SWR SM-400- Getting the Most of It

I don't think the power is over rated.
Louder than my SVT or 3Pro.

I've been meaning to start a thread about this amp.
I have found that the bass control is centered at an EXTREMELY low frequency center
Set the bass at 12:00 and just push the strings in and out toward the pickup and watch to see if your woofers dance.
My suggestion is to turn the bass knob either off or 1 click up, and, if needed use the graphic to boost anywhere between 100 - 180 Hz.; also like the aural enhancer on at about 12:00

Mine is an extremely early production model; wonder if they all have this crazy low EQ on the bass knob?

I had an sm400s for years and recall that I sometimes used the lowest fader to roll off the ultra low frequencies (sub 40 hz) that sucked power and caused huge stage rumble.

Goodness knows the amp was plenty loud... You turn the gain knob up to just below where the clip light blinks (probably around 2 o'clock) and then turn up the master to where you need it. I found that was rarely past 12 o'clock -- leaving tons more volume if you need it.

If you can find an original manual it has a suggested reggae eq setting.

Proper cab selection, matching ohms, is paramount to great SWR performance. One 8 ohm cab on one side (stereo) gives you 160w and that's kind of anemic. One 4 ohm on one side (stereo) is 220w is bountiful. Bridging into one cab at 8 ohms is great. Use good cable and connectors.
Tonally I would go with the suggestions above. Obey the suggestion about the pre gain and master gain!
Actually no limiter, aural enhancer no more than 11, reduce treble a hair, and leave the other eq alone.
What cab are you using?

You have a Henry? Okay then, bridge that SM and we'll duck for cover. Keep the SM in a case with a minimum 1/2 rack space above.
Do not be afraid to turn this thing up. That thing about reducing the low bass do it. These things are capable of amplifying stuff we can't hear or use.

I rarely use mine anymore, but I have never had trouble keeping up with a hard hitting drummer and a 100 watt Mesa. Just used a 410, 115 and my G&L's.

I used to have a Pedulla that had a much weaker output than the G&L's. I had to drive the amp pretty good sometimes using that bass, but never a problem with any other.

Hi.

Count Bassie said:

I just got an SWR SM400, and I understand it's power tends to be overrated with the 400w @ 4 ohms spec. If you have one of these, how do you tweak it to get the most volume out of the available wattage?

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IIRC the one I had years ago delivered the promised 400W to 4Ohm load. 40Hz, before clipping.

woolysock said:

I had an sm400s for years and recall that I sometimes used the lowest fader to roll off the ultra low frequencies (sub 40 hz) that sucked power and caused huge stage rumble.

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I do that with the ST220 too and it sure makes more juice available from the PS for the upper frequencies.

If the amp doesn't deliver full power and/or runs hot, have it biased. SWR's are notorious for shifting bias, but as they have the means for easily to adjust it, it's no biggie. IMHO of course.

Regards
Sam

woolysock said:

I had an sm400s for years and recall that I sometimes used the lowest fader to roll off the ultra low frequencies (sub 40 hz) that sucked power and caused huge stage rumble.

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+1 The relatively low output of the SM400 compared to other amps of its wattage rating always seemed to me to be due to that amp pumping out massive low end, all the way down to the sub harmonic region. Unfortunately, it seems the designer of that amp, rather than including some sort of hi pass filter or other means of deep low end reduction (i.e., rolling off 'useless low end' that any amp is capable of delivering), thought it would be cool to have it go all the way down to 20hz (which I think is part of the story and description of that amp in the manual).

As a result, it seems a lot of energy is used up trying to reproduce those almost 'sub harmonic' frequencies, and that amp tends to piston many speakers to the point of having an issue.

Unfortunately, while many attempted to fix this issue by cutting that lowest semi-parametric slider completely at the lowest frequency, those controls seem to have a tight enough Q that it didn't really help.

The only way I was able to successfully use that amp at high volumes was to incorporate an external hi pass filter, with everything under 30 hz or so rolled off. That resulted in the speaker cones settling down, and the perceived volume output of the amp to increase with no noticeable change in tone.

It's a nice sounding amp, but a classic case of 'too much low end being too much of a good thing'.

IMO and IME.

Count Bassie said:

Okay, a lot of responses here- and I just posted this late last night! Far out, and thanks everyone!

So firstly, the 8x8 is gone. I live in a stone house with rough stone steps UP to the basement, and often have a van full of carpentry tools to unload before I load it with gear... bass or two, rig, maybe drums and small PA for the reggae band..., so there's a major loading morasse of some type whenever I have a gig- which is most weekends. In winter it's a battle with icy rough stone steps, and I've hit the dirt more than once in this hostile scenario... So it's too bad, but the Henry has gone off to play with a nice rock cover band. Modular is the new idea (to me).

