Are Most Amps Designed Like This?

Discussion in 'Amps and Cabs [BG]' started by The Golden Boy, May 5, 2004.

  1. In a different thread, someone had made the comment that the SWR SM-900 is supposed to get hot. It is, I understand that part, but how hot is it supposed to get? After about an hour of running the amp through a 4 ohm 2x15 in bridged mode, the front rack handle is not just 'hot' but more like 'if I ABSOLUTELY HAD TO hold the amplifier by these two handles on the face of the amp I would not be able to because my fingers would be fried right through' kind of hot. It's caused some problems, and it's shut down during shows, and it's blown output fuses.

    I realize my particular amp is what could affectionately be called a 'lemon' but I prefer 'dud,' but I think there's a few certain design flaws which exacerbate the 'running hot' problem.

    The vent in the back is a decent sized vent, but it's just an opening in the case to passively draw air in. The fan inside blows across the heat sink and hot air would exit via a vent hole cut in the side of the chassis. (the right side, as you're looking at the back). The fan is only blowing air that is already inside the chassis across the heat sink towards a small vent on the side.

    Does anyone find this to be a not all that bright design?

    First of all, the amp is sucking a lot of juice, it's going to be getting hot, so use passive air induction? (there is not a fan to force air in or out of the unit, it just wafts in)

    Secondly, and most not brightly... the amp is designed to be put in a rack case. (Hence the rack ears built as a part of the faceplate of the amp) Now why have the exhaust exit the side of the unit, where the wall of the rack case is 1/4" to 1/2" away? Not only does it not really allow for heat to exit the amplifier chassis, but the hot air also stays inside the rack case to be drawn into the passive air induction vent hole again!

    My amp has had most of it's QC issues taken care of during it's warranty period (it's only been in for service 7 times since 2001 :meh: ) but the cooling issue is an inherent design flaw, not really covered under warranty. And besides, the amp is "supposed to run hot."

    To rectify this problem (the amp shutting down in the middle of a set is a big problem) the SWR repair guy modified the fan to run at a low setting all the time, and kick into a higher gear when it reached the point that the fan would normally turn on. I also mounted a 3" or 4" computer fan on the top rail of the rack case to actually remove the hot air from inside the rack case so it doesn't go right back into the amp and heat up more.

    Since those two things were done the amp has neither gone into thermal shut down nor blown a fuse. Which is another issue in and of itself...

    Here's the question-

    While these look like simple common sense issues, and while component design has much to do with the layout- are other "top notch" amplifiers designed this poorly, or am I cursing at the engineers who designed it unfairly?
  2. g4string

    g4string Supporting Member

    Sep 19, 2002
    Melissa, TX
    I do know that SWR uses aluminum for the "face plate" of their amplifiers. The face should be hot to the touch. The purpose of the aluminum is to act as a heat sink and draw heat away from the other components inside the amp. In theory this is good idea. However, I think that this approach is fundamentaly flawed. I feel that aluminum plate acts like an oven. The hot metal keeps the inside compenents of the amp warm/hot all of the time. The only way the front face is cooled is from ambient room tempature air on the outside of the panel.

    Let me ask you this, is your 900 in a 2 space or 4 space rack. I think a four space rack would help as well. If you had 2 shallow rack effects (tuner, compressor, wireless, etc. etc.) this would leave plenty of room for air to flow in and around the inside of your rack case. If you are using a 2 space rack, there really isnt a heck of a lot of room for air to flow around the inside of your rack case. Good luck!
  3. I've never had ANY trouble with my SM900, but I must admit that I do not put the amp in a rack case, I just set it on top of the cab with absolutely nothing on/in front/behind or beside the amp AND I always pull the patch cable from the input and allow the amp to slowly cool down while leaving the power ON so that the fan can dissipate the heat.

    With all that said, perhaps the design could have been worked on a little bit better to allow more ventilation.

    The ONLY bad thing that I have heard about this amp is that it gets hot, hence my set up and tear down routine, but I'm never finished my drink after we play so it works for me. :)
  4. Do all SWR amps get hot? Anybody else with SWR heads or combos?
  5. It's on the bottom spot of a 4 space SKB rack, with a Korg tuner directly above and a Rack Rider 6 or 8 outlet power supply on top. I did switch those around because the tuner is about a quarter as deep as the SM-900, and the power supply is not quite half. Just a bit more air around the thing.

