Friday, March 24, 2023

Attempt #1: How Not To Measure an Audio Amplifier's Gain

I want to measure the gain of the audio amplifier that powers my  Elenco AM/FM Radio Kit build. The instruction manual gives straightforward directions for doing this. Unfortunately, I don't own an audio generator, which the instructions call for, so I can't follow them.

However, I figured could McGyver my way through this process by using tools I have on hand. Gain is the output voltage of the audio amplifier divided by the input voltage. I'm thinking this is simple: I'll feed in an audio signal from my cell phone into the radio and then use my multimeter to measure the input and output voltage. A bit of basic arithmetic would then tell me what the gain was. Easy peasy.

Wrong Way #1

To feed in an input signal, I downloaded the Frequency Generator App, and used the setup described previously to connect my phone to the radio.

Next, I measured the output voltage:

The meter read: 4.51 volts. Fair enough. Now's it was time to measure the input voltage:

And my meter told me: 89.2kΩ. Wait, what? I'm no electronics expert but even I know that's not right. What the heck?

It took me a few days to figure out what's going on here, but in hindsight, it's obvious. At the start of this project I picked up a TESMEN TM-510 multimeter. I knew I'd need a meter and figured a basic one would do the trick. It was cheap, compact and even automatically selected the measuring mode so I couldn't screw that up. I figured any low cost multimeter would get the job done. Apparently, I was wrong.

The meter's automatic mode selection must work by first looking for a voltage. If one is found, then it must guess that I want to measure voltage. If no voltage is detected, then it must mean that I want to measure resistance.

In this case, when I try to measure the input voltage, the meter must not be detecting enough voltage to convince it to properly measure the right value. Therefore, it falls back to resistance and tells me some nonsense value. Looking at the specs of the meter I see it has a minimum voltage of 0.8 volts. If my output voltage is 4.5 volts, and the gain of the amplifier is 150, then the input voltage should be around 0.03. That's way below the minimum voltage this meter can detect.

It never occurred to me to consider this bit of fine print. I'm not sure what role this meter is optimized for, but it's probably not designed for hobby electronics where one may deal with small voltages. That's my guess anyway.

I've since ordered a KAIWEETS HT118A, which is also relatively low costs, and generally receives a good bit of praise on YouTube. This is probably the meter I should have bought in the first place.

The mystery of my audio amplifier's gain is going to have to wait for another day. But the lesson on respecting fine print has been learned today.

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