Have you got pics of these Serge?
I must say, that using the 801 as standard, i.e. passive, the bass is very different to my Meridians, which are bass reflex. So far, I've stripped everything out of the cabinets, and will start repolishing the boxes tomorrow. Then, I'll reinstall the bass unit and mid/top unit and bring out the connections to seperate terminals. Then starts the measuring/listening to get the electronic crossovers set up correctly. The Behringer crossover makes things very easy, so hopefully it won't take long to get them up and running. Then a period of measuring and listening to decide on the best crossover settings.
One thing I will be interested in listening for is whether small adjustments of the crossover frequencies is audible, as I don't expect much measured difference, and whether the fiter slope change between 24dB/octave and 48dB/octave matters audibly.
It would have been nice if you had managed to measure the speaker's response prior to going active.
I find that the treble sounds best when attenuated to it's highest/strongest setting.
P.S.: you could start a thread at Equipment Reviews and Tests
Last edited by tuga; 01-11-2011 at 11:31 AM. Reason: P.S. added
"...man will occasionally stumble over the truth, but usually manages to pick himself up, walk over or around it, and carry on." - Winston Churchill
Here is the first batch of piccies:-
The 801 original.
With the head removed. It uses a 3-pin Bulgin connector as it commons up the ground connections for the tweeter and mid-range. I'm replacing it with a 4 pin XLR connector as I don't want to common up the grounds. If anyone should use a brodged amplifier in the future, it would effectively ground half the amp, so not worth risking.
Three bags full! The box is stuffed with wool, and I removed three plastic bags full to get to the inside and remove the internal crossover.
Here's the passive crossover. I'll keep it in case I should ever need to reinstall it, or I could put it in an external box if ever I want to play passive against active.
Here's the bass unit. The unit is mounted with a rubber gasket an bolted through rubber shock-mounts so it's more or less decoupled rom the box whilst still making a "perfect" seal. Any air leaks will affect the bass, as it relies on the internal air spring as part of the suspension.
A view of the internal bracing of the box. B&W obviously thought it was inadequate, as they then went on the the Matrix construction, although they also abandoned the sealed box. The 801 Matrix had much better bass extension and greater efficiency, but I've had bass-reflex for many years and fancied trying sealed box bass. You can see at the back the 6 holes for the new loudspeaker terminals.
Finally, a view of the top plate. The five white spots are where the old crossover mounted, and I've had to block up the holes with some dowels to maintain air tightness. The holes at the fron are where the head mounts and those are sealed with a rubber gasket which I'll have to make. The gasket both selas the joins and decouples the head from the bass box.
More pics as I get into the project, and some graphs etc.
Nice project Serge,
I spent a bit of time playing with a similar type of project but for a DIY speaker using Behringer DCX2496 and and A500. http://www.hifiwigwam.com/showthread...ghlight=b250.8
If you are intending on keeping the DCX2496 and three A500s in your system how are you going to control volume? It may sound a simple question but I assume you will input digitally in to the DCX, IIRC the DCX likes a decent signal level digitally (and even analogue) so as I can see it you have 2 options.
1) volume control digitally prior to DCX input (possible issues is low input level to DCX)
2) build a 6 channel attenautor stack and locate between DCX analogue out and A500 input.
You could go for analogue operation of the DCX but it gets very noisy. Supposedly the A500 "level controls" are not well thought out and introduce lots of distortion if not set to maximum volume/no attenuation.
If I could have easily overcome this I may have purchased another 2 A500s and gone fully active myself.
Will you use any capacitors on the tweeter/mid to safeguard against switchg on/off thump from the amps. IIRC behringer suggest turning volume controls down prior to switch on/off to avoid a thump. I know mine did thump if I didn't turn them down.
I'll try this first, before I try and get over the practical difficulties of a 6 way analogue volume control after the DCX.
As to capacitors, yes, I was thinking of doing that to protect the mid and top drivers from DC thumps or the amplifier going DC! I've managed to get a circuit diagram of the A500, and it has a peculiar topography I don't fully understand. It looks like the loudspeaker is AC coupled through the power supply capacitors as it has some sort of floating supply as the mains transformer has no centre-tap. If anyone has a good explanation of how this works, I'd like to know, as at the moment it's an infinite baffle.
