2 - Germanium transistor noise with Feedback ITC receiver - by Andrés Ramos

[Andres Ramos]

Encouraged by my first experiences with germanium semiconductors I was looking for means to improve signal quality and S/R ratio (signal to noise ratio). I frequently heared from other experimentators that different kinds of feedback can improve the voice signals. It seems that positive feedback causes a system to become unstable, adding more entropy to the setup the spirits can use to form speech.

The most common form of feedback used in ITC experiments is positive acoustical feedback. I considered if maybe feddback on an electronic level would do as well. Thus I designed the following electronic.

Electronic schematic of germanium transistor receiver with feedback

The circuit is rather simple. A germanium transistor AF137 is used as the noise source. The negatively biased Base-Collector diode of the transistor is the originof the noise. It is routed via C3 to an operational amplifier with a voltage gain of roughly 40. Via the potentiometer R10 the amplified signal is fed back phase synchronized into the transistor by using the emitter. Original and feedback signal "mix" in the germanium crystal.

 

Test and results

In the experiments there was definitely a change is the signal spectra depending on the feedback settings.

Spectrum without feedback=0

 

Spectrum with feedback just before start of self oscillation

 

From an electrotechnical point of view this behaviour is nothing paranormal. Feedback always is changing the frequency response of a signal. All active filters are working this way. I had the impression that with high feedback the intelligibility was slightly improved however certainly more because of the bandfilter properties and not by some paranormal effect.

The recorded signal required a certain amount of post processing and the results were comparable to my previous experiments with zincite, maybe a little better.

A collection of exported audio samples can be found here.

See the attached test reports for more details.

 

ITC-Report 2019-B-01.pdf