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SWR SM-400S with 2 8-ohm cabs?

Lobster11

Lobster11

Supporting Member
Apr 22, 2006
7,729
24,992
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Williamsburg, VA
#1
I've been playing through my buddy's old SM-400S at band practices recently; it's a nice amp and it saves me the trouble of having to haul my own rig every week. He's been using it in bridged/mono mode into a single 8-ohm, 2x10 cab (via banana plug). As I understand it, the amp puts out 400 watts in this configuration and likes it.

I happen to have another 8-ohm 2x10 cab that I'd like to add to the setup. (Specifically, I want to stack the two cabs vertically so I can hear myself better, etc.) The question is how best to hook things up with the second cab. I've been doing lots of searching here at TB and reading up on ohms and watts and whatnot, and I think I'm beginning to understand well enough to try out some things -- but because it's not my amp I want to be certain I don't do anything that might hurt it in any way.

As I understand it, I have three options (not including bi-amping and sending different frequency ranges to different cabs -- not interested in this):

1. Leave the amp in bridged/mono mode with the banana plug out to one of the cabs, and then daisy-chain the second cab to the first. If I understand correctly, this would pump out 500 watts into 4 ohms -- the maximum power that the amp has to offer. The manual states clearly that in bridged/mono mode the cab(s) should be a minimum of 4 ohms -- but will this arrangement push the amp too hard? Does "minimum" mean "You can do this, but you're pushing your luck"?

2. Switch from bridged/mono mode to "stereo" mode, and plug one cab into each channel (1/4" plugs). If I understand correctly, this yields only 150 watts into 8 ohms per side. This might well be enough volume for my needs, but it seems like a waste of a lot of good amp. Seems like this setup would be good for two 4-ohm cabs, but maybe not our 8-ohm cabs.

3. Switch from bridged/mono mode to "stereo" mode, but plug both cabs into the same channel (there are two 1/4" outs for each channel). If I understand it correctly, this would yield 250 watts into 4 ohms -- i.e., 250 watts "per side," though I'd only be using one side. It seems counterintuitive to me that I can get more power into the two cabs by using only half the amp than I can using both halves in stereo mode, but hey, this is all pretty mysterious to me.

So, can anybody familiar with this amp explain to me the pros and cons of each of these options and suggest the "best" solution? As I said, I want to err on the side of caution since it is my buddy's amp, not mine. Thanks in advance!
 
#4
#1 - 500w
the max power, but generates a lot of heat; if it does not overheat, then all should be fine; some say the heat decrease longevity and reliability ...

#2 - 300w
is a lot less power but very stable, if this is loud enough, that's what I would use

#3 - 250w
I see no benefit in do this
 
#5
Try it in #2 first. More stable, easier on a 15+year old amp, should be plenty of volume (another cab, double volume {some restrictions apply}) If that does not work, bridge the sucker. Remember that the additional wattage will not make a significant boost in volume, so if you are out of juice, it may be time to consider even more speakers, or a greater wattage amp. However, if one cab has been serving you well, I can hardly see this being the case.

Also, are the two cabs similar in sensitivity? ie, are they the same brand, model, ect, or two completely different beasts? I ask because if one is less sensitive, it wont sound as loud as the other. Not a huge deal if they are stacked one on top of the other.

Also, try stacking them vertically, so you have them standing skinny, instead of fat. It will raise up the speakers to ear level and make it easier to hear yourself, and in a show context, cuts down on comb filtering in the audience, which is a good thing.

Good luck. Also, your buddy has good taste in amps.
 
#6
Thanks for the advice, everyone. It sounds like I understood things pretty well -- i.e., that #1 was do-able but with some potential risk, and that #2 is safer and probably adequate. I will try #2 first and see how it goes.

BTW, I am indeed planning to stack the cabs end-to-end to create a vertical 4-speaker column (something I've been reading a lot about recently here at TB). And although the cabs are "different beasts" -- an SWR and an Avatar -- it occurs to me that by using "stereo" mode, I'll be able to use the amp's balance control to adjust for any differences in sensitivity between them.

Next practice is Sunday night. I'll report back here Monday in case anyone is interesting in knowing how it goes. Thanks again!
 
#9
I run my SM 400S in stereo between two 8 ohm cabs and added an MXR M80 & a Sonic Stomp, which gives the head a BIG boost and it produces plenty of volume!

Fan runs half the time of when I run under bridged mode and I have to say that the head sounds better while in stereo mode. Cleaner and clearer.
 
#10
The manual says the SM 400S will put out 550 watts into a nominal 4 ohm load - and that's the way I'd run it. The power supply in the amp will be fine as long as you give it adequate cooling (be careful if you mount it in a rack - definitely leave a blank rack space or take it out of the rack).
Rock on!
 
#11
I'll take #2 for $200, Bob.