I'm now, until I get settled down with a new cab thing anyway, using an Avatar 212 (ceramic) and a Joe's 115 ("TL-606") loaded with an EV DL-15X driver. This represents a power rating mismatch, but it'll do for the time being.

I get the low-bass tuning trip on the amp; and KJung, I hear what you're saying about an external EQ. So you just think adding an EQ into the FX Loop will hi-pass that "lows" business out suffieciently? I have an MXR M-108 pedal that might do it alright, I'll have a go with it- and I'll try those recommended settings, thanks JimC and co.

Thought: I also have a Yamaha PB-1 lying around, which has a nice little parametric EQ on it. I wonder if setting that pre all flat and using the parametric would do the trick? Adjustable Q and all that...

It was 'pistoning' the crap out of the Henry speakers at my last gig. I ran the Henry later the next day and it seemed alright though. Tough little bastard, I guess!

I keep a small fan (Target) clipped to the back of the SKB 2-space box it's mounted in. Seems to be alright.

Mine is also an early model, by the way.

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The only way I really found to fix it was a true hi pass filter, but again, matching it with the right cab will help. If that EQ has a low end shelving control that can be set very low (30hz or so), that might do the trick.

Count Bassie said:

Would this be perhaps a capacitor-box of some type- something one could make from parts bought at a Radio Shack? And, where in the signal-chain (where on the amp's signal-flow chart) would I want to place such a device?

Thanks for your help man!

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That's way over my head. Again, it's a great, classic head that has just a touch too much low end extension for some, and it is a bit hard to control. However, a zillion guys have played these with no additional EQ, etc.

I would suggest getting it out on a few more gigs, control the low end as best you can with the EQ on the unit (bass control and low slider), go VERY easy on the Aural Enhancer (that can kill your tone at even modest levels), and let 'er rip for a while with some different cabs that you mention in your original post.

Edit: 50hz is way too high... that will suck the balls from your tone IMO.

Lowest freq on the semi-parametric is 31 Hz; one would think that pulling 15 dB here would stop the pistoning (great description), but it doesn't; possibly because of Q

It's that pot that's labeled bass that controls all the damage; I was amazed that even when rolling all the way off and then going back 1 click, that there was plenty of low end with a Berg 322

In fact, I've been leaving the treble and transparency pots nearly off as well to get a similar EQ curve as my SVT set flat

Seems like Steve Rabe's design goal was to build a high power, super high fidelity amp that goes from DC to daylight, in a 2 rack space container, and did so long before others

I was afraid that my early amp was the only one with this issue; in fact mine did not come with the transparency control; when I called the factory, they said to send it back, added this feature for $35 then paid shipping back; pretty cool

KJung, any recomendation for a cheap passive high pass filter?
I have 1 rack space good parametrics kicking around but want to keep things simple and light weight

If you guys get a minute, set your amp at regular volume with the bass knob anywhere at 9:00 or above and gently push a string toward the pickup while watching the speakers; mine move around like crazy; this doesn't happen with other amps I have (Ampegs, TE, or Acoustic)

Jim C said:

Lowest freq on the semi-parametric is 31 Hz; one would think that pulling 15 dB here would stop the pistoning (great description), but it doesn't; possibly because of Q

It's that pot that's labeled bass that controls all the damage; I was amazed that even when rolling all the way off and then going back 1 click, that there was plenty of low end with a Berg 322

In fact, I've been leaving the treble and transparency pots nearly off as well to get a similar EQ curve as my SVT set flat

Seems like Steve Rabe's design goal was to build a high power, super high fidelity amp that goes from DC to daylight, in a 2 rack space container, and did so long before others

I was afraid that my early amp was the only one with this issue; in fact mine did not come with the transparency control; when I called the factory, they said to send it back, added this feature for $35 then paid shipping back; pretty cool

KJung, any recomendation for a cheap passive high pass filter?
I have 1 rack space good parametrics kicking around but want to keep things simple and light weight

If you guys get a minute, set your amp at regular volume with the bass knob anywhere at 9:00 or above and gently push a string toward the pickup while watching the speakers; mine move around like crazy; this doesn't happen with other amps I have (Ampegs, TE, or Acoustic)

Click to expand...

+1 to all of the above. Yes, that Q on the semi-parametric results in that control just taking a slice out of the deep low end, but unfortunately letting those problematic deep lows still come through.

