DI Box has much higher Output on hot side

[12dbLow]

Hi guys,

a quick question:
I was measuring two passive DI boxes with a scope just for curiosity and noticed the more expensive one has got a much higher output on the hot than on the cold pin. Is it damaged? I checked everything twice. The scope and probe settings were right. The pad and GND lift switches didn't make a difference.

Info for the scope screenshots:

- Signal generator: 1kHz Sinus / 0,775 Vrms / 50 Ohm / connected to unbalanced input of DI box
- Yellow: XLR Output Pin 2
- Blue: XLR Output Pin 3

DI Box 1: Cheap noname "DXI-DI Sirus Pro"
DI Box 2: LA Audio DIX

 

[12dbLow]

Here comes the second Screenshot

 

[RuudNL]

A transformer is used in a passive DI box.
I think it is normal that the capacity of one end of the output winding (to ground) is higher than the other end of the output winding. (Because that is what you are measuring; you are measuring the capacitive leak from the output to ground.)
I don't think this will be a real problem when the output is terminated.
Probably the difference at higher frequencies will be even greater.

 

[12dbLow]

I was so focused on the amplitude that I was totally missing that the signals are in phase! What is going on here? I will have to check that when I'm in the lab again 

 

[Harpo]

The scope and probe settings were right.

obviously not.

The pad and GND lift switches didn't make a difference.

The pad switch will make a difference when probing between the correct pins.

Info for the scope screenshots:

- Signal generator: 1kHz Sinus / 0,775 Vrms / 50 Ohm / connected to unbalanced input of DI box
- Yellow: XLR Output Pin 2
- Blue: XLR Output Pin 3

The transformers secondary winding has (at least) two winding ends that connect to XLR-pins 2/3 of your DI-box for a floating balanced output. You measure the differential AC voltage between these Pins 2/3. Ground/shield/XLR-pin1 is not involved in this measurement.

 

[12dbLow]

The transformers secondary winding has (at least) two winding ends that connect to XLR-pins 2/3 of your DI-box for a floating balanced output. You measure the differential AC voltage between these Pins 2/3. Ground/shield/XLR-pin1 is not involved in this measurement.

With the pad switch not making a difference, I meant it didn't change the amplitude difference on pin 2 & 3. Of course the signal was attenuated. Sorry for the misunterstanding.

I connected scope GND to pin 1. The two probes were connected to pins 2 and 3. In case of the cheap DI Box this showed two signals with pin 3 being inverted to pin 2. Only the LA-Audio DI Box showed strange results with the same setup. Isn't it possible to measure the signals against ground?

 

[Harpo]

With the pad switch not making a difference, I meant it didn't change the difference in voltage on pin 2 & 3. Of course the signal was attenuated. Sorry for the misunterstanding.

I connected scope GND to pin 1. The two probes were connected to pins 2 and 3. In case of the cheap DI Box this showed two signals with pin 3 being inverted to pin 2. Only the LA-Audio DI Box showed strange results with the same setup. Isn't it possible to measure the signals against ground?

Your measuring method shows capacitive leakage of an unterminated winding, already described by Ruud in a previous post.
If you whish to measure signal against ground for whatever reason, you need to include this separate and unrelated potential into your circuit by joining one transformer secondary winding end with this gnd/shield potential, hereby making this signal unbalanced again. Signal for usual is exclusively connected between XLR-pins2/3, so you would measure between these and ignore signal in respect to XLR-pin1. You could measure DC voltage of a maybe +48V phantom supply between pins 1/2 and pins 1/3, but that wouldn't be signal AC voltage.

 

[benb]

The outputs may be "floating" relative to ground, whether it's a transformer output or an electronically balanced output.

Connect a 1k resistor from pin 2 to ground (pin 1, and/or your scope ground), and another from pin 3 to ground. THEN see what the differences between these two boxes are.

 

[joaquins]

  Load the output, for example as benb said, but then you would have a difference depending on the matching of the resistors, nothing to do with the transformer it self.

  If you have a floating output as the secondary winding doesn't make much sense to measure both ends to some random reference ground (even if it's the same DI box or the transformer shield). Instead use a single prove and measure the voltage across the entire winding.

  if you want to measure other properties of the transformer there are proper methods to do so. Jensen always specified their methods on their datasheets, for each specification, how they measure that. (everybody should, specs doesn't mean much unless the method is clear)

JS