Amp clipping...HELP

Discussion in 'Archived Threads 2001-2004' started by TedO, Mar 20, 2002.

  1. TedO

    TedO Stunt Coordinator

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    I just bought a QSC RMX2450. I thought it was a little overkill on the power since I was only going to run a sub in bridge mode. Thats over 2000 watt to the sub.

    Here is the problem:

    I set the amp to "bridge mode", turn the clip limiters OFF, turn the Low feq limiter to Off and I hook up my SVS to the bridged output. (ch1+ to +, ch2+ to -)

    I calibrated it using the RS meter, all channels reading 75, the sub is set to -6db. I have a Pioneer VXS36-tx, sub out from this to channel 1 input on the amp.

    Well anyway, I throw in Pearl Harbor, the attack scene. Well the amp's clip light starts lighting up like crazy,so I turn down input on the amp. Finally I got it all working OK, but dam with this wattage I didn't think the clip lights should light up the way they do. What happens when I build my TW_blueprint_15 SonoSub and need more power?

    Could this be a problem of input sensitivity? The manual says the input sensitivity is 1.23v (+4 dBu) and the Pioneer manual doesn't say what the rated line level out is. The only thing that doesn't make sense is why would the amp clip if it wasn't getting enough signal?

    I'm dazed and confused, and things just don't seem right.

    I do have the option to trade with the guy I bought this from for a Crown CE-2000, it has the option to reset the input to .777v.

    TedO
     
  2. TerryC

    TerryC Stunt Coordinator

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    I'm assuming you have your gains matched from your receiver to your amp? In most cases set the amp to 3/4 or full gain and then use the receivers sub out level to set the sub at 75db which will usually result in your sub level setting in the reciever somewhere between -3 all the way to -11 plus depending on brands.

    You need to remember that just because you have a powerful amp it still takes roughly the same input level to reach full power as a small one.

    Here is a quick run down. Say you have a 200W amp and a 2000W amp of the same brand and use the same input stage and both reach full power at 1volt. When you receiver puts out 1 volt of power then both amps will run full blast.

    Of course there is the issue of having the larger amp needing less input voltage to drive your sub to 75db but its still not going to be a huge difference(lets pretend for the sake of arguement that the two amps have the exact same input stage). For instance, if a 200W amp barely clipped, addding a 2000W amp doesn't mean the output meters aren't going to go past 1/4 or 1/2 level. In fact odds are they will still go to full clipping if you are using the same sub.

    So whats your solution? More subs. That way you are able to presurize the room with less overall gain needed and havbe ample power in reserves for those power hungry peaks.
     
  3. TedO

    TedO Stunt Coordinator

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    "I'm assuming you have your gains matched from your receiver to your amp? In most cases set the amp to 3/4 or full gain and then use the receivers sub out level to set the sub at 75db which will usually result in your sub level setting in the reciever somewhere between -3 all the way to -11 plus depending on brands."

    Yes, I set the amps gain to full, then I set the sub-out level on the receiver to 75db using a RS SPL meter. It ended up around -6db.

    "You need to remember that just because you have a powerful amp it still takes roughly the same input level to reach full power as a small one."

    I understand this, but with the more powerful amp, wouldn't it take less input to drive the sub to the same level as with a weaker amp? In other words, if the receiver's volume nob is set at 1/2 way (after calibration) the weaker amp would be running at 75% max power and the more powerful amp would only be running at 30% power, thus the more powerful amp would not clip at all.

    "Of course there is the issue of having the larger amp needing less input voltage to drive your sub to 75db but its still not going to be a huge difference(lets pretend for the sake of arguement that the two amps have the exact same input stage). For instance, if a 200W amp barely clipped, addding a 2000W amp doesn't mean the output meters aren't going to go past 1/4 or 1/2 level. In fact odds are they will still go to full clipping if you are using the same sub."

    Why wouldn't the 2000W amp clip less, it should be running much farther below it's max output?

    "So whats your solution? More subs. That way you are able to presurize the room with less overall gain needed and havbe ample power in reserves for those power hungry peaks."

    If it clips with one sub, remember it's running in bridged mode, would it clip even more with two? I would have to switch back to stereo mode which would split the power between two subs.

    I do plan on building a more powerful sub. A SonoSub with a Blueprint 1503 driver. I would need even more power to drive the Blueprint sub, and this would lead to more clipping.

    I read in a post ThomasW saying they tested a sub with a Crest LT-1000 and the amp drove this sub (Blueprint SonoSub) with barely lighting the gain leds.

    I do know the input for the QSC2450 is 1.23v, thats higher than the 1450 and 850 RMX's.

