measuring input impedance?

[lindstrom]

i got told to measure the input impedance of my amp and i got told i need to put a sine wave in and measure the current in series with the generator and then divided it by the voltage of the signal going in, BUT! my ammeter is going up and down because its a sine wave so it impossible to get a proper reading.
does anyone one know another way? cheers.

[brett]

Hi
is it a solid state or a valve amp?
Do you have a schematic?

The sine wave should be at high frequency (say 1000Hz), making the voltage and current readings smooth on most DMMs.

[lindstrom]

nah its a simple transistor push pull circuit. yeah with an input of 1KHz my ammeter goes up and down.

[R.G.]

That's one way. A simpler way is to set the generator to a convenient voltage in and measure the AC voltage coming out. You can set the frequency to a convenient frequency so your AC-setting voltmeter measures it steadly, 300-800Hz is usually good. Now insert a resistance in series with the input and read the voltage on the output. A resistance in series with the input equal to the input resistance of the amp will make the output voltage be half of what it was with no resistance in series with the input. You can also use a pot set up as a variable resistor and dial in half the voltage on the output, then measure the resistance.