checking phase on balanced outputs

im looking att xy figures on my scope when sweeping my tube eq build.

before the output tubeamp the signal is unbalanced and on the output it is balanced.

is it ok to only probe one of the leads from the output transformer or must i have a differential probe or something?

  Depends on your circuit, schem is useful to help!

  Usually output transformers secondary are floating, so you can't just probe one end and leave the reference to ground as you would measure the capacitive transfer not the actual transformer output. But as it's floating you don't need a differential probe, you just hook one end of the transformer to ground and measure as usual.

JS

Hi joaquins!

thanks for responding!!

in this thread is the schematic, although i have altered a few resistors.

https://groupdiy.com/index.php?topic=65321.0

so could i connect one end of the secondary to audio ground and probe the other end? or should i ground one end with the ground clip of the probe?

Yes - with an audio output transformer secondary, it's OK to 'ground one end' of the winding and put a cro probe to the other end to take voltage measurements.

Remembering that a CRO probe is 'internally grounded', one can simply put the CRO probe across the audio transformer winding without having explicitly grounding one end.

The process of grounding one end of the audio transformer winding makes it 'unbalanced' or 'single ended' and thus suitable to measure with a typical (non-differential)  CRO probe.

Note that debalancing a transfomer winding and taking a voltage measurement will return  a +6dB level shift over the 'notional' signal on each phase.  ie. V single ended = V+diff - (V-diff) = 2x Vdiff

Of course, one can use the X and Y channels of a dual input CRO, set one channel to 'invert' and use the CRO's dual mode input of type (x+y) - this will function as a true differential probe with no grounding of the winding at all.

Also note that although making sensible 'phase-to-ground' measurements in a differential audio output is not guarranteed to show a pair of 'same-amplitude signals of opposite phase', it is often the case anyway - it depends on the circuit.

For example, a solid state balanced output's '+'  and '-' connection signal will typically be symmetrical about ground, so it's fine to take the expedient single ended readings of each 'phase-to-ground' style.

This stuff is the kind of thing you can verify, for example, when 'lining up' or calibrating, an audio interface (dB) with a CRO (voltage).

ie. you want to measure voltage peak-to-peak on the CRO (or AC true rms voltmeter), and convert back and forth between RMS, dBu (600 ohm load) and so on.

5v333 said:

Hi joaquins!

thanks for responding!!

in this thread is the schematic, although i have altered a few resistors.

https://groupdiy.com/index.php?topic=65321.0

so could i connect one end of the secondary to audio ground and probe the other end? or should i ground one end with the ground clip of the probe?

Click to expand...

Yes, the best way of doing so is probe to one of the transformers terminal, ground of the probe to the other, leave them floating other than the probe ground. That way you will get the smallest ground path and no ground loops, so lower noise for your measurements and no safety issues for your scope.

JS

thanks guys!

i just realized that the name of the thread should have been:  checking phase shift on balanced outputs.

which leaves me to another question.

when having one of the x or y probes connected on the output as discussed (probe ground clip to one end of sec and the tip to the other end), will this give a true measurment of the phase figure in a xy setup? the other probe i intend to connect befor the input transformer of the tube amp.

im trying to learn about phase margin and to know if an amplifier is stable.

  Depending on how you plugged it, if you connect it reversed you will have reversed polarity. Test with a 1kHz sinewave, that frequency usually is taken as reference as being away from both ends of the audio band, so it shouldn't have much of a phase shift. If your input an output are reversed in polarity switch the output around. Then you make your measurements.

  Note that for audio band work you can use the PC sound card and get better measurements than with a scope. Transfer functions to get the bode plots and such. Still, stability issues on audio devices happen much higher, even up to the MHz region, so there you can't do much with a sound card.

JS

thanks for your friendly replies!

when probing the output transformer sec with one end of the winding to probe-ground, i get similar or maybe the same phase response as leaving one end floating. but guess i get the right gain response now.

phase/gain stability is new to me and very interesting and i think im starting to see on my scope why i feel funnny when listening to it

i dont know how to calculate the phase shift from a lissajou figure yet but im looking for 0, 90, 180 degrees and see what the gain is.
do you think this is good enough or must i do proper bode plots?

although theres something fatiguing from listening to it, theres alot of potential in this amp with its lundahl transformers!

so when i scope the amp, it looks like i have good phase margin over individual parts of the amp but not when if i check from the start to the end (input xformer, tube1, tube2, output xformer).

is the whole chain essential when i check the stability? or would each stage be sufficiant to investigate?

  The phase margin is only needed inside a closed loop, unless you have feedback from the output transformer to the input transformer is not a problem having no margin there.

JS

of course you ARE synching the scope with your oscillator signal aren't you.

Otherwise, if the scope is synching on its input signal, you will see NO POLARITY difference probing either +ve or -ve output pin on your XLR.

That's one reason to use a nasty assymetrical test signal instead of a nice clean sine wave to check polarity.

joaquins, thank you so much once again! this takes me further indeed!

hi ricardo!! i actually dont bother with sync so much as long as i get a good reading. but maybe i will try it... i insert my generator in the very beginning of my eq and take measures through all parts. right know im in the outputstage and trying to see how the phase response looks like with frequency and gain, and also transformer ringing and bandwidth.

im not looking for polarity of the phase as my thread might suggest...


thanks again people!!!