DM 70s

[TopMarc]

I've just completed a pretty comprehensive rebuild of a pair of B&W DM70s. My work included:

• Refoam the base units
  Building new HT power supply boards (Cockroft Walton Ladders)
  Strip and rebuild ES panels with new conformal coating, Mylar diaphrams etc
  New crossover caps in both hi-pass and low-pass filters
  Cosmetics on the case veneer and stands.

The job was not without some difficulties but the mechanics have been accomplished and I am confident that I have adhered to good practices.

I am still not blissfully happy with the sound. There appears to be a "gap" in the upper base to lower midrange that makes the speakers sound "hollow". They seem to lack a certain color. The speakers seem to handle a sine wave from 10Hz to well past my upper hearing limits (over 16KHz) without any rubbing of the base coil or harmonic vibrations of the cabinet.

I really need some considered opinions from this forum as to the possible causes of this sonic "hole":

1. Did I short change myself on the crossover caps? - the original low-pass caps were metalized 30uF 150 volt in metal cases - I replaced them with polypropylene metalized film from Illinois Capacitor (2x15uF at 400 volt parallel) They are not particularly expensive but I figured I wasnt going to buy paper and oil at high cost until I could justify it. The original high-pass caps were unidentifable in type - I replaced them with equivalent value 4.7uF polypropylene metalized film again from IC (there were 3 in parallel). Could this be the cause of my "hole". The crossover is at 400 Hz according to B&W - I do have a circuit diagram with values but its from the DM70 Improved and the values differ from what is fitted to my units which are early examples)
   
   Could the base cones have lost their stiffness? - I have read some articles that talk about getting some color back into lack luster base units by "doping" them with white glue. The cones are original on my units and they have not been modified or altered in their life. I refoamed them about 20 years ago when I lived in england and was able to get parts from B&W in Worthing but of course now all OEM parts are gone and forgotten. Do woofer cones loose their rigidity over time (now 36 years old) with high humidity (in Virginia during the summer) or with abuse (these units have seen some action in their life)?
   
   Should I upgrade the wiring in the cabinets? - It never ceases to amaze me that B&W produced such speakers with "bell cable" wiring in them. The main wiring in the psu/ht box is single core of a VERY thin guage. Would I benefit from replacing this with either thicker guage solid core or multi-strand speaker cable? If so, should I also add some meat to the crossover wiring right up to the component leads or am i just dreaming.
   
   One final - Did I get the tensioning correct on the ES units? - If you have seen the DM 70s you will recall that the ES unit is curved. I laminated the Mylar diaphragm to the rear stators on a flat surface then stretched this over the curved wooden chassis then screwed the front stator to the assembly. In doing so I noted that the curve introduced additional tension in the diaphragm in one plane. Could this extra tension be causing the ES units to have a low midrange output thus my "hole" (they still have good top end)? Should I rebuild them with less tension in the horizontal plane and allow the radius to introduce the missing tension or, as has been told to me, should the airgap spacers bars reduce the effects of this additional tension.

In my opinion some or all of these could be combining to cause my audio "hole". Please, let me have your thoughts.

Thanks for putting up with such a long post from a newcomer.

Marc

[john beresford]

Hi Marc,

I have no experience with B&Ws But ERAudio specialise in new & repaiting ELS panels & associated electronics. They have a very good reputation. You could try posting on the ELS board too.

John

http://www.eraudio.com.au/index.html

[coolhandluc]

What you are describing is an inherent "feature" or design flaw of the DM70, and most probably why B&W did not persue hybrids, it is very difficult if not impossible to coherently blend a large dynamic woofer with a very small Esl panel/s.
All DM 70's exhibit this characteristic "hollowness"....but you may be able to minimise it with careful system matching, room placement etc....

[Steve M]

Yes, I agree with coolhandluc on the reason for the midbass suck-out. The 12" woofer cannot reach up nor can the small ESL diaphragm reach down to that critical midbass/lower mid area.

A friend of mine with DM70s had good success with playing around with the stuffing in the speaker, which makes the speaker a bit dead sounding overall (over damped). He took the factory stuff out and stapled thin rubber floor mats to the internal walls and left the cavity free, and he reckons the bass gained in both texture and punch.

Regards,

Steve M.

[TopMarc]

Thanks for your replies.

