I would like a simple setup for testing the input impedance of a solid state amp. It does not have to be perfect, a figure within 10% or so would be fine.
Actually, what I am really interested in at the moment is finding the input impedance of soundcard, SB 16, for which I have misplaced documentation. I believe it was 10K ohms, but I would like to check it.
I figured the method will be the same. Also, since I am interested in passive line level filters put between preamps and power amps, the best thing would be to learn how to measure the input impedance for the power amplifier, and apply it to the sound card.
I have a digital multimeter, an analog multimeter, and an Elenco function generator.
Any assistance would be greatly appreciated.
Actually, what I am really interested in at the moment is finding the input impedance of soundcard, SB 16, for which I have misplaced documentation. I believe it was 10K ohms, but I would like to check it.
I figured the method will be the same. Also, since I am interested in passive line level filters put between preamps and power amps, the best thing would be to learn how to measure the input impedance for the power amplifier, and apply it to the sound card.
I have a digital multimeter, an analog multimeter, and an Elenco function generator.
Any assistance would be greatly appreciated.
soundcard should be enough to do it
make up 1 or 2 series R of 10K or 100K with rca connectors and use RMAA in loopback test mode to measure with no R and your test R in the loopback cable
you need to do some dB and R divider math but the 1 KHz (or wherever its flat in fr) differences should let you figure the impedance
make up 1 or 2 series R of 10K or 100K with rca connectors and use RMAA in loopback test mode to measure with no R and your test R in the loopback cable
you need to do some dB and R divider math but the 1 KHz (or wherever its flat in fr) differences should let you figure the impedance
Even easier - use a variable resistor (potentiometer with only centre and one outside leg connected), adjust until level is reduced to half, measure resistance = input impedance.
The variable resistor and the soundcard input impedance form a voltage divider. When the voltage at the soundcard input is half what the generator is supplying, the two values are equal.
Cheers
Rob
The variable resistor and the soundcard input impedance form a voltage divider. When the voltage at the soundcard input is half what the generator is supplying, the two values are equal.
Cheers
Rob
Use your funktion generator setup for sinus ~100Hz 1V and a multimeter that have good AC low voltage reading and a potentiometer of lets say 100KOhm.
Connect generator to input of soundcard with potentiometer in series (use only runner and one outer end). Adjust potentiometer until half voltage is over potentiometer an half over input to soundcard. Disconnect potentiometer and measure its value, reading will be same as your soundcards input impedance.
Connect generator to input of soundcard with potentiometer in series (use only runner and one outer end). Adjust potentiometer until half voltage is over potentiometer an half over input to soundcard. Disconnect potentiometer and measure its value, reading will be same as your soundcards input impedance.
Thank you jcx, rpapps and 4fun.
I shall use the potentiometer method, even though I have RMAA, both because it seems so simple, and also because years ago I saw it in an old electronics book and the method appealed to me as being so quick and fun.
Thank you all. 🙂
I shall use the potentiometer method, even though I have RMAA, both because it seems so simple, and also because years ago I saw it in an old electronics book and the method appealed to me as being so quick and fun.
Thank you all. 🙂
Hi,
so the method has given us the resistive impedance. Thank you all.
Is there an equally simple method for estimating the reactive impedance of the unknown input stage?
so the method has given us the resistive impedance. Thank you all.
Is there an equally simple method for estimating the reactive impedance of the unknown input stage?
The reactive impedance should show up when measuring frequency response with the series resistor connected ? In theory the frequency response will be no longer flat if input impedance behaves reactive.
Mike
Mike
Hi Mike or others,
if the input is loaded with a series resistor equal to the resistive input impedance and the frequency reponse of the overall stage changes from the unloaded condition, then what formula/method estimates the reactive component?
For the sake of an example let's take a 20K resistor and 20k input impedance, the high frequency roll-of changes from -1db @ 50kHz to -1db @ 40kHz. What conclusion can be drawn? How do we get there? Or does the response go the other way?
if the input is loaded with a series resistor equal to the resistive input impedance and the frequency reponse of the overall stage changes from the unloaded condition, then what formula/method estimates the reactive component?
