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THE AM BULLETIN BOARD => Technical Forum => Topic started by: Opcom on January 07, 2022, 10:54:54 AM

Title: ground loop issues
Post by: Opcom on January 07, 2022, 10:54:54 AM
I'm having issues with ground loops. There are 2 PCs and a SWL receiver input. It's not in the shack itself but is in the house and uses a 100W/CH audio amp and hi-fi speakers. It was fine with one PC but adding the other PC and shortware radio have caused noises that don't appear when any one of these things is connected directly to the amplifier.

These are all unbalanced and have been reduced to RCA-jack level, about 300mV max. The impedance has also been dealt with so they are similar, since they are headphone outputs. An attenuator was added so the output can drive a 600 ohm load without mismatching to the source device. I could make a drawing of this but it is really simple. It is a series Rx of the value needed to cut the few volts from each individual source down to what will provide the 300mV across 600 Ohms, and then a 600 Ohm resistor from there to its common GND. -its apparent chassis GND.

I have a switching box whicxh has RCA jacks on it. All the RCA grounds are connected together in this box, in fact it's well made box with some shielding and has never ccaused issues when I used to use it for tape decks and other audio stuff in a stereo system.

The problem seems to have come into play when more than one source was added. Even though every piece of equipment is grounded via the same rack and its power cord, the problem persists.

I eliminated the switching box as the trouble by using an RCA-RCA female to female connector between the audio power amp and any one source, and the noise disappeared.

But, when I touch the shield ground part of an RCA connector from any other souece to the shield ground of the RCA cable going to the amplifier, a noisy hum appears. It's not a smooth 60Hz but garbagey and also changes character when the PC is doing 'stuff' whether increased load or moving the mouse. I am sure this is a noise from the PC mainboards, regardless of their grounding or anything else. It's real annoying.

To confirm, there is none of this issue when only one source's RCA cable is connected to the amp.

Is the usual way to eliminate ground loops to put a good quality 1:1 audio transformer between each source and the switch?

That seems to be what's inside some of the ground loop prevention boxes, that and a huge price tag for the magic. But I have a pile of quality (UTC/Thordarson, not cheap modem units) 600:600 audio transformers that would seem up to the task.  

Will it be also important to try to use transformers with a faraday shield, or should the simple isolation between the windings be enough considering the 600 Ohm circuit and the low level signals?

Title: Re: ground loop issues
Post by: WA2SQQ on January 07, 2022, 01:28:39 PM
In a past position I had to troubleshoot these type of problems almost every day. Ground loops usually occur when you have multiple pieces of equipment some of which that have three prong AC cords and others that have two prong AC cord. First step is to check your AC source measure between common and Ground and make sure that you have absolutely no voltage between them. The next step in the troubleshooting is to use one of those ground cheater plugs which isolates the third round pin. If the use of the plug cures the ground loop then youíre obviously referencing two different grounds. Iíve seen several cases where these power surge protector strips have caused a ground loop.

Title: Re: ground loop issues
Post by: K1JJ on January 07, 2022, 01:51:24 PM
Hi Pat:

There was a time when I was fine-tuning my station to eliminate the last vestiges of AC hum in the audio. I have five audio boxes used for processing; one is a DSP unit.

The bottom line is no matter how I grounded them, placed them or any other thing I could think of, there was always some low level hum left.  What solved the problem was using a 240 > 120VAC isolation transformer, rated at about 3KW, to power all the low level audio gear and other related equipment. By experimentally plugging the various boxes in or out, I was able to get the hum down to non-existent when running full strap with the 4-1000A plate modulated AM rig.

An AC isolation transformer is sometimes the only way to go when really digging down for the last hum gremlins.


Title: Re: ground loop issues
Post by: WA2SQQ on January 07, 2022, 04:53:13 PM
So the isolation transformer effectively removes any difference in potential between ground and the common wire. Until I corrected some grounding issues in my own home on some outlets I was actually able to measure up to 2 V between earth ground and common.

Title: Re: ground loop issues
Post by: Opcom on January 07, 2022, 07:24:14 PM
This guy did a successful demo using audio transformers, looks like I can try it first.

also saw in ebay a board sold for the purpose but has very low quality transformers. The secondaries have a 680 Ohm resistor in each lead on the output going to the amp.

Title: Re: ground loop issues
Post by: KA3EKH on January 07, 2022, 09:18:33 PM
PC has the worst audio output stages of anything ever produced. The High Z ports on the PC are useless for doing anything beyond local speakers or microphones. For twenty years I have had to work with integrating computers to professional audio equipment in broadcasting and have found that unless you want to pay for real good sound cards that have balanced inputs and outputs you have to use an external matching box that will convert High Z to 600 Ohm balanced. Once you have balanced audio lines you can send audio several hundred feet, between buildings or anywhere else as long as you maintain the balanced system. A balanced line by design is not capable of ground loops or external interference but non balanced High Z lines will cause issues any time you try to go any distance or integrate into any system.
You should know this from your military training, think of the tie lines between shelters, CP and the like.
Bogen makes a little transformer called the WMT-1 that transfers from 10 K High Z to 600 Ohm Balanced that works great. The only disadvantage is that it has no gain so if you try to do 600 Ohm 0 dB thatís a issue coming from a -10 dB consumer device.
ART Cleanbox will give you two way stereo audio from something like a consumer CD player or a computer and they are cheap, have gain and you can buy them off the Bay for reasonable price and they are like $40 at Sweetwater. Think I may have used dozens of them for cheap broadcasters who wanted to use desktop computers for running things like Audition and doing audio work and with that you can use a off the shelf computer in a high RF environment without issue.

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands