Do your lights dim when your amp powers up?

I would add that the experiment of using a long extension cord can cause voltage drop, which in turn may increase current draw. 

Hey @whart - I'm afraid your understanding is a little inconsistent.  You are correct that voltage drop would occur due to the extension cord, but this would cause a decrease in current.  Let me work through the math with you. 

Let's assume a 300W at idle amplifier.  At 120VAC this is the equivalent of a 48 Ohm resistor which is drawing 2.5 Amps. 

Let's check the basic math: 

Watts = A x V , or Watts = AxAxR

so: 

300W = 2.5A x 120V

We also know that V / I = R

120 / 2.5 = 48 Ohms. 

So, summarizing: 

Wall voltage: 120V

Amp power : 300

Amp current: 2.5

Equivalent R : 48 Ohms. 

Putting this in terms of current: 

2.5A = 120V / 48 Ohms

So, everything is consistent no matter how you look at it.  What happens now when we add the resistance of an extension cord?  We should avoid using cheap lamp cord extensions except for lamps, but let's discuss a really long or really bad one which has a whopping 2 Ohms of resistance.  Honestly that's a lot even for a super cheap extension, but it makes our math easier. :) 

So now the total series resistance = 2 Ohms + 48 Ohms = 50 Ohms. 

As we showed above, A = V / Ohms, so 

120 / 50 = 2.4 Amps.  It's dropped, not increased! 

Now, in terms of "voltage drop" what we mean is that literally a voltage drops across a resistance due to current.  So, taking our really bad lamp cord: 

2.4A x 2 Ohms = 4.8V of drop! 

And here's the real problem of "voltage drop."  Instead of your amplifier getting 120V it sees: 

120 - 4.8 = 115.2 V ! 

It's the same if we calculate it using A and Ohms:

2.4 A x 48 Ohms (for the amp) = 115.2V. 

So, you were a little mistaken.  You were right about the voltage dropping but so does current.  What is true though is that on startup the amp won't finish charging for a longer period of time due to the reduced current flow. 

There is an exception!! If you used a voltage regulator, current would rise when voltages dropped, but what's happening then is the voltage regulator is a variable impedance, it's no longer a fixed R value like we modeled here. 

PS - I almost forgot one of the most important lessons which relates this all to the need for proper wire gauges in the walls. 

Our cheap extension cord now has the following electrical properties:

4.8V across it

2.4 Amps of current

Which also means: 

4.8V x 2.4A = 11.5 W !!!!!!

That is, our cord is warming up, and releasing heat all by itself. If it can’t dissipate the heat fast enough it will melt.  I’ve you’ve followed my previous post and this one you know fully understand why even cheap extension cables have a power rating and why it must be respected!

l would have thought all high quality well designed amps would have slow start up circuits. To avoid a switch on thump, and to protect the electronics. Even my 12 volt  DC phono amp has it. 

@erik_squires - you are right, I was thinking about how I tried to use a low gauge extension to an iso transformer to power my  tone arm air compressor and it blew 20 amp fuses on the transformer, given that those are motors. Thanks for correcting. Bill

If your lights dim you need either a higher 200+ amp service at your breaker box, 

And or a dedicated  20 amp  dedicated line for your amplifier or line conditioner .