Some of the test equipment I'm looking at have balanced inputs, like the Sound Technology 1700 series, while others have just single-ended inputs, like the HP 8903A or the Heathkit IM-5248 or the Heathkit IM-22.
If I have an amplifier which has balanced outputs, what's the safest/best way to make measurements with single-ended test equipment?
If I have an amplifier which has balanced outputs, what's the safest/best way to make measurements with single-ended test equipment?
Run either the unit under test or the test equipment on an isolated mains supply, so they no longer share an earth connection, now the ground side of your scope can be one side of the balanced. Or use a 1:1 transformer to isolate the test gear in the input path.
Doesn't the HP8903 have a balanced input? Turn off the ground link switch, and the input "floats".
Doesn't the HP8903 have a balanced input? Turn off the ground link switch, and the input "floats".
You can float the input and output on the 8903 for a balanced signal to your source and measure your output not referenced to ground. Look at the float switches like Enzo mentioned.
Thanks! The 8903B is the one with balanced inputs, the 8903A has just single ended. I figured that was what the float switch was for if it's there. Some test equipment doesn't have one though. For those, if I'm measuring line signals I think I can put together a balanced-to-single ended converter, right?
Power amplifiers with balanced output and monkeying with grounds make me nervous. Will a Variac do or am I looking for something more like these?
Power amplifiers with balanced output and monkeying with grounds make me nervous. Will a Variac do or am I looking for something more like these?
As a quick fix you could measure the outputs separately. This is a pessimistic estimate of the true balanced performance, as common-mode signals (noise, distortion, power supply ripple...) will not be rejected.
Building a differential line receiver at the 8903A performance level should not be too difficult. I'd suggest a "superbal" (Balanced Line Technology figure 4) with a LT1469 or even a NE5532. Rise R1/R2 to get the desired attenuation and input impedance.
Samuel
Building a differential line receiver at the 8903A performance level should not be too difficult. I'd suggest a "superbal" (Balanced Line Technology figure 4) with a LT1469 or even a NE5532. Rise R1/R2 to get the desired attenuation and input impedance.
Samuel
In post #4 you mention POWER amp outputs. Are you talking about measurements of the speaker signal on a bridged output stage? I had assumed you meant the balanced output of some line level source like a preamp.
Power amp outputs connected to the 8903 inputs, with the inputs set to float. I have not bridged an amp into my 8903, but the same idea applies. The analyzer input path is not referenced to ground when in float. The top BNC is high and the bottom BNC is low. The manual talks about this in a somewhat clear way. Forget the section at the moment, but it is there.
Sorry yes mostly right now I'm concerned with measuring a bridged amp at the moment. All my line components are single ended but my Winsome Labs Mouse is a bridged chip amp. For example, I'm really curious to measure its IMD with my Heathkit 5248 but it doesn't have a float switch so I dont want to electrocute myself or destroy the amp or meter...
soundcards are at least stereo - so pseudo differential is possible
have to calibrate the attenuator and channel gain to get much CMR
the Heathkit seems possible to match with a soundcard
have to calibrate the attenuator and channel gain to get much CMR
the Heathkit seems possible to match with a soundcard
Hmm. The Tektronix "Floating Oscilloscope Measurements… and Operator Precision" technical brief says, "WARNING […] Historically, floating measurements have been made by knowingly defeating the built-in safety ground features of oscilloscopes or measurement instruments in various manners. THIS IS AN UNSAFE AND DANGEROUS PRACTICE AND SHOULD NEVER BE DONE!"
And it specifically calls out isolation transformers and defeating grounds as "WARNING! This Is An Unsafe And Dangerous Practice And Should Never Be Done!"
So I'm a bit leery of the idea of isolation transformers. Perhaps the answer is, get a real differential probe for my scope, and only use test equipment with balanced inputs (or a designated float feature) for measuring any bridged amp.
And it specifically calls out isolation transformers and defeating grounds as "WARNING! This Is An Unsafe And Dangerous Practice And Should Never Be Done!"
So I'm a bit leery of the idea of isolation transformers. Perhaps the answer is, get a real differential probe for my scope, and only use test equipment with balanced inputs (or a designated float feature) for measuring any bridged amp.
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Hello,
Do not connecting earth of oscilloscope allow to measure floating voltage, but of course it's very dangerous because oscilloscope enclosure is at your measured potential !
The safe and cheap way is to use isolation transformer for powered your DUT, but not your scope !
If, for example you test a SMPS, when you connect it through an isolation transformer so you can measure voltage with your standard scope probe without risk (of course if the voltage level is high, it stay a risk !).
It is important to keep in mind that all oscilloscope alligator clips is at same potential
and you must be careful to avoid short circuit with others potentials when you use
many probes.
Regards.
FRex
Do not connecting earth of oscilloscope allow to measure floating voltage, but of course it's very dangerous because oscilloscope enclosure is at your measured potential !
The safe and cheap way is to use isolation transformer for powered your DUT, but not your scope !
If, for example you test a SMPS, when you connect it through an isolation transformer so you can measure voltage with your standard scope probe without risk (of course if the voltage level is high, it stay a risk !).
It is important to keep in mind that all oscilloscope alligator clips is at same potential
and you must be careful to avoid short circuit with others potentials when you use
many probes.
Regards.
FRex
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