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

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

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!

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.

Albert H wrote: ↑Tue Nov 28, 2023 1:09 am 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.

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.
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.

Thanks Albert.