HIGH IMPEDANCE HEADPHONES
(2019)
KLIK HIER VOOR DE NEDERLANDSE VERSIE
With the box on the right you can turn all those modern low impedance
in-ear headphones into high impedance headphones, as was used with tube radios!
High impedance headphones!
High impedance headphones! I immediately thought of my student time and Alex. Alex stood there, barefoot, with very antique headphones with cotton-insulated wires. He had connected those headphones to a self-made tube radio. Direct current had to be supplied because of the required pre-magnetization. The banana plugs made a poor contact, the creaking hurt his ears! And the banana plugs had a screw that you could touch and that had a high voltage on it!!!
Alex was always barefoot. Why should you wear shoes when you can walk barefoot? It's cheaper, simpler and feels even better! I loved such a minimalistic simple lifestyle! I had to try that too! Barefoot student! Freedom, happiness, a minimalistic, simple and carefree life! Alex was crazy about radio tubes! TV's and radios with tubes, that's what he was always working with! He had just repaired an old black and white tube TV. Hit by a lightning strike, only the power switch was broken, completely burnt away! Solid tubes, cannot be broken!!! He did that repair for free, because it was for a poor student!
Alex was crazy about radio tubes!
And... Alex was always barefoot.
Alex was a real collector, his whole house was cluttered. He had another pair of antique headphones for me! But no, I did not want them, I wanted as little possession as possible, just a very minimalist life as a barefoot student! A simple crystal phone was enough! Unfortunately! Now I regret not being able to try out such old high impedance headphones. Because there are many radio circuits that require high impedance headphones! It's fun to try out some of those old circuits, but how do I get high impedance headphones? There is a whole box full of modern in-ear headphones here, but they are 32 or 64 ohms, that impedance is far too low!
Radio circuits for which you need high impedance headphones!
Above you can see some examples. I'd like to try that UN-9. I made it once before, but the feedback did not work smoothly. Could I have connected the 1.5 volt supply for the glow wires the wrong way around? Or does that not matter? Why did the feedback not work properly? Nice to give it a try! And suddenly I had an idea to turn these modern in-ear headphones into high impedance headphones!
This is what you need to turn modern low impedance in-ear headphones into high impedance headphones!
In audio systems for buildings "100 volt" audio transformers are used, they are inexpensive and available everywhere. These audio systems are so-called 100 volt systems. The audio signal on the cable has a level of 100 volts. A speaker is connected to the secondary side and can be 4, 8 or 16 ohms. The primary side has taps with a specified power. By connecting the correct tap to the 100 volt line, the volume (power) can be set.
And we can use such a transformer to make high impedance headphones! We connect the two parts of the in-ear headphones (usually 32 ohms, but at 64 ohms per piece, it also works fine) in parallel to the 16 ohms secondary. And by choosing the right tap we can now set multiple impedances, not just the 2000 ohms that such antique headphones usually have!
The impedance can be calculated as follows:
Z = U*U / P (P is the power of the tap), U = 100 volt so: Z = 10000 / P. We can make the following table:
10 watt: 1000 ohm
5 watt: 2000 ohm
2,5 watt: 4000 ohm
1,25 watt: 8000 ohm
0,625 watt: 16000 ohm
The tap can be selected using a rotary switch. So now we have high impedance headphones that we can set to impedances from 1000 ohms to 16000 ohms! In practice, turn the switch until you get the best result. This can be the least distortion or the most noise!
And this is it!
Two diodes 1N4007 are connected in parallel on the secondary side, so the voltage over the headphones can never exceed 0.7 volts. This is a hearing protection, so that loose plugs etc. do not hurt your ears as was the case with Alex's antique high impedance headphones! The direct current through the primary is so low that it does not saturate the transformer. There is no need for direct current for the pre-magnetization of the headphones. And... those modern in-ear headphones are much more comfortable than such old headphones! The transformer is also an insulation between the sometimes high voltages on the primary side and the in-ear headphones on the secondary side. But... do not use banana plugs with touchable screws with a high voltage!!!
A simple drawing says more than 1000 words!
Index PA2OHH