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Author Topic: Measuring output impedance - what am I doing wrong?  (Read 162 times)

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Offline Loquah

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Measuring output impedance - what am I doing wrong?
« on: March 30, 2021, 02:39:04 PM »
Hi BH clan,

I feel like I'm losing my mind because I am having huge difficulty measuring output impedances today having successfully done it in the past. My method is:

  • 3.5mm plug with wires off the ring and sleeve
  • Play test tone at 1kHz
  • Measure voltage across the wires (AC voltage) - e.g. 0.562V
  • Apply a known load - I have a 100 ohm and a 6.8 ohm resistor here, have tried with each of them
  • Measure again with the load in place - I am using the hook connectors on my multimeter to hold the resistor leads against the stripped wire from the headphone jack. My problem is I keep getting voltages of 0.002V even though I haven't altered the output volume

What am I doing wrong??
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Offline 2wo

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Re: Measuring output impedance - what am I doing wrong?
« Reply #1 on: March 30, 2021, 05:43:19 PM »
Connect a pot, instead of a resistor. First see if you can vairy the voltage then set it for 1/2 the starting voltage then disconnect and measure the pot.

I think I remember that right...John
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Offline Loquah

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Re: Measuring output impedance - what am I doing wrong?
« Reply #2 on: March 30, 2021, 10:23:59 PM »
Connect a pot, instead of a resistor. First see if you can vairy the voltage then set it for 1/2 the starting voltage then disconnect and measure the pot.

I think I remember that right...John

Happy to try that, but not sure it will change anything given that my resistor is just a fixed version of the same thing. Happy to try though while we wait for anyone else to chime in with my error  ;)
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Offline Doc B.

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Re: Measuring output impedance - what am I doing wrong?
« Reply #3 on: March 31, 2021, 05:32:24 AM »
Is your meter capable of reading up to 1kHz AC?
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Offline Loquah

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Re: Measuring output impedance - what am I doing wrong?
« Reply #4 on: March 31, 2021, 11:57:20 AM »
Is your meter capable of reading up to 1kHz AC?

I'm not certain, but it IS showing a reading with no load. It's only when adding the fixed load that things get strange. I can always run 50Hz through it - would that cause any inaccuracies for measuring Zout?

P.S. yesterday was a bit stupid so I haven't tried with the pot yet
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Offline Paul Birkeland

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Re: Measuring output impedance - what am I doing wrong?
« Reply #5 on: March 31, 2021, 11:59:09 AM »
Yes, in an amp with global feedback (feedback that includes the OT), you would expect the output impedance to be a bit higher at 50Hz than it would be at 1kHz.  I would imagine your meter could resolve 120Hz though.

If you wanted to check, you could use a phone and an 1/8" to RCA cable, then use a signal generator app to about 100mV at 50Hz, then start turning the generator up and see when the AC voltage reading on your meter starts to deviate significantly.
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Offline Doc B.

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Re: Measuring output impedance - what am I doing wrong?
« Reply #6 on: March 31, 2021, 12:21:45 PM »
There are pretty cool little digital scopes for dirt cheap these days, that could be useful for this kind of thing. I'm considering taking one to Bonneville with me to dial in my nitrous controller's pulse width.

https://www.amazon.com/dp/B081Q2KDJT/?coliid=I3V3Z8L6ZYWVS9&colid=AWNK7QD0RTKE&psc=1&ref_=lv_ov_lig_dp_it
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Offline Loquah

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Re: Measuring output impedance - what am I doing wrong?
« Reply #7 on: March 31, 2021, 01:08:50 PM »
Yes, in an amp with global feedback (feedback that includes the OT), you would expect the output impedance to be a bit higher at 50Hz than it would be at 1kHz.  I would imagine your meter could resolve 120Hz though.

If you wanted to check, you could use a phone and an 1/8" to RCA cable, then use a signal generator app to about 100mV at 50Hz, then start turning the generator up and see when the AC voltage reading on your meter starts to deviate significantly.

I've got a signal generator on my PC - I'll set that up and test your suggestion. Thanks for the input, guys!!
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Offline Loquah

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Re: Measuring output impedance - what am I doing wrong?
« Reply #8 on: March 31, 2021, 01:09:18 PM »
There are pretty cool little digital scopes for dirt cheap these days, that could be useful for this kind of thing. I'm considering taking one to Bonneville with me to dial in my nitrous controller's pulse width.

https://www.amazon.com/dp/B081Q2KDJT/?coliid=I3V3Z8L6ZYWVS9&colid=AWNK7QD0RTKE&psc=1&ref_=lv_ov_lig_dp_it

Oh wow! How cool!!!
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Offline Loquah

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Re: Measuring output impedance - what am I doing wrong?
« Reply #9 on: March 31, 2021, 09:06:44 PM »
Looks like frequency was the issue. Thank you, Paul!

Signal generator at 125Hz solved the problem  :)
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