Maybe I should have said at the outset that I have noticed the sonic "hole" has appeared since the rebuild. I have owned the speakers and been more than satisified with their sound for the last 20 years.

A couple of years ago I retired them from service because the diaphragms would become waterlogged in the humid summer weather in Virginia rendering them useless and there were a couple of holes burned through them because of arcing.

I purchased the kit of parts from eraudio and rebuilt the ESL panels and Rob and I have conversed on this issue. He also helped me diagnose low HT issues which I have now fixed but this only really solved the top end. There is a gap which sounds like either the ESL panel is not reaching down or the woofer is not reaching up. I dont have too much in the way of test gear to check which one it is but its around the critical area for vocals which sound hollow and lifeless. I put it down to one or other of 2 things - the ESL panel is too tight after bending it round the chassis causing it to lose sensitivity at its low frequency end or the woofer cone has gone soft and has no color at the top of its range.

Which one is more likely? -- Do paper cones lose integrity over time and become soft and mushy or have I just got the diaphragms too tight?

I have new Mylar (stretched in the vertical plane only) already stretched and ready to go on the workbench and dammar varnish and brushes ready to apply dope to the cone. My quandary -- which one to do first (and secondarily - on which board to post the results - after all they are hybrid speakers :o)

Thanks for the ideas of removing the damping - I hate the glass matting that B&W used. I might try some sound deadening foam I have left over from another job and staple it to the inside of the enclosure.

Marc

[Andersonix]

I think the woofers can lose some of their midrange with age, especially if they also become waterlogged from high humidity. But wouldn't you have noticed this happening before you repaired the esl section? Maybe they need a coat of latex or some kind of doping to stiffen them up again. But not until you can pinpoint the problem.
Did you make several changes/repairs at the same time? Have you played them at loud volume for a few days to break in everything? I suppose you could try to put the xo back to its original condition, but that may not be easy. Do the speakers sound identical left to right when you switch the esl sections around?

When you repaired the esl section, did you make the esl larger to better use the available space inside the top enclosure?

Another small possibility is that the tension in the rod connecting the front and rear face has changed.

I wouldn't bother with changing the wiring.
I think you should play with room placement to tweak the sound. I found these quite sensitive to small toe in and rear wall distance changes.
And please keep us updated. Perhaps this thread should go in the esl section for better suggestions?

[Steve M]

Marc,

Now that you've explained it more fully, the problem could be the ESL re-build. With the ER Audio kit, did you replace the membrane with the same thickness of mylar as originally provided by B&W. Rob prefers to use 3.8 micron mylar which is very thin by industry standards, it provides excellent speed and transient response but may not be giving the lower mid/upper bass that a thicker membrane provides.

It may simply be that the thinner mylar gives a frequency response that you are not used to, perhaps lacking the body in vocals that you are accustomed to with normal DM70?

BTW, I am an ER Audio ESL-3 electrostat user, see my System Pics below...

Regards,

Steve M.
System Pics: http://forum.audiogon.com/cgi-bin/fr...146370182&view

[TopMarc]

Originally posted by Steve M
Now that you've explained it more fully, the problem could be the ESL re-build. With the ER Audio kit, did you replace the membrane with the same thickness of mylar as originally provided by B&W. Rob prefers to use 3.8 micron mylar which is very thin by industry standards, it provides excellent speed and transient response but may not be giving the lower mid/upper bass that a thicker membrane provides.

It may simply be that the thinner mylar gives a frequency response that you are not used to, perhaps lacking the body in vocals that you are accustomed to with normal DM70?

Ah ha.

You might have hit the nail on the head. The ESL panels sound perfect on their own and so do the base drivers. When paired they leave a gap. I was searching for a reason and I think you may have it, the diaphagm is thinner and has moved the response curve up in frequency. I feel an experiment coming on!

So what thickness should I use? 6 micron seems to be the choice of the quad guys for treble panels and 12 for the base. Which would be better given the 400 Hz crossover? Could I get 12 micron to handle up to 18/20 KHz? What tension should I try? Where would I go to get it in the USA? Area 51?

I love you guys....

[Salas]

You may ease up on the woofer crossover coil's inductance a bit on the other hand...

[coolhandluc]

SteveM, please contact me via email re: aussie audiophilia