Don't want to trash the speakers because they are quite good especially for the price I paid. In fact I get pissed when snobs turn their nose up at these because they are "old'.
The Only issues I have is what I perceive to be a peakiness in the lower mid and slight bass control (speed/mudiness). Again this is slight so what I will do is recap it and maybe replace R1R2 with a higher value to reduce the midrange a bit. ( I assume a lower value C2 would make the XO point slightly higher and reduce the lower mid a bit) BTW, I am an engineer and have experience building speakers but have not in quite some time. I am a returning DIY who needs to retrain myself. My gratitude to everyone at this board. Everyone has been very nice and it's quite refreshing not to hear trash talk as we are all enthusiasts. Thanks again all!!
The Only issues I have is what I perceive to be a peakiness in the lower mid and slight bass control (speed/mudiness). Again this is slight so what I will do is recap it and maybe replace R1R2 with a higher value to reduce the midrange a bit. ( I assume a lower value C2 would make the XO point slightly higher and reduce the lower mid a bit) BTW, I am an engineer and have experience building speakers but have not in quite some time. I am a returning DIY who needs to retrain myself. My gratitude to everyone at this board. Everyone has been very nice and it's quite refreshing not to hear trash talk as we are all enthusiasts. Thanks again all!!
Disconnecting either R1 or R2 should lower and broaden the mid response. I'd play with that first. Putting them in series would spell it out.
The aim, as I see it would be to find out whether the mid issue is a matter of mid driver level, or mid cone breakup, or maybe the tweeter resonance. Have you tried listening to them off axis?
Muddiness might have something to do with positioning.
The aim, as I see it would be to find out whether the mid issue is a matter of mid driver level, or mid cone breakup, or maybe the tweeter resonance. Have you tried listening to them off axis?
Muddiness might have something to do with positioning.
Check out for phase connections reversed on mid, as this was also of a speakers problem with in others forums.
AudiogoN Forums: B&W DM640 - is there anything wrong?
AudiogoN Forums: B&W DM640 - is there anything wrong?
When you look at the schematic the tweeter and Mid are supposed to be OOP with the woofers. It is intentional. Are you saying try it in phase?
For me the polarity of the mid and the tweeter is OK, negative polarity.
As said replace only capacitors in series, c2 85uF -->82uF c4 7uF-->6.8uF C5 16uF-->15uF by film capacitors. You will have a great improvement.
If the electronics are OK,to improve speed, see if you can, if possible use a shorter vent ? Or damp a little the vent.
I still think this speaker is hard to drive for an amplifier.
There are only a couple of fine points that haven't been mentioned:
When replacing an electrolytic with a film cap, the ESR (equivalent series resistance) will change (probably to a much lower value). ESR varies with frequency so which section it's in would affect the effect of the ESR change. It might not be a dramatic change, or even be significant, but just be aware that it will change the crossover frequency and slope.
When replacing inductors/coils, if you don't use one that has the same DCR (D.C. Resistance), the crossover frequency and slope will be changed, and in the bass section, for example, it would also change the relative output level, possibly significantly. A lower DCR for L1 might give the amp better control of the bass driver, which could be a good thing. But then it would be louder than it was, relative to the other drivers. Again, it might not be too significant. Just be aware.
Another note about coils: If there is any iron-based metal in the core, then there WILL be hysteresis, which WILL cause distortion. But I don't know how significant it might be. I have heard many reports from people who said that changing to an air core bass coil caused a very noticeable improvement. But I haven't tried it myself, yet, and they might have been hearing the differences that were caused by a much lower DCR. Another potential problem with iron-based cores is saturation of the core if the current gets too large, which would be "a bad thing", but I would hope that the coil would have been sized to preclude that.
I would definitely replace ALL of the electrolytics with metallized polypropylene films. And I would probably also replace all of the inductors with air cores, even if I had to wind them myself to keep the cost down. This is a one-time upgrade of a long-term piece of equipment and the cost will be spread over many years so I would want to build it with sufficient quality and reliability.
Here is a nice air core inductor calculator: Pronine Electronics Design - Multilayer Air Core Inductor Calculator
Just make the coil length some whole number of wire diameters, for each wire gauge you try, and try first with the cross-section of the wires as approximately a square shape. Then try different coil lengths and wire gauges while keeping an eye on the resulting DCR and total wire length needed (as well as the overall dimensions of the result). And also remember that you won't want the inner radius to be too small if you are going to use very thick wire. I would also use a radius that will be easily available, such as the radius of a piece of PVC pipe that you would have available to make the coil form.