The A500 volume control isn't a simple attenuator across the input, but a gain control that seems to give high distortion at around 70% rotation. I was intending just to leave the controls at 100%. With digital equipment, I know it's theoretically better to drive the ADC hard and keep the level up, but with a 24 bit system, it may not matter than much. I can always attenuate the input to the A500 with an external attenuator so that I drive the digital side harder. The trick then is to adjust the gain structure of the whole system so that I achieve maximum volume level without clipping anything.
Lots to do!
"My god, it's full of arse"
I love it, Serge worshipping at the altar of Hobby
Sounds an interesting project.
Prof. Chaos 2 - Electric Boogaloo
A bit more progess:-
I've now rubbed off the old varnish and waxed the bass box, but I'm not very happy with my woodwork. I may have the cabinets reveneered at some stge in the future once I'm happy with the overall loudspeaker performance.
I've rewired the socket for the mid/treble head and started putting the three bags full of wool back in the box.
One can see the fixings for the plate that holds the head, and the XLR plug I've fixed to the plate replacing the three-pin Bulgin originally fitted.
Here's the box fully stuffed, with the cables for the bass unit left out. I've left an indentation in the wool to accomodate the bass unit, as recommended by B&W in their Service Manual for the 801. I'm using 15 amp mains cable for the wiring, in the head I wanted something a bit more flexible so I used some CAT5 cable with each twisted pair soldered together. 4 flexible wires, job done.
Bass unit with the wires soldered on. The bass unit then dropped in the hole, and was bolted up with 6 rubber isolated bolts. Now to give it a try.
With nothing done to the crossover except dialling-in the crossover frequencies and slopes. 380Hz and 3kHz, 24dB/octave Linkwitz-Riley. Put on the power amps, with 16dB attenuators at their inputs so that the +22dBu output of the Crossovers when at 0dBFS, the amps can't clip. 16dB gives me 110 watts per amp from 0dBFS input, and the amps are good for about 120 watts, so there's no risk if the amps clipping. When I get into it further, I'll probably set a limiter on the treble amp limiting the output to around 35-40 watts as I don't want to risk a blown tweeter when playing loud. The original crossover had Overload protection that switched off the 'speaker if any drive unit was overloaded, and that came on at around 36 watts for the tweeter, much higher for the midrange and bass, but limiting the amps to 110 watts, I won't be exceeding these drivers' capabilities.
So, what does it sound like? Pretty horrible! There's been no attempt yet to level the crossover, or match the sensitivites of the drivers, so it's squawking wonderfully. Still, it proves it works! Now the fun starts, measuring and listening and tweaking.
One further thing to address is the question of volume control. Using my analogue 201 pre-amp, the volume control reduces the analogue level into the ADC. Consequently, most of the time, the digital level is somewhere between -40dBFS and -20dBFS. This makes the noise level somewhat higher than I would like, there's a steady noise, not audible from the listening position, but still higher than I would like. I may have to invest in a 6-channel remote volume control if the hiss gets to me.
The next thrilling installment will have some measurements and graphs. Tune in, same channel, if not same time.
Good stuff Serge
Let me know how you get on with the hiss and 6-way remote. I am planning a similar thing with my horns, although would probably need an 8-way (or maybe 10) to cover all the drivers. I'd be keen to hear how this actually worked in practice.
Have you deconstructed the passive XO to ascertain the padding, as well as the XO points and slopes?
Following with interest.
Serge - You seem to have noticed the same issue as me with volume control / noise.
I ended up sending digital from my squeezebox touch (SBT) in to the DCX behringer and then attenuating the input with the behringer globally (via DCX input level adjustment settings) so that 100% volume on the SBT was "bloody loud but tolerable" and 70% was normal listening level. All noise/hiss was eliminated.
You are probably going to start and lose resolution from 2 lots of digital attenaution but both SBT & DCX are 24 bit attenuating a 16 bit signal so there is some margin there and it still sounded better than a shedload of hiss!
One thing I wasn't sure of and I think got me confused and chasing my tail.... When crossover slopes are applied via the DCX I assume that no phase shift occurs unlike in passive crossovers? Building one cross-over and measuring in comparison to a fully active on the other channel got me chasing my tail somewhat because of this.