Author Topic: Ohm questions  (Read 3958 times)

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Offline touchdown

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Ohm questions
« on: March 16, 2008, 11:37:41 am »
I played a show last night; my first with the new amp (Sunn Coliseum 300)

I ran it through a 4x10 (8 ohms) and a Sunn 1x18 (4 Ohms), which is a total impedance of 2.667 ohms if I am not mistaken.
Will this load make my amp run at 300 Watts (as it is supposed to at 2 ohms) or 200 (@ 4 ohms)? It definitely did not sound like I was pushing 300 watts last night. I have another 175 watt amp that sounds just as loud through this cab setup. Your thoughts?

Also, what negative consequences am I suffering by running through this cab setup? I've heard various things, but never gotten a concrete answer for what problems are caused by a 2.667 ohm load.

Should I consider getting a new 4x10? Or perhaps see if I can rewire it to 4 ohms?

Offline EdBass

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Re: Ohm questions
« Reply #1 on: March 16, 2008, 12:50:41 pm »
Your load should be safe for an amp capable of 2 ohm operation. It should produce between 200 and 300 watts at a 2.667 ohms. I personally don't know of any practical way to rewire an 8 ohm 410 to anything but 2, 8, or 32 ohms, and I'm sometimes referred to as "Mr. Impedance" around here.
If you had a 4 ohm 410, you would get a little more juice out of your amp, but IMO the best benefit of doing that would be that the 410 would be getting an equal amount of power as the 118, but I don't think it would really get much louder. You would probably get better mid and high end response, and depending on what sound you're trying for of course, it may have better presence in the mix.
Uhmm, was that your question?

Offline basiklybass

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Re: Ohm questions
« Reply #2 on: March 16, 2008, 02:56:50 pm »
When in doubt, always, always, always consider getting new gear.

However.......


Mr. Impedance knows his stuff, so......consider getting a new cabinet...but buy it only if you can really afford it 'cause you really won't hear much difference purely based on the impedance of the cabinet. Cabinet desgbn, speakers inside, etc all will have more effect....but always consider new gear. Half the fun is learning about this stuff, searching, trying, shopping...Oh My Gosh....did I say shopping? Yep, I can spend a few hours in GC without any trouble at all. As long as there are not rows of clothing.....or shoes.....or furniture....or my ex-wife....sorry.....that just slipped.

Offline touchdown

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Re: Ohm questions
« Reply #3 on: March 16, 2008, 06:48:45 pm »
yeah, i'll see if i can trade this one for another 4x10. hopefully by throwing in some cash i can get something decent.

Offline touchdown

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Re: Ohm questions
« Reply #4 on: May 07, 2008, 04:27:44 pm »
revisiting this topic because i am a little aggravated about the matter.

now, the amp sounds wonderful through the aforementioned setup, but it's just not as loud as i feel it should be (a 350W, 4 ohm amp running at about 5 on the master volume will overpower this setup easily).

i took the amp in to a tech and he looked it over. said there wasn't much wrong, fixed a few things, and said that there should be nothing inhibiting full power output.

will there be a notable volume increase if i get a 4 ohm 4x10 as opposed to the 8 ohm one i have now. I've been told that by running an 8 ohm cab and a 4 ohm cab together, the 4 ohm cabinet will be getting much more power (and thus will overpower) than the other. however, it certainly doesn't sound like this as i run them now. the 4x10 can keep up with the 1x18 very easily.

some advice would be great. I'd hate to have to scrap this setup and get a bunch of other stuff just to keep up with my other setup, because this rig does sound great! it's just not quite loud enough.

Offline Isaac

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Re: Ohm questions
« Reply #5 on: May 07, 2008, 05:32:33 pm »
The answer to your question is a lot more complicated that you'll probably like.

First, it's true that the 4 ohm cabinet will be drawing more power than the 8 ohm cabinet. And no, there's no practical way to rewire the 8 ohm cabinet for 4 ohms. But drawing more power doesn't necessarily mean it will be louder. To conmplicate things further, it might well be louder, in that it's putting out more acoustic energy, but not sound as loud due to the energy being in a part of the spectrum where our ears are not very sensitive. No way to tell without more information.

If the 4 ohm 4x10 is similar to the 8 ohm cabinet you presently have, then the difference in volume is not likely to be audible. However, a second 8 ohm 4x10 similar to the one you already have most likely would be.

