Ohms - How do they work?

[Bassamatic]

The early posts are correct - You likely cannot hear the difference between a 4 and 8 Ohm cab that are the same efficiency. The 4 Ohm cab WILL be actually louder, but not enough to tell. The way our ear work, your AMP power has to be TEN TIMES greater to seem like it is twice as loud (perceived loudness). So don't worry about that. However, doubling the cone area WILL be twice as loud when you add a second cab.

If you get the 4 Ohm cab, and your amp is 4 Ohms minimum, you're done - you can't add any more cabs - another reason to get the 8.

The efficiency rating of the cab - How many dB it generates at 1W input at 1 Meter distance - lets you compare the loudness difference between cabs. The impedance is irrelevant here.

And - the amp is not working "harder" at 8 than at 4. That's just the way it works. Amp doesn't care as long as it is set for 8 (tubes). SS amps don't care at all.

 

[Fletz]

They clear the mind when doing yoga

 

[40Hz]

If you amp is rated for 4ohm and you run an 8ohm cab the amp is not working harder or hotter. Is actually working less hard and cooler. Less ohms means more current more heat more power. Amps are rated for a minimum load usually 4ohms . If you run higher ohms you'll just never get the full power rating . But in reality your never using the full power rating anyway unless you're running wide open on 11 .

Thx for the correction. After all these years I was bound to get some part of wgat I remember being told wrong.

 

[40Hz]

And - the amp is not working "harder" at 8 than at 4. That's just the way it works. Amp doesn't care as long as it is set for 8 (tubes). SS amps don't care at all.

People actually use something other than tube amps for bass? (kidding!)

 

[CL400Peavey]

Sounds like we agree on ohms but not the analogy.

The 4 vs 8 cylinder analogy works perfectly for this situation. You dont know how much power either motor is making, or any of the other particulars. You know one number, and that isnt enough to make a worthwhile guess.

The loudness of a speaker is more a factor of it's sensitivity rating. I.E. a 92db speaker (or speaker cabinet) will play significantly louder than one rated at 87db. Considering they both have the same ohm rating and with the amp set at the same volume level.

And that is only part of the equation. You need to know all the other particulars to judge a speaker's true output.

 

[monsterthompson]

However, as others have pointed out, if you use the same amp to power the cabs, the amp will produce less power into the 8-ohm cab than into the 4-ohm cab and, at the same level control on the amp, the 4-ohm cab will be somewhere around 2 or 3 dB louder.

If I'm not mistsken, this is not true of tube amps, but a characteristic of the commonly available solid state offerings on the market.

 

[HolmeBass]

Most SS and class-D amps can drive more power to a 4 ohm load than into an 8 ohm load. That's the reason for the "4 ohm cabs are louder" which is an over-simplification. All other things being equal, a 4 ohm cab and an 8 ohm cab will be equally loud when driven with the same number of watts. It's just that a 4-ohm cab allows you to use the full power of an SS amp, so more watts means more volume. It's because of the way SS amplifiers are made, not speaker cabs.

Power (watts) = volts x amps. In any amplifier, there are physical limits to both the max.voltage and max current (amps) that can be shoved out the speaker terminals. With an 8-ohm cab, a typical SS amp will run out of voltage (and start clipping) before it runs out of current. With a 4-ohm cab connected, the voltage limit is the same but a 4 ohm cab will pull more current and can use the 'full power' of an SS amps' output.

I when graduated to a 'grown ups' bass rig, I got a PF-500 and a 4 ohm 4x10 G-K cab. Yes, I know I can't add another cab and I'm OK with that. The 4x10 provides more than enough volume for my uses, and I never have to dial the master above 1 o'clock.

+1!!

Also I will add that twice the power gives almost +3 dB of output. If that is "barely noticeable" to you you are deaf! I am so very tired of this particular TB meme. The JND is sensibly right around 1 dB, not 3.

That doesn't change the truism that if you really want more volume, add more speakers, but it also denies the truth that sometimes just 3 more dB is actually enough.

 

[Craig Fones]

SS amps will typically increase Wattage output as it goes down in Ohms. How much a SS amp will increase in wattage for each drop in Ohms is a good indicator of an amps power reserve. Some high-end amplifiers will even double their wattage for each halving in Ohms. For example, I have a [Invalid or Expired Link Removed]each rated at 600 watts @ 8 Ohms, 1200 watts @ 4 Ohms, and 2400 watts @ 2 Ohms. But that doesn't necessarily equate to any increase or decrease in loudness from the speaker. However, it does maintain plenty of headroom. There are other variables that will determine that. Some amps can handle a change in Ohms while others cannot. Best to refer to the amp's manual for that answer.

