Help please - Crossover recap

dogfonos

Well-known Member
My speakers are probably twenty five years, (+), old. I've already played about with them by removing the passive radiators

Seems like B&W agreed with you because they replaced the passive radiator with a powered drive unit in subsequent versions (i.e. S2 & S3) to make a 2.5 way speaker - and now it seems the current B&W 603 is a three-way with two powered bass drivers.

I have thought about buying some drivers to replace the passives, but understand, (from reading), that this could get a bit complex and so I backed off, although I'm interested to hear others views on this.

It's true that replacing a passive radiator with a powered drive unit is experimental and not entirely predictable because there are so many variables. I guess it depends how strong the urge to tinker is (surely we all get that feeling from time to time?). Good thing about adding a powered driver is that the changes made could be easily undone and the speaker returned to it's current build.

Incidentally, is the passive radiator hole in the cabinet blocked/sealed up or have you left the cutout hole open?

There are a few things to be considered - though there are a good few lesser factors too...

Check the nominal impedance of the speaker 'as is'. If it's currently 6 Ohms or less it's probably unwise to attempt the modification because the added bass drive unit would likely lower the modified speaker's nominal impedance to below 4 Ohms which most amps/receivers won't appreciate (though a few will cope admirably). The chosen bass driver should have a nominal impedance of at least 8 Ohms.

The existing crossover probably won't need altering but a secondary low-pass filter to feed bass signals to the new bass driver will be required. The new secondary low-pass filter (with a centre frequency in the region of 200Hz to 300Hz - typically, determination of the optimum crossover freq. for a particular application would require measurements and trial-and-error) needs to be connected in parallel to the existing bass/midrange driver. This will turn the speaker into a 2.5 way speaker where both the existing mid/bass driver and the new bass driver will share bass duties. Doing this mod. will lower the speakers impedance in the bass region but assuming the DM603 is an 8 Ohm speaker and you add a bass speaker with 8 Ohm impedance (or greater) and your amp/receiver is happy to drive a 4 Ohm load, then all should be well. This example is the type of filter required (x2) although filter (centre) freq. seems a bit too high at 350Hz. Nearer 200Hz - if there's one out there - would have less sonic impact on the rest of the speaker:


Some, possibly many, 2.5 way speaker designs have a separate enclosure for the dedicated bass driver built into the cabinet so each driver is presented with the optimum loading. I suspect that if you chose a dedicated bass driver that has a similar efficiency (or lower) compared with the current speaker and is of similar size, this won't be too important. However, some bass speakers are designed for sealed box and some for reflex port whilst others are suitable for either type of bass loading. As a generalization, bass speakers with a lower resonant frequency usually suit sealed cabinets, like the DM603.

Personally, as this exercise would be an experiment, I wouldn't spend big on a dedicated bass driver. Worth looking at Electromarket who sell some half-decent cheap bass drivers, such as:


The above drive units are examples and not recommendations.

And there's always ebay sellers. It's likely that some woodworking will be required as different drivers (even ones of the same nominal size) often have different cutout hole sizes and fixing requirements.

Regarding the capacitors, would it be worth a change? Or is it possible to test the existing capacitors to understand their condition?
'Fraid I haven't a clue.
 

vm1451

Active Member
@dogfonos Thanks for the detailed reply.
Pictures attached as to what I have done, and it is fully reversible. To be honest anything that I try would be reversible just in case it didn't work!

The DM603s are in fact 8 ohm and rated at 120W. The AVR is rated at 170w @ 8 Ohms or 220w @ 6 Ohms. Cabinets are front ported but I have those plugged, as again, measurements show this to be better, (to be fair, I can also hear the difference although it's not night and day).

Some research has thrown up how to test capacitors and it's fairly straightforward, so I'll give that a go out of interest.

Fitting drivers is not an issue, the electronics more so, (for me). Wiring and soldering hold no fear, but the maths aspect does! So basically I get confused as to what I need to do to keep the overall impedance of the speaker at 8 ohms

My understanding is that throwing another driver in the cabinet and wiring it in parallel will reduce the impedance, but no idea what this will do to sensitivity.

To make my life a little easier I was looking at pre-built XO boards such as:-
I think the existing XO is around 3Khz, so was thinking along the lines of a three way 500Hz/3Khz; dropping the existing 180mm driver to the bottom, (where the passive was mounted), and introducing another B&W driver of around 145mm-150mm, (seems a little pointless to introduce another 180mm driver?). The thinking being that if I use a B&W driver it will blend better with the existing two than another manufacturers, (the trick here is to find a pair I guess!).

