Ohms - How do they work?

[jmattbassplaya]

I was curious if anyone could explain how ohms work and their role in amplification. The biggest question I have is if they play any role in how loud a cab can get. For example, say we have two identical cabs and amps but one is 4ohm and the other is 8ohm. Would the 4ohm cab be louder?

Thanks!

 

[Robroy]

I'd recommend googling it. You'll get everything you need.

 

[Michael B]

You have to match the cab ohms to the amp. The rating of watts on the cab and amp tell you what is bigger wattagewise.

Generally here is an example using my Fender 200 combo.

Running it stock it is using the one 8 ohm speaker in the combo. It runs at 140 watts.

It accepts an 8 ohm extension. Adding the 8 ohm extension means the amp is using 2 8 ohm speakers in parallel (one in the combo and the one in extension cab). When the head has 4 ohms it runs at 200 watts.

The most important thing for knowing what an amps volume range is is look at wattage rating for amp. And connect inly supported speakers to it.

You need to know about series and parallel. Getting this wrong can ruin an amp.

Don't connect a 4 ohm speaker setup to an 8 ohm connection or vice versa.

 

[jmattbassplaya]

You have to match the cab ohms to the amp. The rating of watts on the cab and amp tell you what is bigger wattagewise.

Generally here is an example using my Fender 200 combo.

Running it stock it is using the one 8 ohm speaker in the combo. It runs at 140 watts.

It accepts an 8 ohm extension. Adding the 8 ohm extension means the amp is using 2 8 ohm speakers in parallel (one in the combo and the one in extension cab). When the head has 4 ohms it runs at 200 watts.

The most important thing for knowing what an amps volume range is is look at wattage rating for amp. And connect inly supported speakers to it.

You need to know about series and parallel. Getting this wrong can ruin an amp.

Don't connect a 4 ohm speaker setup to an 8 ohm connection etc.

Thanks for the explanation this part I actually do already understand. I guess my main question is regarding the question of loudness between a 4 and 8ohm cab.

 

[Michael B]

Thanks for the explanation this part I actually do already understand. I guess my main question is regarding the question of loudness between a 4 and 8ohm cab.

They are setup as 4 or 8 (or 2) to allow different configurations and combos of speakers.

So to sort of answer your question my head in my combo uses 200 watts when run on 4 as opposed to 140 watts on 8. This is by design and also a function of ohms law.

 

[Robroy]

Enzo's post here:
Invalid Link Removed

 

[two fingers]

The short answer to your question is this.

Yes, the 4 ohm cab will be louder all else being equal.......but not by much.

Here's the gamble you make when getting one cab that matches your amp's minimum ohm rating. You let go of any flexibility and ability to add another cab in parallel (which is by far the most common way to run cabs).

Here's my advice, which is worth exactly what you payed me for it.

Find the head you like. Then find a cab you like. If your head is 4 ohms minimum, get two 8 ohm cabs exactly alike. Why? The difference in volume between a single 4 ohm cab and a single 8 ohm cab will be small..... almost negligible. But the difference between a single 8 ohm cab and TWO 8 ohm cabs will be huge. Because you are not only using the amp to its fullest wattage potential, but you are increasing speaker cone area by a factor of two. That will move lots more air.....which equals lots more volume.

So, then you can grab one cab for your smaller gigs, and both when you need to shake the drywall screws out of the walls.

You can leave one at home and one in the rehearsal spot so you only transport your head.

It's the most versatile setup.

Hope that helps.

 

[Bass_Pounder]

Simple answer:

The volume difference between two otherwise identical cabs (only difference is one is 8 and the other 4 ohms) will be barely detectable.

Increasing speaker area trumps additional wattage every time.

 

[Crater]

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.

 

[Michael B]

^ best answer if it you follow the technical explanation.

^^ great explanation of why you may want to move more air.

^^^ explains why some amps are rated differently at different loads.

