measuring Input impedance of an Amplifier

pinkmouse · 22nd May 2002, 11:42 AM

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?

Cheers

Al

Jocko Homo · 22nd May 2002, 01:54 PM

You are on the right track. If this is DIY stuff, you could design the crossover into the amp's input circuitry.

Jocko

peranders · 22nd May 2002, 02:41 PM

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.

pinkmouse · 22nd May 2002, 07:04 PM

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!).

Cheers

Al

p.s.
Your site is great, and very informative, but it would not load in Explorer 5.1 on the Mac.

Nelson Pass · 22nd May 2002, 08:18 PM

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.

pinkmouse · 23rd May 2002, 08:15 PM

Ok folks,

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

Al

22nd May 2002, 11:42 AM	#1
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	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? Cheers Al

22nd May 2002, 02:41 PM	#3
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	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. __________________ /Per-Anders (my first name) or P-A as my friends call me Tube Buffered Gainclone in work \|Thread

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22nd May 2002, 01:54 PM	#2
Jocko Homo Banned Join Date: Feb 2002 Location: As far from the NOSsers as possible	You are on the right track. If this is DIY stuff, you could design the crossover into the amp's input circuitry. Jocko

22nd May 2002, 07:04 PM	#4
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	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!). Cheers Al p.s. Your site is great, and very informative, but it would not load in Explorer 5.1 on the Mac.

22nd May 2002, 08:18 PM	#5
Nelson Pass The one and only Join Date: Mar 2001	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.

23rd May 2002, 08:15 PM	#6
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	Ok folks, I gave it a go, and does 6k2 ohm sound right for a trannie amp at 100Hz? Al