Converting Ampeg Cabs From 4 Ohms To 8 Ohms

[Vince Klortho]

No, that will not work the way you think it will. You will still have the same issue but in different locations. This is called a wave interference pattern. Waves from two sources will add together in some places and negate each other in other places. I don't think it's that big of a deal though. If it sounds weird in one place then move a few feet over and it will be different. Just make sure you have your mic in a place where it sounds good. I see lots of bands do this on stage so it can be made to it work if you really want to.

Personally, I would keep my stuff on one side and rely on the side fills as needed. If they can't handle it then have the sound guys get better ones and have your monitor guy dial 'em in better. Also, you might want to look into an in-ear monitor system. No side-fills needed with those and you will have a consistent sound everywhere.

[Downunderwonder]

Originally Posted by TimSterling1981

Correct me if I'm wrong though, but can't I just adjust the phase of one set of cabs to correct this problem?

NO.

Delaying one stack to suit one position for "E" rotates the patterns to another note. Like if you had a wheel of rainbow colours to signiify the different dropouts and blooms and rotated that. I would have to run around the room in time to the music to hear all the notes equally no matter what you did.

You can test it for yourself or take our word for it.

Same deal for house subwoofers and your bass cab on stage.

[Vince Klortho]

No, it does not rotate the patterns to another note. It just shifts the locations of the peaks and valleys of the interference pattern. The pattern will exist for every note - just in different locations depending on the frequency and, therefore, the wavelength.

This page show this graphically and is essentially what happens in a horizontal plane at the height of your ears.

[JimmyM]

Originally Posted by TimSterling1981

Oh, I was already prepared for a bombardment of replies that had nothing to do with providing me with an actual answer. LOL. Thanks for a quick one, though!

I discovered from Ampeg that they actually made two SVT18 cabs...the SVT18 is a 4 ohm cab, while the SVT18E is an 8 ohm cab.

Yes, I am biamping, but I'm using one head to do so; the maximum load I can put on one side is 2 ohms. I need the ability to expand to four cabs per side (4 top cabs for highs/mids, 4 bottom cabs for the real subterranean lows) in the event of a massive outdoor show.

Yes I realize it's overkill to most. Yes I understand the physics behind why 10" cabs sound better to most or move more air or whatever the reason of the week is. But it's all about the tone I'm after (and tuning down to low G on a 5-string, I need that extra girth in the low end). If something sounds good and works and fits within the tonal spectrum of the band, then to hell with physics.

Besides...I don't hear anyone telling Mike Inez that his custom SVT 2x18"s sound like garbage. If you know how to properly run and EQ that type of cab setup, and you know how to tweak it from venue to venue to sound good in that particular environment, then it will sound just as good, if not, better than any head-thru-8x10" rig. It's all about proper EQing and tweaking the mix and volume levels of each set of cabs to that particular venue, be it a massive outdoor stadium gig or a small club deal. The average bass player (not to slag anyone) wants a quick setup; just plug & play. But the great bassists know how to fine-tune a rig to the point where it may seem like an oddball to most (look at the way Billy Sheehan runs his amps).

Thanks to all for the input, and I apologize if any of my words came off offensively...just trying to be lighthearted with the physics players who will argue about this kind of setup.

Our Pal Johnny April of Staind runs a similar rig, though about 4x as big Two 810's, two 218's, with big huge JBL tweeter horns from the 70's on top. He loves it. Could work out for you as well.

[Downunderwonder]

Originally Posted by Vince Klortho

No, it does not rotate the patterns to another note. It just shifts the locations of the peaks and valleys of the interference pattern. The pattern will exist for every note - just in different locations depending on the frequency and, therefore, the wavelength.

This page show this graphically and is essentially what happens in a horizontal plane at the height of your ears.

potato, potato, you're looking at the note and I'm looking at relative response at fixed postion in the room. Carry on the good work.

[Sartori]

For "subterranean lows," I'd stop looking at full range bass cabs (which the Ampeg 18" cab still is), and start looking at actual subwoofers.

[anderbass]

Originally Posted by TimSterling1981

I discovered from Ampeg that they actually made two SVT18 cabs...the SVT18 is a 4 ohm cab, while the SVT18E is an 8 ohm cab.

There were more differences between those 2 models than just the ohm rating. The 8-ohm/SVT18E cabs were much shallower (only 16" deep), plus their speakers were only rated @175-watt.

[packhowitzer]

Originally Posted by Vince Klortho

Everyone has an opinion so here's mine. If you are going to run four cabinets off of each side of your amp make sure that you have a big one. Like at least 1000W per side at 2 ohms big.

If you are going to bi-amp (which I would if I were you) I recommend using a subwoofer. You can get them with amps and crossovers built-in (active) or passive so you will have to use your own amp and crossover. Lots of companies make them like Peavey, Yamaha, Bag End, Carvin, ElectroVoice, JBL, Cerwin-Vega, QSC, etc. Here is a link to what guitar center has. If you are going to use an active subwoofer then your amp does need to be nearly as big.

Originally Posted by Sartori

For "subterranean lows," I'd stop looking at full range bass cabs (which the Ampeg 18" cab still is), and start looking at actual subwoofers.

+1 to both of these.
It sounds like what you want to do is build a PA just for bass guitar. If you want to send different frequencies to different cabs, then don't use a bunch of full-range bass guitar cabs with different size drivers. Use a bunch of purpose built cabs that might happen to have different size drivers.

I think that just because some amp manufacturers include a bi-amp feature on their heads, it doesn't mean that the full range cabs they make will necessarily sound that great when used with them.

I hate to admit it, but maybe my dum-dum story will help keep others from the same fate. In the 90s i used an SVT IVpro biamped with 410s and 115. It sounded ok, but not nearly as good as just running dual mono. And ultimately, all i did was blow 2 15" speakers before i wised up. Switched to 2 410s and never had another issue. and my rig would melt your face off.
ymmv

[Bass 45]

So Tim, which cab do you have - SVT18 or the SVT18E?

I owned the SVT18 and I can tell you - it is a sub-woofer. It was not full range - but it could accomplish punishing lows - even far superior to the Ampeg SVT410HLF down low.

I tried the SVT18 (the big box with the 4 ohm speaker) in conjunction with three different 410 cabs that I owned at the time - and it killed with any one of them. There is something to be said for hearing combos for yourself rather than relying on the unscientific/scientific guess work presented here.

If you do have the 8 ohm SVT18E, sell it before you blow that poor speaker out of it's box.

[dincz]

Originally Posted by TimSterling1981

Correct me if I'm wrong though, but can't I just adjust the phase of one set of cabs to correct this problem?

You can correct the phase difference at one frequency but that will introduce an error at another frequency.

[Downunderwonder]

Originally Posted by dincz

You can correct the phase difference at one frequency but that will introduce an error at another frequency.

While simulataneously introducing the same phase error at a different location.