AH Small Tech Limiter

[s2000]

Just wondered if anyone has built the Albert H small tech limiter that was on the old radionecks forum? (schematic attached). I have built it on a breadboard and it appears to be working, the audio coming out is high quality going straight into my hifi amp. The only thing I can't work out is what the 47k variable resistor is for? Mine doesn't appear to do anything whatever way you tune it. I have checked the components to make sure everything is in place and is of the correct value. If I pull the VR out of the breadboard it carries on without it lol I would of thought it was to control the level of either the input or output? Or maybe the attack/delay times? Problem is the BA3312n datasheet is all in Chinese so not very helpful. Seems that pin 2 and 9 associated with the VR are going to the inverting input of the built in op amps (inside the BA3312n).

I have got the input connected straight to my cd player via a potentiometer so I can control the audio level going in. If I set the audio level in to 1v peak to peak the output appears to be 2v peak to peak. If I turn the pot right up so the input is getting the full level of the cd player (sometimes 6v peak to peak on the oscillocope!) the output level seems to remain the same 2V p-p so it does appear to limit well. The only thing I have noticed is occasionally I am seeing audio peaks outside of this 2v span for a split second. Is that normal with a limiter or will that overmodulate a carrier more than 75khz if connected to a transmitter?

[s2000]

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[Albert H]

You shouldn't be able to overmodulate - that's what the diodes are for around the 5532 - they're level clamps and clip anything that gets past the 3312. The original idea of the input pot was to provide a crude input sensitivity control - these days I'd just put a fixed 10k resistor in place of the pot and the 4k7.

I remember getting the 3312 for the first time, and lashing up the limiter you see here. I was very pleasantly surprised by the quality. It's worth playing around with the values of the 4µ7 and 2µ2 capacitors at pin 4. Smaller values give faster response, but if you go too small, you get really gross bass distortion. These were the values that I found to work well, but as the Yanks would say "Your Mileage May Vary".

Bear in mind that this is a very basic device - it's capable of pretty good results, but you'll have to construct something much more complex to get the limiting you hear on commercial stations.

I have another old circuit that I used to use, with two dual op-amps and an NE(or SA)571. This also had a clipper - and clip indicators - and gave a very good account of itself. One of the Yank TX companies copied the circuit (mostly) and offered a kit for it back in the 80s. I think that I've got the Veroboard and PCB layouts for that one too. I'll have to look through a couple of old hard drives.

[Albert H]

That's a busy breadboard!

[s2000]

Thanks for the response Albert, yes the breadboard is packed I had to stretch some of the components from pin 4 right over as had run out of space near the BA3312n lol Tbh I am having a few probs with the breadboards I have got, they were made in China and not giving me reliable connections on some of the components.

Will definitely have an experiment with the values. Ideally just want something to stop me overmodulating the carrier. Ive just got myself a modulation meter so will be experimenting with this and a small transmitter...