Testing an output transformer … but it’s not balanced?

[sponzorb]

I was hooking up a circuit earlier that’s basically an API 312 with a THAT 1206 instead of an input transformer. I’m planning to use this as a line amp. I wired up a Cinemag CMOB-2 as the output transformer with Brown connected to the output of the 2520 and Red to ground. On the other side of the transformer I see the signal on Orange but there’s 0V DC at yellow. I expected to see the inverse on Orange. I’m measuring this with a scope. Is this expected? Do I need a load between the hot and cold of an output transformer?

 

[gyraf]

You "see" the signal.. referring to what?

A transformer output is floating, and thus only makes sense when referred to itself.

 

[sponzorb]

You "see" the signal.. referring to what?

A transformer output is floating, and thus only makes sense when referred to itself.

I see signal referring to ground of the 312-like circuit.

 

[ruffrecords]

I see signal referring to ground of the 312-like circuit.

Because you are using a scope to look at the signal you are effectively grounding one side of the transformer which is why you see signal apparently referred to ground.

Remember, in a balanced output, the signal exists only between the hot and cold pins. Ground is not involved in any way whatsoever. In many active balanced outputs the hot and cold are referenced to ground but there is no requirement for this to be so neither is there a requirement for the signals to be equal and opposite. The ground connection is really just a connection for the screen of the cable to help reduce interference. It is not involved in carrying the signal and does not need to be connected at both ends.

Cheers

Ian

 

[sponzorb]

Because you are using a scope to look at the signal you are effectively grounding one side of the transformer which is why you see signal apparently referred to ground.

Remember, in a balanced output, the signal exists only between the hot and cold pins. Ground is not involved in any way whatsoever. In many active balanced outputs the hot and cold are referenced to ground but there is no requirement for this to be so neither is there a requirement for the signals to be equal and opposite. The ground connection is really just a connection for the screen of the cable to help reduce interference. It is not involved in carrying the signal and does not need to be connected at both ends.

Cheers

Ian

Oh duh that makes sense now. How could I verify the transformer is correctly hooked up then? Could I put each end of the probe on each end of the transformer and flip the orientation with the second probe?

 

[CJ]

Use an ohm meter to pair up the leads.

 

[moamps]

I was hooking up a circuit earlier that’s basically an API 312 with a THAT 1206 instead of an input transformer. I’m planning to use this as a line amp. I wired up a Cinemag CMOB-2 as the output transformer with Brown connected to the output of the 2520 and Red to ground. On the other side of the transformer I see the signal on Orange but there’s 0V DC at yellow. I expected to see the inverse on Orange. I’m measuring this with a scope. Is this expected? Do I need a load between the hot and cold of an output transformer?

Connect one probe to the primary brown wire. Second probe connect to orange secondary wire, and yellow wire connect to ground. The sinusoidal forms seen on the scope should be in phase. Then connect the orange wire to gorund and measure the voltage on the yellow wire. The sinusoidal forms seen on the scope should be out of phase.
What you saw on the scope in your experiment without connecting one wire of the secondary to ground is most probably the result of the influence of parasitic capacitances between the primary and secondary windings.

 

[Matt Syson]

Because the scope input is high impedance AND referenced to ground you must rethink your testing strategy.
Others have given clues but have not mentioned stray capacitance (coupling) that will give incorrect readings into a high impedance like the scope. thus you must test with a real output 'load' (resistor of say 10K Ohms as a starter) AN?D carefully consider what your observations really
mean. the term 'balanced' is both precise and imprecise UNTIL you define the degree of balance which is then related to real world usage. THEN it will also depend on the frequency and ALL external connections, even down to how wezll the wires are twisted together on the ' balanced' side.