Here is one good place to purchase bulk magnet wire with large diameters: cPath_9 | Applied Magnets : Magnet Wire & Magnet Wire Spools - discount wholesale prices.
Also look at the DIY coil-winding comments on the North Creek site. They say that too much tension of the wire is potentially not good, and also say that epoxying the wires and layers together as each layer is wound onto the coil will greatly improve the sound quality of the resulting coil.
Sorry to have blathered-on for so long about all of that.
When replacing an electrolytic with a film cap, the ESR (equivalent series resistance) will change (probably to a much lower value). ESR varies with frequency so which section it's in would affect the effect of the ESR change. It might not be a dramatic change, or even be significant, but just be aware that it will change the crossover frequency and slope.
When replacing inductors/coils, if you don't use one that has the same DCR (D.C. Resistance), the crossover frequency and slope will be changed, and in the bass section, for example, it would also change the relative output level, possibly significantly. A lower DCR for L1 might give the amp better control of the bass driver, which could be a good thing. But then it would be louder than it was, relative to the other drivers. Again, it might not be too significant. Just be aware.
Another note about coils: If there is any iron-based metal in the core, then there WILL be hysteresis, which WILL cause distortion. But I don't know how significant it might be. I have heard many reports from people who said that changing to an air core bass coil caused a very noticeable improvement. But I haven't tried it myself, yet, and they might have been hearing the differences that were caused by a much lower DCR. Another potential problem with iron-based cores is saturation of the core if the current gets too large, which would be "a bad thing", but I would hope that the coil would have been sized to preclude that.
I would definitely replace ALL of the electrolytics with metallized polypropylene films. And I would probably also replace all of the inductors with air cores, even if I had to wind them myself to keep the cost down. This is a one-time upgrade of a long-term piece of equipment and the cost will be spread over many years so I would want to build it with sufficient quality and reliability.
Here is a nice air core inductor calculator: Pronine Electronics Design - Multilayer Air Core Inductor Calculator
Just make the coil length some whole number of wire diameters, for each wire gauge you try, and try first with the cross-section of the wires as approximately a square shape. Then try different coil lengths and wire gauges while keeping an eye on the resulting DCR and total wire length needed (as well as the overall dimensions of the result). And also remember that you won't want the inner radius to be too small if you are going to use very thick wire. I would also use a radius that will be easily available, such as the radius of a piece of PVC pipe that you would have available to make the coil form.
Here is one good place to purchase bulk magnet wire with large diameters: cPath_9 | Applied Magnets : Magnet Wire & Magnet Wire Spools - discount wholesale prices.
Also look at the DIY coil-winding comments on the North Creek site. They say that too much tension of the wire is potentially not good, and also say that epoxying the wires and layers together as each layer is wound onto the coil will greatly improve the sound quality of the resulting coil.
Sorry to have blathered-on for so long about all of that.
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Indeed, I thought about mentioning some of those points which is why I had originally only recommended replacing the caps.
I don't know quite how much effect changing the DCR resistance will have on the overall performance of the crossover and without knowing the DCR of the current inductors we will never know. Like you say though this could very well be quite insignificant for the most part (usually it is unless the DCR is abnormally high or ridiculously low).
The effective ESR of an electrolytic cap though is another thing entirely and I have once experienced some utterly useless electrolytics that had a ridiculously high ESR to make them pretty much useless. Either that or they were defective. If you're going to replace what would be C3 in the 640i schematic it might be worth buying some extra values for R3, perhaps a 1.5, 2 and 2.5 ohm resistors. This should allow you to fine tune the response a little so it's more to your liking. Altering the value of R3 alters the output of the midrange driver around 2khz. Having the resistor value too low will result in a peak being present here that could make the loudspeaker sound far too forwards.
Just to be safe, if you're going to change the inductors jim then I'd recommend buying a few extra resistors for experimenting with the level of the midrange driver. Going for a lower DCR inductor could increase the midranges output by a bit, but the most significant change here is that decreasing the DCR results in some peaking occurring at both ends of the midrange drivers output, that is around 2khz and 300hz. Adding some additional resistance here might actually be quite beneficial, so again you might require some extra resistors for tweaking things. Perhaps buy 0.5 and 1 ohm for this position.
