High Impedance Sound Powered
(Balanced Armature) Phones
Here is a very interesting article written by Dejan Momirov. Balanced armature phones are found in sound powered phones, but are usually low impedance. Dejan has made his own set of high impedance phones. Since his pair of phones has a 56k impedance at 1 kHz, a matching transformer can be eliminated for some crystal set applications.
Please drop Dejan an e-mail if you would like to discuss with him. His address is firstname.lastname@example.org
Many thanks to Dejan for his great article and the many hours he spent on this project.
REWINDING THE DLR-5 To HIBAPHONE (High-Impedance-Balanced-Armature Phone)
By Dejan Momirov
This is a description of the procedure I used trying to make a high-impedance, high-sensitivity earphone set. The idea was to combine the advantages of high impedance vintage phones with the high sensitivity of a balanced-armature construction. I was more than satisfied with the results, and I would like to share my experience with other crystal set DX enthusiasts.
I have several DLR-5 earphones, and I decided to rewind the spools of one pair. First I tested their characteristics, and here they are (per phone element):
AC-impedance at 1 kHz............280 Ohms
Sensitivity at 1 kHz..................0.13 picoW
It wasn’t easy at all, but with patience and enthusiasm, you can also do this!
Before you start, keep in mind:
Don’t rush, take your time!
Don’t panic if something goes wrong! Almost everything is reparable (see also the section troubleshooting).
Keep dust and small particles away.
Control every step as it is done
Never solder or de-solder the coil contacts with the permanent magnet in place, because of the strong alternating magnetic field of the soldering iron.
Use magnifying goggles.
Part 1 – Disassembling The Element
In Fig.1 you can see the front and rear of the original DLR-5 ear piece, inside and outside the housing.
When taking the element out, it is better to unscrew the contact screws than to desolder the leads in such a small space and so near the magnet. After loosening the three screws on the top, first remove the magnet and then the upper plate of the magnet unit. It is very difficult to work within the strong magnetic field and there is the risk of damaging the armature.
Pull the leads out of the eyelet of the lead holder. Do not desolder the leads from the coil yet! The ends are wound around the coil contact plate. Unscrew the tiny nut from the drive rod using fine but strong forceps like those shown at Fig. 2. (Bent micro-surgical or the depilation forceps).
Remove the coil, with the armature still inside straight upwards, to avoid damaging the drive rod. The bottom plate of the magnet unit has to be removed only if you plan to clean off the hardened varnish. However, it is important to clean it because the varnish can later interfere with the reassembling the coil and can damage the wire. It could also break, and the particles could fall into the diaphragm space or between the armature and the coil. You should leave in place the diaphragm, drive rod and the second nut. Try not to touch the lower nut, because it determines the central position of the armature.
All parts are shown in fig. 3.
Important: the two tiny ”U”-shaped parts, marked with the arrows at the photograph (fig 3), are the only two fixation points of the armature. They also determine the distance between back end of the armature and the poles of the magnet unit. Remember the position of the two pieces before removing the armature from the coil form, and don’t lose them!
After reviewing the Figures below, clink on the link on the left side of this web page to go to Part 2.
Part 2 –Preparing The Coil Form
At the coil form you will see two contact-plates with lead contact and coil-wire contacts. Using two pairs of needle nose pliers, as shown at fig. 4, bend them straight or a few degrees further. Don’t hold the coil; the form can break from the bending!
Now remove the original wire. (You can keep it, just in case you decide to undo the whole job!) Clean the varnish from the coil form. Also, clean the soldering points.
Remove the insulation from the contact plates and polish them as well as the edges of the form with the fine sand-paper (# 360 or higher). The whole inner side should be smooth and without any obstacles, otherwise it could catch and break the delicate wire later.
Fig. 5 shows the form ready for winding.
Part 3 – The Wire
The idea of the whole project was to get higher impedance and that means as many turns as possible. To achieve that, you need the thinnest wire you can get and the maximum space you can get for it.
The wire I used was enamel coated 0.05 mm from a reed-relay activating coil. Another way to get the wire is to order it via Internet. I ordered some by CONRAD (www.conrad.nl), order nr. 60 75 09-06, cost 3.55 Euro +p.p. One spool (cc 1500 m) is enough for pair of phones.
Anyway, the wire must be solderable. The wires I use are shown at fig. 6. Available space is limited by the mounting rods, as shown at fig. 7. You cannot stuff the whole form because of the 18 mm clearance between the two mounting rods.
The drawing also shows how you have to bend the contacts from their original position (marked 1.) to the temporary winding position (marked 2.). After winding, bend them back to the position 1. See also fig. 4. The area shown as 3. is the available wire-space.
Click the link at the left to go to Part 4.