B&W DM70's - top end lacking

I just took out the two power/ energiser units and was surprised to find that there were many voltage selections (250, 240, 230, 220, 115, 105) and that they were set to 250V.
When this was changed to 230V which is the nearest to my mains ay my current address, the treble units came to life and now produced output up to 14kHz. That is still not as it should be, so the next thing is to send the units to One Thing who repair these (Change all the diodes and whatever else may need doing).

If the sound is still not correct I will get the Panels repaired by One Thing, who say that they have made a jig for these.
I do wonder if the panels were last repaired with too thick a membrane and I do also know that if it is too thin there can be a bit of a gap between mid and bass. It may be for this reason that they always had a falling response above 16kHz.

Regarding the bass, I have now stuck a door mat to the rear panel of the main box (inside). This seems to have tamed a resonance in the upper bass, so now I will do the same with the other unit.

ct0517, I hope you are enjoying yours

Iscm

from the manual.

Impedance
Nominally 8 ohms but rising throughout the mid frequencies to some 20 ohms, falling at 20 khz to 4 ohms.

The manual specifies the following for frequency response.
better than plus or minus 4db - 50 hz to 15khz
plus or minus 5db - 40hz - 20khz

This tells me they need really good setup or people will hear 50hz - 15khz (or less) with an average setup..

The manual also states the ESL panel has a removable pad to control the rear radiation pattern. Make them more dipole. This would be very important for a full midrange and keeping them a few feet at least from the front wall. You don't have a lot to work with - its a small panel. Everything helps. 

My feeling is if B&W kept producing them, the ESL panel would have grown in size - taller, as ESL Panels become more efficient with size. Interesting that the Quad 57 treble & bass panels are vertical. The DM70 mid/treble panel is horizontal. From what I remember the DM70’s have a bigger sweet spot over the 57’s when the 57’s are kept on the ground on their stock feet.

Mentioned earlier I was thinking of taking them out of the stored spot and setting them up. But this needs my son’s lifting help so maybe when he visits. The power supply finding is interesting. Will check the setting on mine before they do get turned on again. I have multiple dedicated lines in the house and live in the country so the demand is not high - Its a solid 121 volts. So the 115 setting it is, and it is low.

ct0517:
This is what I have on the DM70’s so far from different sources.
https://1drv.ms/f/s!AsdR4rYZVpE4sUzCKIvZoEG9f5Ag
Sorry, but there may be duplicates and it includes what I got using the link that you provided

I like this in particular, as there is a very long explanation about their development:
https://1drv.ms/b/s!AsdR4rYZVpE4sDU7GJjlvhBf1_2N
I note that a later equivalent for 1974 is much shorter and more simple.

This tells me they need really good setup or people will hear 50hz - 15khz (or less) with an average setup

My treble issues also show up when I use an amp that previously worked OK with these speakers, so it seems that there is some issue with my speakers.
I talked to One Thing Audio who suggested that I get the ’energiser’ units serviced as a first step.
They said that it is very common for the diodes to fail and that they would replace them all as a matter of course. If that does not work I will send in the panels for repair.

My feeling is if B&W kept producing them, the ESL panel would have grown in size - taller, as ESL Panels become more efficient with size.

I am not so sure, as making the panels taller would change the dispersion pattern.
Their horizontal dispersion is also quite good due to the curve used.
B&W did say, however, that a vertical arrangement was tried.

Hope this is of interest
Ian

I have owned two pairs of these.One pair sounded a bit dull but the other pair had lovely extended treble so it really depends on their condition.

One pair sounded a bit dull but the other pair had lovely extended treble so it really depends on their condition.

I think that either my energiser units need an overhaul of the panels need redoing.
First I will get the energiser units serviced and if I still have issues I will move on to getting the panels done.

I did not think that the electrostatic panels should sound dull.