Left: headphone Omega, right: Telefunken EH333
2000 Ohm headphones.
In the early days of radio (about 1920) 2000 Ohm headphones
where often used for listening to crystal receivers.
With these 2000 Ohm headphones the DC resistance per speaker is
about 2000 Ohm.
The 2 speakers are often connected in series, so the resistance
is then 4000 Ohm.
The impedance at audiofrequencies is however much higher, and
strongly depends on the audio frequency.
In the next table we can see this, there are two headphones
tested, one Telefunken model EH333, and one from brand "Omega"
with unknown modelnummer.
| Frequency Hertz |
Telefunken: EH333 Impedance |
Omega Impedance |
| Direct current | 4.2 | 3.7 |
| 100 | 4.7 | 3.8 |
| 200 | 6.3 | 4.3 |
| 500 | 11.6 | 7.6 |
| 1000 | 20.3 | 13.4 |
| 2000 | 31.6 | 22.8 |
| 5000 | 61.0 | 40.6 |
| 10000 | 77.4 | 72.6 |
Left: headphone Omega, right: Telefunken EH333
The soundquality is better then the crystal earplug but not yet optimal, around 1 kHz there is a peak in sensitivity, other frequencies are less loud.
In the picture above you see how to connect the headphone to a
detector circuit.
The function of capacitor C is to remove RF frequencies over the
headphone, the value of C can be e.g.1000 pF.
The diode is not connected to the top of the detectorcoil, because then the LC circuit is loaded too heavy by the headphone, and selectivity and sensitivity wil reduce.
The diode is connected to a tap on the coil where the
impedance is about the same as the impedance of the headphone.
The impedance of the headphone depends on the audiofrequency as
wee have seen, but you can use the value of impedance at 1 kHz.
The dioderesistance Rd must also
have about the same value.