Isolation transformer question

[homebrewed]

Some time back I bought a 50VA N-68X Triad isolation transformer and recently encountered a problem that it may solve. I think I have a ground loop via a couple of switching-mode power supplies and it's injecting a lot of 60Hz hum into a headphone power amplifier I designed/built. It isn't ripple on the power outputs because the amp is very quiet when nothing is plugged into its input jacks: and it isn't my phono preamp because I'm not hearing any hum when I connect it up to my laptop. It only shows up when the preamp is plugged into the headphone amp. It may be the chassis-ground connection between the turntable and preamp, but that connection MUST be there to keep hum out of the preamp.

Ohming-out the power supply outputs revealed a capacitor-like signature between the output pins and the earth ground on the 3-wire power plugs, so that is another reason to suspect a ground loop issue. The ohmmeter momentarily shows a low resistance value that quickly rises into the megohm range, pretty typical for a decent sized capacitor.

The N-68X is a bare isolation transformer so needs power connectors on either side. The input side has three wires: two for the AC, and one for an internal shield: and that's my question. How is that shield connected? Is it connected to the input earth ground? The cold side of the input line? The "earth ground" line on the output's 3-wire power connector?

 

[Weldingrod1]

Modern power supplies have X and Y capacitors among the line, neutral, and ground leads to minimize noise. They are pretty small value and aren't likely your problem.

This picture is sorta representative of a good line filter.

You need to ground the case of the transformer and any internal shields.
Your audio stuff's grounds should all terminate here.

You need to tie one side of the isolated 120 winding to ground also. If it's center tapped you COULD ground the center tap, but that's a super weird transformer. This is a safety thing. You COULD put a small resistor here. A couple of Ohms, and rated for real power. Like 100 Watts... a few in parallel to up the power rating and the reliability would actually be safer!

If you want to be really over the top, verging on cray-cray, score a solid state decoupler (like ones from Dairylands Electrical). That will totally block -small- DC Voltages but still conduct in a fault. They are used on cathodic protection systems. Basically back to back high current diodes.

I would also put a line filter on the power side of the transformer to block high frequency garbage. Like the one in the photo, actually!

https://www.dairyland.com/product/ssd/

Sent from my SM-S911U using Tapatalk

 

[markba633csi]

I think commonly it's taken to the input earth ground, but you could experiment and try it on the output side instead
Don't know if the shield will make a lot of difference as far as the problem you are having

 

[Reddinr]

The shield should be connected to the chassis ground. The shield does help by creating improved electro-static isolation between the input and output of the transformer. That leads to lower leakage current from the primary to the secondary and hopefully, less hum. One leg of the transformer secondary should generally be tied to chassis as well.

 

[whitmore]

The ground loop might not be in power wiring at all; most phono preamps ground the cartridge LEFT shield, and cartridge,
and the cartridge RIGHT shield connects to the phono preamp only (so the shields don't make a ground loop).
Phonograph motors are usually well-shielded and separate the power wiring away from the signal.

Position of transformers (including isolation transformers) can create problems with hum
this might be some good advice.

 

[MikeInOr]

"An isolation transformer does not have a "common ground" between its primary and secondary sides; the key feature of an isolation transformer is that it completely separates the power line ground from the load circuit, effectively creating a "floating" ground on the secondary side, meaning it is not directly connected to the earth ground, thus preventing ground loops and providing galvanic isolation."

So your amplifier has a floating ground but your turntable and preamp have an earth ground. I believe this is where your ground loop is coming from. If you want to isolate your amp from powerline noise I think the amp, preamp and turntable should all be connected to the secondary side of the isolation transformer and their grounds should be tied together BUT they should not be tied to earth ground.

If your main goal is to reduce powerline noise I think a EMI/Noise filter like @WelingRod1 posted is a better choice than an isolation transformer.

 

[homebrewed]

The problem is hum. 60Hz. I have an EMI/noise filter I can try but not hopeful it will do much in this particular situation.

These kinds of problems can be tricky to nail down. I'll do some experiments based on suggestions here and report back.

Thanks all!