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Amp advice

Discussion in 'Sound, Music, and Intercom' started by gafftaper, Jun 10, 2008.

  1. gafftaper

    gafftaper Senior Team Senior Team Fight Leukemia

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    Disclaimer: I'm am a competent sound op but not an expert in the noise department.

    We were advised by a theater sound system designer to purchase a QSC CX502 to power a JBL ASB6118 Sub.

    Amp:
    Bridged Mono at 8 ohms 1100 Watts

    Sub Specs:
    Transducer (AES) Power Rating:1200 W (4800 W peak), 2 hrs
    Long-Term System Power Rating:800 W (3200 W peak), 100 hrs

    We cranked it up and BAM distortion on all the big bass notes. The installer says theres not enough head room and it's distorting when it peaks, so I need a bigger amp.

    I'm hoping to return it and upgrade to a QSC CX902 which hits 2000 Watts at 8 ohms, bridged. Does this sound to you like it will fix the problem?
     
  2. avare

    avare Active Member

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    Looking at the specs for the speaker and into consideration you wrote about a theater sound system designer, I have one question immediately. How loud are you driving the system?

    Going from 1,200 W to 2,000 W is less than a 3 dB increase. If 3 dB is the difference, try first reducing the output by 3 dB and see if stops the major distortion.

    Andre
     
  3. David Ashton

    David Ashton Active Member

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  4. DaveySimps

    DaveySimps CBMod CB Mods Premium Member

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    I agree that proper gain structure is key to a good sounding system. As a rule you want your amps to be 1.8 to 2x the watts of your speakers RMS rating. For instance, if you have a speakers rated at 500W than your amp should be at least 900W or 1000W so that you can handle the peaks in your program material. If you go to big with your amp, you greatly increase blowing your speaker.

    Looking at your spicific case, I would inclined to get a QSC CX1102 (if you want to stay in that line of amps). It offers a wattage in the range I listed above. The CX902 might still be a little weak on those peak program signals. The 502 that was recommended to you is definately under sized for your usage in my opinion. This could definately cause the distorted sound you were hearing on the "big bass notes" since these represent more peaks in the program material you were sending to the speaker.

    ~Dave
     
    Last edited: Jun 10, 2008
  5. avkid

    avkid Not a New User Fight Leukemia

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    I tend go on the lower end of the scale at 1.5-1.8 x RMS.
     
  6. DaveySimps

    DaveySimps CBMod CB Mods Premium Member

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    Even based on your preferred formula, that amp that was recommended is still undersized. If you go with the 1.8 value, he will find a workable solution.

    ~Dave
     
  7. gafftaper

    gafftaper Senior Team Senior Team Fight Leukemia

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    Just got word that they will take it back with a 15% restocking fee but I need to upgrade within the QSC product line. I'm really happy because it's been sitting here waiting for install for 6 months so I was sure they wouldn't take it back. I'm checking on the price for that 1102... that's double the power and one of the most powerful amps QSC sells. Sounds like every watt helps.
     
  8. DaveySimps

    DaveySimps CBMod CB Mods Premium Member

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    It especially makes a difference when you are talking about subs.

    ~Dave
     
  9. museav

    museav CBMod CB Mods Departed Member

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    As Andre noted, if you change to the CX1102 you are talking about a maximum 3dB increase and that is only relevant to the maimum peak level if you really push the sub.

    Before you go getting a different amp, I would first verify some other potential factors. Are all the amp settings correct and is it wired correctly, especially in regards to the bridge mode operation? Has the installer set the system gain structure? Are you clipping the amp input or anywhere else in the signal chain? Have they tuned the system and can someone confirm that appropriate filtering and processing, including high pass filtering for the sub, has been applied? Any of these could also be related to the distortion you noted and you probably want to eliminate these potential causes before spending more money on a different amplifier.

    Also, just because you can go potentially go that loud does not mean that was the intent. You might want to find out if there was some target level that was defined, it may be that the system is working exactly as intended.
     
  10. gafftaper

    gafftaper Senior Team Senior Team Fight Leukemia

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    We were running it loud, but speaking loudly, I could still be heard by someone three feet away. Yelling I could be heard 30 feet away in the booth. I would say it was no louder than you would hear in a movie theater. It was definitely not concert volume. The audio being played was a Blueman Group 5.1 audio DVD so it had some big bass in places, but also wasn't loose hip hop bass... we heard the "bad sound" while listening to "Hotel California" too. The technician doing the install was quite certain it wasn't his gain settings.

    The installers are bringing a bigger amp in to test in the next day or two. Just to verify what's wrong.

    Thanks guys.
     
    Last edited: Jun 10, 2008
  11. Eboy87

    Eboy87 Well-Known Member

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    I assume an RMX4050 is out of the question? If you're staying in the CX series, I'd go for the CX1102 if budget allows. Just to be sure, and I'm sorry if this sounds like it's from the department of Duh, but you are using a crossover for this, aren't you?

    I tend to go for the higher side on power for subs, so things like a Powerlight 6.0 or the 4050 are what I'd go for, though you most likely need a 20 or 30 amp circuit to use them to their full potential.
     
  12. gafftaper

    gafftaper Senior Team Senior Team Fight Leukemia

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    It's all going through a Bi Amp Audia Flex DSP. The crossover is set to send everything from all channels below 100hz to the sub. The alternative amp should tell us a lot. The CX 1102 is what I'm going for... looks like it's going to add about $900 to my system cost. Which isn't as bad as it could have been.
     
