UK Vintage Radio Repair and Restoration Discussion Forum

In my final year at a technical college, the class was given a project, to design develop, build and put onto a PCB a given electronic task, I chose the Audio Limiter.

The spec given to me was it must have 600 ohm impedance on both input and output. It must have an overal gain of unity up to the limiting point and be linear to 1V peak to peak.

Until that time i had never come across a limiter, let alone seen a circuit for one.

In fact I had seen a diagram for a compander in Practical Wireless some years back and used this as a basic building block.

I got an top marks for my efforts, if I remember i nearly came top of the class, a far cry from my school results.

I submitted the project to Practical Wireless in a somewhat modified form and it was published in June 1980.

The original didn't have any negative feedback, but did have a 600 ohm resistor strapped across the input. The way the outut transistor is configured gave an output impedance of approx 600 ohms.

This was considered acceptable by our tutor.

I have scanned the articles in and saved them as PDF's as below.

The variable gain stage is centred around transistor TR1 and FET TR2.

What took quite a bit of experimenting and a flash of inspiration was the 'switch' the circuit which detected that the limiting point had been reached.

To do that I used a transistor whose emitter was held at a constant voltage. Audio was fed to the base but unless that audio was of sufficient amplitude as to exceed the emitter voltage bu 0.6V, there would be no a/c voltage at the collector and therefore no voltage fed back to the FET.

The output from this switch was amplified, rectified and fed to the gate of the FET.

The PCB design and laboratory tests were carried out by practical wireless prior to their accepting and publishing the article.

Opening up my mixer and finding a version in there brought back a flood of memories.