's volume but more importantly it does not change the distance from the centre of the 6.5" mid bass to the end of the line (top panel). Pipe will need stuffing :hand: and possibly a hole cut in the back panel for aperiodic working. Volume of the pipe will give a qtc below bessel.

There won't be any speaker level xover with this speaker :naughty: :naughty: :naughty: The filters will be done passively at line level in the input section of my nva amp boards. All it needs is the change in value of two capacitors. No extra components needed :guiness; :guiness; :guiness; as the framework is already there. No padding resistors needed, nothing between amps and both drivers but lovely LS6 speaker cables.

More later .........
DIY inspired by Richard "The Doc" Dunn RIP

User avatar
karatestu
Posts: 4790
Joined: Sun Jan 08, 2017 4:40 pm
Location: North Yorkshire
Has thanked: 1048 times
Been thanked: 616 times
Great Britain

Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

For the full range driver I initially went straight to the Mark Audio range for no other reason than I have read a lot about them and they are very well regarded.

Their smallest driver is a 3" affair with no spider :shock:. Sensitivity 85dB, Vas 1.7 Litres, Qts 0.5. All sounded good for a closed enclosure. Best Vb turned out to be 2.6 Litres which was more than I could stretch to for a butterworth or lower Q. And I had only 5mm to spare once mounted on the front baffle. I decided to look for something physically smaller which also required less volume.

After much research I found the SB acoustics SBA65WBAC25-4 (rolls off the tongue) 2.5' full range. Perfect for my needs at only 65mm wide, flat FR and an enclosure volume of only 1 litre. I bought two and they arrived yesterday.

Image

Image

Image

Image

So now I have to chamfer the back of it's cut out for good airflow, rebate it into the front panel and stuff the pipe with fibreglass- more dense as you go away from the driver to the vack of the enclosure. Faffing with the stuffing will probably be the most time consuming part.
DIY inspired by Richard "The Doc" Dunn RIP

User avatar
slinger
Posts: 6870
Joined: Wed Aug 22, 2012 4:30 pm
Location: The Garden of England
Has thanked: 1351 times
Been thanked: 1326 times
EUROPEAN_UNION

Re: Modifying my 24 year old B&W P4

Unread post by slinger »

I have no idea what 95% of what you are saying means, I'm glad you're having fun though, Stu. :grin:
These users thanked the author slinger for the post:
karatestu (Fri Sep 17, 2021 8:38 pm)
Amps - NVA P50, AP30, A40, Stanislav Palo Tube Headphone Amp BB 85
Speakers - Monitor Audio Silver RX2
Cables - NVA LS1+LS3, SSC, Gotham S/PDIF, IBRA Optical
Digital - NAD C516BEE, SONY ST-SDB900 DAB TUNER, TEAC UD-H01 DAC
Analogue - Pro-Ject Debut Carbon Esprit SB, Graham Slee Gram Amp 2 Phono
Cans - Grado SR80, ATH-M50X

User avatar
karatestu
Posts: 4790
Joined: Sun Jan 08, 2017 4:40 pm
Location: North Yorkshire
Has thanked: 1048 times
Been thanked: 616 times
Great Britain

Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

Five years ago I would have thought similar - knew bugger all about designing speakers. In the grand scheme of things I still know bugger all about speakers :lol:

I wanted to utilise these speakers again that I loved for 15 years before getting my head turned. I look back and think I would have been better off just settling for what I had. I am very glad I decided to look in at HFS, join and start posting. What I wasn't expecting was to be sucked into a spiralling whirlwind of endless subjective experimentation, the addiction of trying new things and the overwhelming thirst for more knowledge.

This little project is a world away from my other speaker project. Doc would have crucified me for posting the above twaddle. I am expecting lightning bolts from above.

Repeat after me

Thou shalt not have speaker filters
Thou shalt not rejoice in point and squirt
Thou shalt not covet ports or any other holes In a speaker enclosure
Thou shalt not even consider stuffing anything inside a speaker

I will be breaking all the above commandments. Strike me down. Its not even high efficiency or the size of a wardrobe. It will be something different, fun and something to compare with apart from the pair of 2 way standmounts I have. The spherical, multi driver, bipole, filter dodging doped up affairs will supplant these I am sure (when I get them finished :roll: ).
These users thanked the author karatestu for the post (total 2):
slinger (Fri Sep 17, 2021 9:09 pm) • CycleCoach (Sat Sep 18, 2021 2:59 pm)
DIY inspired by Richard "The Doc" Dunn RIP

User avatar
CycleCoach
Posts: 416
Joined: Tue Nov 15, 2016 5:20 pm
Location: Kings Cliffe
Has thanked: 494 times
Been thanked: 86 times
EUROPEAN_UNION

Re: Modifying my 24 year old B&W P4

Unread post by CycleCoach »

There are no rules other than "if it sounds good TO YOU it is good."

