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Old 22nd May 2002, 11:42 AM   #1
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Default measuring Input impedance of an Amplifier

I am planning to cobble together a cheap sub with bits and pieces I have lying around and could do with some advice.

I have built the box with no problems at all, and I had lying around an old 150w mono power amp built by a friend years ago to power it, but I need some help.

Because buget is a major factor, ( and I also like to play around), I would like to use a passive 2nd order line level crossover to run at about 80-90 Hz to drive the amp from the lfe output of my Arcam Alpha 10 Dave system.

But, as far as I can see, I need to know the input impedance of my power amp to properly calculate the component values in the crossover.

So, my question is this... Is there an easy way to measure input impedance using basic tools, such as a multimeter and pc based tone generator in the same way you can measure drivers, or do I need massive amounts of test gear?

And if that's too easy, any way to modify passive line level crossovers to give a sweepable crossover frequency for room tuning?

Anyone any Ideas?


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Old 22nd May 2002, 01:54 PM   #2
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You are on the right track. If this is DIY stuff, you could design the crossover into the amp's input circuitry.

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Old 22nd May 2002, 02:41 PM   #3
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Yes, it's easy!

Take a variable resistor (a pot) in series with the input and the signal source. Measure the output with increasing resistance. When you have got half the signal, stop there and remove the variable resistor. Measure the resistance. It's the input RESISTANCE.
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Old 22nd May 2002, 07:04 PM   #4
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Hi Per

Thanks for the tips, I'll give it a go this evening and see how get on.

Sorry about the resistance/impedance confusion, I had searched many pages whilst doing research, and I think my brain was overloaded! ( as a stage lighting tech, normally I use 3 phase power at several hundred amps, and this small stuff gets confusing!).



Your site is great, and very informative, but it would not load in Explorer 5.1 on the Mac.
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Old 22nd May 2002, 08:18 PM   #5
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Do it at a low frequency to get just the resistance,
say at 100 Hz, otherwise you might find yourself
also measuring the cable and input capacitance.
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Old 23rd May 2002, 08:15 PM   #6
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Ok folks,

I gave it a go, and does 6k2 ohm sound right for a trannie amp at 100Hz?

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