A very obvious impedance/output question

[Chienmortbb]

11 minutes ago, bremen said:

Not quite. Few amps are ideal voltage sources, there's generally a bit of sag.

It’s true that at the extreme, the voltage may drop a little but in a well designed power supply, it will only be at very high power, close to the rated maximum. 
 

Sag is really used to describe the phenomenon of  older valve/tube designs that used thermionic diode rectifiers. In this case the voltage drops considerably rather than a small percentage. It was once thought to be an intrinsic part of “tube/valve sound”.

Edited June 25 by Chienmortbb

[bremen]

OK we could go on forever as a "well designed power supply" is a matter of degree and isn't necessarily a feature of many bass amps, mostly they're just found in high end class d power amps.

 

Point is though, anyone expecting more than a marginal increase in loudness by choosing 4 over 8 is going to be disappointed. Confirmation bias aside, and of course that's always worth a few dB!

[Phil Starr]

Like many of such questions the real answer is that it depends on which speakers you are talking about.

 

In the real world you'll see many amps rated at 300W into 8ohms and 500W into 4ohms. So the power supply in those amps is clearly unable to supply 600W. However that is at full power, at lower volumes (very roughly up to 75% of full power) you will get double the power. Another consideration is that the 4ohm speaker will be carrying more current and the speakers will be running hotter, at high powers the resistance in the coil will rise and the output will fall as a result of thermal compression.

 

The real stumbling block to a yes/no answer though is that the two speakers will have different voice coils and so the speakers won't be identical as a result. It would depend upon how the lower impedance was achieved.

 

I wonder why the OP is asking though, they say "if I had" so is this a purchase they are considering or just a theoretical idea?

[Marky L]

1 minute ago, Phil Starr said:

Like many of such questions the real answer is that it depends on which speakers you are talking about.

 

In the real world you'll see many amps rated at 300W into 8ohms and 500W into 4ohms. So the power supply in those amps is clearly unable to supply 600W. However that is at full power, at lower volumes (very roughly up to 75% of full power) you will get double the power. Another consideration is that the 4ohm speaker will be carrying more current and the speakers will be running hotter, at high powers the resistance in the coil will rise and the output will fall as a result of thermal compression.

 

The real stumbling block to a yes/no answer though is that the two speakers will have different voice coils and so the speakers won't be identical as a result. It would depend upon how the lower impedance was achieved.

 

I wonder why the OP is asking though, they say "if I had" so is this a purchase they are considering or just a theoretical idea?

 

Thank you. 

It's a theoretical question that may lead to a purchase at some time, or maybe not. I was mulling over the idea of getting a 4ohm 4x10 to use instead of the 8ohm 4x10 I have. I was pondering if it would leave me a little more headroom and not be pushing the amp as hard.

[Chienmortbb]

Just to add that the reason the amps cannot reach double the output is that they are current limited. As @agedhorse has described, SS amps are constant voltage, once the maximum current is reached it cannot supply any more current/power. It is more expensive to build a bigger powers supply. At 4 ohms, you are trying to draw twice the current.

[pete.young]

42 minutes ago, Marky L said:

 

Thank you. 

It's a theoretical question that may lead to a purchase at some time, or maybe not. I was mulling over the idea of getting a 4ohm 4x10 to use instead of the 8ohm 4x10 I have. I was pondering if it would leave me a little more headroom and not be pushing the amp as hard.

If you have the vehicle to transport 2 cabs, it would make more sense to me to get another 8Ohm 4x10 and run them both together.

[Marky L]

8 minutes ago, pete.young said:

If you have the vehicle to transport 2 cabs, it would make more sense to me to get another 8Ohm 4x10 and run them both together.

 

It is an option but I was thinking of easy set up and lugging. I do have a 2x10 which works nicely with it so I'll likely stick with that as an option. 

 

Normally I run a 1x15 with the 2x10 on top and I do get on well with that. I did try the 8 ohm 4x10 on it's own at a gig this weekend and I had to push the output another 3 clicks round before I was happy(ish) but I do wonder if that may also be because I didn't have speakers closer to my ear level and so I wasn't directly hearing what I was expecting.

[Phil Starr]

2 hours ago, Marky L said:

 

Thank you. 

It's a theoretical question that may lead to a purchase at some time, or maybe not. I was mulling over the idea of getting a 4ohm 4x10 to use instead of the 8ohm 4x10 I have. I was pondering if it would leave me a little more headroom and not be pushing the amp as hard.

I think the answer is not. Firstly not all 4x10's are the same and there will be a lot of overlap with some 8ohm units being louder than some 4's, you'd have to try them next to each other to be sure. The increase in power is not going to give you much more sound. Going from 300W to 500W will only give you an extra 2db where 1db is the smallest change noticeable.

 

I think your comment about ear level is a good insight into what is going on. 4x10's are usually very loud for the audience but are notoriously directional, like a torch beam, with your ears a long way from the 'spotlight' everyone else is hearing your sound better than you. If you like the sound of your 2x10 then a second identical one stacked vertically will significantly increase your sound levels and the top speaker will be pointing at your ears, not the back of your legs

[Downunderwonder]

18 hours ago, Marky L said:

I was pondering if it would leave me a little more headroom and not be pushing the amp as hard.

The cab with the 8 ohm impedance could easily be much more sensitive. In that case you would have to use more amp power to get the same volume from the 4 ohm cab even though the knob position said otherwise.

 

I once plugged in an old 15 and a 12 of the modern uber variety, both 8 ohm. Couldn't hear a peep out of the uber cab because of the sensitivity deficit.

[agedhorse]

On 25/06/2024 at 02:38, bremen said:

Not quite. Few amps are ideal voltage sources, there's generally a bit of sag.

In general, most amps, even some tube amps, the sag is almost zero until reaching maximum power. The original question was regarding doubling the power in which case +3dB minus any power compression of the driver is the correct answer.

 

 

On 25/06/2024 at 04:50, bremen said:

OK we could go on forever as a "well designed power supply" is a matter of degree and isn't necessarily a feature of many bass amps, mostly they're just found in high end class d power amps.

 

Point is though, anyone expecting more than a marginal increase in loudness by choosing 4 over 8 is going to be disappointed. Confirmation bias aside, and of course that's always worth a few dB!

A well designed power supply is a big part of any well designed amp isn't it? 

 

It is likely that a 4 ohm driver (not a 4 ohm speaker made up of 8 or 16 ohm drivers) is likely to have a lower sensitivity. This would be a more significant factor in comparative volume, but I did make the clear assumption that all else being equal, therefore +3dB is still the correct answer.

 

On 25/06/2024 at 06:36, Phil Starr said:

Like many of such questions the real answer is that it depends on which speakers you are talking about.

 

In the real world you'll see many amps rated at 300W into 8ohms and 500W into 4ohms. So the power supply in those amps is clearly unable to supply 600W. However that is at full power, at lower volumes (very roughly up to 75% of full power) you will get double the power. Another consideration is that the 4ohm speaker will be carrying more current and the speakers will be running hotter, at high powers the resistance in the coil will rise and the output will fall as a result of thermal compression.

 

The real stumbling block to a yes/no answer though is that the two speakers will have different voice coils and so the speakers won't be identical as a result. It would depend upon how the lower impedance was achieved.

 

I wonder why the OP is asking though, they say "if I had" so is this a purchase they are considering or just a theoretical idea?

Correct, hence my qualification that all other parameters were assumed to be identical.