How To Test For Input Impedance?

[kelticwizard]

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.

[jcx]

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

[rpapps]

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

[AKN]

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.

[AKN]

rpapps

I am beaten, one must be fast here

[rpapps]

Great minds think alike

[kelticwizard]

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.

[AndrewT]

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?

[MikeB]

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

[AndrewT]

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?