Effect of limiting audio peaks using diodes?

[AA7EJ]

In theory, what would be an effect of using two small signal diodes, 1N4148 variety, on the "quality" of audio used as input to SSB? Same principle as limiter in FM receiver chain.
The objective is to control the input level to some degree.
73 Vaclav

[WB2WIK]

Creates audio harmonics when the diodes start to conduct and clip.

I wouldn't do it.

[KE3WD]

BAD idea.

Diode clipping is abrupt and would introduce audio distortion as well as possibly generating undesirable harmonics as the clipped waveform could easily venture into squarewave territory, complete with the odd-order harmonics that entails.

Beasides that, there are several good IC choices available today that perform the Audio Compression task for communications superbly well and are almost as easy to implement.


73

[K7JEM]

Speech clipping has been used for decades on SSB radios as a way to increase average power. But they use RF clipping, where the RF signal is clipped, then filtered. It can make a big difference, but is is not easily done at audio. You could implement a small amount of clipping and not get too many bad results, but you probably wouldn't get any really good results either. All FM radios use clippers and filters right before the modulator, to limit maximum deviation. A nominal amount of clipping sounds good on FM voice, a lot and you get distortion.

As suggested, get an external compressor or an external RF clipping circuit if you need beefed up audio. This will require some on the air tests to get it right, but should work OK.

Joe

[KA1MDA]

A really cheap way to limit line level audio is to get your hands on an OLD BELL style telephone heandset. If you unscrew the cover and remove the earpiece, you will find a small device across the earpiece terminals. I believe this is some type of varistor, and it tends to kick in somewhere around 0 to +4 dBm (if I remember correctly). I have used them in the past to limit audio levels in audio paging systems, and they work great. Excellent limiting with no audible distortion.

Tom

[KA9JLM]

I have seen 1N34A types used in audio line gear.

Normally there are many diodes depending on the audio level needed.

Flat topping audio is not a good thing, but does work for overdrive protection.

[WB2WIK]

A really cheap way to limit line level audio is to get your hands on an OLD BELL style telephone heandset. If you unscrew the cover and remove the earpiece, you will find a small device across the earpiece terminals. I believe this is some type of varistor, and it tends to kick in somewhere around 0 to +4 dBm (if I remember correctly). I have used them in the past to limit audio levels in audio paging systems, and they work great. Excellent limiting with no audible distortion.

Tom

Might work sometimes, but I wouldn't recommend those.

They are actually a bidirectional diode and are used as "click suppressors," so when someone hangs up on the circuit, the resulting click doesn't make a huge popping noise in the earpiece. Very, very old technology and not dynamically controlled in any way.

Also intended strictly to be across a 500 Ohm balanced line.

I supplied those to AT&T, Northern Telecom and others for many years.

[K5JY]

Steve, be sure to put in your unauthorized autobiography that you invented the "bidirectional diode." I'm trying to think of a way to use one of those in a circuit with a bunch of zero-ohm resistors

[WB2WIK]

Steve, be sure to put in your unauthorized autobiography that you invented the "bidirectional diode." I'm trying to think of a way to use one of those in a circuit with a bunch of zero-ohm resistors

I didn't invent it, Bell Labs did.

A bidirectional diode is two junctions formed at the same time using opposing polarity P-N diffusions.

The ones used in telephones are pretty "sloppy" and have a lot of capacitance, making them unsuitable for higher-Z circuits; they were intended to work across a 500 Ohm line and load. But they can be made to work fine at RF if you add opposing polarity signal diode diffusions in series with them to reduce capacitance without changing the clamping characteristics.