A Handy Headphone Matching Unit

[Sparks]

I have now completed it by adding the label, and photographing it in daylight.    As I have said (twice) this unit is for use in "small valve receivers," and by that, I mean one or two valves.   Once you get into 3 valves, they should drive a loudspeaker.    The "small receivers" that I am talking about, work on 45 Volts HT.    In any case, I really don't see how I would end up with even 45 Volts "across my head," if the transformer failed.     If the primary went o/c, the receiver would stop working, if the secondary went o/c, the receiver would stop working!    If the primary and secondary both went o/c and by a million to 1 chance the ends connected across, connecting the low impedance headphones straight into the anode circuit, the headphones may burn out, but even then, I cannot see how I would get  45 Volts "across my head," because the headphones are made of plastic, and the wires insulated.     I am not so much a dinosaur as to use the old-fashioned headphones that had exposed terminals on the outside of each earpiece!     This unit should not be used on mains sets!      In the interests of safety,  don't build one if you don't think it is safe.    
Bob

[pwdrive]

Mike made a good point regarding that transformer's reflected impedance, eg: typical optimum load for DL series battery operated valves and other comparable valves can vary from 5k to 20k, doesn't take much to figure out that there will be a large power loss.

Also an effective low impedance anode load in a pentode regen won't do much for it's performance, a triode regen will fair better but the load still needs to be reasonably high, that's been my experience.

Lawrence.

[Sparks]

I don't doubt it, but what you say doesn't actually mean a great deal to me! It works, and it works well, as far as I am concerned. I feel that most of you who frequent this forum will have a far greater knowledge than I do, so when it comes to safety, I naturally expect anyone who makes one of these matchers, to use it sensibly, as I suggested, for small valve projects. Anyone who is almost totally lacking in knowledge would not even know what an LT44, or even a valve was, and would not even wish to make one of these units anyway! They are less likely to kill themselves with my simple projects than buying and ancient valve mains set on a car boot sale and "having a go!"

With me, it is no longer an obsession, and I only gain pleasure from experimentation, trial and error, and the theory can "go hang!" I can go for weeks or months without even thinking about radio.

I built this thing on a whim yesterday, and I am completely happy with its looks, safety, and performance!

Bob

[pwdrive]

Whatever suits, no offence Sparks but understanding how a valve or transformer works is as important as understanding Ohms Law, which I suppose one could class as theory but no doubt you will have understood the relationship between voltage, current and resistance in that context.

Lawrence.

[Sparks]

No offence taken from anyone        I knew (or thought I knew) about all these things long, long ago., but have since "defragmented" my mind by erasing what I considered to be superfluous!   I do know how a valve works (or I think I do).     As far as transformers are concerned, mutual inductance between one side and the other.   More windings in primary than secondary, less comes out of secondary and vice-versa!    Ohms law (Not copied from anything, but typed straight from the "hard drive" of my mind from my first week in radio college, 1959!
"In any closed circuit of constant temperature, the ratio of voltage applied, to the current set up by that voltage is constant.   That constant is known as the resistance of the circuit!"  
At the time, I didn't really understand it (but I do now!).    
You may have guessed it, I qualified by learning off-by heart anything that I did not understand, and that was an awful lot!      It was the only way for someone such as myself to qualify, and although the method may be considered "questionable," it worked for me.     As far back as 1973, I walked out of an Advanced Marine Electronics Course, deciding  that I had had enough of theory and book learning as far as electronics were concerned  (Southampton Technical College).     I was assured by the principal that my career was ruined, but it wasn't and I soon overtook many more qualified colleagues over the next 20 years, after which I tired of it, and left the sea, taking voluntary redundancy!     I then taught myself model shipbuilding and writing because neither activity needed much "brain work!"       I like to keep my hand in with practical electronics, and believe it or not,   I can usually repair things even though I don't necessarily understand how they work!
Bob

[pwdrive]

No worries, the formula for transformer turns ratio/impedance transformation is widely available on the web or in books, also plenty of online calculators for the said, just punch in the numbers and the result spews out....

Lawrence.