For the sake of an example let's take a 20K resistor and 20k input impedance, the high frequency roll-of changes from -1db @ 50kHz to -1db @ 40kHz. What conclusion can be drawn? How do we get there? Or does the response go the other way?
I think this method measures input impeadance at the selected frequency, no matter if it is reactive or not.
The result is just the resistive equivavlent of the impedance at the signal frequency.
The result is just the resistive equivavlent of the impedance at the signal frequency.
kubeek said:
Agree!
My choice of a rather low frequence of 100Hz was because of the instruments he had in hand, but still well inside audio passband. I think standard frequency to measure impedance is 1kHz.
Hi,
assuming the actual input impedance is, for example 20k//1200pF.
Then putting a further 20k series test resistor in place will create the measuring system for checking the input conditions.
When one measures the normal frequency response, it is normal to load the input with a low source impedance, often in the range 50r to 600r.
If the frequency response is measured with the test resistor in place then the filter formed by the high source impedance (resistance) and the // capacitor will roll off some treble.
I think this is what Mikeb is referring to
What would be nice is if some expert in these things could tell us how to do the reactive part of the tesing and calculate the reactive impedance.
It may be possible to avoid doing the full frequency repsonse analysis and just measure the input conditions to find the reactance present on the input.
As I said earlier, give us a simple method.
assuming the actual input impedance is, for example 20k//1200pF.
Then putting a further 20k series test resistor in place will create the measuring system for checking the input conditions.
When one measures the normal frequency response, it is normal to load the input with a low source impedance, often in the range 50r to 600r.
If the frequency response is measured with the test resistor in place then the filter formed by the high source impedance (resistance) and the // capacitor will roll off some treble.
I think this is what Mikeb is referring to
and having had my attention drawn to the possibility, I have had a bit of time to realise he is probably (certainly) right.
What would be nice is if some expert in these things could tell us how to do the reactive part of the tesing and calculate the reactive impedance.
It may be possible to avoid doing the full frequency repsonse analysis and just measure the input conditions to find the reactance present on the input.
As I said earlier, give us a simple method.
Doing the series potmeter test with a low freq gives you the resistive component as described.
Then replace the series pot with a largish resistor (say 1M to simulate current drive.
Then check the upper 3dB roll-off freq. That is the freq where the impedance of the input capacitance equeals the resistor found initially (20k in the example). So, C = 1/(2. pie. 20k . f)
Jan Didden
Then replace the series pot with a largish resistor (say 1M to simulate current drive.
Then check the upper 3dB roll-off freq. That is the freq where the impedance of the input capacitance equeals the resistor found initially (20k in the example). So, C = 1/(2. pie. 20k . f)
Jan Didden
>What would be nice is if some expert in these things could tell us how to do the reactive part of the tesing and calculate the reactive impedance.
Hint:
Measure the phase angle and then calculare 🙂
Hint:
Measure the phase angle and then calculare 🙂
Okay, here's another wrinkle.
I've got the Elenco tone generator up in my work area, but the cables and power cord for it are down the basement tangled up with about a gazillion other cords and cables. 😀
I wonder if I can use an online tone generator run through the output of my soundcard, into a walkman to walkman 1/8" stereo cord plugged into the line level input, with the potentiometer spliced into either the positive or negative lead.
David Taylor's Tone Generator is the one I had in mind.
www.satsignal.net => Audio Tools
I've got the Elenco tone generator up in my work area, but the cables and power cord for it are down the basement tangled up with about a gazillion other cords and cables. 😀
I wonder if I can use an online tone generator run through the output of my soundcard, into a walkman to walkman 1/8" stereo cord plugged into the line level input, with the potentiometer spliced into either the positive or negative lead.
David Taylor's Tone Generator is the one I had in mind.
www.satsignal.net => Audio Tools
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