Audio and Chassis Grounding

[exlabs]

Or try running your laptop on battery…. I had exactly the same issue today

 

[Markw4]

A few years later, they actually did it.

I have one of those. Its called an isolator. Two built-in probes. Bandwidth is limited though. It optically isolated, good for maybe up to 3kV floating. Don't recall the exact spec at the moment.

 

[Bonsai]

Bonsai, The chassis are not close to eachother, the PC is at least 2 metres away from the amplifier. There is no real option to connect all chassis together, since I have a lot of measuring equipment (signal generators, AC volt meters, wattmeters, etc.).

Note: The amplifier (KT150 PP) itself is very quiet. I check for “noise” and “hum” by connecting the amplifier output (from a dummy load) directly to a 50 Watt AKAI amplifier (line input). Of course there is a little noise with this extreme amount of amplification, but it’s very limited and without hum.

For HF noise, you just need to join the chassis. This keeps HF common mode noise to a minimum. This is a different mechanism to the ground loop discussed earlier.

 

[Bonsai]

Get rif of that 10 Ohm! Not a single commercial unit uses this because it would never get through any safety certification. If you need it it means your grounding scheme is wrong.

A HBR, if implemented as shown in post #8 has no effect in safety and is a widely used technique in commercial equipment.

 

[Bonsai]

Ah, an instrumentation issue, supposedly my major as an EE. How Tektronix screamed and cried when I tried to float one of their battery powered scopes - which would illuminate all these warnings about how it MUST be grounded on screen upon power up. The corporate powers wouldnt ultimately let me us an RS232 isolating device to float that scope. What's with the battery, then? To go dead and sell you a new one? My Fluke 123 doesnt seem to care, neither does their DMMs. The 123 even has an isolated RS232 cable.

We had the same issue doing bench measurement of power converter noise and dynamic response. All the bench stuff with its ground, the DUT with its ground. We ended up using diff probes, which are not within the budget of most DIYers. Some Yokogawa scopes had floating inputs, which we tried also.

One time Tektronix had a "what product would you like" meeting with us. I told them give me a probe with a fiber optic cable to the scope. They looked at me like I had 3 heads. "Why would you want that?". A few years later, they actually did it. I was gone by then, so never got to use one. Part of the tech in that probe was they send a laser beam down a fiber optic cable, converted to DC, to power the electronics in the probe tip.

It is to wonder how the USB isolator gets the power across isolated, to the DAC / ADC. You may want to examine that, in the particular device chosen. I'd expect the device on the isolated side to have its own power source, which sounds like a float to me, for Ground, DC and RF. Battery powered sound card.

One of the issues with isolation barriers like this is you can have very high HF common mode voltage across the barrier that couple capacitively into the input electronics side. So, although these techniques provide a great way to isolate high LF voltages and/or DC, you have to be aware of capacitive coupling. On my QA401, I connect the housing to the laptop (battery powered) USB shroud at the laptop, and then from the QA401 housing to the amplifier housing and then the amplifier housing to the load housing. The wide and noise floor is very low.

 

[Havoc]

A HBR, if implemented as shown in post #8 has no effect in safety and is a widely used technique in commercial equipment.

I'd like to see that in the schematic of a commercial item.

 

[thimios]

Try this if an isolating transformer is available.
Use the isolation transformer to power your p.c.
Use another wall socket(directly ,not isolating)to power your amplifier or whatever you want to test.
If you haven't a success try the opposite.