B&C DE250 1" compression driver - looking inside

Previously I introduced the B&C DE250 as a great value good sounding compression driver. I have since had more time to use this driver in various projects, and I've also taken it apart and measured its dimensions carefully to understand how to design a waveguide for it as part of my point source horn project.

This section shows what I was able to measure. I didn't have access to everything, so there is some guesswork with the phase plug and pole. 

Most of this is just the standard compression driver design that has mostly remain unchanged for a very long time. The phase plug has concentric slots and a conical exit tube. I estimate a 12 degree included angle on the exit tube. It was difficult to measure accurately and I believe Earl Geddes has stated that it is in fact 6.5 degrees to the normal (13 degree included angle). The phase plug (shown in purple) is a one piece cast plastic unit. There is a gap at the end that is significant and should be filled when mounted. 

This rear cap comes off when the diaphragm needs replacing:

It appears to be cast aluminium and there is a small amount of foam. Tweakers might like to add some more material here, although it's difficult to get much in before crowding the small space. For critical applications I'd like to see a much bigger chamber with a lot more foam.

Diaphragm front and rear view. It's a 44mm dome with a copper voice coil and flat ribbon lead in wires. This is a second hand unit, the usual spring terminals have been replaced with a soldered cable. The previous owner apparently has a tendency to tweak everything. 

 You can see it's seen some action already. You can also see the gap around the exit tube (right). You can see the concentric slots on the phase plug (left). Then you can also see the VC gap.

Overall, this driver could have been made better. The diaphragm lacks a spigot to centre the VC so there is no way to be sure that you have it positioned correctly. This could well mean a bigger than necessary VC gap to allow for this. Far better to simply design it to centre the VC so that you can't get it wrong. The exit tube gap is quite poor as well. Despite these complaints, it's a very good sounding driver and I put that down to the polyimide diaphragm which is well damped and the fact that overall it's well designed in terms of its acoustic performance. The response is smooth and flat and I'm yet to hear a better sounding compression driver.

See also

BMS 4550
Faital pro HF10TX