AM audio processing

[Nige from Brum]

Hi Guys,

Can anyone point me in the direction of a simple but effective audio processor circuit for using with a round the house AM transmitter.

There have been a few circuits suitable for FM and stereo published on here recently, but I'm aware the audio bandwidth will need filtering at least.

It needs to be able to run from a 12v battery, as it will be sited in my shed, and preferably a very simple circuit.

I'm not expecting it to sound like an Optimod etc

Nige

[eiradioguy]

Stereotool is a software-based audio processor that apparently runs on a raspberry pi, maybe worth a shot?

[Albert H]

Stereotool never sounds particularly good to me, and isn't really appropriate for AM.

There are many good circuits around for AM processing - just how complicated do you want to get? I've designed simple single-band and dual-band AM processors and even big, complex commercial processors - most of them can be run from car batteries!

I can put something fairly basic up here shortly. The one I came up with recently for a friend uses just one TL074 quad op-amp, and provides fixed EQ, compression, clipping and a lowpass filter on the end to keep the bandwidth reasonable. I lashed together the prototype on a bit of Veroboard, and my pal has been using it on his little MW station for a few months..... Circuit to follow!

[Albert H]

It just occurred to me - you want a 12V powered job. The TL074 circuit needs a dual supply rail and plenty of volts for the headroom it needs....

I've also got another (older) circuit that uses the NE571. We used it on battery-powered AM rigs with great success. Circuit to be transcribed from my notebook and will be up here tomorrow.

[Nige from Brum]

It just occurred to me - you want a 12V powered job. The TL074 circuit needs a dual supply rail and plenty of volts for the headroom it needs....

I've also got another (older) circuit that uses the NE571. We used it on battery-powered AM rigs with great success. Circuit to be transcribed from my notebook and will be up here tomorrow.

Thanks Albert,

Look forward to seeing it.

[Albert H]

This is the compressor part that I've used for years for MW rigs. The values are mostly not critical. The timing capacitors (pins 1 and 16 to deck) are made up of paralleled capacitors - I found that 22µ was too slow, and 10µ was too fast (just), so this was a compromise - you might want to tweak to your taste.

MW Compressor.png

The 430k resistors on pins 2 and 15 are the "gain / noise reduction" resistors as recommended in EDN about 30 years ago - without them, the gain of each stage rises forever (until you hit the noisy maximum) when the mod is quiet.

You'll notice that there is a facility included for taking a stereo signal and making it mono. I found that the 150Ω resistors were about right for the Walkman player we used to use up blocks. It's not a multiband compressor, but it sounded pretty good!

I'll jot down the clipper part of the circuit (goes after the compressor) and post it tomorrow.

[Nige from Brum]

Thanks Albert.