Back to the index
If we want to use a low impedance speaker or headphone in a
crystal receiver, we need to use a audiotransformer.
The transformer converts a high input impedance (=primary impedance) to a low output impedance (=secondary impedance).
100 Volt line transformers which are used for 100 Volt
loudspeakersystems are very suitable for this.
In 100 Volt loudspeakersystems the input of the transformer is connected to a audiosignal with a maximum amplitude of 100 Volt. The transformer converts this to a lower level, which drives the speaker.
Via several tap's on the transformer primary winding, you can select how much power the speaker gets.
The power of the tap determines the input impedance of this
In the following table are some standard values for power and the corresponding input impedance.
We can calculate this with the formula: impedance = (100Volt x 100Volt) / power.
|Left on this picture: audiotransformer brand: Visaton
The input has tap's for 1, 2, 4, 8 and 16 k.Ohm.
The output has tap's for 4, 8 and 16 Ohm.
Right on this
picture: audiotransformer brand: Adastra model: 952.431
Connection of a audiotransformer in a crystal receiver.
Capacitor C1 is for removing radio frequencies behind the diode
Toghether with the transformer primary impedance (Ztr), C1 forms a lowpassfilter with a frequency of f=1/ (2.pi.Ztr.C1).
This frequency must be minimal 4.5 kHz, than we have no reduction of audio quality.
The DC resistance of the transformer is much lower than the
impedance for audio frequencies. That is why resistor R is added,
the value of R must be about the transformer input impedance.
Capacitor C2 passes all audio currents, so there is no loss of audio signal across R. The full audio signal will stay across the transformer.
When R and C2 are not added, strong distortion can occur when receiving strong stations.
C2 and R form a highpassfilter with frequency f=1/(2.pi.R.C2)
This frequency must be lower than 50 Hz.
In the following table are some useable values for C1, R and C2 for a given transformer impedance.
impedance in k.Ohm.
|640 k.Ohm||47 pF||680 k.Ohm||10 nF|
|320 k.Ohm||100 pF||330 k.Ohm||22 nF|
|160 k.Ohm||220 pF||150 k.Ohm||47 nF|
|80 k.Ohm||390 pF||82 k.Ohm||100 nF|
|40 k.Ohm||820 pF||39 k.Ohm||220 nF|
|20 k.Ohm||1.5 nF||18 k.Ohm||470 nF|
|10 k.Ohm||3.3 nF||10 k.Ohm||1 uF|
Series connection of primary windings.
|Schematic 2||Schematic 3|
If we need a higher input impedance, we can connect some
transformers with the primary windings in series, the primary
impedances can than be added.
The values of C1, R and C2 must correspond to the total input impedance.
In the schematic the black dots indicates the common connections of the transformers.
The secundary windings can be series connected like in
The secundary impedances can than be added, so two 8 Ohm winding become one 16 Ohm output.
The secundary windings can also be parallel connected (schematic
The output impedance wil now be divided by 2.
4 trafo's in series.
The output windings are via a combination of series
and parallel connection connected to the loudspeaker.
The power going into the primary winding must also come out of
the secondary winding.
But in practice the transformer wil give some loss of power.
The transformer efficiency is equal to the output power
divided by the input power.
In the ideal case the efficiency is 1.
Bigger transformers have in practice a better efficiency than smaller transformers.
For crystal receivers we need a efficiency of at least 0.60
The efficiency is sometimes also expressed as a loss in decibel (dB).
Loss (in dB) = 10 LOG efficiency.
The transformer 952.431 from Adastra has a efficiency of 0.79 which is a loss of 1 dB
Connecting the primary windings in series (schematic 2, 3 ,4) has no effect on the efficiency.
Back to the index