A Handy Headphone Matching Unit

[Sparks]

A HANDY HEADPHONE MATCHING UNIT
This headphone matching unit was devised and built today.    It is for matching normal low-impedance headphones with a stereo jack plug to, small valve receivers, in order to remove the need for a matching transformer in each project.
Basically, it is an LT44 audio transformer with the high impedance leads going to a mono 3.5mm jack plug.   The low impedance leads (one half of the centre-tapped section) goes to a 3.5mm stereo jack socket wired for mono.    
I had to rack my brains for a suitable container, but when my glue stick dried up, it occurred to me that it was perfect for the job.     I sawed off the end, and pulled the insides out, and washed it thoroughly.      I fitted a wooden plug in the open bottom end, and filled in the gaps with P38 car paste.   The whole thing was then rubbed down with wet &  dry paper to obtain a suitable surface for painting.      I drilled a hole in each end, one for the input lead, and the other for the output jack socket.      I sprayed it with aerosol grey matt primer car spray from Halford's.   After wiring it up, I packed each end with a bit of bubble wrap for insulation, and snapped the top (containing the 3.5mm output socket), back on.
In the image, it appears to have a pinkish colour, but that is because I photographed it in artificial light.   It is a light, matt grey in the light of day.     It has quite a professional look to it, is light and handy.   No-one would imagine it was a glue stick.     Works a treat.
Bob

[Mike Watterson]

The LT44 is only a 10:1 impedance ratio, 20K to 1K + 1K
so if you put 32 Ohm load on 1/2 the secondary, then it's only 160 Ohms on primary.
Also it can't take much DC current, it's a driver transformer for germanium transistor feeding 2 x germanium push - pull output.
The LT44b is rated 10K to 500  + 500 Ohms.

The LT700 is rated for 1.2K + 1.2K to 3.2 Ohms, so is far more suitable to adapt a 600 Ohm to 4K Earphone socket to 16 Ohms (32 +32 stereo phones).

However neither can cope with Anode current.  Both are abysmal bass response and modern 32 + 32 or 8 + 8 stereo phones need an amp  as the transformer reduces the signal level. Both 600 Ohm + 600 Ohm (headsets using telephone inserts) and moving iron 2000 + 2000 Ohm headsets are quite sensitive compared to many modern earphones, though not hifi due to moving iron (steel disc).

PDF

  LT44-LT700.pdf (Size: 73.3 KB / Downloads: 7)

[Mike Watterson]

I'd use a 3VA 220V to 9V mains transformer. It will manage DC of anode current of battery or low power mains Anode out.
A 12VA transformer for 2W to 6W valves. (Derate the power due to lack of air gap for DC).
Never use Toroid except on matched push pull. If Push-pull, then you can actually "uprate", i.e. a 6VA 110 + 110 primary will manage near 8W to 12W as the power capacity of a transformer rises with frequency. Hence some USA 60Hz 117 or 120V radios or amps with cheap transformers can't handle as much power on 50Hz, so run them at 110V.

More spec LT700, the output type for small speaker or headphones for push pull transistor (150mW). The various LT44 types are interstage transistor use, like in pocket radio, not suitable except to drive 2K phones!

  lt700.pdf (Size: 89.62 KB / Downloads: 1)

[Sparks]

I knew there would be something wrong with it     Once again, I must bow to your superior knowledge.      But all I can say is that I have been using LT44s for years as output transformers for  "small valve receivers," and never had the slightest problem with them.   None have ever burned out, and the quality of sound reproduction appears to me to be as good as it can get.   But after years of listening to morse, I probably can't tell the difference anyway, although I still have very acute hearing!  and I only use the cheapest headphones that money can buy!    
The reason I first began using them was when I purchased a large box full of them on the car boot sale, still in their individual packets, for 10p each!  
As you already know, theory washes over my head and disappears out of the scuppers of my mind!  
I hope you haven't put everyone off building this handy little matcher!
Bob

[Mike Watterson]

I've been using them since 1970s ... for transistors.
They are very poor compared with mains transformers robbed from old clock radios (£2 in Charity shop, or about £5 new in Argos/Tesco from time to time, other useful parts).
I tried four kinds of those small transformers, the LT44 was only nearly usable with a 64 Ohm speaker and 30V HT. Much poorer than any small mains transformer.
They are just too small, too low an inductance and too low a ratio. They are not very cheap now, either. Buying up S/H clock radios is cheaper. If you need a 4 gang AM/FM (or LW/MW & SW) poly varicon, small speaker, volume controls, 10.7 & 455 ceramic filters, MW ferrite rod and a miniature mains transformer, then a new "budget" clock radio is very good value indeed for parts.
So, I'm going on theory, practice and cost.

[Sparks]

Well I could hardly get cheaper than a boxfull at 10p each, could I?      And as they work perfectly for me, there is not much point in looking elsewhere for small matching transformers.     As I have said before, I also use the small tape recorder PSUs as a source of cheap transformers (that work for matching as well), and they can usually be obtained for about 50p each on the car boot sale, but of course one often has to use a certain amount of force and cunning to get them open without damaging them.    In this case, I wanted a small light-weight matcher that would be easy to open if a wire broke inside it.   Consequently, the glue stick is ideal, and no special tools are required to make it.    I like it - I think it is great!    
Bob

[Refugee]

I made a matching transformer to test amplifiers with a mains signal generator without an earth loop. I salvaged a transformer from a broken toy walkie-talkie.
It fitted into a capacitor can.

[Sparks]

Brilliant!     It never ocurred to me to use an old capacitor can, I have loads of them of various sizes.     The transformer looks very similar in size to my LT44.     I find that my lack of theoretical knowledge often helps when undertaking what others would consider to be "non-starters!"      
Bob      

[Mike Watterson]

This isn't a conversation in a club or pub, it's on the Internet. LT44 are expensive and for transistors. Any random person reading is better to start experimenting with small mains transformers unless they already have a box of these little transformers designed for transistor use. They also have low voltage interwinding breakdown so a hazard to life, especially if added to an un-isolated mains set, but 90V HT across your head can be lethal. Many small headphones have metal gauze and might not be isolated from jack.

Try some small mains transformers. I'll even send you one free from a clock radio so you can hear the difference. Also they are 2KV minimum isolation.

[pwdrive]

Reading through the posts I can't find anything that says what the impedance of Spark's headphones are other than low impedance....Would be handy to know when trying to "match" ?

Lawrence