    TedO
     
  4. TerryC

    TerryC Stunt Coordinator

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    "I understand this, but with the more powerful amp, wouldn't it take less input to drive the sub to the same level as with a weaker amp? In other words, if the receiver's volume nob is set at 1/2 way (after calibration) the weaker amp would be running at 75% max power and the more powerful amp would only be running at 30% power, thus the more powerful amp would not clip at all. "

    That is where you're incorrect. Say you have two amps with the exact same input gain but one is 200W and one is 2000W. It seems that you are thinking the 2000W amps gain will be 1/10 the setting for the same output as the 200W one right?

    That's not how it works. Lets just say it takes 20W for the sub to reach 75db in your room for pink noise. That 20W input is such a weak input level for both amps that they gain settings will actually be roughly the same. Yes, I said the same or close enough. Not 1/10 difference. The bigger amp just ramps up to more power. Many subs can't take this extra power and disperse it as heat. If a sub can use it you'll get louder output.

    Adding a second subs considerably lessons the strain on your amp as you are doubling the SPL right off the bat.

    Say you have a 200W amp with one sub. You add a second sub, you gain 3db although both subs are only getting 100W each. Now if you have power in reserves and the subs can handle it you get 4-6db more by adding a second sub. That's about the same as quadrupling amp power.

    Hope this helps.

    I've played with a variety of amps including 3 Mackie models from 800W to 1400W, a 1400W QSC and a 1500W Crown amp and all have been easy to clip.
     
  5. TedO

    TedO Stunt Coordinator

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    Terry,

    So, given the option which would be better?

    Add a second SVS+ or build a Sonosub with a Blueprint 1503.

    In prior posts, everyone seemed to be saying one Bluprint 1503 Sonosub with a 1000w+ amp would outpreform two SVS+ with a 500w per channel Samson.

    TedO
     
  6. Wayne A. Pflughaupt

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    Ted,

    Gice us some details on your sub. Like what is the impedance?

    Regards,

    Wayne A. Pflughaupt
     
  7. Dan Hine

    Dan Hine Screenwriter

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    Wayne,
    I know you asked Ted but I'll answer for him ( I hope he doesn't mind [​IMG]). I am 99% certain that all SVS subs have a 4ohm impedance.
    Regards,
    Dan Hine
     
  8. Vince Maskeeper

    Vince Maskeeper Producer

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    Something is wrong here.

    It's not a matter of input sens- in fact- if you are clipping the amp it's because you're getting too much juice into the input stage.

    If you're getting clip, means you're getting max input, and thus max output. Bridged at 4ohms, you're putting out 2400 watts plus. The driver should have dropped bottom.

    Something is not right here.

    I'd try disconnecting the sub and seeing if the input still shows clip flying with no speaker connected (sometimes improper load will make things go nuts). Make sure the sub output is wired right- those RMX have the ch1&2 posts reversed from one another- so the hots are the middle two posts, not posts 1&3.

    DOuble and triple check the dip switches- don't assume anything. Check the manual to make sure they're right- and flip them back and forth to make sure they're actully flipped all the way. Those dips are a pain, and I have seen tons of people have problems because of them.

    Does the RMX have an input level indicator or just an input light (been a while since I've seen one)?? I'd be curious to see where you're at on the scale with test tones.

    Again- something is very wrong here. THe more subs answer isn't one I would give- not yet anyway- because this is not a question of headroom really- this is a question of why 2400 watts isn't doing what it should be doing... and I'd guess it's because despite the amp's clipping, you're not getting 2400 watts.

    -Vince
     
  9. TedO

    TedO Stunt Coordinator

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    Vince,

    I think you are right about the input. If this amp was giving me 2450 watts my driver would be toast.

    I have tried the dipswitches in a couple of different configurations, all with the same results. I have tried stereo and bridged with the same results. Yes I hooked up the sub correctly in both stereo and bridged mode, I even unhooked the sub and the amp still showed it clipping. The one odd thing is QSC has reversed the polarity of the ¼ phono jack in their input plug. They did this for a noise suppression reason and said to reverse the polarity on the speaker. What I haven't done is to set the clip limiters to ON. In the manual it says most people running subs leave this off for an extra punch with things like a kick drum.

    The reason I bought such a powerful amp is to get the headroom and extra power for when I build my Blueprint 1503 Sonosub. The amp continues to light the clip light on strong bass passages. I talked to a tech at Crown and he said the QSC's weren't stable bridged into a 4 ohm load (2 ohms pre channel in bridged mode). So I think I might be better off trading this for a Crown while I have the chance.

