Read the comments below. Elektor (or whoever ripped off the original Mark Weiss circuit) fouled up the diode clipper.

This was part of the AMN airchain developed in the 1990s by Mark Weiss (aka "The Peg-legged Bass Pig"). His AMN website (available on the Wayback Machine) has some great circuits for a PLL exciter, an intermediate FET amplifier and a final PA delivering around 300 Watts. There was also a stereo coder, limiter / clipper, audio filters and a sibilence limiter. He used to have the best quality pirate FM signal on the eastern side of the USA.

High precision preemphasis 50µs 75µs

rigmo post layout

Having so many issues with this amp and any kind of amplifier for that matter. It just picks up charging noise from the laptop or pc. There must be something wrong. Iv even tried adding a transformer in between the audio line to take some of the noise out. It works slightly but there’s still noise getting through. Could it be an impedance problem ?

Do you use Audio Signal Isolation Transformer 600:600 Ohms 1:1 EI14 Isolation Miniature?

rigmo wrote: ↑Thu Jan 19, 2023 4:04 pm Do you use Audio Signal Isolation Transformer 600:600 Ohms 1:1 EI14 Isolation Miniature?

Yes but still a high pitched noise on the carrier.

My setup is RDS, Pre Emp/Amplifier/5v USB Charger
Iv also tried Pre emp first then RDS into that but then it amps up the RDS aswell.

whats happening is the pre emp amp is also amplifiying the crud coming from the 5v regulator for charging. Iv got all the usual capacitors on input and output and its all grounded to the case to stop ground loops.

Is the isolation transformer an inline type that plugs into the cable, or are you putting it inside the case?

If inside the case make sure the ground on the audio cable from the pc is only connected to the transformer, not to the chassis. If you're grounding it to the chassis then it defeats the isolation transformer and you'll still get a ground loop. If using phono plugs this means you need to isolate the socket from the chassis. You can get sockets with plastic rings for this, or mount the socket on a piece of plastic or phenolic board.

But I think the real answer is a better quality power supply. Usb chargers aren't designed for running audio circuits, they're way too noisy. Your best bet would be to build a linear supply with a proper transformer and regulator.

Also are you powering the preemp circuit from 5v? Not sure which circuit you're using but 5v is too low for any of the ones on this thread. They all want 12v supplies.

How can i isolate the phono socket from the chassis though? then it wont work. and all the boards are grounded to the chassis anyway.

and yes im using 12v for the preamp

Run one wire from the phono centre pin to the transformer primary live, and the other from the transformer primary ground back to the solder tag on the phono. Phono is mounted on a piece of plastic so its not touching the chassis, and there should be no connections from the transformer to chassis/ground. That way the input circuit is completely separate from everything else which will break any ground loop.

Or skip all of that and just use a transformer plugged in line with the cable like this: https://www.amazon.co.uk/product-reviews/B07RK2MHF7/

But I'd still say try a different power supply before going to all that effort

Are you certain it isn't RF breakthrough causing the hum. Is the noise the same with the rig running into a dummy load ?

No even at 300mw it was doing the same thing. Iv given up on it atm.