High Impedance Sound Powered
(Balanced Armature) Phones
Part 4 – Winding
When everything is ready, the real work and excitement begins! I recently obtained a winding device for old 8mm films, shown at fig. 8. You can also use place the form in the chuck of a hand-drill, as I did for years (fig. 9).
It is very important to center the form perfectly, otherwise the windings can spill over the form edge.
Besides wearing magnifying goggles, have a bright spot light, and put white paper under the coil and the
wire. It is important to see the wire and how you feed it. You can see better by looking with only one eye.
Choose the direction of winding, so that the wire comes from the upside of the coil. The source-spool must also spin easily, without any resistance.
With your left hand, rested at a comfortable height, you lead the wire, organize it within the coil, and keep it under a slight tension. See fig 8. Try to organize the windings as well as possible. Don’t do a criss-cross pattern; it takes up too much space. First try a few layers to get some experience and to feel the maximal tension before breaking the wire.
Next cut a few small strips of adhesive tape. You will need them to hold the wire at the coil if you have to take a break. Secure the start by making a threefold loop of 4-5 cm and twist it. Bend it around the soldering contact (do not cut it!) and make the first few turns with this twisted loop. Next continue with single wire layers. Do the same with the end.
Mark the paper every 100 turns if you do not have an automatic counter. You will later want to know how many turns you have put into the coil. You can calculate the total number of wire turns by multiplying the number of drill-handle turns with the turn-ratio.
After finishing the winding, solder the ends and protect the windings with a 7mm wide strip of thin elastic adhesive tape, or a thin layer of a transparent nail-polish.
Control the clearance for the two mounting rods; it should be 18 mm. Also use the Ohm-meter to see if the contacts are good. The DC resistance of the contacts should be between 1500 and 2000 Ohms.
Part 5 – Reassembling The Element
Clean and check all parts prior to reassemble the unit in opposite sequence. That means start with the bottom plate first.
Slide the armature in the coil with the small hole facing the contact side. Than put the coil (with the armature inside and the two small fixation “U”-shaped parts in place), vertically into the bottom plate, so that the poles fit into the special grooves on the coil form. The drive rod must pass through the small hole of the armature, and the two fixation pieces must lie at the pole of the magnet unit bottom plate.
Now put back the upper nut of the drive rod, and fasten the armature. It is of crucial importance to check if the armature is free and equidistant on both sides within the central slit of the coil form!!! If not, adjust with both nuts, and tighten it. Remember that the vibrations of the armature have to be transferred to the drive rod and finally to the diaphragm. When you put back the upper plate, adjustment would be very difficult, if not impossible.
When everything is OK, put back the upper plate so that the poles fit into the upper grooves of the coil form. Put the screws back, but don’t fasten them yet. First slide the magnet back and than fasten the screws.
Pull the leads through the eyelet of the lead-holder and check once again if the armature is at central position. If you look through the coil against the light, you should see equal spacing between the armature and the poles of the magnet unit. Also check if the lead contacts are not touching the adjacent metal parts.
Now put back the contact-screws and fix the element into the housing.
Connect the flexible cable and the head-band and...finally, you have it! (Fig 10)
Now click on the link at the left to go to Part 6.
Part 6 – Testing The Phones
To check the expected characteristics, you can run tests on the phones or simply compare with another set you have.
I rewound two elements and tested the headphone-set using the following instruments: PHILIPS LF Generator PM 5100, PHILIPS dual oscilloscope model PM 3207, Digital AVO-meter Conrad Electronics M830B and analogue Velleman Multitester 40000 Ohms/V.
There are many procedures described on the Web to test the AC impedance and the sensitivity. I applied the one described by Dick Kleiert, with a small variation: for R1 I used 50 kOhms instead of 10 kOhms for better reading of the input voltage (200 mV).
Here are the results:
Windings per element................5300 + 5000
DC Set resistance...................................3870 Ohms (2050 + 1840 )
AC Impedance at 1 kHz.......................56000 Ohms (pair)
Threshold sensitivity at 1 kHz...........0.00028 picoW (pair)
The term “threshold sensitivity” means that I was just able to hear the 1 kHz signal in compete silence. Of course, to understand the speech or music, higher input power is needed.
Part 7 – Troubleshooting
In spite of all the attention and care there is always something that can go wrong. Here are some situations, and how you can manage them:
- One of the two “U”- shaped armature-fixation parts jumps off, and you cannot find it any more. * Take a 0.2 mm thin sheet of a non-magnetic metal and cut a strip 2mm wide and 5 mm long. Bend it around 0.6 mm, (that is the thickness of the armature) in a U-shape.
- The coil form brakes. * Cut a piece 3 x 8 mm of thin (0.15-0.2 mm) pertinaks or similar hard non- metallic material, and glue the patch from inside (wire-side) with cyanacryl glue (super glue).
- The wire breaks. * Fix the last layer at the coil with the piece of adhesive tape, and twist both broken wire-ends together. Solder the joint, cut it off to the length of about 2 mm and continue winding. there is not need for any insulation (it occupies the limited space). Just cover it with the next layer of wire.
- You lose the tiny nut off the drive rod. * Try to get one from a watch repair shop, from an optician, or from a hobby shop. Take the element with you to check the size.
It is a good idea to have one extra (third) phone element, just for the spare parts or to start over again if you encounter serious problems.
Part 8 – Epilogue
Perhaps this approach will not meet the demands for the most serious crystal DX work, but it is fun to try something new. It is possible that I made some mistakes, in approach or by the testing, so if anyone has done something similar, I would be grateful for any comments, corrections or confirmations.
Click the link to the left to see the Appendix, comparing several earphones.
And finally, fig. 11 shows my hobby-corner where I did all the work.
TESTING MY EARPHONES
MODEL R/DC IMPEDANCE SENSITIVITY Ohms at 1 kHz pico W U.S.I Model A260 124 1500 0.024 SP Bal. arm ( pair) Western El. Co. 90 500 7.2 Model HS-30 ( pair ) DLR-5 I.T.B.A.5 27 280 0.13 single element balanced arm. DLR-1 Magnetic 50 280 0.23 Single element DLR-5 “H.I.B.A.” 3890 56000 0.00028 (High Imp.Bal.Arm.) home rewound, pair Continental RAG 4000 22000 0.3 vintage magnetic pair Rothermel Brush *** 33000 1.2 crystal, pair Taivan ear plug *** 15000 0.15 crystal single element Hi-Fi 600 Ohms 46 560 46.0 mini-speaker element pair