That said I have the XO in the AVR at 120Hz, (work in progress!), and if the XO in the speaker is set to 3Khz, it means the existing 180mm driver is covering frequencies from approx 120Hz-3Khz?

I have no idea what I'm talking about so happy to listen to ideas and talk them through! Is this the right place to be doing that? Or should I start a thread in the DIY section? o_O
 

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dogfonos

Well-known Member
The DM603s are in fact 8 ohm and rated at 120W. The AVR is rated at 170w @ 8 Ohms or 220w @ 6 Ohms
My understanding is that throwing another driver in the cabinet and wiring it in parallel will reduce the impedance, but no idea what this will do to sensitivity.

Unless the AVR states a power output into a 4 Ohm speaker/load, I wouldn't attempt the mod I suggested in post#26 (i.e. turn the DM603 into a 2.5 way speaker) because the modded speaker's impedance, likely to be in the region of 4 Ohms, may be too difficult for the AVR to handle and could damage the AVR.
I admit I didn't notice the front port in the images of the DM603 I saw. :facepalm:It's a rare design that employs both a passive/auxiliary bass radiator and a port. Odd.

Converting the 2 way DM603 into a 2.5 way won't affect overall speaker sensitivity but will give more (bass) output below the low-pass filter cutoff frequency. That said, I don't think it's a wise mod to make due to the AVR not specifying an output into a 4 Ohm load. Shame.

I think the existing XO is around 3Khz, so was thinking along the lines of a three way 500Hz/3Khz; dropping the existing 180mm driver to the bottom, (where the passive was mounted), and introducing another B&W driver of around 145mm-150mm, (seems a little pointless to introduce another 180mm driver?). The thinking being that if I use a B&W driver it will blend better with the existing two than another manufacturers, (the trick here is to find a pair I guess!).

Mods as significant as this need more specialist advice which, I suggest, would be better served by joining the DIYAudio.com forum. Well worth a look for anyone considering DIY mods or builds.

One quick point: Speaker crossovers usually contain more than just crossover circuitry. Many (most?) bass/mid drive units exhibit peaks in the higher part of their working range. Such peaks are frequently removed using electronics contained on the crossover circuit board. Different drive units have different peaks (in terms of magnitude, Q-value [sharpness of peak(s)] and frequency) so it's a minefield to start switching drivers.

Note that the link you provided to the Dayton Audio product is in fact a low pass filter rather than a crossover because it doesn't actually provide a 'crossover' function. But this is the type of filter that would be used to feed the bass driver in a 2.5 way design. See how the price increases as the filter frequency reduces.

An off-the-shelf, generic three-way crossover won't have any additional circuitry to remove any peaks in drive unit responses so, ideally, extra circuitry capable of doing this would need to be added.

Whilst you could start a new DIY thread on avforums, I suggest DIYAudio would be more appropriate. Lot's of very knowledgeable users. Good luck with your build.
 

vm1451

Active Member
Cheers fella. I think I may just drop the idea, it was a bit of a pipe dream and something to keep my mind active while caring for the wife.

I've also just realised that the XO in the speaker is not being employed anyway as they are bi-wired. Duh!

I will take a mosey at the site you've linked to though. Thanks for the chat. :)
 

david1961

Active Member
Hi Curtis Difference between MK1 &M2 Tag F1 speakers

Was a tweeter upgrade and possibly a small adjustment to the crossover as far as I know

Doubt there would be much difference in listening to the human ear
 

dogfonos

Well-known Member
I've also just realised that the XO in the speaker is not being employed anyway as they are bi-wired. Duh!
The crossover is still being used when bi-wired. The crossover has simply been physically split into two separate filter sections: a low-pass filter to feed the bass/mid driver with signals below the XO frequency (around 3 kHz or whatever) and the other section is a high-pass filter to feed the tweeter with signals above the XO frequency.

All the best.
 

vm1451

Active Member
Just to follow this up quickly, are the capacitors on the existing XO employed then? Or is it just the coils?
Also I missed out the fact you mentioned the link I had posted was for a filter and not an XO, that's because I posted the wrong link!! Meant to post this:-

Anyway, thanks for the advice and I've signed up to the DiyAudio pages so thanks for that link too.
 

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