Reactive Load Question

[martin manning]

You can do as shown above, and use a multimeter to measure RMS voltage instead of a scope. For the series resistor, 22k is far too big. Since you are expecting impedance in the 10 to 200 ohm range, I would make that ~100 ohms.

[Cathode Ray]

You can do as shown above, and use a multimeter to measure RMS voltage instead of a scope. For the series resistor, 22k is far too big. Since you are expecting impedance in the 10 to 200 ohm range, I would make that ~100 ohms.

So, the only piece of gear needed that I do not have is a signal generator(function generator) ?

[martin manning]

Yes, and you could use a phone app for that.

[Cathode Ray]

Yes, and you could use a phone app for that.

Holy moley !

That's awesone!

So I'm d/ling the app now.

So I take an 3.5mm audio output off the phone's headphone jack adapted into 1/4" guitar cable => into the load box's input jack.

then the mulitmeter trick you mentioned measuring the output(with the resistor) ?

[Colossal]

You can do as shown above, and use a multimeter to measure RMS voltage instead of a scope. For the series resistor, 22k is far too big. Since you are expecting impedance in the 10 to 200 ohm range, I would make that ~100 ohms.

My apologies to OP and othwrs if that diagram caused confusion. That is the setup I've used to characterize guitar pickups where the DCRs are much higher. I should have scaled that resistor in the drawing.

[Cathode Ray]

You can do as shown above, and use a multimeter to measure RMS voltage instead of a scope. For the series resistor, 22k is far too big. Since you are expecting impedance in the 10 to 200 ohm range, I would make that ~100 ohms.

My apologies to OP and othwrs if that diagram caused confusion. That is the setup I've used to characterize guitar pickups where the DCRs are much higher. I should have scaled that resistor in the drawing.

No worries!

I appreciate all the advice!

I have the Function Generator app running on my iPhone. Just need to put together a cable that can go from stereo 3.5mm to mono 1/4" (guitar cable) to feed the signal into the load box.

[martin manning]

You can also use a Bluetooth speaker to drive the load if it has ou you can make an aux output.

[Cathode Ray]

You can also use a Bluetooth speaker to drive the load if it has ou you can make an aux output.

The only bluetooth speaker I have does not have an AUX output, so I still need to hobble together a cable.

May have one in a box of parts somewhere. Will dig it up/or make one this weekend and do this.

You guys rock !

[martin manning]

Note you can’t measure as in Collosal Dave’s diagram above using a multimeter because there will be some phase shift in the reactive load. Not to worry though, you can measure the voltage across the series resistor and calculate the current through it, and then use that current and the voltage across the load to calculate the impedance of the load.

[Cathode Ray]

Note you can’t measure as in Collosal Dave’s diagram above using a multimeter because there will be some phase shift in the reactive load. Not to worry though, you can measure the voltage across the series resistor and calculate the current through it, and then use that current and the voltage across the load to calculate the impedance of the load.

So I'm clear on this, can you show me on the schematic I posted - where i place this resistor and multi-meter leads ?

Thx.

[Aurora]

Basically, all you need to measure and generate an impedance plot, is a PC with soundcard ( laptop works OK) and free software, REW is one example... and some reading to do, of course, on "how to.."

[Cathode Ray]

Basically, all you need to measure and generate an impedance plot, is a PC with soundcard ( laptop works OK) and free software, REW is one example... and some reading to do, of course, on "how to.."

I had wondered about that sort of setup as well.

I've seen online stuff that can generate audio signals and (obviously) send them out via your soundcard.

It's how to hook into the circuit and perform a reading that I'm foggy on.

[martin manning]

Note you can’t measure as in Collosal Dave’s diagram above using a multimeter because there will be some phase shift in the reactive load. Not to worry though, you can measure the voltage across the series resistor and calculate the current through it, and then use that current and the voltage across the load to calculate the impedance of the load.

So I'm clear on this, can you show me on the schematic I posted - where i place this resistor and multi-meter leads ?

You’re going to be driving the load through a series resistor, say 100 ohms (located where the 22k is in the drawing Colossal posted). At a given frequency, measure AC volts across that resistor and calculate current I = V/R. Then measure AC voltage across the load and calculate impedance R = V/I.

[Cathode Ray]

You’re going to be driving the load through a series resistor, say 100 ohms (located where the 22k is in the drawing Colossal posted). At a given frequency, measure AC volts across that resistor and calculate current I = V/R. Then measure AC voltage across the load and calculate impedance R = V/I.

So then, the AC current I measure across that 100 Ohm resistor correlates to Hz ?

Then I can plot points on a grid, as I change frequency of the signal being input.

Those point can then be used to draw a curve revealing the resonant peak ?

When measuring AC volts across the load - this would be the reactive load itself (not the load resistor), and not involving the transformer driven line out ?

In other words, I could not take this reading at the line out tap where the signal comes out as I do when in normal use.

Basically it's this arrangement, but substitute the speaker for the reactive load box:

The frequency counter and oscilloscope are replaced by a multi-meter and some math.

Thx!

[martin manning]

Actually, I think you can avoid making any special hook up other than finding some way to get your phone app signal into the input of an amp, and plugging the amp’s output into the load box as usual. Use a clean setting, and adjust the volume to get a couple of volts AC at the output. You already have a series resistance in your load box, which is 16.5 ohms (the 2x33 ohms in parallel), which you can use to sense current. So, you can clip one multimeter probe to the load’s input (hot), and measure voltage at the far end of the 16.5 ohms and also at the load input ground with the other probe for each frequency. To find the resonant peak clip the meter leads across the 16.5 ohms and adjust the frequency to minimize the voltage reading. That will be the frequency at the impedance peak. Then measure the voltage across the load input at that frequency and calculate the peak impedance. I’d expect the peak to occur at about 80 Hz, with a value around 100 ohms.