home audio: How much current flows through my speaker amplifier's output?

Question

In a recent project I used relays to electronically control a multi-room speaker selector. I wired these relays in parallel with the physical switches in the box.

When I was designing it, I wasn't sure how to analyze how much current could be conducted. The amplifier is rated for 2x 25W RMS, speaker impedance 8 ohms; thus the theoretical maximum current (given by P = I^2 x R) is sqrt(25 W / 8 ohm) = 1.77 A. In practice, when I attempted to measure AC current at high volume with a multimeter, I saw <100mA. I didn't have a scope at the time, but I imagine that because it's not a sine wave, it'd be better to use an oscilloscope and measure the voltage across a shunt.

I ended up using relays rated for 2A contact current and random ~24 AWG wire I had on hand. The project works very well, but I'd like to better understand what is actually going on.

1. Is the theoretical current above correct? Why is the measured current so much lower?
2. Each room has an "impedance matching volume control." Does this mean that they simply put a resistor in series with the speakers, so that when multiple channels are on, the amplifier doesn't see <8 ohm impedance? (I can't find much information about how these work.) That could explain the low current.
3. I'm frankly a bit surprised that a small amplifier like this can drive 5 rooms/10 speakers at a pretty high volume. The speakers and in-wall volume controls were professionally installed before we moved in. Is it likely that there is actually some powered amplification hidden in each room?

Answer 1

You have asked a lot of questions. I'll try to get all of them.

Your power calculation is correct. You will see about 1.77 Amps RMS current whey your amplifier is delivering its' rated power of 25 Watts into an 8 Ohm load.

1) The reason your measured power is so much lower is that most of the time, the amplifier is delivering only a fraction of its' rated power. In addition, your multimeter isn't responding fast enough to show the peaks. An oscilloscope monitoring voltage across a series shunt resistor will give you a better idea of the peak current.

2) Your speaker volume controls are most likely "L-Pads". This is a wire-wound potentiometer with two resistive elements that change simultaneously. They are configured to keep the impedance as seen by the input terminals (the amplifier) reasonably constant as the attenuation is changed. Wikipedia has a good description.

3) Speaker efficiency varies greatly. Some speakers can deliver significant volume levels with tiny amounts of input power, some speakers require large amounts of input power to achieve even modest volume levels. It all depends on the complete speaker design: drivers, cabinet, crossovers (if any).

It is not likely that there are hidden power amplifiers driving your speakers but is it possible.

Answer 2

The installer may have used Powered Speakers instead of passive speakers. If so, your amplifier power rating (watts per channel RMS) wouldn't matter that much. Example: if every 2-way powered speaker connected contained a 200-watt RMS amp driving the woofer and a 75-watt RMS amp driving the tweeter, odds are very high that your system will be loud in every room, no matter the watts per channel rating of your small amplifier.