Enjoy!
These users thanked the author CycleCoach for the post:
karatestu (Sat Sep 18, 2021 3:16 pm)
Current System: Linn LP12 Lingo Ekos Troika, NVA Phono 2, Cambridge Audio CXC thru Cheapo Chinese DAC & PS, NORD streamer, NVA P50sa, NVA A80 x2 (they're special ones.), NVA SSP Mk2, NVA LS6, NVA Cube1

User avatar
karatestu
Posts: 4790
Joined: Sun Jan 08, 2017 4:40 pm
Location: North Yorkshire
Has thanked: 1048 times
Been thanked: 616 times
Great Britain

Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

CycleCoach wrote: Sat Sep 18, 2021 3:00 pm There are no rules other than "if it sounds good TO YOU it is good."

Enjoy!
I have no idea (yet) if it will sound any good even to me (and I'm easily pleased). :lol:
DIY inspired by Richard "The Doc" Dunn RIP

Daniel Quinn
Posts: 8431
Joined: Thu Jul 12, 2012 7:16 am
Has thanked: 17 times
Been thanked: 288 times

Re: Modifying my 24 year old B&W P4

Unread post by Daniel Quinn »

I'm loving what you do. But there's only me that loves genesis, so I'm all in favour of point and squirt, as you put it.

Also I'm convinced you'll end up with a 12 inch bass driver. There really is nothing like it.
These users thanked the author Daniel Quinn for the post:
karatestu (Sat Sep 18, 2021 10:19 pm)

User avatar
karatestu
Posts: 4790
Joined: Sun Jan 08, 2017 4:40 pm
Location: North Yorkshire
Has thanked: 1048 times
Been thanked: 616 times
Great Britain

Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

Daniel Quinn wrote: Sat Sep 18, 2021 7:24 pm I'm loving what you do. But there's only me that loves genesis, so I'm all in favour of point and squirt, as you put it.

Also I'm convinced you'll end up with a 12 inch bass driver. There really is nothing like it.
Thanks for your comments DQ.

Off topic alert for the following :roll:

You may have forgotten I have four Doc modded 12 inchers :grin: I will probably revisit them at some point. We will see how these little 2.5" full range drivers work out with the 6.5" transmission line. If I like them and I can live with the electrical first order xover at ~500Hz then I will try them with the 12 inchers :dance:

With the 12 inchers I had issues with trying to get the box volume down to a reasonable size without causing a bass hump in the response. Looking back, part of the issue was I also had two 5.25 inchers doped and running full range. All that overlapping bass production from three separate drivers will not have helped my cause, especially as there was a bipolar element which contributes the whole 6dB of baffle step compensation. BASS OVERLOAD :angry-steamingears:

So a simple two way comprising isobaric 12 inchers with the little 2.5" full rangers may be the way forward and will give yet another flavour. The 12 inchers would have to be in up firing clamshell isobaric configuration to keep the box volume down and they will have to be aperiodic for the same reason. The baffle will be 35- 40cm so the middle of the baffle step transition (-3dB point) will be in the 300-400Hz range. That would be the xover frequency between the 12 inchers and the 2.5" FR. Done 1st order passively at line level of course.

The 2.5" FR could be mounted on top of the magnet of the up firing outer 12 incher. That would help to time align the two drivers and keep them within a quarter wavelength distance at the xover frequency- so they appear as one driver.

I intend to try these 2.5" full rangers up firing into a diffuser at some point and could also try that with the upfiring isobaric 12 inchers.

Got too many options going around in my head now. Nothing will get finished at this rate :roll:
DIY inspired by Richard "The Doc" Dunn RIP

User avatar
Toontrev
Posts: 153
Joined: Mon Aug 22, 2016 12:48 pm
Location: Chester le Street
Has thanked: 19 times
Been thanked: 22 times

Re: Modifying my 24 year old B&W P4

Unread post by Toontrev »

Looking forward to another great project, keep up the good work Stu.
These users thanked the author Toontrev for the post:
karatestu (Mon Sep 20, 2021 7:01 pm)

User avatar
karatestu
Posts: 4790
Joined: Sun Jan 08, 2017 4:40 pm
Location: North Yorkshire
Has thanked: 1048 times
Been thanked: 616 times
Great Britain

Re: Modifying my 24 year old B&W P4

Unread post by karatestu »

Toontrev wrote: Mon Sep 20, 2021 10:45 am Looking forward to another great project, keep up the good work Stu.
Thanks Trev :guiness;

I need to find an inverted chamfer router bit but the buggers are impossible to find in this country. It would have to be a very small router to get in via the hole for the 6.5 incher so I can chamfer the back edge of the mid tweeter cut out with a standard bit.
DIY inspired by Richard "The Doc" Dunn RIP

Post Reply