Hi there,
I’m testing some transformer coupled/ balanced out audio gear and I would like to take some measurements with different loads (other than the QA403 100k input impedance). Say, 600, 10k, 20k, etc…
What would be the correct way to do this? For the 600-ohm load example, is it better to connect two 600 ohm resistors from the BNC pins to the cells or just one 600-ohm resistor across the two BNC pins?
I made a wonderful drawing that I’d like to share with you…
So, what’s the correct approach? The Left input or the right input?
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As long as you’re using both + and - inputs in differential mode, the right channel is the correct way. Keeping the common mode impedance high will provide better CMRR.
If you don’t know the configuration of the output stage, you might load one side to ground to investigate.
Dale
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Thank you daleshirk
So, for a transformer output device (i.e differential output) the right connection would be more suitable, right?
I’m only asking because on the “DAC Output Impedance” guide, Matt recommends the left connection so I got a bit confused.
When loading the DAC outputs, you will want to use a load that is a shunt to ground, in parallel with the QA40x audio analyzer inputs. For example, if you have your balanced DAC outputs going directly into the QA40x audio analyzer, then the test loads you apply will resistors tied from the L+ input to ground, and from L- input to ground.
Sorry this reply is so much later.
Solid state outputs such as DACs rarely if ever require loads. Transformers on the other hand will self-resonate usually a couple octaves above their intended passband and need a load to measure correctly.
BTW, your left channel is a 1200 Ohm differential load
Thanks daleshirk and no worries about the late reply. You’re not obliged to answer… 
The gear I want to test already has a zobel network and a termination resistor on the secondaries of the output transformer. What I’d like to measure is how it would behave when the following device has input impedance of 600R or 10k or 100k, if that makes sense.
Since the QA403 has 100k input impedance, I guess I should strap a 600R or 10k as shown on the right channel, correct?
