[Elecraft] Audio Hum in data mode

[Conrad PA5Y]

Gentlemen I have cured the problem.

The answer came as a result of Jim's comments. 

1. It was magnetic and it was hum
2. All my shack has a single ring main and share a common earth, so it was nothing to do with that
3. 'Magnetic coupling is proportional to the area of the loop', this pointed to the problem

So let me describe what the problem was. Some time ago I bought a lot of benches and cables from a company that made video equipment for the broadcast industry. They were down-sizing and moving premises. Some of the cables were dual link DVI cables, they were quite long and looked to be of the highest quality. I was the length of the cable that caused the problem. I was using it with a large monitor to get full resolution. I also had this monitor in portrait mode - I can swivel it.  By going to landscape mode and changing to an HDMI cable, being much more careful with the supply routing and video routing I was able to eradicate the problem. The longer original monitor cable was picking up magnetic fields and coupling into the audio transformers.

The hum is now inaudible, I should mention that I was running the 2nd RX without AGC and used an attenuator so that the receiver was close to clipping. This shows up any nasties on TX audio. If I put the AGC on this masks such problems to some extent. Now there is nothing audible. 

73

Conrad PA5Y




-----Original Message-----
From: elecraft-bounces at mailman.qth.net <elecraft-bounces at mailman.qth.net> On Behalf Of Jim Brown
Sent: 28 September 2019 03:32
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] Audio Hum in TX Data Mode

On 9/27/2019 2:26 PM, Conrad PA5Y wrote:
> What did make a difference was moving the radio away from sources of magnetic fields such as monitors.

First, an important clarification -- is it HUM (pure 50 Hz) or BUZZ (harmonics of 50 Hz)?  You description sounds like hum. The important thing is that the causes are different. Hum is, indeed, magnetic coupling of the 50 Hz field to some loop in the audio wiring.

Magnetic coupling is proportional to the area of the loop. Suggestions. 
First, get power for all interconnected equipment from outlets that share the same protective earth. Second, bond chassis-chassis of every piece of equipment in the audio interconnection path. Third, try to figure out what the loop is that is doing the coupling and reduce its area. In some recording studios where a strong magnetic field exists, the solution is to run shielded twisted pair audio wiring in close proximity to power wiring.

Also try to find or eliminate the source of the magnetic field. In North America, neutral must be bonded to earth at one, and ONLY one point, usually where it enters a building. A second bond between neutral and the same protective earth can create a magnetic field. Is it possible that there is a second bond somewhere in your system, perhaps associated with power to your EME setup?

The I/O board for the original K3 uses un-shielded audio transformers that are a sitting duck for magnetic fields. My neighbor K6XX is an engineer at Elecraft. When he was testing an early K3 prototype, he set it up almost on top of his 1.5 kW power amp. When he tried to run RTTY, The AF circuitry was set into regeneration by the 60 Hz field! The solution I suggested was a steep high pass filter for AFSK-A, and that solved it.

Many years ago, a Chicago FM station whose engineers I knew well moved their studios from a location downtown to another location that is common to a TV studio with whom they had a relationship. When they first moved in, a strong magnetic field was coupling into the mic input transformers of their music mixing desks. The cause was an error in mains wiring -- the neutral for the distribution panel for the FM studio was exchanged with the neutral for a distribution panel feeding lighting in a different part of the building.

73, Jim K9YC
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