BNC Scope Adapter For Balanced Signal

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

iangomes

Well-known member
Joined
Feb 17, 2008
Messages
199
Location
Toronto, Canada
So I've been buying some vintage gear lately and need to do some calibration. I want to convert a balanced signal to a single ended signal so I can look at it on the scope (while only using one of the channels). Is this a problem that you guys have run into before? Do you just make a little unbalancing board? with an XLR input and BNC output?
 
The answer is you can't do that directly, for all balanced signals.

If your output has two hot legs, the scope only has one hot input so you need to add a black box between your gear and the scope to convert two hots to one hot.

This black box can consist of a transformer, or active balanced differential to single ended circuit.

Inserting your black box between the DUT (device under test) and your scope, means any errors introduced by this black box will affect your measurements.

JR

 
I made a XLR to BNC cable that I use - it connects XLR pin 2 to the BNC center, and XLR pin 3 to the BNC ground. The scope is then showing the difference between pin 2 & 3 of the balanced signal. The XLR chassis ground is not connected.
You might run into problems since it is not ideal, but generally it works fine. 
 
okay, good. At least I know I'm on the right track then. If I'm just using it for level, then a high precision part like a ssm2141 could turn my balanced signal into a single ended signal. I think I'll use this solution. Is it likely I'll get a high enough quality output to make it accurate?
 
I usually connect a bnc to banana plug on my scope. I have made various balanced AND unbalanced to banana cables which makes interfacing a breeze.  on the xlr side just wire as normal. On the other side lift pin 1 and wire pin 2 to the red and pin 3 to the black. works perfect. Also can use the same cables to interface a DMM to your gear.
 
I soldered a XLR to 2 BNC adapter last week. Having a dual scope helps, of course. And it's great for checking signal balance. Biggest problem was getting the two coax leads into the Neutrik XLR jack.
 
Professional oscilloscopes often use 3 wire line cords with the BNC shell grounded to earth ground. If you connect pin 3 to BNC shell, you may be shorting an active output line to ground.

Many of us learned this the hard way (by blowing stuff up).

JR
 
That's a good point - I've done this working with transformer balanced equipment, but I'll have to pay attention for anything with an active balanced out. My scope does ground the BNC shell.
Quite a bit of equipment now is spec'd as unbalanced or balanced with TRS jacks, so a TS plug connects (-) to ground, my audio interface, for example. And it is an active out. Wonder how that is done? A series resistor to protect the chip?
 
There are two basic approaches for active balanced (both hot) outputs.

The simple approach is just to use a second inverted output with enough of a series buildout resistor that it doesn't release it's smoke when shorted.

The more complex approach uses cross connected negative feedback, that detects if either active output is shorted and shifts the gain to the other output. This is a better mimic of transformer outputs and only downside to shorting one leg, is losing 6 dB of output swing. Shorting one side of the cheap approach can make things hot, and will dump that short circuit signal current into grounds, that could cause crosstalk, or smoke release.

JR

edit-  BTW if this whole exercise is about measurement and calibration, it seems unwise to short one active leg that could cause as much as a 6 dB error with some equipment.  /edit
 
Is it for simply for level calibration? I'd use a dmm or get a floating ac voltmeter.
Or do you want to connect it to the scope to see the signal output?
In that case I would use a scope on one of the hot outputs and properly terminate the other one (connect/disconnect)
 
well it was for looking at two balanced audio signals at a time. My scope has a differential feature where you can put the + on channel 1, the - on channel 2. I was hoping to find an unbalancing solution so I only needed one channel and could view the correct level.
 
iangomes said:
So I've been buying some vintage gear lately and need to do some calibration. I want to convert a balanced signal to a single ended signal so I can look at it on the scope (while only using one of the channels). Is this a problem that you guys have run into before? Do you just make a little unbalancing board? with an XLR input and BNC output?

You can buy differential 'scope probes from the usual suspects. You will need a lot of space on your credit card, though. Do note also that differential 'scope probes are designed for high-speed signals (think picosecond rise times), which explains the expense. Also their input range may have limits lower than what you'd like for line-level audio.

-a
 
I built a box with a Jensen bridging tranny, 10k :10k I think. Then padded it so it introduces exactly 1 dB of loss. It's worked great for 20 years or so. Also put a 20 dB pad on it for hot signals.
 

Latest posts

Back
Top