  13. SHARYNF

    SHARYNF Well-Known Member

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    Keep in mind that most consumer dvd units have a boost on the low end on the 5.1 dolby setups. It might be worth also looking at how the bass management of the LFE signal is set up. Again might not be the case, subs typically need lots of reserve power, so you might be on the right track, also it is worth checking the ac power feeding the amp.
    Sharyn
     
  14. museav

    museav CBMod CB Mods Departed Member

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    Which goes back to the issue of the system configuration. If you are using an LFE output that typically already involves an 80Hz crossover, depending on how the system processing with the 100Hz crossover is configured, you might be applying two crossovers and creating a hole between 80 and 100Hz.

    I do not know your Contractor and they may be completely right, but it seems worth verifying some things rather than just saying they're okay and suggesting you spend more money. Have they verified the amp wiring and settings, I have seen improper wiring or switch settings for bridge mode operation on more than one occasion? Do they have a high pass on the subs at maybe 25-30Hz to keep out very low frequency garbage? Is the distortion associated with any clipping? How are the subs mounted and might that be affecting them (e.g. not having the half space loading JBL assumes in their numbers or getting anomalies from nearby barrier reflections)?

    Based on the ASB6118 98dB/1W/1m sensitivity, you should be getting around 100dB peaks 100' from with the existing CX502, which it sounds like you are not. And going from a CX502 (1,100W bridge mono into 8 Ohms) to a CX1102 (2,200W bridge mono into 8 Ohms) will net you at the most a 3dB gain in peak output while it sounds like you are missing a lot more than that. I would want to make sure you are getting what you should from the existing system before making any changes.
     
  15. SHARYNF

    SHARYNF Well-Known Member

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    I agree with Brad, before paying to swap out to a new amp identify what is the current problem. Sure for max performance the 1.5-1.8 factor is a good guideline for amp power for subs BUT from your description this might not be the problem.

    One common problem in addition to all of the above is back to gain structure. It is easy to forget that amps may have volume controls on the input BUT usually these are AFTER the input stage that can still overload. the problem could be simply that when you are raising the level you are sending a signal to the amp that is overloading the input stage. The degree of distortion would lead me to check this out carefully, the installer should have gotten this right but you know how it goes


    Sharyn
     
  16. gafftaper

    gafftaper Senior Team Senior Team Fight Leukemia

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    We tested the sub today with a 3400 watt amp and it worked perfectly so it was just a barking due to lack of headroom. Now to order the biggest amp I can afford.
     
  17. SHARYNF

    SHARYNF Well-Known Member

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    possibly but not guaranteed, I still would go back and check the levels going INTO the amp and of course check another copy of the same amp.

    Sharyn
     
  18. museav

    museav CBMod CB Mods Departed Member

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    I agree Sharyn. Maybe the situation was misunderstood and the problem was not as extreme as it seemed but going from 1,100W to 3,400W should be a maximum 4.9dB increase and it at least sounded like you were looking for more than that. If you actually got more than a 5dB difference by swapping the amp then that that indicates there might be other factors involved such as a greater maximum input level on the other amp or something else. There are two other things that concern me.

    It sounds like you may not be addressing headroom as much as normal operating level. Headroom is basically the difference between the average and peak levels, for systems headroom is the difference between the average operating level and the peak level possible. If the old amp did not work for normal use and the new amp provides a maximum 4.9dB greater output then you have a system headroom of less than 5dB, which is less than the 10-12dB that might be more typical and thus may indicate other problems.

    Also, 3,400W is 4.25 times the 100 hour long term power handling and 2.8 times the AES rated power handling of your sub. Especially if you are running it that loud on a normal basis, every operator is going to have to be very careful or you are likely going to be replacing drivers regularly.

    Overall it sounds like there may currently be a very fine line between "not enough" amp and "probably too much" amp. Again, this indicates that something may be wrong on a different level, be it gain structure or processing issues or simply the wrong speaker for the application.

    That leads to the question of where is your Consultant? A Consultant is typically involved throughout the project including during the system installation, testing and training. They should also be responsible for their design. It sounds as though the Consultant has not been included in this and they probably should be involved, at least before any final decisions are made.

    I really hope there is such a simple fix but it sounds like there may be more to a real solution than simply throwing the biggest amp you can find at it.
     
  19. gafftaper

    gafftaper Senior Team Senior Team Fight Leukemia

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    In their infinite wisdom, the college cut the sub and amp from the building budget to save $3,200. I'm currently out about $2200 for the amp and sub. I'm out $1900 for hanging (we wanted someone else to take the liability), resetting the DSP, and renting their own Genie lift (we can let them use ours due to liability). Now I've got a 15% restocking fee to pay and another $1000 to upgrade to the new amp which I hope will do it.

    Also when the college cut the amp from the system we also cut the support of the consultant. The current sub and amp were quickly suggested to me by the consultant but he wasn't paid for it, it was just a courtesy in a conversation during a site inspection. So I've been hosed on several levels to save $3,200 on a $4.5 million building. :wall:
     
  20. gafftaper

    gafftaper Senior Team Senior Team Fight Leukemia

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    So Brad Sharyn and others... going back to the original speaker specs and Brad's comment about the amp needing to fit into a delicate balance with just the right amount of headroom without too much power...

    Sub Specs:
    Transducer (AES) Power Rating:1200 W (4800 W peak), 2 hrs
    Long-Term System Power Rating:800 W (3200 W peak), 100 hrs

    Does it sound to you guys like the QSC CX 1102 running 2200W at 8 ohms would hit in that middle sweet spot?
     

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