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Old 26th January 2006, 11:26 PM   #1
Wizard of Kelts
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Default How To Measure Input Impedance Of Solid State Amplifier?


I have an amp that I am interested in designing some inline filters for.

I need to know the input impedance of my solid state receiver.

Here is what I have so far:

An online tone generator which I downloaded from this site:
www.satsignal.net => Audio Tools

An inexpensive cable from my computer sound card to my amp that I am willing to alter if necessary.

An inexpensive multimeter.

I remember doing this some years ago, and there was something about a variable resistor or potentiomenter from Radio Shack and taking voltage measurements, but the details escape me.

Measurements do not have to be super exact to the last ohm. I just need a decent idea of my receiver's input impedance so I can design inline filters for it.

Any help would be greatly appreciated in this.
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Old 27th January 2006, 12:19 AM   #2
poobah is offline poobah  United States
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Open the amp

Trace your inputs

After the coupling cap, find the first resistor to ground... that is close enough.

OR:

Put a 50 - 100 K rheostat (pot) in series with your sound card and the amp (pos lead)

Power up the amp and apply a sine wave... 500 - 1000 Hz, 2.5 V ACrms

Adjust the pot until the ACrms across the pot and across the amp input are equal.

Power down, disconnect, and measure your pot value; that is your Zin.

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Old 27th January 2006, 12:25 AM   #3
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Here's a simple way to do it (but it won't tell you complex input impedance, just input resistance):

Do all of this while the input source is not loaded to anything. Take your input test source (your tone generator), put it at midband frequency (let's say 1kHz for audio), and make its amplitude a nice nuumber (1V peak is fine). Once you have your input source adjusted to these settings, do NOT change them.

THEN, place a potentiometer in series with the source:
Tie the wiper of the pot to the source, and one end of the pot to the input of your audio amp (you can leave the third terminal open). Your source is now connected to the amp input through the pot.

THEN, put your oscilloscope probe at the input of the amp. Turn the amp on. Keep dialing the potentiometer wiper until the measured voltage at the input is HALF of what you started with (so if you started with a source that is 1V peak, adjust the pot so that the voltage at the input of the amp is .5V peak). Once this is done, turn off everything, and then measure the resistance of the potentiometer. The resistance of the pot will be equal to the input resistance of your amp.

In order for this method to work, you have to make sure your pot's maximum resistance is greater than the input resistance of your amp.
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Old 27th January 2006, 12:27 AM   #4
poobah is offline poobah  United States
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rtarbell,

great minds.........................................
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Old 27th January 2006, 02:49 AM   #5
Wizard of Kelts
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Quote:
Put a 50 - 100 K rheostat (pot) in series with your sound card and the amp (pos lead)
Any reason we can't use the ground lead?

Reason I ask is that the easiest way to do this would seem to be to simply shred an inexpensive 1/8" to RCA cable, and it might be easier to use the ground lead than the "hot" lead.

Would it make a difference?
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Old 27th January 2006, 12:24 PM   #6
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Quote:
Originally posted by kelticwizard

Any reason we can't use the ground lead?
Greetings from Norfolk

The only problems likely are

1. Possible bypassing of pot due to both source and amp input being commoned via 'earth' connection elsewhere

2. Possible noise pickup, in particular 60 Hz, (50 Hz in UK) which could mask or spoil measurement.

Good earthing /0 V practice should always be followed, even if it means making up special leads !
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Old 27th January 2006, 03:05 PM   #7
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Gandalph:

And if splice the potentiometer into the positive wire instead of the negative wire of the cable, these concerns become minimized?
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Old 27th January 2006, 03:14 PM   #8
poobah is offline poobah  United States
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Keltic,

Right... you don't want ground separation involved here... You could measure more stray, loop induced, 60 Hz than signal. These are single ended signals. These assume (no matter how incorrectly) that ground is ground is zero volts.

Do this trick in the positive lead and you'll be good to go.



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Old 27th January 2006, 10:39 PM   #9
Wizard of Kelts
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Thank you all, Poobah, rtarbell and Gandalph. I have the info I need to do this now.
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Old 28th January 2006, 08:42 AM   #10
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Default Almost the same idea, with different words Poobah is rigth

This method works fine.

And you all are rigth, and singing the same song, each one with your personal characteristics.

regards,

Carlos
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