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.
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.
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.
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.
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.
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.
kelticwizard said:
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 !
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.
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.
All this cutting and splicing and tracing of leads and stuff is confusing to a simple fellow like me. Might I suggest sticking a .1uF cap in parallel with the input of the amp with alligator clips, running a frequency sweep and seeing where the response falls off. From there you calculate the Zin and stick in the real value. The test cap need not be of audiophile pedigree--use a humble polyester. The tighter the tolerance, the better.
Grey
Grey
Hi Grey,
You need to know the output impedance of your generator for that to work accurately.
Measure the open circuit voltage from the generator/ sound card. Increase the resistance until the signal is 1/2 what it was, measure at the input terminal of the amp. Measure the series resistance. That's what it is.
Works every time.
-Chris
You need to know the output impedance of your generator for that to work accurately.
Measure the open circuit voltage from the generator/ sound card. Increase the resistance until the signal is 1/2 what it was, measure at the input terminal of the amp. Measure the series resistance. That's what it is.
Works every time.
-Chris
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