    TedO

    P.S. correct the SVS is 4 ohm SVC
     
  10. Vince Maskeeper

    Vince Maskeeper Producer

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    YEah, if this is new, I'd return it and try another.

    -V
     
  11. TedO

    TedO Stunt Coordinator

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    Picked up the Crown this afternoon, switched the input sensitivity to .773 and hooked it up. I coudn't hook the sub yet, need a connector, but drove the amp anyway. NO CLIPPING!! Don't know why but I think this will work better than the QSC.

    The fan hasn't kicked on yet either, so it's quiter, It's made in the USA and has a better warranty. The choice is a no-brainer.

    TedO
     
  12. jeff lam

    jeff lam Screenwriter

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    Ted,

    run the gains wide open(if you haven't done so already) and see if you can clip the amp.
     
  13. TerryC

    TerryC Stunt Coordinator

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    Vince,

    The sub should be able to take 2500W for brief periods. I know I've thrown aa sustained 1500W 20hz steady sine wave into my Shiva without killing it. What sub/amp combination do you have at home? The reason I ask, many DVD's have insane amounts of headroom in the LFE track, when watching movies such as the haunting or Phantom Menace I every amp I've scene ends up clipping a tad or close to it.

    The thing to keep in mind is the SVS is likely maxing out SPL wise at lets say 500-1000W and anything after that goes up in heat not extra output. If the sub doesn't have any extra headroom doesn't matter if you throw 5000W to it, its not going to get any louder. Subs do have limitations even SVS.

    It will be interesting to see if the Crown clips when the sub is set to the same 75db using the same calibration technique. I know my buddies Crown K1 clips on The Haunting. PS- That Crown rep is full of propaganda, many on this forum and others are using QSC @ 4ohms wihtout issues including Dan Wiggins of Adire.

    Ted, am I reading you right, you are just running the Crown with no load connected. Don't bother with that as it means nothing at this point.

    One question we've all failed to ask, when your watching your bassy scene how did it sound it sound up to that clipping point? Loud as hell? If so I doubt you had a problem.
     
  14. Vince Maskeeper

    Vince Maskeeper Producer

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    I dunno- I'm running a QSC powerlight with only 700 watts to my SVS. Calibrated @ ref, I have never once clipped the amplifier (small room, so I'm really not pushing that hard to get ref). I have however, with only 700 watts, bottomed the driver a few times.

    I assumed with an amp able to produce 2500 watts peak, showing a steady clipping indicator, that at least the driver would have bottomed if in fact the amp was supplying 2500w.

    Certainly TPM or the HAUNTING will drive a sub much harder than the average film, and thus a peak here an there in those soundtracks is certainly not the end of the world-- but it sounded like the original poster was getting clips often on majority of titles.

    This points to either the original poster is seing the signal light and thinking it's the clip light--- or that there is something not right with that amp.

    Regardless of what soundtrack were dealing with: I would say a 2500 watt amp on a SVS @ 4 ohms with no limiting should easily run out of headroom on the driver (and make it bottom out) before it runs out of amp headroom and clips. Or, one would assume. IN this case he's getting "clipping" at the amp without slamming the driver-- which makes me wonder immediately.

    -V
     
  15. TedO

    TedO Stunt Coordinator

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    The amp is now fully hooked up (to the sub) and calibrated. I calibrated to the receivers test tones, all speakers to 75db. What seems to work best was the amp’s gain control is set just above half way and the receivers level for the sub is set to +.05db. I did not change any of the other speaker settings. My receiver also has a max bass limiter. When you set this it runs a bass signal (not sure what tone) into the sub. You continue to turn it up, and then back it down a number or two. This sets the max bass signal the receiver will send to the sub to protect it. With the Crown this max setting went from 11 to 8, big difference in SPL, the extra wattage helps the sub put out more SPL before it distorts.

    I have been using the scene in Pearl Harbor were the USS Arizona blows up as my base line for performance. This scene would always clip the amp and make the sub distort if the volume was up too high. With the Samson S1000 the max volume for this scene would be –5db on the volume control and the amp’s clip light would flicker then light during this scene. Now with the Crown, I can run it at 0 without the sub distorting or the clip light even flickering.

    Obviously, I either had it setup/hooked up wrong, the amp had a problem or the QSC just didn’t work well with my Pioneer.

    My recommendation would be to buy/use a Crown CE in your HT because:

    1)Very attractive price, $599 ($399 CE-1000) new at zzounds.com

    2)Adjustable input sensitivity to .773v

    3)Lots and lots of power, 1950 watts @ 4 ohms bridged

    4)3 year no fault warranty

    5) The fan is quiter than the QSC (not as quite as the Samson)

    6)MADE IN THE USA

    TedO
     

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