How does one determine impedance of given gear?

Sorry if this is a noob question.

I have some gear with unbalanced outputs which I would like to convert to balanced outs. They are Trident S80 channels, Audix 35102 channels, and Sphere 9 band graphic EQs.

It would be cool if someone knows the impedance at output of this gear, but I'd like even more to know how to calculate the output impedance myself.

Measure the output voltage with no load. Then add a high-value potentiometer to the output, hooked up as a rheostat (a simple variable resistor). Decrease the resistance... When the output drops 6dB, remove the pot, measure it... and that's your output impedance.

Do not try this with any circuit that can deliver significant power, such as a power amp.

Would this be incorrect in this situation...
Take AC volt and current measurements and solve for Z=E/I?

...oh wait...I think I know the problem with that... but clarification would be wonderful nonetheless.

-Ethan

That's great if you want to know the impedance of the LOAD. It doesn't tell you the source impedance.

Dave,

Would it be totally inappropriate to tell you that I love you :grin: :guinness:

Yes there is.

Connect a high-value variable resistor in series with a signal generator. Adjust the resistor to zero and connect to your input. Increase the resistance until the signal drops 6dB. Disconnect the variable resistor and measure it.

Note that you'll have to use a really high-value variable resistor to get 6dB drop with some inputs. Even then, you might not be able to drop 6dB, and might have to calculate it out using the voltage divider formula for whatever value of drop you did get.

Would it be totally inappropriate to tell you that I love you

Click to expand...

Steady, Ethan....

:green: :guinness:

I have always used this method, but remember seeing on Jensen website somewhat different approach, IIRC. I browsed their website for awhile, but did not seem to find it. Anybody has the link?

OK, found it:

http://www.jensen-transformers.com/as/as048.pdf

Dave , your solution seems to be about working out the i/p impedance, I think that the question is how to detrmine the output impedance.

I would put a sine wave to the i/p so that a reading can be taken on the o/p. Measure this output off load (Ve). stick a variable resistor across the output & adjust so that you get 1/2 Ve across the resistor (this is Vl) Take the resisitor out of circuit & measure it (Rl)

Your output Z is (Rl(Ve-Vl))/Ve.

or output Z = Rl(Ve/Vl -1)

This does not take into account the tolerance of your meter. When measuring the o/p volatges care should be taken with the frequency of the i/p signal. Most DMM`s have problems reading accurately above about 400Hz on AC measurements.

[quote author="Marik"]OK, found it:

http://www.jensen-transformers.com/as/as048.pdf[/quote]

Nice link, :guinness:

Can I assume the 1% tolerance is due to 1% resistors? :?: How accurate would it be with a precision resistor?

What are the exact reasons why you can't use the method on pwr-amps, are they related to the 1% tolerance?

Thanks,
Justin

Btw, link is very dry

Was it written by Whitlock or Jensen? :wink:

J

Read again, Rob. I gave two methods: one for determining output impedance, as per the original question, and one for determining input impedance, in response to a followup question.

The method you outlined is just a restatement of the one I already gave in my first post.

The Jensen method is unusual; they have you inject a signal back into the device's output terminals.

The reason this wouldn't work with a power amp is because the impedance looking back into the output terminals is so low that you wouldn't see anything there unless you were using a sensitive microvolt meter.

[quote author="thermionic"]Can I assume the 1% tolerance is due to 1% resistors? :?: How accurate would it be with a precision resistor?

What are the exact reasons why you can't use the method on pwr-amps, are they related to the 1% tolerance?
[/quote]

No, it's because we are using 10vac with the 100k resistor to approximate a 1ma constant-current source. Since the output impedance of the line amplifier would add itself to the total impedance seen by the signal generator, the accuracy of the reading gets thrown off a little bit, but "still close enough for government work". Somebody who likes math a whole lot more than I do could come up with the proper correction factor.

You CAN use this method with power amplifiers. Since the output of most power amplifiers is very low-impedance, you can change the 100kR to 10k and have a direct relationship between millivolts and output ohms. Or use 1k and have 10mv represent 1 ohm. You can adjust the resistor value to get whatever scale you need.

Hello all,

I've been reading my Audio Cyclopedia and Dave's and others' posts about measuring input/output impedance, but the diagrams I've seen seem to show unbalanced connections. Is there a particular way to measure impedances for balanced gear? Maybe it's as simple as connecting one of the balanced leads to ground? Or not... ? Thanks!

Best Regards,
Jesus

Just do the measurement floating--in other words, do not reference your signal generator or voltmeter to the chassis of the gear you're testing.

Thanks, Dave. :guinness: Now off to the garage...

--Jesus

floating... as in dont connect voltmeter ref to anything?
sorry if this sounds dumbassed, but i love things clarified.
-bryan