# Audio interface for impedance measurment



## kinser (Dec 6, 2016)

Hi all,

A while ago I purchased a Behringer UCA202 that turned out to be fake ( eBay).
I tried to use it with my DIY jig (using two 5 ohm non-inductive resistors in series) and I could not get the thing to work without noise.
Eventually, I went to a friend to use his Dayton DATs to get out the T/s specs. 

Since then I kind of dropped the whole thing altogether, But I still want the fun of doing the measurements at home.

What is a fairly good and non-expensive audio interface I can use to do impedance measurements?
I was looking at the Behringer UMC202HD - would this be sufficient?

I've got a Macbook Pro running macOS as well as Windows 10 so drivers should not be an issue.


Thanks,
David


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## JohnM (Apr 11, 2006)

That is a fairly low resistance value, many soundcards would struggle to properly drive the overall load unless they had a decent headphone output and you used that. The unit you mentioned may well fit the bill for that but I don't have any experience of it. Steinberg's UR22 MkII is very good but quite a bit more expensive.


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## kinser (Dec 6, 2016)

I was using the headphone output on the UCA202 to power the driver.
Only reason I used the 10ohm load was because I read it was best to use non inductive resistors.
I can replace it with a standard 100ohm 1/2w resistor, but will that be sufficient?

The UR22 looks like a good piece of hardware but its way over my budget.
I think I'll pull the trigger on the UCM and see what it's worth.

As for amplification - I have a bunch on single rail TDA2030 amps laying around - would using one of them be good enough for a few watts of power? They are china kits and have a 470uf decoupling cap on the output but might be a tad noisy, unless I would be okay with running off the line out of the sound card (UCM)
It might be worth converting to dual rail for better response.


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## JohnM (Apr 11, 2006)

Running through an amp can work well, but if it has gain probably best using some dividers to protect the inputs of the soundcard.


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## kinser (Dec 6, 2016)

I can alter the gain of the TDA2030 to keep it fairly low. But I do plan to add a divider to the inputs of the sound card anyway if using an external amp.

How are you running the circuit on the UR22? I assume you are running unbalanced input/output via TRS?
Do I need 2 ring plugs(+/-/ground) on the input or can I use a single ring plug (+/ground)? I don't want to short anything out.


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## JohnM (Apr 11, 2006)

I haven't used the UR22 for impedance measurement. I've used single ring (TS) plugs on the UR22 inputs in its combination sockets without any issues.


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