Topic: DLR No.1 headphones project

[andromedanwarmachine]

This is hopefully going to be a thread about my DLR No.1 Headphones which I got recently at a car-boot sale for £5.

I intend to take them from 1940 and make them useable and useful in 2010.

[andromedanwarmachine]

I have not found out much about these cans, although a web resource developed by a collector of these things on your side of the pond points them to be a crystal-radio set wartime-comms headset of some kind.
Now- I'm not a maniac when it comes to destroying old things, and the first thing I do in this kind of a situation is to see how ubiquitous they are. A cursory stab at eBay reveals a few floating around, so I don't feel too bad about having "my way" with them.

Needless to say a few things will have to change to make them practical in 2010.

1 new drivers
2 new cable
3 3.5mm minijack connection
4 stereo rather than mono.

[andromedanwarmachine]

I daresay as a reader of this thread so far, someone may know EXACTLY what these are and their relative (historical) worth and I would be very interested to hear any details from such an informed person.

However, most bizarrely, and I REALLY want to know about this, the drivers themselves are not speakers but vibrating plates. An arrangement akin to a solenoid is assembled in each housing magnetically holding a disc in place which acts as a diaphram.

At this point it is my intention to remove this arrangement to make way for more modern low impedance drivers.

[andromedanwarmachine]

The modern drivers I have in mind for this are the ones used in Sennheiser PX 100-IIs.

which may be the right diametre to go in without too much hassle.

[stevetoney]

I daresay as a reader of this thread so far, someone may know EXACTLY what these are and their relative (historical) worth and I would be very interested to hear any details from such an informed person.

I have no clue about what they are or their historical worth.  However, I am hearing challenged (thus the handle 'tonedeaf') and those look a hella lot like the headphones that audiologists have been putting on my head for the last 45 to 50 years...the phones that are connected to the (usually prehistoric) machine that generates tones of a certain frequency and volume...in order to generate an audiogram of my hearing spectrum.  They're about as comfortable as stabbing your head with a fork.

[andromedanwarmachine]

really?!

well, I'd say that's a good sign- as to their ability I mean rather than comfort!-

Yes, relative to my HD 25s, they're not exactly like sitting on a comfy sofa- more a stone floor!

45-50 years of audiology, yeah? If it's any consolation- at 37 I don't go above 6.3K in my left and 10K in my right...

JimP

[andromedanwarmachine]

Further reading has revealed these are British military low impedance "sound-powered" balanced-armature headphones.

And that "DLR" was a type manufactured during WW2 by S.G. Brown in the UK.

Still not sure as to a date of manufacture as the more popular No.5s are from 1944!

Although the providence of the "D" prefix is in doubt- it may mean "dynamic", the "LR" appears to mean "low resistance".

[stevetoney]

Further reading has revealed these are British military low impedance "sound-powered" balanced-armature headphones.

And that "DLR" was a type manufactured during WW2 by S.G. Brown in the UK.

Still not sure as to a date of manufacture as the more popular No.5s are from 1944!

It's cool that you were able to trace the details back.  It almost makes me want to go to a yard sale and find some old electronics that aren't worth more than a dollar just so I can conduct a history search to see if I can determine origin.

[andromedanwarmachine]

well, thanks for saying that!

I think alot of what's on here is so quality concious that no-one would see the point of this little adventure! I'm not under any illusions about their ultimate usage- even with Sennheiser drivers in, I just fancy a modern useable retro set of cans- they certainly won't look like anything else on the street!
(not that I ever use a Walkman/iPod actually!)

[andromedanwarmachine]

and so the cable.

I managed to source some "2-core" 0.75 "chandelier cable" from an eBay supplier for next to nothing and when it arrived it turned out to be 3-core! Which wasn't a bad thing as obviously I need the 3-cores for stereo (unlike the original 2-core mono configuration). There was a choice of colours so I went for "bright silver"!

So an inline ground (or "-") bond had to be accomodated within the lead at the Y-split. I got a 2:1 reduction Y-transistion piece of heatshrink from Farnell which worked quite well I think. I've had to Superglue every braided breakout to stop the cut fibres fraying up the cable.

The minijack was more of a challenge- a nominal 6mm being required to accomodate the three braided cores. I had originally envisiged a step-down cable transistion before the connector, but after a bit of "work" I managed to get a result in one go!!

Headphone end next...

[andromedanwarmachine]

next up would be the connections to the driver-housings themselves.