[Mike Watterson]

The impedance ratio is the square of the voltage ratios. The voltage ratio, current ratio and turns ratio are identical, though current is inverse.. I.e a 3.16 to 1 turns ratio is 10:1 impedance.   316V AC primary is 10V secondary, 10mA primary is 316mA secondary. Assuming no power loss, this has to be true. As power is unchanged, the impedance ratio has to be a square of the current or voltage ratio.
V= IR
Power = I x I x R
Power = (V x V) / R


However there are several other nasty things about transformers:

1. The saturation DC current of the core. One trick is to fed an opposing current in another winding, that's why push pull transformers don't have the problem. But if turns ratio is say 4:1, and the Anode current is 5mA, then you can "cancel" that with 4 x 5 = 20mA on the secondary, in antiphase. Then load has to be capacitively coupled.
   
2. The Inductance. This and the core limits the lower frequency response.
   
3. AC core saturation. The AC power handling. 1000KHz needs a tiny core compared to 27kHz, which is tiny compared to 50Hz. 50Hz needs a slightly bigger core for same power as 60Hz. Hence SMPSU, originally at 15KHz to 30KHz to use a tiny cheap transformer, now 1MHz is common.
   
4. Capacitance. All those wires with thin insulation.
   
5. Coupling factor, how well the primary and secondary share all the magnetic flux. Hence Pot cores and toroidal transformers.
   
6. Insulation. Any transformer with more than 48V must have extra insulation between primary and secondary. Transformers in mains gear (audio out or mains transformer)  or connecting to phone line have to provide usually 2kV isolation.
   
7. Other factors
   

[Kalee20]

 I find that my lack of theoretical knowledge often helps when undertaking what others would consider to be "non-starters!"      
Bob      

Many of the posts in this thread, to me, do seem to be pouring cold water on Bob's achievement, which he has taken the trouble to share with us. Which is a pity really.

At the same time, lack of theoretical knowledge can be rather worrying. Insulation in these little transformers isn't up to more than 50V or so, if they withstand anything higher then it's got to be a bonus. So taking a transformer, without an appreciation of its construction, and connecting to a 90V circuit probably should be a non-starter.

The rest, about turns ratio and impedance match, well given that Rob bought a load of them cheaply, if it works then stay with it! It may not be optimum, who cares? (I do but then I'm a pedant and a perfectionist). Rob isn't claiming he's found the optimum, he's saying he has a cheap low-cost thing which makes a bad match into not so bad.

[Sparks]

Thanks Kalee20,
Actually, despite what I may say (often jokingly) about knowing next to nothing about such things, I was, once upon a time, very highly qualified for the era when I began my professional life (radio communication)     In later years, my ever-increasing "practical experience" carried me safely across any shortcomings in "book-learning!"     Nowadays, I have intentionally  regressed to my schooldays when radio (an obsession with it, in fact) was the be-all and end-all of my existence, and because I didn't know much about it at that time, discovering new things was a pleasure and an adventure.      All the theory that is trotted out to me these days, just goes over my head, and I pay it no heed (unless I want to, which sometimes I do, sometimes I don't).      Rest assured that I am as aware as any of you of all aspects of safety, and the risks of getting 90 Volts across my head doesn't cause me much lack of sleep, because, believe it, or not, I do know what I am doing!      I tend to use about 54 Volts for small receivers.    Larger ones don't matter, as they drive loudspeakers.
Also, don't worry about me taking offence at anything - A good discussion is the "spice of life" and I am enjoying my time here immensely.        
If I disappear for a while, I am not "sulking," it is just that my radio activity comes in waves.      At the moment, I am in the process of dismantling old projects, and packing things away in a smaller space.  
Here I am at the peak of my career in 1983!

Bob

[unrealdave]

Also, don't worry about me taking offence at anything - A good discussion is the "spice of life" and I am enjoying my time here immensely.       

Well said Bob, I agree completely.  I have enjoyed this thread, I like your project , and also some of the more technical bits from Michael are teaching me a lot, although I am still going over them

Its a rare thing to find people with good technical and theoretical knowledge who are prepared to share.  As a (relative) beginner I have come across lots of BS from 'experts'

Its fair to say that the greatest gift you can give is knowledge so keep it up guys