It's a unique amp, and can work very well for some. However, per your comment about the design goal, which I agree with, I call it 'a brilliant execution of a bad idea'.

I am not up to speed on hi pass filters. The only ones I'm familiar with are the large rack mount parametric EQ's like the Rane that have a hi pass filter included in their features.

K

Edit: That 'upper treble' transparency control, from what I remember, was an option that had to be requested on the 400. I think it might have become standard on the later 500 model, but not sure. That was a cool control, and if I remember correctly, just impacted what was coming out of the tweeter, WAY up top.

Glad to help Count and thanks for the confirmation KJung
I quit using this amp for 10 years and only brought it back into service as the 3 Pro is in the shop and current gigs / rehearsals don't merit carrying an SVT; am liking the SWR now although the tone is much cleaner than the Ampeg sound I love; then again, the last gig with the SVT was in a room with horrible acoustics and having a super clean and transparent bass tone could have been a good thing

Misery does love company; great to know that the noise you hear in your head just might be coming from your amp; OK perhaps this is the only time, but at least it happend once!

The SM400 is anything but hi-fidelity. Running flat, the preamp shows a signal with boosted hi mids, a 5-6db cut at 250hz, and lower bass than the hi-mid hump below that. The hi-mids are what makes it sound so clear. The amp is rated to reproduce signal down to 10hz, not 20hz. That fits with what was going on at the time(ah, the synthesizer days) .

Keep in mind that I play 4-stringers, so that is how I relate to this amp. Also, this is related to the original SM400 without the Transparency knob. The Transparency knob was added later after requests along with a fan, ground lift on the XLR, and an increase in power. That is the SM400s model.

I run the lowest semi-parametric knob down 6-9db at 31hz. This is the best way to free up power for signal higher up and adds punch. With a 4-stringer(at least mine), there's not much below that to interfere. Keeps the mud out also on live stuff. At home, the bass knob is turned UP, not down, for practice. When going out to play, you just turn the bass knob down as you turn up. I agree with KJung's thought on using a hi-pass in the effects loop, although I've not found a commercially available unit strictly for that purpose. There are hi-pass filters on some eq's though. There is a user here on TB named fdeck that makes and sells a hi-pass filter. You may want to get in touch with him.

The Aural Enhancer activates the second gain stage of the tube up to about 1-2 o'clock on the knob. So all you are doing up to that point is adding gain from the preamp tube. From that point on up, it starts cutting a deeper notch at about 250hz.

This amp is made to run hot. The aluminum case is also the heat sink for the amp. So the louder and longer you play, the hotter it's going to get. Don't sweat it. Follow the previous recommendation to leave at least a 1/2 rack space above for ventilation. I leave 1/2 above and below when in a rack just to be sure. I don't exactly remember but I think the heat flow is top to bottom even though that doesn't sound right. A small cheap flexible fan is a good idea. Also, if you use it in a rack, cut small blocks of wood to support the rear end of the amp while in transit. The circuit board is connected directly to the front panel and may flex and crack solder joints. The rear supporting blocks will prevent this. Just make sure you don't cover the vents.

Your wattage in stereo should be about 200watts@4ohms in stereo mode. Mine runs at 148watts@8ohms. In bridge mode, you should get about 400 watts at 8ohms and 4ohms. Whatever you do, DO NOT run this amp at 2ohms in bridged mode. Yeah, there's someone out there that has done it with no problems but you will be at high risk of frying your amp. The optimal configuration for the amps is to run 4ohm cabinets in stereo mode or run an 8ohm cab in bridged mode. So, cabs and configuration, as always, is very important.

Most people that say it's not enough power do not know how to operate it correctly or what it's optimal parameters are. They may also be trying to make it do something it's not built to do. Use the Bass and Treble knobs for initial large sweeping changes in tone, then the semi-parametric for tweaking. It's a tone monster and should be plenty for most of your gigs.

If the SM400 has the same response curve and similar Q on the semi-parametric controls to the SWR GP, then the flattest response will be had by adding 8dB boost @ 250Hz. The default sound is heavily scooped here plus slightly boosted lower and higher, which makes the amp sound much quieter than you'd expect. Add that 8dB and it sounds far louder and fatter even once you turn the volume down to compensate.

Everything iammr2 has said is in keeping with my findings with the SWR Grand Prix - I was amazed to find that the aural enhancer just added another stage of tube gain for about half its travel! The GP had a switchable highpass @ 30Hz filter which the SM400 sadly doesn't have - a very valuable feature.

Alex