 

[bobyoung53]

I was curious if anyone could explain how ohms work and their role in amplification. The biggest question I have is if they play any role in how loud a cab can get. For example, say we have two identical cabs and amps but one is 4ohm and the other is 8ohm. Would the 4ohm cab be louder?

Thanks!

With a SS head the 4 ohm cab will be louder, the amp will put out more watts with a 4 ohm vs an 8 ohm. With a tube amp you need to match the output with the speaker, most tube amps have a switch on the back for that, old SVT's had a different arrangement. With a tube head if it's mismatched it doesn't get any louder it just makes the amp work harder although it's better to go too low than too high, with a SS head it's the opposite. Tube amps have output transformers with taps for usually 4 and 8 ohm outputs, An SVT has 4 and 2 ohm outputs. With a SS head just make sure it's rated for 4 ohms and virtually any fairly new SS head will be.
Ohms are a measure of resistance to current, the lower the ohms (impedance) the more current can flow, so if you use a cab with too low of an impedance for an SS amp it will be trying to work too hard, if you use a higher impedance the amp will loaf and not put out as much power. You can run a SS head with a higher impedance speaker than it's rated for but not a lower impedance. Again tube heads should always be matched, they won't put out any more power with too low of an impedance but will put out less with too high impedance. A short circuit is 0 ohms for ex. and an open circuit is infinite ohms. Feel free to correct me if this is confusing or misleading because it's been many years since i studied this.

 

[ArtechnikA]

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

 

[jmattbassplaya]

So to add one more piece to the puzzle, I ask because I can get a good deal on an 8ohm cab. My Aguilar DB751 can work with that just fine, but I'm considering getting an SVT. I was curious if I would be better off going for the 4ohm model over the 8 due to the SVT only being rated at either 4 or 2ohms.

 

[Michael B]

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

That is the wrong question.

Is an 8 ton load heavier than a 4 ton load?

 

[bobyoung53]

So to add one more piece to the puzzle, I ask because I can get a good deal on an 8ohm cab. My Aguilar DB751 can work with that just fine, but I'm considering getting an SVT. I was curious if I would be better off going for the 4ohm model over the 8 due to the SVT only being rated at either 4 or 2ohms.

I would definitely get the 4, SVT's do not like 8 ohms, ask me how i know, haha!

 

[Jules Meuffels]

If you play large venues 4Ω is the best, it draws the most current from your amp and is therefore louder. However, as Twofingers mentioned, best option is to have 2x 8Ω cabs. For many reasons.
-You can indeed choose between 1 and 2 cab setup according to venue. (full stack 2x8Ω=4Ω)
-Transporting SVT is a B!@#$, unless you have roadies. (assuming you want a 8x10 or so)
-...etc.

 

[jmattbassplaya]

I've schlepped an SVT before for a couple years. It never really bothered me all that much. But it's good to know that a 4ohm cab might be better for that

 

[MJ_Sotti]

thing is re: SVTs, 4ohms. there's a 2 ohm selector on back for 2 4ohm cabinets. never ran a single 8 on it, some guys here will say it's ok...lots won't. I never have.

but it sounds like you know this...at least 2 8s

it does allow the amps to run wild.

 

[Downunderwonder]

If you play large venues 4Ω is the best, it draws the most current from your amp and is therefore louder. However, as Twofingers mentioned, best option is to have 2x 8Ω cabs. For many reasons.
-You can indeed choose between 1 and 2 cab setup according to venue. (full stack 2x8Ω=4Ω)
-Transporting SVT is a B!@#$, unless you have roadies. (assuming you want a 8x10 or so)
-...etc.

did you not read the other replies? Getting all the power out of the amp is very seldom necessary these days. In fact it is more likely to overpower the single 4 ohm cab.

In the absence of any other criteria the SVT situation dictates best to get a 4 ohm cab.

 

[friendlybass]

This can be a very complicated question. Simple answer, read the wicked long sticky on the amp forum page and do a quick couple of searches. Its explained in great detail.
Basically 4 ohms yields some more power, and you work the amp a little harder. With solid state amps this means very little. With tube this can be a rabbit hole of tone conversations.