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sound amps and bass amps whats the differnce

Discussion in 'Amps and Cabs [BG]' started by quincsta, Aug 26, 2005.


  1. quincsta

    quincsta

    Apr 11, 2005
    I'm lookin at probably buying a new head soon. I started looking into a sepereate amp and preamp. The fender mb1200 is extremly nice. Now i do alot of pro sound stuff ad i know alot and im continously learning and reading. I am wondering whats the difference in an amp used to power foh or live sound speakers and an amp made by fender to power bass cabinets? I mean like the fender cost like 1000 something and up depending on the store and I could probably get a QSC hat could put out the same power for much cheaper.
    Thanks,
    Quincy Knowles
     
  2. uglybassplayer

    uglybassplayer

    Aug 24, 2001
    New Jersey
    I'd love to get me one of thos QSC hats myself, where's Bob Lee when you need him :D ;)

    On a serious note... in theory, there really isn't much of a difference between a "bass" power amp like the MB1200 and any other power amp (other than the Fender is a mono amp compared to the QSC which is a stereo amp). I said "in theory" because even power amps introduce some level of "color" into the signal chain, and it's possible that the Fender may be somewhat less transparent than the QSC. I wouldn't worry so much about that however. I'd make my decision based on 1) whether or not you want/need a stereo amp, 2) the reputation/reliability of the manufacturer and servicability of their product, and 3) does it meet your needs. Oh yeah, last but not least 4) price.

    P.S. Having owned a PLX1602 for a number of years, I can tell you first hand that you can't go wrong with QSC. Having access to Bob Lee on this board doesn't hurt either :D

    - Ugly.
     
  3. Question (hopefully) in the spirit of this thread. I always liked the idea of a separate power amp / pre amp for bass, but never liked the stereo thing. Bridging an amp seems to have some detriment to the sound, and on most amps limit you to an 8ohm load in bridged mode, although I guess that's changing (however..... those that say they go down to 4, which I think results in the equivalent of a 2ohm load per side always make me a little nervous)

    I would love to see 1/2 of one of the big Stewart's, for example, put in a single space rack unit... in other words, a good, solid, honest 800-1000 watts mono that would run cool and totally safely into a 4 ohm load (so you wouldn't need an extra rack space between the amp and pre... keeping the total size to 2 spaces, depending on the preamp, of course). I know Epifani has a large mono power amp in the works (at least it's listed on their site at the moment).

    Is there anything out there like what I'm talking about?
     
  4. quincsta

    quincsta

    Apr 11, 2005
    ok so what would i gain by having a stereo amp rather than a mono. I was lookin at the qsc RMX2450 and it says it'll run at 1200 watts @ 2ohms stereo. Does that mean that i have 1-2ohm cabinet on each end or a total of 4 ohms on each end for a total of 2 ohms altogether.
     
  5. uglybassplayer

    uglybassplayer

    Aug 24, 2001
    New Jersey
    KJung, good question. Sure would be nice to have a lightweight powerful single rack mono amp.

    Quincsta, one of the advantages I could think of would be that since a stereo amp act as two distinct separate amps, you can use one side to power your bass, and the other side to power something else (like another instrument). In the case of any of the QSC amps, you can alway use the amp in bridged mode (which essentially transforms the two amps to work as a single amp), which will make it appear as a mono amp. Note that while the mimimum impedance of each channel in stereo is 2 ohms, but in bridged mode the mimimum would be 4 ohms.

    - Ugly.
     
  6. Stereo power amps also yield themselves well for bi-amping i.e. using a crossover and splitting your tone to different cabs.
     
  7. That would be what I was talking about in my post.... some of the newer stereo power amps can run either 2ohms per side (i.e., having two 4ohm cabs per side, running in stereo), or a combined total 4ohm load when bridged.... I have run into a number of manufacturers who state their amps will run safely into 2ohms a side or 4 ohms bridged, but have heard some horror stories... at the very least, they will run very hot so that you would need to leave an empty rack space to help with cooling.

    Regarding biamping.... that doesn't seem to be very much in vogue anymore, with all the amazing full range cabs out there, etc.

    I think as bass players we are, for the most part, stuck with the stereo power amp thing if we want to go component. And I guess there aren't enough of us to make it economically viable for a company like QSC to product a nice single or two space high power mono power amp that would be the 'bees knees' for a modular bass system.

    I really would like to get half of the Steward 2.1, or whatever it is in a single space mono... would that be awesome.... it would run cool into 4 ohms and put out a ton of power! Oh well...
     
