An AGC - or rather AVC - for rigs without one.

LED/LDR audio limiter in uBitx

A common problem with many kit rigs, such as the QCX and uBitx, is that they have no AGC. When a strong signal is encountered, it can be hard on the ears.

I recently put together a uBitx v6 as a winter project. I’d had a v3 some years ago and it presented quite a few challenges. I was curious if things had improved. They have, but the passband is still much too wide for CW and of course there is no AGC. My goal was to make it “CW-worthy.”

I added a Hi-Per-Mite filter, which helped immensely. However, because the filter seems to peak signals, it increased the need for some kind of auto volume control.

VK3YE has several YouTube videos outlining a simple and ingenious analog design that takes few parts and requires minimal connection to the rig’s circuit. Curt, WB8YYY, who adapted it to the uBitx, calls it “non-invasive” because of this. Although it is labeled an AGC, it is really an AVC or more accurately, an audio limiter as it evens out the peaks (and without the distortion one often gets with digital solutions).

The design uses a bright LED that on strong signals activates a photoresistor (or Light Detecting Resistor – LDR) connected to the volume control, decreasing the resistance and thus the volume on peaks.

In the WBYYY version, the signal is sensed at the volume control and the LDR is also across it. I wanted to change that for two reasons. First, I wanted to sense the signal AFTER the CW filter; otherwise, the limiter might be acting on other signals in the wide passband. Second, I didn’t like the idea of bringing down the overall resistance across the amplifier stage, and a voltage divider would have meant cutting a trace in the PCB.

My modification was to take the sensed signal after the filter, from the speaker, and to put the LDR across the volume control wiper and ground. This of course means that the limiting action is affected by volume. Thus I mounted the trim pot on the front panel rather than on the PCB.

I've found that it is very convenient to tweak the limiting as needed for a specific signal, providing a second layer of audio control. However, if you don't have a filter to bypass, you may want to sense at the high end of the volume control instead of the speaker. That way the limiting will be less affected by the volume control setting.

Watching the LED activated by volume is quite entertaining but it is also helps keep track on what is happening to the signal, so I figured out a way to make it visible when operating.

I don’t think the parts are very critical. I had a number of photoresistors (CDS cells) in my junk box and selected one with a high resistance so it wouldn’t affect the overall volume much when the LED was off. WB8YYY used a 580 ohm resistor for R4 but it found that the 4.7K in VK3YE’s design seemed to give a little more “head room” to the control. Any general purpose NPN transistor will probably work. A bright LED is required, and should be mounted directly in front of the photoresistor; I took mine from a little give-away flashlight I had lying around.

I’m very pleased with this solution so far. The uBitx is much more pleasant to use.

(Note: With the uBitx V6, all volume control connections can be accessed with the multi-pin connector on the PCB under the encoder.)