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.

[andromedanwarmachine]

The driver housing were closed with a small piece of the acoustic foam from the front face of the donor Sennheisers left inside to act as a dust catcher for the rear port.
However, the work now turns to the more subtle art of tuning or analysis of sound quality.

It was interesting to observe through initial listening tests that the classic telephone sound of old phones was not entirely due to speaker/balanced armature/carbon diaphram frequency response; The ability of these Sennheiser drivers is considerable but they are completely strangled in this application due to the earpiece design. It's fascinating to think that at the time of manufacture, so little must have been understood about response, that the earpieces reduced even further the ability of the original armatures.

Anticipating this to a degree, I had already drilled out the surrounding three holes to 4.5mm.

[andromedanwarmachine]

However, even with the considerably enlarged centre section of the screw on earpieces, the response was still considerably compressed.
The following shots show each earpieces pink noise response on the front of the driver-housings. (Note that the earpiece shown lying next to the DLR driver-housing in each image is NOT the one being tested.)

[andromedanwarmachine]

And so it was obvious that to achieve even a passing attempt at a neutral reproductive curve that there was going to have to be less earpiece and more driver!

But at what expense to originality- or the perception of it? It would be straight forward to remove the entire centre section of each earpiece which would give total "transparency" to the grille within but that would be innapropriate; producing a very visible butchering. A kind of headphones-hotrod!! After all- if I really wanted the uncompromised sound of Sennheiser £30 headphones- I wouldn't be doing this!- I'd just be wearing them! So a result in context is called for.

Measurement of the donor Sennheiser grilles showed twelve slotted holes for sound within the surface area of the driver underneath. The drivers themselves have a diametre of 35mm. Advanced computer modelling techniques allowed for this pattern to be transposed onto the front face of the earpieces themselves so as to produce a matching set of holes that would align when tightened with the grilles below.
A hole size of 5.5 mm on a diametre of 25mm was chosen.

[andromedanwarmachine]

well, the jury is in on the playback quality of these things...

Two or three things are apparent. Firstly there is a general bathroom-mid-frequency-early-reflections kind of component to the sound.
The bakelite earpieces themselves were of considerable thickness; more than 7mm (for whatever design purpose I can't imagine) and each hole, which matches to a corresponding gap in the donor headphone grille, is acting like a little reverberant tunnel for the sound to pass down. Bakelite is I suppose halfway to being a ceramic compound and the effect is very much like a micro-bathroom sound with a muddling of the mid-frequency information. Because of the small distances involved between driver surface and ear the reflective influence is slight.
I've got to say; I've never listened so critically to anything before and have been establishing all sorts of shortfalls in material I have been testing with! The sound is not unpleasant and entirely within the expectations of many domestic ear phones but it is apparent that the earpieces are having an effect.
Secondly, these cans are really efficient and have great bottom/low mid response!! I have not even approached 70% of their ability but the DLR driver housings are very sturdy "cabinets" for the donor drivers to fire from. The considerably sized "port" which I added to each rear piece (hidden under the strain-relief) acts as a small sound O/P itself and plays back a much attenuated version of the content within!
And finally, thanks to the dozen 5.5mm holes drilled in each earpiece- they are quite uncomfortable!! The centre-in concave bevel of the earpieces themselves is not enough to remove the drilled surfaces from the ear cartilage and as such parts of my ears touch these drilled surfaces. Although not sharp, these holes have an edge and I expect if worn for long perios my ears whould bare octopus-style marks afterwards!!

Analysis responses and final pictures to follow.

JimP

[andromedanwarmachine]

"bump" as they say...

for the benefit of anyone looking for this from The "Mics in the King's Speech" thread..

and if not- sorry!

[yousef]

Absolutely fascinating stuff - sorry I missed this at the time.

Excellent work there - and I'd be interested to know if these mutant cans see much use.

[gkatz]

this is cool stuff, can you post a picture of the whole thing?

[andromedanwarmachine]

Wow- well; thanks for asking!

to answer your question yousef; No- they actually don't see much use! Primarily because I'm not an iPod/personal stereo wearer and I don't think the donor Sennheiser drivers (although they are really good) are quite up to the standard of my HD 25's for recording monitoring on location.

I had to consider an alternative headphone cord as the braided twisted 3-core flex which I made for them kind of has a mind of it's own! I added an old lapel-mic clip to the cable to attach it to the collar of whatever I'm wearing to try and keep it under control! The original DLR/S.G. Brown lead was much more flexible- I might try to source another set of the more ubiquitos DLR No.1's from eBay to get the cable to make a combination stereo lead for them which is appropriately sourced!

I went out to try and find a modern braided cable to have an alternative, which turned out to be much smaller and not twisted and found a really cheap pair of nasty cans to take apart; the quality of the cabling and plug; playing a significantly lesser role in overall quality than the drivers themselves...

I then had to make new epoxy glass FR4 mountings for the new gauge of cable.

Thanks gkatz, I will get a shot of the completed article tonight...

JimP

[andromedanwarmachine]

So, the revised three core lead was too unwielding.

I found a cheap-cheap pair of in-ear headphones that had a great woven-cable which would be perfect.

[andromedanwarmachine]

relative to what had passed before, it was a small task to get the cable from these cans and make new FR4 mounts to bond them too. Due to the miniature nature of the cable I decided to drill the FR4 pieces a couple of times to thread the cable through them.

[andromedanwarmachine]

so that worked out fine, but then DISASTER!

As must be the case with many things of antiquity used in a modern context; a part broke. In this case; a folded wire mounting for one of the drive unit housings snapped at the bend. Fortunately I was able to fabricate another.

[andromedanwarmachine]

The final detail concerned the actual comfort of the things on the ears; The bakelite surfaces press with some strength into one's lobes and the drilled holes which were added to the ear-cups to improve speaker>ear coupling have sharp edges. Despite countersinking, these edges become painful overtime and it became obvious that an earpad of somesort would be required.
I came across some adhesive blackfelt which would be perfect and be again an "invisible" modification to the headphones as seen externally. A scalpel removed every individual cut out inline with the holes beneath.

Result!

[yousef]

Just wow...

As someone who can't even repair a guitar lead without much profanity, burnt fingers and ruined furnishings I am just in awe of what has clearly been a labour of love for you.

Any plans for similar projects?

[andromedanwarmachine]

thanks for that yousef!

I now have plans for the next thing that I have to make, but it won't really be a project as such. More of a build.

I need to make a new twin XLR to 3.5mm minjack socket adaptor to match the connector mounting pitch of my new location recorder. I prefer not to use inline leads for this sort of thing and like to encapsulate all the connectors and wiring involved in a single epoxy resin package.

It'll be short, but this thread will appear in Battery Boxes & Pre-amps...

Thanks for your interest!

JimP