  8. I doubt that running a quality amp at 2ohms would be a problem these days. The car audio industry has stepped up to making amps 1ohm and 0.5ohm stable in the most extremes of heat and cold, I would imagine the pro audio world could make something work at 2ohms. If you put your poweramp in an SKB style rack there should be some space on either side, front and rear, and if you put it on the bottom rack spaces, space underneath.
    Peavey makes a single rackspace stereo power amp that will run 500watts per side at 4ohms and 1400w bridged at 4ohms. No mono amps though.
     
  9. The stereo channels in bridged mono don't "know" they're bridged, they're just amplifying a signal, and driving a load.

    Getting the high output voltages required for high power is simply easier to attain with a stereo amp bridged than for a mono amp. You can get away with half the supply voltage in the power supply for instance, the output stages run under 1/2 the voltage also.

    So do not fear the "bridge", it is the best way to cross over to the higher powered side of the river. It just sounds exotic. All you're doing is feeding the same signal to both channels (one channel is phase inverted) and that's it. The fact you take the output off the "hot" leads of both channels instead of the "hot" and "ground" leads is irrelevant. That's how you get a 220 V line out of 2 out of phase 110V lines. Same principle.

    And you have the flexibility of using it as a stereo amp for pa usage. A mono amp would be limited to just instruments, not pa.

    But since the amp simply doesn't know the difference between running stereo @ 2 ohm load and running bridged @ 4 ohm load, there's no advantage to producing a mono amp with limited marketability, when a stereo amp can do it all, bass and stereo PA.

    That said, a bass amp power supply needs higher sustained current capacity to reproduce predominantly low freqs without sagging, where a PA amp could get by since bass is a much smaller portion of the freq spectrum it is reproducing. So some amps are more suitable for bass than others.

    Randy
     
  10. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Pro power amps are generally designed to be neutral, to introduce no audible coloration, et al … in essence, to be analogous to a "straight wire with gain." Most well-designed amps do a pretty nice job of this, although some are better behaved than others at extremes, like clipping. A preamp, used to boost the instrument's small signal level up to a line level to drive the amp, will provide coloration, EQ, and other tonal manipulations.

    Instrument amps may be designed for neutrality, or they may not. It's all a matter of preference; there's no "right" or "wrong" other than what you and your audience like.

    If an amp is well-designed and not defective, running it in bridged mono will not degrade its sonic quality at all.

    I have a QSC hat, but I have a big head and it doesn't fit well. My TB toque fits a lot better. ;)
     
  11. quincsta

    quincsta

    Apr 11, 2005
    so i can have a combination of example... two 4ohm cabinets on channel one which will produce 2 ohms on channel 1 and the exact same on channel 2 whicn will also be 2 ohms on channel two. So if the amp says it runs at 1200 watts stereo that means i am getting 1200 watts out of channel 1 and another out of channel 2.

    or does it mean that i can put a minimum of 4 ohms on channel 1 and a minimum of 4 ohms on channel 2 and the amp will be powering 2 ohms altogether?
     
  12. I'm going to defer to a TBer who is familiar with that amp... usually the specs talk about the stereo wattage being the separate power at a given load on each side, vs. the bridged power at a certain load being into one cab with the amp bridged.

    So, if the spec says, stereo, 1200 watts per side at 2ohms, then your first example is correct.... two 4ohm cabs driven by each side (i.e., 4 cabs in total), with the amp NOT bridged..
     
  13. quincsta

    quincsta

    Apr 11, 2005
    nice. thanks alot this really helped.
     
  14. What he said. You'd get 2, 4ohm cabs (making it a 2 ohm load) on channel 1 getting 1200 watts, and 2 4 ohm cabs (making it a 2 ohm load) on channel getting 1200 watts, total 2400 watts.

    Note that this is EXACTLY the same load on the amp as running it bridged into a 4 ohm load, presumably 2 8ohm cabs and you'd get 2400 watts max out of that setup.

    That's assuming the amp spec is 1200w per channel@ 2 ohm loads.

    Randy
     
  15. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Putting 4 ohms on channel 1 and 4 ohms on channel 2 amounts to 4 ohms on each channel.
     
  16. Ian Perge

    Ian Perge Supporting Member

    May 11, 2001
    Evansville, Indiana
    I'm assuming that toque keeps your (rack) ears warm as well? ;)
     
  17. uglybassplayer

    uglybassplayer

    Aug 24, 2001
    New Jersey
    Eh', Take off you hoser! ;)