Oscilloscopes and balanced signals

[johnnyc]

In practice it's not terribly necessary. I just use some cables that go from 1/4" and xlr to bnc and just look at the signal single ended. It helps to know what you dealing with though so you can use/build the right cable. For example with transformer output gear you can usually ground pin 3, other gear might not like this and perhaps even damage it.

Alternatively you could build a cable that used 2 inputs of the scope and then look at their difference.

[Marcocet]

I've run in to some trouble using a scope between pins 1 and 2 and ignoring 3. If you tie it to ground that should solve the problem, but I like to check out both. Some gear has strange variances between + and -, which I'm still just beginning to understand.

[brianroth]

Many times, you can do a "cheat" by using a "3 pin to 2 pin" plastic adapter on the scope's AC mains power cord, thus floating the scope's ground.

Somewhat imperfect, but often works for testing gear which has an "electronically balanced" output.

Bri

[PRH]

Thanks for the input, guys. Seems like there might be a couple of approaches. Need to grab a scope and experiment!

Paul

[Radardoug]

Some scopes allow you to mix the two input signals and invert one, so you can use the two probes, one on each leg. That would work fine for active balanced outputs, but with transformer balanced outputs not so good. But then, with a transformer balanced output, just ground one leg.

[ritelec]

thanks for the blast from the past.

my story is 40 yrs old and could be self incriminating .

oscilloscopes .....................thanks for the memories.

[tINY]

...and please, don't pull the ground pin off a scope and go poking around tube equipment...


-tINY

[Lotus 7]

Originally Posted by tINY

...and please, don't pull the ground pin off a scope and go poking around tube equipment...


-tINY

You mean it's not a good idea to plug the scope power cord into a 3 to 2 wire adapter to float it and then connect the probe ground pin to the B+ center-tap on the output transformer primary to view the signal across the primary?

I always thought it was a ideal example of Darwinian selection.

[brianroth]

As I mentioned, the ground-lift idea is usually suitable for scoping the electronically balanced line level output from a piece of gear, such as a recording desk. That should NOT be taken as a blanket suggestion for other measurement tasks.

Bri