At the moment these are twisted "rings" of bare wire which bolt on. The cable before them is woven onto a paxolin shape which acts as a strain relief. First up would be the remake of the paxolin strain relievers in the modern equivilent- epoxy glass FR4 which is PCB material. A rough outline was drawn to establish a shape- which did not turn out to be symettrical!
This was cut along with a matching shape for the other side from the material and then the two checked for bolt pitch. This established, holes were drilled for the driver-housing bolts and then the two FR4 pieces were bolted together to allow for matched finishing.
Once filed to satisfaction, the cables were tie-wrapped temporarily to the strain relievers and then bolted to the driver-housings to establish breakout length. This established, a variety of heatshrink (and superglue to prevent fraying of the ends) was used to take the place of the 1940's weaving to hold the cables onto the strain relievers...

The image of the cables tie-wrapped onto the strain reliever bolted to the driver-housing also shows one of my Sennheiser HD25 foam earpads tempoarily added for evaluation.

[andromedanwarmachine]

following this, the next stage would be the removal of the original drivers from the chosen donor Sennheiser PX 100-IIs.

The specs for these showing a quoted 15-27000Hz response with 114dB sensitivity making them less of a size only choice and more of a performance orientated decision!

Despite their budget pricetag of £30, these cans showed really high quality manufacturing details, with almost every part screw-removable. In the end- the drivers themselves were bonded with four adhesive spots through holes to the rear housing. I was able to break these bonds with a twist of a small flat bladed driver.

[andromedanwarmachine]

following this- the most terrifying part of the whole enterprise and the part which could bin the entire project-

Modifying the bakelite driver-housings themselves to accept the new Sennheiser drivers.

These bakelite housing being made up of three parts- an unscrewable telephone style earpiece and a front & rear section held together by three bolts.

Having removed the balanced-armature "mechanism" a lot of headscratching went into how to best add the new drivers to the old shape. Would it be best to mount the drivers themselves, removed from their front grille, or with their front grille still attached. Would it be best to attempt to Dremel out the rear of the driver-housings to fit the depth of the new drivers to mounthem with no external modification to the front section, of remove the front section of the housing themselves to accomdate the grille?

In the end it was decided to confront what must be done head on. After all if the front section was damaged, things would be tricky but hideable behind the earpiece, if the rear sections which also attach to the headband and cable were damaged- game over! The Sennheiser driver's and grilles had a combined diametre of 39mm. A 38mm hole cutter was obtained...

[andromedanwarmachine]

as it happened, the bakelite drilled very well!

The only problem being the jamming of the 38mm hole cutter on a metal lined vent/pressure membrane which was part of the front section. Drilling was stopped and this feature Dremelled out. The resulting ring of bakelite then filed and Dremelled further to gain the required diametre to accomodate the Sennheiser driver & grille.
At this point, I realised that I had not made any measurements of the response and ability of the original DLR armatures and so worked stopped for a night to allow analysis of the remaining original housing before it would be dismantled forever.

Cursory pink-noise plots revealing (impedance offset aside) the sound of 1940!!!

[andromedanwarmachine]

The final shot of the previous post shows (although you'd probably have to zoom in to see it) the outer sides of the donor Sennheiser driver grilles having been "slotted" with a junior hacksaw in repeated places round the circumference and the inside of the bakelite front section "rings" grooved with the same tool.
This was to provide keying for the Araldite epoxy resin to bond the two together by interference fit. It is a common misconception that epoxy resin is a "glue", it is very good at making shapes and if you can make shapes that will interfere with two parts; chances are they'll stay together!
Once the drivers and grilles had bonded to the DLR driver-housing front sections, attention was turned to the rear sections. The threaded solder lugs that are used to attach the headphone cable were unscrewed and removed. These were cleaned of old 1940's solder and retinned for 2010 lead/colophony free variety. A short length of Van Damme "ultra-pure silver plated" mic cable was stripped out to give linking conductors between the solder lugs and the new drivers.
The rear of the donor PX 100-II driver housings had been open to allow air to the rear of each speaker. The DLR housing are closed tight. In order to lend an acoustic sympathy for the intended operation of the donor drivers, I decided to "port" the rear section of the DLRs with a hole not visible once the strain reliefs are attached.

Offering the front and rear sections together- the three original bolt holes which pass between the parts were redrilled to pass through the Sennheiser grilles. At this point the original BA (?) gauge hardware was discovered to not be long enough to hold the sections together any more and so M2.5 x 25 countersunk bolts were used.