wattage and multiple amps

[mynameisjonas]

completely random question: will the output level of two 15W amps plugged into the same 4x12 cab (stereo, 2 speakers each) be the same as plugging in a 30W amp into the same cab (mono, all 4 speakers)?

[chrisjedijane]

I reckon that the 30W using all 4 will be louder as the wattage is higher, and there's more air being pushed by those four speakers. There will definitely be an increase in apparent volume. I guess the only way to find out is to try it

[mynameisjonas]

yeah i guess so

[the older brother]

Jonas McFly

[1946dodge]

The current in each speaker is the same for each configuration. About 1.3693 amps,  if all speakers are connected in parallel and are 4 ohms each.
I think you would get the same loudness for each configuration.

Watts = current squared times resistance. For two 4 ohm speakers in parallel you have 2 ohms equivalent so with 15 watts you get current out of the amplifier  of square root of 15/2 or square root of 7.5 . Half of this current goes into each speaker so this is 1.3693 amps each.
For 30 watts into four 4 ohm speakers in parallel you have 1 ohm equivalent resistance so with 30 watts the current  out of the amplifier is square root of 30. Each speaker gets one fourth of this current which is 1.3693 each.

So in each configuration each speaker gets the same current.

The voltages are the wattage divided by the current out of the amp. The 15 watt configuration gives voltage of square root of 30 and so does the 30 watt configuration.
Each speaker gets 7.5 watts.

So the loudness is the same in each configuration.

***************************************************************
For a 100 Watt amp into four 4 ohm speakers you get 10 volts and 10 amps out of the amp and 2.5 amps per speaker and each speaker uses 25 watts. I guess that must be why they use at least 25 watt speakers in cabinets.

I never went through this calculation before and it was helpful. Thanks for posting this question.

[soundhack]

btw - make sure the two amps are in phase... otherwise you could have some bass cancellation.

[1946dodge]

btw - make sure the two amps are in phase... otherwise you could have some bass cancellation.

I have one of those BBE Sonic Stomp boxes that claim to correct the phase of the signal all across the band. I wonder if you could put that in before the stereo box that feeds the  2 amps and correct the phase of the two amps.
I dont know how the Sonic Stomp works so I dont know if it would.
Anybody?

[soundhack]

that should work (it should be a simple signal inverter), but i think you could also correct the phase by inverting the speaker leads from one of the amps? (correct me if i'm wrong here)

[1946dodge]

that should work (it should be a simple signal inverter), but i think you could also correct the phase by inverting the speaker leads from one of the amps? (correct me if i'm wrong here)

I honestly dont know, since I dont know how exactly a signal inverter would correct the phase. If each frequency in the band has its own phase delay, then wouldnt they try to correct them all so that the phase across the band is the same? How would a signal inverter do that?
I may not really understand the problem.
Of course when I did the calculation above I assumed the two amps signals were in phase. If you put a phase shifter on one of them you could tune it  until the two of them gave maximum output.

[soundhack]

in the studio (my day job), we have buttons for phase inversion on mic pres. this is usually just a signal inversion, and its used for two reasons:

1 - you put a mic on either side of a drum, so one mic feels pressure when the other feels suction

2 - you have electronics which invert the signal, and you need to correct that.

the reason for number two: in audio electronics you typically have a number of amplifier stages, and each amplifier stage typically inverts the signal. now if you have an odd number of stages, you have an inverted signal. (most good electronics has a even number of inverting stages).

now i don't know why the signal inverter button is called the phase, except that it what it does looks similar to a 180 degree phase shift.

[blacktiger]

Back to the original question; it's going to depend on the individual amps which is louder.  Amps rarely put out exactly what the manufacturer claims, sometimes it's more and sometimes less.  Healthiness of the components, including the tubes, will effect output.  Also, isn't apparent volume effected by the frequency curve in some way? 

I don't think that you can know the real answer to this question unless and until you do an actual side by side comparison of the two set-ups.   

[1946dodge]

Ah Soundhack, you have lifted the fog from my eyes.

So the phase shift they correct is 180 degrees, which is a signal inversion.

I thought they tried to correct any phase shift like by varying the delay between the amp outputs until a maximum occurs.

I guess they only correct modulo 180 degrees.

I am too used to thinking in terms of phased array antennas where they try to correct each element's phase so that the resultant signal will add constructively at some angle you choose.