 

[BioWeapon]

If I'm not mistsken, this is not true of tube amps, but a characteristic of the commonly available solid state offerings on the market.

That is correct, tube amplifiers would work the same way were it not for the output transformer.

 

[CharlyG]

Ohms don't work, they just are.

 

[Craig Fones]

↑
The loudness of a speaker is more a factor of it's sensitivity rating. I.E. a 92db speaker (or speaker cabinet) will play significantly louder than one rated at 87db. Considering they both have the same ohm rating and with the amp set at the same volume level.

And that is only part of the equation. You need to know all the other particulars to judge a speaker's true output.

True but with all else being equal, the sensitivity (efficiency) rating will give you an ideal of how much loudness a speaker will produce for one watt of power @ one foot away from your ear or mic. There are many other variables that will effect the overall loudness but the db spec will allow you to compare speaker to speaker or speaker cabinet to speaker cabinet.

 

[Craig Fones]

That is correct, tube amplifiers would work the same way were it not for the output transformer.

And for reasons I don't quite understand, tube amps seem to put out a lot more power than their wattage rating would suggest. As compared to SS amps.

 

[Michael B]

The 4 vs 8 cylinder analogy works perfectly for this situation. You dont know how much power either motor is making, or any of the other particulars. You know one number, and that isnt enough to make a worthwhile guess.



And that is only part of the equation. You need to know all the other particulars to judge a speaker's true output.

All 4 cylinder cars I've owned were dogs. Even the beat up old 8s drink their milkshakes. Sure you could make a 4 work better but that would be an exceptional case.

 

[BioWeapon]

And for reasons I don't quite understand, tube amps seem to put out a lot more power than their wattage rating would suggest. As compared to SS amps.

I have a theory for this but I'm not sure if it's right - and it has to do with distortion. Solid state amps aren't pushed into distortion seeing as that isn't such a pleasant sound. Meanwhile, tube amps can be pushed and create soft clipping, which sounds nice and is generally less noticeable. A clipped sine wave of the same amplitude as a normal sine wave carries more power. If I'm not mistaken, amps are given power ratings according to a percent of distortion in the output (ie 250 watts at 1% total harmonic distortion) - meaning that a tube amp can be pushed above it's wattage rating and still sound nice.
Anyways that's just what I think is going on.

Note that a watt is a watt, and that doesn't change with respect to output device type.

 

[The Chuck]

Is an 8-cylinder vehicle faster than a 4-cylinder vehicle ?

YES!

 

[Craig Fones]

Is an 8-cylinder vehicle faster than a 4-cylinder vehicle ?

NOPE !!!

 

[Jack Butler]

Is an 8-cylinder vehicle faster than a 4-cylinder vehicle ?

If each cylinder is the same displacement and all else being equal, yes. Is it twice as fast? No.

 

[Downunderwonder]

The early posts are correct - You likely cannot hear the difference between a 4 and 8 Ohm cab that are the same efficiency. The 4 Ohm cab WILL be actually louder, but not enough to tell. The way our ear work, your AMP power has to be TEN TIMES greater to seem like it is twice as loud (perceived loudness). So don't worry about that. However, doubling the cone area WILL be twice as loud when you add a second cab.

If you get the 4 Ohm cab, and your amp is 4 Ohms minimum, you're done - you can't add any more cabs - another reason to get the 8.

The efficiency rating of the cab - How many dB it generates at 1W input at 1 Meter distance - lets you compare the loudness difference between cabs. The impedance is irrelevant here.

And - the amp is not working "harder" at 8 than at 4. That's just the way it works. Amp doesn't care as long as it is set for 8 (tubes). SS amps don't care at all.

Still no.

Double loudness is defined as + 10dB on the dB scheme. It's like agreeing the sky is blue and gravity sucks. You don't get to redefine dB.

Doubling up on both cab and amp power nets potentially +6dB at the most.

An old SS gets closer to doubling power when halving impedance than modern class D in general. Tube amps put out same power to the various impedance taps.

Perfectly coupled cabs gain +3dB over one with the same power applied.

Depending on how much of your headroom is soaked up using one cab just playing along, a whole enchilada 6dB might be a pretty huge boost, or it could be a waste of carting the cab.

 

[Munjibunga]

If I'm not mistsken, this is not true of tube amps, but a characteristic of the commonly available solid state offerings on the market.

Correct. I was remiss in not mentioning that.

 

[Hand slap]

Some things to keep in mind. To increase you're volume by 3db it take doubling the watts.

Good point, IMHO, I like 8 ohm cabs, cause in theory you can add another 8 ohm cab, you have more options.