BNC to RCA adapters

Hello, I’m still using a QA400 on a windows 7 machine. Waiting patiently to upgrade to the 403. It’s a great piece of gear!

Anyway, my question:
Is it ok to use regular bnc to rca connectors like below to measure the output of a audio equipment?
I was getting much different frequency response results with the same preamp going straight into the QA400 vs an amplifier, and people on DIYA were speculating on the load used into the analyzer.

I assumed that rca to bnc was fine and the preamp would see the 100k input impedance of the QA400, but I also see that a bnc adapter sometimes says 75ohm or whatever, and assumed that comes into play only at high frequencies. If someone could kindly shed some light on the adapters and also what might be happening in my application (see below links).

StarTech RCABNCFM RCA to BNC Adapter - F/M https://www.amazon.com/dp/B00027BVUO/ref=cm_sw_r_cp_api_glt_fabc_YAFW7S5Q4T5CAQ91Z1CT

I have been using those adapters with the QA402 and have found them to be fine for audio work and the 75ohm rating would come into play more at RF.

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Thanks! That was my understanding as well but I wasn’t sure.

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Hi @matt. I just read your response about measuring amplifiers here: Attenuator for high RMS signal tests

I am wondering if connecting a preamp directly to a the QA400, rather than differentially through the resistor network in your article, allows hum to creep in to the measurement. ( I realize the QA is not differential).
Basically, I get much better results through the preamp–>amp–>QA400, than through preamp–>QA400 directly. Can you shed some light on what might be happening?

In my question on this thread, I mention frequency response is improved when measuring preamp–>amp–>QA400, vs through preamp–>QA400 directly, but I also notice a reduction in 60Hz when inserting the amp between the preamp and the QA400 as well.

Hi @anchan, it’s hard to know precisely. What works on one amp might be different on another amp. There are so many amp and pre-amp topologies I don’t think there can be one hard and fast rule for everything. Some pre-amps might run from a single rail, and use AC coupling on inputs and outputs. Heck, some amplifiers (class D) often run that way. TI’s monster TPA3255 idles the outputs at supply / 2, so if you have a 50V supply, both speaker outputs are sitting at 25.00V when you have no incoming signals.

In your case, it could be the power amp presents a true ground (tied to wall), while the pre-amp is using a wall wart that doesn’t have any ground reference at all.

Try this: Setup your pre-amp → QA400, and then touch the ground lead from a scope to the BNC shell of the QA400 and see what happens. If that changes things, then that says your setup has a tenuous ground. And that explains the amp coming in and making things better.

I don’t think the QA400 was isolated, which adds another dimension to the mix. In that case, if the pre does have a ground, you are adding another ground through the computer. Or, if you are using a laptop, you’ve got another very tenuous ground