    Where is the exhaust on other "high end" amps? Is it on the side? Or is this just an engineering aberration?
  6. I guess the cooling system wouldn't be too bad IF cool air was available to the amplifier. As it sits, all the hot air generated by the heat sinks (the fan blows directly on the heat sinks- located in the middle of the amp) does not exit the amplifier chassis because of the size of the exhaust vent and the proximity to the rack wall. Because of the enclosed nature of the rack (only hot air is in the back of the rack) hot air gets drawn in to "cool" the heat sinks. Vicious circle. Feedback loop. Catch 22. Et Cetera.
  7. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    I think running 4 ohms bridged (same load as 2 ohms stereo) is the culprit.

    I have found that my SWR Baby Blue runs very hot when used at 2 ohms. After trying it a few times, I stopped doing it. On one gig where the amp was stacked on top of an extension cabinet I could feel the heat on my back radiating from the front panel two feet away :meh:

    By comparison, my A.M.P. heads (which also use the case as a heat sink, have small side vents, no cooling fins and no fan) are just warm at the end of a gig run into a 4 ohm load (the specified minimum) and my Walter Woods heads (which have top vents, cooling fins and no fan) are barely warm at the end of a gig when using the minimum rated loads (4 ohms).

    I love my two Baby Blues but consider the 2 ohm rating optimistic at best, an emergency operating mode only.
  8. It most definately is. Running the same cab through one channel of the stereo is fine.

    I guess why I'm so stubborn about this (running the Big Bertha in bridged) is that it is the FIRST RECOMMENDED cab in the manual for bridged mode.
  9. Mickey Shane

    Mickey Shane what goes here?

    Feb 23, 2003
    Denton, Texas
    My Ampeg SVT-4 is designed as you described. After I got it back from being repaired, I drilled holes in the SKB 4 space rack handle that lined up perfectly with the heat exhaust vent in the side of the head (ruining the waterproofing ability of the case). You could feel some heat coming out of the holes, but it was still pretty hot above the head inside of the rack case. I took out the tuner that was above it and installed a vented 1 space blank cover. Then I put in an AC computer fan, in the back on that side, that blows outward.

    Finally, I can run it hard, for long periods of time, and the top of the amp barely gets warm. Those rack case mods really helped.

    Someone should design a rack case that has a removable side panel. However, there might not be enough demand for one. I certainly don't need one now.
  10. BassIan

    BassIan Supporting Member

    Apr 27, 2003
    Cupertino, California
    This is a surprising problem. My Fender 800 Pro and my Mesa Walkabout both have rear-to-front air flow. The fans blow air from the back of the head, through the heat-sink that the MOSFETS are mounted to, and out the front panel. I always feel a good amount of air exiting the front, and on my 800 Pro, it's just somewhat warm when I've been running the thing hard for 2 or 3 hours. I run it at 2.67 ohms (lowest allowed is 2).

    If you've solved the problems, don't worry so much, though I'd certainly make it a criteria in your next purchase. I'm surprised to hear about this type of problem affecting these relatively high-end products.
  11. ihixulu

    ihixulu Supporting Member

    Mar 31, 2000
    getting warmer

    Considering how much you go on about that head I would've thought you'd be rid of it by now.....

    The heat generation is part of the design. We may not like it but it is how the SWR line was designed. At my practice space there are several amps that I get to go through. The Musicman HD130, after about an hour or so of steady loud playing, gets hot to the touch (faceplate); the Acoustic head (450 or something) doesn't get hot at all... except for the heatsinks on the back which once gave me a blistering burn when I brushed against them. I've seen amps go into thermal protection while the metal parts of the case was still cool to the touch...

    If it was a design flaw, the amp would have failed miserably in the marketplace long ago. As it is, it just shows the age of the SWR design.
  12. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    The ideal is to design with the air vented out the front. That is my preferred design approach if the amp is gonna get hot. Most PA amps are done that way.

    One problem with that idea: the front panel is "accessible", you can touch it.
    The safety folks who do the rules and regulations for the "CE" mark have limits on how hot "accessible" surfaces can get. In fact, temperature of other surfaces may also be limited, depending on the exact unit.
    Anyhow, a fan out the front blows hot air and heats the front, maybe too much to pass the tests. No fan out the front, less heat to the front, maybe the unit will pass the testing.