Things affecting loudness include (but are not necessarily limited to) sensitivity, that is, how much sound the speaker puts out for a given input; Vmax, which is the maximum amount of air the speaker can move at full travel; power handling; the type of cabinet the drivers are installed in; power compression; the linearity of the magnetic field in the voice coil gap.

You say it's not loud enough. Not loud enough for what? "Not as loud as my other rig" isn't a very good answer, because we don't know how loud your other rig is, nor what you're using them for, so please be specific as to why you need so much volume. 250 watts or so into one 18" and a 4x10" should be quite enough for most applications, and if you need more, well, that's what sound reinforcement systems are for. Your situation may be different, though.

"A 350W, 4 ohm amp running at about 5" doesn't tell us much, either. It's quite possible that, even with the master on 5, that the 350W amp is already putting out all it has, and may be well past its rated output. What kind of amp is it, and what speakers is it running into? For that matter, what are the 1x18" and the 4x10"?

Based on the little bit of information I have, here's my guess: the 350W amp has a response curve that cuts off a lot of the low bass, leaving more power available for the low mids and mids, giving it more output in the range our ears are most sensitive to, so it sounds louder. The technical term for that is that it has a greater apparent volume. You can do something similar with the Coliseum: cut the low bass, and leave the other tone controls on 5. Then turn up the volume. It's apparent volume should be much greater than before. Once it's loud enough (assuming it gets there), you can add in some low bass until you like the sound or it starts to distort more than you like. Of course, this might not be necessary. You might like the sound you get with the low bass turned down. You can also adjust the tone using the other tone controls to make it more to your liking. However, IIRC, the Coliseum tone controls are set up like an equalizer, essentially a separate volume control for each frequency band - like a graphic equalizer, but with rotary controls. If that is so, then you'll likely get your maximum volume with the controls set flat, with the exception of the low bass. Low bass uses a lot of power.

One other thing, which could make everything else moot. Are your two cabinets the same polarity? If you put a 9V battery on the input to the cabinets, do they move the same direction? If not, they'll fight each other, kill the volume, especially in the lower frequencies.
Isaac

Offline touchdown

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Re: Ohm questions
« Reply #6 on: May 07, 2008, 06:22:00 pm »
here is a picture to illustrate my "setup"; hopefully this will help in figuring out just what is going on here, if anything.


The amps pictured are outdated. I run the cab on the left (a Peavey 2x18 + 2x10, rated 400 W @ 4 ohms) with an Ampeg SVT 350H. The Coliseum 300, not pictured, runs the stack on the right side. The 4x10 is a Crate BX410E rated 400W @ 8 ohms.

My primary concern here is that the Coliseum-powered stack can barely keep up with a 50W tube guitar amp, while the other stack can overpower a 100W tube guitar amp. My settings on both amps are more or less the same: Bassier ranges set right around 0 (that is, right in the middle), Mid ranges at +3 or 4, highs at +1 or so.  I run that Peavey bass through a bass EQ (which has about the same settings as i set my amps at) and a Proco Rat distortion pedal. In my band, we run all these things at once with use of an ABY pedal, just so that is clear.

I will check into the polarity business; unfortunately that's a bit difficult due to the design of the 18" cab (but damn if it doesn't sound wonderful).

So my main question here (in case I wasn't clear about it before) was: By running the Coliseum through a 2.667 ohm load, is it going to put out less power (just by virtue of the ohm difference) than it would if it were at a solid 2 ohms? I realize these things are much more complicated than a yes or no answer, and I appreciate your help a great deal. My knowledge of electronics is extremely limited so you'll have to bear with me. Thanks so much for your help!





Offline touchdown

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Re: Ohm questions
« Reply #7 on: May 07, 2008, 07:20:23 pm »
i checked the polarity, and all of the speakers (in both the 18" and the 4x10") are moving forward.

Offline Isaac

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Re: Ohm questions
« Reply #8 on: May 08, 2008, 09:47:39 am »
Quote
So my main question here (in case I wasn't clear about it before) was: By running the Coliseum through a 2.667 ohm load, is it going to put out less power (just by virtue of the ohm difference) than it would if it were at a solid 2 ohms?
Yes.

