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PostPosted: Sun Oct 28, 2012 4:18 am 
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Joined: Thu Apr 14, 2011 1:10 am
Posts: 228
I have tried a number of automatic volume control and limiter circuits mostly for regenerative receivers. Here is the circuit that worked best for me:
http://litetec.hubpages.com/hub/Automatic-Volume-Control-Circuit
Since a regenerative receiver has no agc the dynamic range of the audio output is very large. You really have to turn up the volume to hear weak signals. The problem with that is you simply get blasted out of it when some strong impulse noise comes along or when some other strong signal comes along.
The AVC(L) circuit solves that and makes using such a receiver much more comfortable. You can also use the circuit for many other applications such as voice compression for a transmitter (add a low pass filter to the output of the AVCL circuit). Or for an intercom circuit for example.


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PostPosted: Mon Oct 29, 2012 2:41 pm 
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Joined: Sun Nov 02, 2008 7:15 am
Posts: 679
Location: NJ, USA
If you go through the effort of building a custom photo-coupler with LDR then it makes sense to use it as a high linearity RF attenuator in a regen receiver. AF-derived RF AGC greatly helps with SSB/CW pull effect (verified it on a tube regen). For AM-only the audio level AGC is fine.


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PostPosted: Mon Oct 29, 2012 4:55 pm 
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Joined: Fri Jun 19, 2009 8:30 pm
Posts: 473
Location: The Netherlands
At http://www.elektronik-labor.de/Labortagebuch/Tagebuch1010.html there is a subject about Automatic Gain Control in the audio part of a regenerative receiver. It works the same like the AGC in a cassette recorder.

In the first schematic below the right transistor amplifies the LF signal which is rectified by two Ge diodes and smoothed (integrated) by an elco of 100u. The left transistors works as a variable resistor. Together with the reistor of 10k in the signal path it forms an adjustable voltage divider.

Image

Image


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PostPosted: Tue Oct 30, 2012 4:40 am 
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Joined: Thu Apr 14, 2011 1:10 am
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With the AC-linked differential pair regenerative receivers I have made I never had a problem with pulling. They will only pull at maybe 10Hz or 20Hz from the carrier. Also I have almost never had any problem with overloading. Perhaps that is due to my location which is kind of remote from high power transmitters, except that I do get very many Chinese SW stations blasting in but they are generally at least 1000km away. For me the automatic volume control works just great in the audio chain only. I guess if you were having problems with overloading then you can do RF attenuation with an LDR.
Maybe there is something about the regenerative design I use where the mathematical Tanh gain curve of the AC-linked differential pair together with mathematics of regeneration, together with the square law detector I use balance each other out to reduce the possibility of overloading.
I am getting very good results with the JFet AC-linked differential pair circuit that I posted to this forum. It would be good to experiment with some different Fets such a the 2sk222 (low noise audio) or BF245 (vhf).


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PostPosted: Tue Oct 30, 2012 4:50 am 
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Joined: Thu Apr 14, 2011 1:10 am
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I modeled the circuit at http://www.elektronik-labor.de/Labortagebuch/Tagebuch1010.html a while back with LTSpice. Yes, it does work (according to LTSpice) but it didn't look too good. There are lots of different AVC circuits on the internet. I would say use one that uses an LDR to control negative feedback around an amplifier. Or use a jFet circuit if that is what you have available.


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