    If your front panel gets too hot, meaning you wouldn't want to take a shower in water that hot, or can't hold your hand on it, there may be a problem with your unit. The unit was probably designed to stay cooler than that.

    Another problem is with some things, there are enough front panel controls etc that there isn't any place for a vent. So it goes on the side, the top, etc.

    The side is at least half reasonable, since hot air can get away from the side if there is at least a couple inch space. Sometimes the case handles make a tight spot, and it would be good to put the amp where the handle is not in the way.

    If the intake is on the back, or the opposite side, it pulls air that hasn't been heated. That assumes you have some ventilation.

    And now to the main point.....leave the back cover off the rack when using! Surprising how many folks I have seen who don't, or who have a little bitty cable exit hole on the back. That's not going to work, the heat has to get out somehow.

    If the rack back is nice and open, you should be OK unless you have several amps going in the rack. Hot air can get away and cooler come in.

    Some amps have much less heat generation due to efficiency. The K2 is an example, no fan, no heatsinks, 94% or so efficiency, hardly any heat output.

    Same with some of the newer Ampeg products, high efficiency power amps, hardly any heat, no fan, or very small fan. You will probably be seeing more of that from all manufacturers in future.
  13. Im a sock

    Im a sock

    Dec 23, 2002
    Central MA
    I own an SWR SM-500, and I keep it in a 2-space rack, and it does get quite hot, but I've never had it shut down or blow fuses or anything like that. I push that sucker pretty hard too.
  14. What kind of speaker cab are you running and how many ohms?
  15. Cost of worrying about this all the time: priceless

    Cost of modifying your amp and other things by drilling holes etc in them: Unknown till you sell it.

    Cost of a cheap high-throughput AC fan on a gooseneck clip to direct air at the back and top of your amp (assuming your rack back panel is off): $10 or so at Walmart.

    KISS, as my wife says to me (a lot). Blow enough air and it'll be cool.
  16. BenF


    Mar 29, 2001
    Boston area
    On my old SM-900 the vents are on the top and on the side, a very poor design. I've modified my rack with vent holes, but if I'm going to be running at 4ohms bridged for several hours I always remove the amp from the rack and use a small fan to cool the back and top of the amp. If I'm using it stereo I can leave it in the rack.
  17. Fred312b

    Fred312b What if I want to play jazz precisely? Supporting Member

    Apr 23, 2002
    Chicago, IL
    i think we're all being a little unfair here about this design. it's a 900 watt amp in a 2 rack space design. there's only so much room for everything in that small space. you have it packed into a 4 space rack, without a lot of ventilation room. the heat needs to escape somewhere, and it's being channeled(?) through the amp chasis. i think you just need to give the amp some room to breathe. this might mean cuting up your rack, as has been suggested, or maybe getting a bigger one and leaving an empty rack space (or two) above it. when i got my bass 350 like 2 years ago, i emiled swr to ask them about rackmounting and they recommended leaving a rack space above it empty to aid in cooling. i'm no engineer (but i did stay at a holiday inn express last night) but i know with my home electronics (dvd player, receiver, etc.) i have them on seperate shelves with at least 2 or 3 inches above them and space on the sides so they have air flow and i haven't had any problems with them, and they get MUCH more use than my (or most people's) amps. :bassist:
  18. How hot does your Bass 350 get running your 8ohm cab? What about if you were running a 4ohm load?
  19. Fred312b

    Fred312b What if I want to play jazz precisely? Supporting Member

    Apr 23, 2002
    Chicago, IL
    honestly, i've only used it once with a band (i know, i suck :rolleyes: )for about two hours, driving a goliath 3 (8 ohms) and it was free standing. it got warm, but not hot, and only on the top, above the heat sink. :bassist:
  20. You're totally right

    I'd really like to find an amp that I love the sound of, and has the power I would like that I can afford at this time. I do bitch about it, and I'd like to get a new amp but it's sort of like going out with a really hot chick who's really tempermental, you don't really like her a lot, but she's really hot. You'll be willing to put up with her issues, but if a normal hot chick is available you'd ditch the bonkers one in a heartbeat.

    Right now I have the girlfriend and the kids to worry about before I can think about another amp, unless I find someone who wants to trade even up...