Noob power amp question

Discussion in 'Amps [BG]' started by tgriley62, Jan 8, 2014.


  1. tgriley62

    tgriley62

    Joined:
    Jan 25, 2011
    Location:
    S.E. Mo
    Home sick today surfing the 'web and thinking of getting my first pwer amp. The one I checked out is the Crown XLi 800. The manual states that is 300w @ 4ohms x 2 or 200w @ 8ohms x 2. I thing I have a grasp on ohms and such but correct me if I am wrong on my idea and questions?

    1. My plan is to get two 8 ohm 210's. Now according to the specs above I would get 400w if I connected one cab per channel but, I think I read that a amp is more efficent at a lower ohm rating and if this is correct then even by lossing 100w it would be best to connect both cabs to one channel for a 4 ohm load

    2. I know that mixing cabs is not ideal for a mono amp but, this does not apply to multi channel ampes does it?. My plan is to start with the two 210's on one channel and if needed add a second 210 or maybe a 115 on the other channel

    Thanks
     
  2. Troph

    Troph

    Joined:
    Apr 14, 2011
    Location:
    Kirkland, WA
    Of those two options, I would recommend the two-channel 8 ohm operation, since it utilizes both sides of the amplifier and produces more total power (200 + 200). You'll have more headroom and slightly more volume.

    Amps running at 4 ohms have to produce more current than at 8 ohms. So running one channel at 4 ohms would be pushing one side of the amplifier hard (~8.7A rms max), and leaving the other side unused. Running both channels at 8 ohms would mean each channel was delivering less current (~5A rms max), and the utilization would be symmetric.

    However, my favorite configuration for a two-channel amp would be bridging the two channels into a single "massive" channel to get the most power possible from the amp, since you really only need one channel and don't need stereo separation. However, that particular Crown amp is a bit odd in that it has a minimum 8-ohm load when bridged, so you couldn't use two 8-ohm cabs. The Behringer iNuke series will permit bridged operation at 4 ohms, and so do several others that seem to be recommended. So I'd consider looking around to see if you can find a different one in your budget that will run bridged at 4 ohms. (I have used a Crown DriveCore XLS series bridged at 4 ohms, but I wouldn't recommend that option because I don't like the relatively high input voltage level required by that model.)

    Mixing cabs is a bad idea regardless. While driving a 2nd cab with a 2nd channel is easier because the 2nd volume control allows you to volume-match the output of the two different cabs, it still doesn't fix the sound quality problems.

    When you add the frequency response of two different cabs together, the resulting response can be muddy and/or unpleasing/harsh, even though each individual cabinet might sound great individually. Occasionally you might find two cabinets that work well together, but more often than not, the result is less pleasing than simply adding another identical cab (which doesn't change the frequency response, so the "sound" doesn't change much).
     
  3. tgriley62

    tgriley62

    Joined:
    Jan 25, 2011
    Location:
    S.E. Mo
    Thanks. I was almost certain that I had read that the lower the ohm the more efficient the amp. Anyway in my situation 2 8ohm 2x10's at 400w is probably more than I will ever need & if I do need more I will just add a third 2x10 for a 500w 6x10 setup
     
  4. swamp_bass

    swamp_bass I love it when a groove comes together Supporting Member

    Joined:
    Nov 20, 2013
    Location:
    North Cackalacky
    Good explanation from Troph; I'm kind of curious as to your overall strategy and budget, though. It sounds like you're building from the ground up, but taking an unorthodox approach to amplification for bass.

    Now I have nothing against unorthodox; I like to test the norm and follow up on my own ideas too. But I'm curious as to what you goal is in this PA amp approach vs. a more traditional bass amplifier head. Care to share?
     
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  6. tgriley62

    tgriley62

    Joined:
    Jan 25, 2011
    Location:
    S.E. Mo
    First I am not in a band as of yet so this is just an idea. Second I have read on here numerous time that using a pre/power amp gives you various options over a standard head/cab set up
     
  7. Troph

    Troph

    Joined:
    Apr 14, 2011
    Location:
    Kirkland, WA
    As I see it, the advantages are:
    1) flexibility to replace just the preamp or just the power amp individually
    2) the power amp can fill in as a PA amplifier if you aren't using it for bass
    3) potentially massive power: in bridge mode, some power amps crank out 2000 watts at 4 ohms, which you're unlikely to find in a standard bass amp
    4) in two-channel mode, you can drive two 4-ohm cabinet loads (or even two 2-ohm cabinet loads on some power amps). Frankly this isn't as common as you might think, as most smaller cabinets are 8 ohms anyway, and few players truly need the stage volume produced by two refrigerator-sized 4-ohm cabs.

    But the disadvantages are:
    1) the form factor is unwieldy compared to a micro head; you'll almost certainly end up putting the power amp in a 2-4u rack case
    2) some power amps aren't well suited for bass amplification; either they just don't produce lots of low-frequency power, or they have low input sensitivity (require high input voltages) which further limits your preamp options.
    3) standalone preamps can get very expensive; there are only a few decent "budget" options, like the BBE BMAX
    4) bass players rarely need a two-channel amp, and running the amp in bridge mode for single-channel operation requires a special speaker cable (which you can make yourself, but you mustn't forget to use!)
     
  8. Stumbo

    Stumbo Wherever you go, there you are.

    Joined:
    Feb 11, 2008
    Location:
    Los Angeles
    You might want to rethink your plans:

    From the manual:
    Electronically balanced RCA & XLR inputs; binding post and Speakon outputs

    Here's a very good amp head for a decent price with 5 star reviews that will take two 8 ohm cabs: http://www.zzounds.com/item--FEN2315800

    There's also a 350 watt version available.

    The both come with effect send/return, aux in, headphone out, XLR Line out (DI), variable input, blendable distortion, 4 band eq.
     
  9. Troph

    Troph

    Joined:
    Apr 14, 2011
    Location:
    Kirkland, WA
    Not sure what you mean. Is there something here which you think won't work? It has unbalanced RCA and balanced XLR inputs, and even an input sensitivity level switch, which should cover all the bases...

    The Fender Rumbles are OK, but nothing special in my opinion. If you decide to look into integrated amps, there are tons of good options, and lots of threads out there talking about them to read. :)
     
  10. walterw

    walterw Supportive Fender Gold Supporting Member

    Joined:
    Feb 20, 2009
    a low-wattage power amp that won't bridge into 4Ω is a waste of money. i'd look for something else.

    also, you wouldn't be "mixing" cabs with two of the same kind of cab; that's matching cabs, a good thing.
     
  11. Troph

    Troph

    Joined:
    Apr 14, 2011
    Location:
    Kirkland, WA
    Yep, I agree, as I pointed out above. Bridged mode is really the most useful operating mode for a 2-channel amp running most bass cabs... And 4-ohm operation is really a must these days.
     
  12. Downunderwonder

    Downunderwonder

    Joined:
    Aug 26, 2009
    Location:
    New Zealand
    As far as twin 2x10 goes your plan is fine, 200w each is ample. That can more than keep up the stage volume you need to play with other GOF's who sensibly use PA for making the big noise.

    Adding a third 2x10 stuffs your dispersion, too wobbly and tall for a vertical stack.
     
  13. dincz

    dincz

    Joined:
    Sep 25, 2010
    Location:
    Czech Republic
    Something wrong there. Balanced RCA inputs are a physical impossibility.
     

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