Tweakers' Asylum

How can I measure the input impedance of my amp?

198.208.159.17

The specs in the user manual don't specify the input impedance of my amp. Can I measure it with a multimeter? if so, how? should the amp be turned on or off for the measurement? or what would you suggest me?

I'd appreciate any help you can provide.

Thanks
Jov

Follow Ups:

What is the amp? Someone may already know. Also, if the load resistor follows immediately... - jeffreybehr 22:18:32 08/01/07 (0)
...the input jack, that is if there no intervening capacitor in series or parallel, the input impedance is the value of that load resistor which is directly measurable by the Ohmeter of your multimeter...from the inputjack's center conductor to its groundshell.
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Tin-eared audiofool and very parttime fotografer.
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RE: How can I measure the input impedance of my amp? - rick_m 15:09:28 08/01/07 (1)
The easiest way is to determine what resistance in series with the input reduces the output by half.

You will need your multimeter, a constant signal source such as an audio generator or a CD with a test tone track, and a pot. Usually a 100Kohm is adequate as most amps have an input impedance between 10K and 100K. A 1Mohm pot might be needed if you have an especially high input impedance.

I recommend unhooking the speakers if you have a solid state amp as you reduce the chance of blowing anything is something goes awry.

Turn the pot fully counterclockwise and measure the terminals with your multimeter set to ohms, hook one end to the center and find which of the other two measure <10 ohms. These are the two you want to use.

Connect those two in series with the hot lead from the preamp or signal generator to the amp. Make sure the volume is turned down on the preamp or generator.

Set the multimeter to AC voltage and adjust the preamp output for 1V out from the power amp. Now rotate the pot until the voltage drops to 0.5V.

Turn off the amp, unhook the pot and measure between the two terminals you
were using with the multimeter set to ohms. The reading is the input impedance of your amp.

Sounds like a pain, but it's easy after the first time.

Rick
- RE: How can I measure the input impedance of my amp? - jdec 19:22:30 08/01/07 (0)
  Rick,
  
  I appreciate your detailed response, thank you very much. I'll folow your instructions.
  
  Jov

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