However, it's not a significant difference. Estimating around 250 watts for 2.667 ohms, and 300 watts at 2 ohms, the difference is only 0.8dB. It's highly unlikely that you could ever hear it, under any conditions.

Seeing your speakers, I suspect now that the main issue is that the Sunn 118VH is a horn loaded cabinet. Due to the polar response of such cabinets, it is never going to sound loud to you if you're standing in front of it. The sound is hitting you in the calves, and you have no ears there. Additionally, the Acoustic amp shown is, I think, of the type I described, with limited frequency response in order to maximize volume. Same with the old Peavey, I think.

Try this: run the Coliseum head through its stack, then, maing no changes other than the speakercords, run it through the Peavey cabinet. See how that works. Another thing to try is the Acoustic through the 118VH by itself, then the coliseum through the 118VH by itself. That will give you a direct comparison between the amps. However, you may have to lay down on the floor to really hear what the 118VH is doing.
Isaac

Offline touchdown

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Re: Ohm questions
« Reply #9 on: May 08, 2008, 11:06:42 am »
Yes.

However, it's not a significant difference. Estimating around 250 watts for 2.667 ohms, and 300 watts at 2 ohms, the difference is only 0.8dB. It's highly unlikely that you could ever hear it, under any conditions.

Seeing your speakers, I suspect now that the main issue is that the Sunn 118VH is a horn loaded cabinet. Due to the polar response of such cabinets, it is never going to sound loud to you if you're standing in front of it. The sound is hitting you in the calves, and you have no ears there. Additionally, the Acoustic amp shown is, I think, of the type I described, with limited frequency response in order to maximize volume. Same with the old Peavey, I think.

Try this: run the Coliseum head through its stack, then, maing no changes other than the speakercords, run it through the Peavey cabinet. See how that works. Another thing to try is the Acoustic through the 118VH by itself, then the coliseum through the 118VH by itself. That will give you a direct comparison between the amps. However, you may have to lay down on the floor to really hear what the 118VH is doing.

Actually, I do have ears on my calves.
As I said, I no longer have that Acoustic amp. I use an Ampeg SVT 350H with the Peavey cabinet. And I would do as you suggested, but won't any audible differences be a result of differing impedances (2.667 through the 18/410 stack, 4 through the Peavey). The Coliseum only runs 200W at 4 ohms.

Offline Isaac

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Re: Ohm questions
« Reply #10 on: May 09, 2008, 10:05:09 am »
Quote
Actually, I do have ears on my calves.
My apologies. I'm impressed by your anatomical advances!

Yes, 200 watts at 4 ohms, which is why I estimated 250 or so at 2.667. The difference between 300 watts and 200 watts is only 1.76dB, which is pretty insignificant. Impedance as such makes very little difference to anything. There is a measurable but usually inaudible difference in power, and that's it, assuming you don't go below the minimum impedance for the amp and let the magic smoke out.

Differences in cabinet sound are due mainly to frequency response and sensitivity. Using the same amp, with the same settings, on different speakers will give you a very good idea of the differences between the speakers. The difference in the amplifier's output power will be insignificant in comparison.
Isaac

Offline basiklybass

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Re: Ohm questions
« Reply #11 on: May 12, 2008, 06:53:13 pm »
Isaac, you have shared a great deal of great information again, thanks.

How does it sound 50 feet in front? The folded horn cabinets really shine out in the venue...and really suck on stage. After all, they are designed to move a large amount of air at very low frequencies. I know my 115BH shakes the stage but I can barely hear it when I stand in front of it.

Offline Johnny Guitar

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Re: Ohm questions
« Reply #12 on: May 12, 2008, 10:50:07 pm »
Great explanations guys!
As mentioned in another thread - I went to hear some friends play and the Bass player was only using an Acoustic folded horn cabinet and didn't take his 215 front loaded cab he used at practice because the club wasn't very big.  I was sitting about 15 feet in front of the stage and there was no Bass.  When I got up to leave the glasses & bottles on the bar were rattling like crazy and the Bartender was furious!  Takes a little room to form the wave from a folded horn cabinet.
Johnny Guitar

Offline Isaac

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Re: Ohm questions
« Reply #13 on: May 15, 2008, 12:39:04 pm »
In a small room, direct radiators are almost always a better choice.
Isaac