The loss of any DCR resistance in the 5mH inductor that feeds the bass drivers would also act to increase the overall sensitivity of the design and most likely improve the bass quality. If both the midrange and bass drivers experience a small increase in output due to the lower DCR resistance of the new inductors they might perhaps balance each other out a little. This could perhaps improve the overall tonal balance as most people tend to prefer a slightly turned down treble.
Yes, changing C2 to a lower value will increase the cut off frequency slightly, but I would wait on altering any of the crossover values until you've replaced those old old electrolytics!
I believe Inductor is merely suggesting you make sure that the drivers are connected with the polarity indicated in the service manual.
As is usual with these kind of things, it's hard to say exactly what effect altering the components will have, but buying a few extra resistors to slightly alter the effective series resistance of the new parts would probably be a very good idea.
OK Update, I only changed the 7uf and 15uf caps. I also put a small poly cap in parallel cross the 85uf in series with the Mid. Only got to one speaker so it is too soon to tell but one thing that DIDNT work was lifting the lead on R1. The change from 4 ohm to 8 ohm lowered the output of the mid too much. I'll reconnect and move on to finishing the other speaker.
No one expected you to hear any difference with the premium parts, at least I didn't. What I expected was for the loudspeaker to perhaps start functioning properly as the old and possibly dried up electrolytics could have been way out of spec, if they were far out the change would have been obvious.
The area where you will alter things is with the large inductor going to the bass drivers. If this has a significant DC resistance then lowering it to a low DCR cored inductor will alter the system Q and possibly improve the quality of the bass - it could also make it worse - this is unlikely, but it's a possibility, it depends how B&W designed the box originally.
The area where you will alter things is with the large inductor going to the bass drivers. If this has a significant DC resistance then lowering it to a low DCR cored inductor will alter the system Q and possibly improve the quality of the bass - it could also make it worse - this is unlikely, but it's a possibility, it depends how B&W designed the box originally.
Parts for DM640i
Don't know if anyone else has these units but one of my right bass drivers got killed by my daughter hitting the volume knob. B&W parts do not have a replacement in stock (DM640i bass part #ZZ08915) here in Mass so i'm stuck... anyone have a pair of these in their basement that they can sell the driver to me?
Don't know if anyone else has these units but one of my right bass drivers got killed by my daughter hitting the volume knob. B&W parts do not have a replacement in stock (DM640i bass part #ZZ08915) here in Mass so i'm stuck... anyone have a pair of these in their basement that they can sell the driver to me?
Yes. I didn't calculate it but effectively the resistance went from 4 ohm to 6 ohm. I camn tell you that despite the change in xover point it was a big improvement. The speakers were just too forward now they sound great.
Resurrecting the thread: B&W DM 640 woofers
Hello, I just acquired an old, but pristine looking pair of B&W DM 640's.
Two of the 8" woofers have succumbed to spider sag, and the coil is rubbing on the magnet. When I remove those two drivers, the remaining drivers sound fine.
Should I find two replacement vintage drivers and keep it original, or should I replace all 4 woofers with new drivers and modify/upgrade the crossovers to suit? The two original drivers sound good, but only have a 1" coil and a medium sized magnet - thinking I can find better quality woofers for about $200 total and end up with more responsive and accurate bass that might last another 20 years?
Thanks in advance - Six - Minneapolis.
Hello, I just acquired an old, but pristine looking pair of B&W DM 640's.
Two of the 8" woofers have succumbed to spider sag, and the coil is rubbing on the magnet. When I remove those two drivers, the remaining drivers sound fine.
Should I find two replacement vintage drivers and keep it original, or should I replace all 4 woofers with new drivers and modify/upgrade the crossovers to suit? The two original drivers sound good, but only have a 1" coil and a medium sized magnet - thinking I can find better quality woofers for about $200 total and end up with more responsive and accurate bass that might last another 20 years?
Thanks in advance - Six - Minneapolis.
It's really up to you, as the main difference will be the work involved. I know some people might completely redesign the crossover even if the drivers were in good order, there's potential for improvement there. There might be some improvement in replacement woofers as well.
But, if the current woofers can keep up with your needs there is probably less to be gained in the lower range given our sensitivity at these frequencies, and the greater issue of room interaction. Finding original replacements should get you in the right place.