#1
Guitar feedback regenerative heterodyne receiver.

I did something more useful with my guitar amp and my guitar since I suck so bad at playing guitar and it was something for me to do while I was bored on spring break.

You don't have to answer my questions of this radio thread, you can ignore the questions (sigh of relief for you). This is because my questions are being answered on an electronics forum on this thread: http://www.electronicspoint.com/help-understanding-snr-radio-reciever-t258517.html


I learned in the Wikipedia article about the regenerative receiver that it uses feedback oscillating at the desired RF as an oscillator for tuning, and you tune it with a pot that controls the gain which thus controls the feedback.

I learned in the Wikipedia article about the regenerative heterodyne receiver that it is exactly a guitar amp over driven to feedback and the feedback whine is called a heterodyne tone and when the guitar amp is energized by a morse code RF signal it beeps on and off in morse code and the oscillation of the feedback is also a tuner in addition to generating the beeping.

So I thought I would try this at home. Here are the results.

I was able to tune to a repetitive morse signal (an aviation navigational beacon in the VHF band I think).

The signal is _ ._ _ _ _ _ _ . . _._ _ _ _ _ _ . . _._ _ _ _ _ _ . . _._ _ _ _ _ _ . . _._ _ _ _ _ _ . .

K O B K O B K O B K O B K OB


The repeated word is "OBK".

I look up aviation navigational beacons here at



and I find that "OBK" is the name of a beacon in Northbrook, IL only about 8 miles away from my house. Its frequency is 113.0 MHZ AM.


Pretty cool?


This is my guitar strings pressed against my guitar speaker so the feedback occurs when the guitar amp is energized by a morse signal turning it on and off whining on and off.


Here is the video:
http://www.youtube.com/watch?v=ds7OP3n9vX0


I suppose it would work better if I had an antenna.

Would would be a good place to connect an antenna to the guitar?

I saw that inside my guitar there is a wire that is stripped at the end that makes contact with the inside of the guitar. This is the ground wire, right?

You connect an antenna to the ground wire, right?

I tried holding some wire to that loose wire as an antenna and all that accomplished was "beeping" on and off in cadence with my body shaking while holding the wire and the beeping stopped as soon as I wound the two wires together instead of holding them together.


Would this feedback loop be better to accomplish with electronics rather than with acoustics?

A problem I had is I have to get the gain just right to tune to a station right before my amp starts feedbacking in a continuous loop, so often I mess up and I have to turn the gain down to start over because mechanical resonance builds in my guitar speaker.

Would using electronics instead of mechanical resonance achieve the same feedback loop and eliminate the high decay time of mechanical resonance?

I mean by high decay time I mean as soon as a few beeps happen then the speaker goes crazy with continuous feedback.


by using electronics I mean I would use an audio mixer as the detector, the mixer output is connected to the line in of the guitar amp, the headphone line out of the guitar amp is connected to a Y splitter, one part of the Y goes to headphones, and the other part of the Y goes to the mixer forming the feedback loop.

anten. --> ground wire of guitar --> guitar output jack --> detector-mixer --> amp --> line out

^ V
^ V
^ V
^
^ Y splitter
^ V
^ V
^ V
^ V V
^ V V
^ V V
<<<<<<<<<< V
V
headphones


You can see the correct formating of my diagram if you quote my post.


Or perhaps would it be more effective to leave the feedback loop to the mixer only and connect the guitar amp to where the headphones currently are in the diagram (after the guitar amp has been removed from its current position in the diagram)?


anten. --> ground wire of guitar --> guitar output jack --> detector-mixer --> >>>>

^ V
^ V
^ V
^
^ Y splitter
^ V
^ V
^ V
^ V V
^ V V
^ V V
<<<<<<<<<< V
V
audio amp

You can see the formating of the diagram correctly if you quote my post.
#2
Round 2.



In the last video for guitar regenerative heterodyne reciever I used the guitar amp as the regenerative detector and since my guitar amp doesn't have multiple line ins I had to use acoustic feedback loop of the guitar strings right next to the speaker cabinet, but in this video I achieved the feedback loop with electronics instead of acoustics for better results.

Also the last video had no antenna and the strings were subject to picking up background noise but in this video an antenna is added and the strings are muted with cloth towels since electronics instead of acoustics are used.

I made the feedback loop with one of those short 8 inch long cables used to connect guitar effect pedals together connected to an audio mixer.

The mixer line in 1 is connected to the guitar. The mixer line in 2 is connected to one part of a Y splitter. The audio line out to the audio amp is connected to the other part of the Y splitter. The input of the Y splitter is connected to the 8 inch long cable. The 8 inch long cable is connected to the line out of the mixer. Forming the feedback loop of the regenerative detector.

The antenna is coupled to the pickup contacts. Tuning is accomplished with the pickup selector switch, which can only tune to 2 settings. When both pickup sets are selected (more coils = lower frequency) it has too much mains hum, so I only use selecting one pickup set (less coils = higher frequency).

More tuning is accomplished with the channel 1 and channel 2 gain pots on the regenerative detector.

Gain on the regenerative detector below oscillating feedback squeel is AM and very close to oscillation is heterodyne CW morse code.

Currently mostly only mains hum is heard on AM mode so we are only interested in CW mode.

The SNR of CW mode is increased by decreasing the band width to the audio frequencies that CW uses. This is accomplished with the tone controls on the guitar amp. Eliminating bass (not used by CW) attenuates mains hum and attenuates booming radio voices that are unintelligeble due to mains hum and lack of frequency selectability.

The frequency selectability over the last video is improved. Now 2 CW stations are heard at the same time and each one can be attenuated by adjusting the gain of the regenerative detector.

The CW station heard in the last video is still the loudest station received in this video, so that means that it is the closest.

Adding a vari cap made out of toliet paper rolls, aluminum foil, and some wire could improve the frequency selectablity if the vari cap was coupled in series with the pickups and antenna.

There is no vari cap here, but I will try that in the next video.


The slightly higher heterodyne frequency sounding beeping (and quiter) is

_ _ _ . _ . _ . .

O R D

O'hare beacon.


The slightly lower heterodyne frequency sounding beeping (and louder) is

_ _ _ _ _ . . _ . _

O B K

Northbrook beacon.


Google says the frequencies of the beacons are:
ORD: 113.90 MHZ
OBK: 113.00 MHZ

So they are very close in frequency so that is why they are bleeding through into each other and they both start with _ _ _ so it makes it confusing.
Last edited by dietermoreno at Mar 27, 2013,