Here’s a batch of audio cables and stuff I built. Unfortunately I haven’t taken time to really dive deep in the measurements yet.
As to going unbalanced output to differential, RCA tip to + and RCA shell to -, BUT, to keep the - from becoming a large antenna, you will then need to separately ground the unbalanced DUT, and use the analyzer differential input to subtract out the difference between the DUT’s ground and the analyzer’s ground. While the analyzer has a fairly high impedance from the BNC shells to USB power from the computer, all analyzer BNC shells are solidly connected together, so if your generator output is driving an unbalanced input, you’ve already connected the grounds together. It’s all part of why unbalanced sucks.
Hi @daleshirk. I find the mode of measurement you propose interesting. I have always followed the modes suggested at this link: Measurement-with-the-QA401 Download and have always obtained good results in noise reduction. But I would be curious to try the mode you propose. Could you, when time permits, make a diagram of the connections? Thank you
Hi @Claudio. The post to which I was responding by @HawkerAMpT concerned a processor with unbalanced RCA outputs, which would almost certainly be an ungrounded device, therefore my recommendation to ground it when using a differential input. 'Tis but the first step, not an alternate to the methods described in the paper you referenced.
The paper you linked looks like an excellent resource. It certainly references impeccable sources, Ott, Whitlock, Joffe, el.al. He gives an excellent analysis of the stray capacitance and coupled inductance modes of noise coupling. Oddly, he ignores shared impedance coupling and indeed his bonding scheme may (or may not) improve with a star connection layout, especially to the USB shield. Why should its current couple to the other devices. His use of a low-Z attenuator to reduce Johnson noise is excellent. His assumption that the black speaker terminal of the amplifier is connected to chassis ground is perhaps forgivable considering the date and hifi orientation, but it is in no way a given in pro sound today. Also he oddly bypasses the benefits of twisted pairs or coax in favor of tied bundles, but I lack personal experience to comment further on that. None the less, it is an excellent resource which I shall download and save. Thank you.