If you look at SWR's manual for that amp, you'll see that the amp will run at a 4ohm MINIMUM in Bridged mode. But it also says that it will run OPTIMALLY at 8ohms. I think you can read that to mean it has a touch more headroom or oomph when you gotta push it. Also, the casing for the amp is the heatsink meaning it will get hot no matter what. It will run harder and therefore hotter at 4ohms than at 8ohms. Heat does shorten the longevity of electronics, so... I run mine Bridged at 8ohms and Stereo at either 8ohms or 4ohms(the optimum in Stereo mode).
 
#12
I'd go with number one if the cabs are identical,

In bridged mode the amp is less likely to clip and cause speaker damage.

if the cabs are different brands i'd go with number two, the cabs may be out of phase with one an other, or just not as efficient in which case running stereo will allow you to balance the cabs. if they are uot of phase you'll need to reverse the bananna plugs on one cab... you can test phase with a nine volt battey. connect the 9 volt battery on the speaker cable straight in to the cab.
 
#13
My suggestion is to run stereo unless you're running out of headroom. Then use bridged mode; there should be a very noticeable increase in available volume that way.

Frequently those with the older stereo Eden WT heads and two 8 ohm cabinets find that they not getting quite enough power running stereo, while their amp is 8 ohm minimum bridged, so using mono bridge isn't an option. Thankfully, the SWR will operate into 4 ohms to get the most out of those two 8 ohm cabinets. Also, because there is more available power, paradoxically, often you don't need to crank the amp as much in bridge mode. So, it doesn't necessarily mean that you'll be working the amp harder (unless you're pushing it right to the limit).
 
#14
I've been running my SM400 at rehearsal bridged mono into two 8 ohm Cabs. The cabs are an SWR 2x10 Golight and 1x15 Golight. Not had any problems with heat or stress.

I thought the amp puts out 440 at 4 and 420 at 8 bridged?

Not sure what an S is and not sure what mine is but it's old.

I use a special cable I made to hook it up. I soldered two heavy duty (one to each wire) solid copper spades to one end of a heavy duty speaker cable and have a Speakon at the other end. Then I use a Speakon to Speakon cable to run from the top cab to the bottom cab. I put the spades under the screw terminals and tightened it up.

This amp works well for rehearsal and allows me to not bring my Shuttle 9.0 and two Berg HT1x12ER Cabs. Which I greatly prefer but it sure is convenient not to have to haul gear. I also leave a Bass there as well.
 
#15
Invalid Link Removed

The power amplifiers in the SM-400S can be used individually to provide full biamp capabilities when used with the built-in adjustable electronic crossover or can function as stereo, 250 watt power units. If you need more power, the amps can be bridged for 400 Watts @ 8 ohms or 550 watts @ 4 ohms.
Click to expand...
 
#16
You'd better do #2. Running two decent 8ohm 4x10 cabs like this gives me enough volume to bury two tube powered 100w half stacks if they're set "pretty damn loud".

#1 will most likely leave you with a blown main power fuse. My SM-400 will NOT run at 4ohms, even at a lower volumes. My model is fan cooled- I've read some of them aren't, mounted with top and bottom breathing room and auxiliary fan to keep the whole rack cool.

The one and only time I tried to run this thing 4 ohms was live in front a large crowd. 45 seconds into the first song, I had no power fuse left.
 
#17
I ran mine in a #2 configuration for years. Always plenty of volume and the amp ran cool. I remember a phone conversation with Steve Rabe about my setup and he told me it was a "good way to run that amp".

/rick
 
#19
The additional bit of power you get by running the amp in mono doesn't mean that much in terms of output. The big reason to run the amp in mono is because you double its slew rate which will give you better transient response. As long as you don't put the amp in an enclosed rack, you'll be fine running mono into 2 x 8 ohm cabinets.
 
#20
Thanks again to everyone for your help and advice. I don't know if anyone cares, but I did promise a report so here it is. Unfortunately, we had two consecutive band practices canceled -- first due to a snow storm, then due to my having the flu -- but we finally had a couple of practices a few days apart and I got to try out the two-cab thang:

So, I stacked the two 2x10's (an SWR on top of an Avatar) -- end-to-end to create a tall 4x10 array -- and ran the amp in stereo mode. (As I mentioned at the beginning of this thread, I wanted to be super-careful because it's my friends amp, not mine.) This gave me (far) more than enough power, and it sounded great. At the second practice I put a Sansamp VT Bass in line, which allowed me to increase the input into the amp substantially (from my passive Jack Casady bass) and gave me the thick, fat, SVT-into-Ampeg-refrigerator sound I was angling for. Given how happy I am with what I've got now, I probably won't need to try bridged mode, unless we get a gig at some point that requires the extra power. (Plus, the slower transient response anticipated by Gizmot isn't an issue given the particular sound I'm looking for.)

BTW, the stacked 4x10 array works just as I'd hoped it would: I can hear myself SO much better now that I even got to hear those words all us bass players long to hear: "The bass isn't loud enough -- turn it up!" Needless to say, they didn't have to ask twice. Happy happy!
 

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