Audio lowpassfilter

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In some crystal receivers we can hear a 9 kHz tone (**see note) interfering the audio signal, the strength of the tone can vary during listening.
This is caused by the fact that more then one radiofrequency can reach the detector diode, the diode works as frequency mixer.
After the diode we don't only have the audio signal, but also the difference between the station carrier frequencies, and this is 9 kHz.
Maybe the diode gives also frequencies of 18 and 27 kHz etc. but this is too high to hear.
Even strong local stations can at night be interfered with the 9 kHz tone.

To remove the tone, we can increase the Q-factor of the receiver e.g. by decreasing the coupling with the antenna, but then also the sound volume wil decrease.
It can also be that the load resistance (speaker, transformer) is too low, or the diode is not a suitable type.

If we want a high volume, so much coupling with the antenna, we can also use a lowpassfilter between transformer and loudspeaker.

The picture above shows a lowpassfilter made of two capacitors (C) and one coil (L).

When using a 16 Ohm speaker, or a headphone with two 32 Ohm speakers parallel, use the following values:
L=0.56mH C=4.7uF (both C's the same value).

When using a 32 Ohm speaker, or a headphone with two 64 Ohm speakers parallel, use the following values:
L=1.2mH C=2.2uF

All frequencies to 4.5 kHz will be passed by the filter, this is exact the audiobandwidth of medium wave stations.
higher frequencies are attenuated. The attenuation at 9 kHz is 22 dB, which is sufficient to remove the tone.

**note: here in Europe the spacing between stations is 9 kHz, but in some other parts of the world, like the U.S.A it is 10 kHz, but you can use the same filter.

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