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amp for speaker ...

Discussion in 'Pro Audio Equipment' started by perik, May 11, 2007.

  1. perik

    perik Guest

    Hi!

    Im going to buy the following studioamplifier:
    Alesis RA-100, 2x100W without fan, 2u

    I shall use the amp for studiomonitors like event 20/20 and for a small PA.

    My question is will the amp be powerful enough to run the speakers? What shall I think of? The room we are rehearsing in is not more than 12 squaremetres so it doesn’t have to be that powerful, but just enough to run guitar, bas, song etc. through the mixer.

    How many W is suitable for the speakers to that amp?
    How does the ohm affect?
    What is SPL and what is a good value in SPL?

    Will the alesis amp be able to run a pair of speakers that looks like this:
    200W 8 Ohm Max SPL 117dB

    Thanks for your help

    Perik
     
  2. dementedchord

    dementedchord Well-Known Member

    an ra100 will be able to push some 20/20's alright for mixing on but IMO i wouldn't want to use it for a pa... not sure what the alesis is costing nowadays... but for around $300 you can get into the lower end crowns and qsc... how much are you going to use it as PA? how big are the rooms you figure to be working??? what kinda level are you expecting???? it aint exactly rocket science but it takes abit more info than you're thinking i suspect... think it through alittle more and get back at us...
     
  3. perik

    perik Guest

    ok, thanks for your reply

    I have a small mixing studio at 12 squaremetres. My plan is to buy a amp to drive my monitors when Im mixing and to switch to the speakers when we are rehearsing. We are using a jembre drum, and through the speaker through a mixingconsule song, bass. guitar and an unamped accordion. the unamped accordion gets in quit good level unamped together with the amped instruments and song, so there you have got the approximate level Im going to need. enough to mix nicely together with unamped accordion.

    do you think that amp will be able to run the speakers at a satisfying level?

    and if you know some about these to. im not very good at electronics

    How many W is suitable for the speakers to that amp?
    How does the ohm affect?


    thanks

    Perik
     
  4. dementedchord

    dementedchord Well-Known Member

    it will be more than enough for the mixing... you really dont use all that much at that stage... and if it's only for practice for instance it will be ok in all likelyhood but will be a bit underpowered if you will be taking it out for performances...
     
  5. Kapt.Krunch

    Kapt.Krunch Well-Known Member

    That setup is more for mixing than live rehearsal. It's to capture all the loud amps and vocals and stuff, and mix them at a manageable level for playback.

    If you don't care if nothing is very loud while you play, and you all sit close enough to those near-field monitors, and you run everything direct, you can probably use it for that...if you want it to sound kind of like you are mixing live.

    Once you add a drum set or a loud guitar amp, all bets are off for hearing those things.

    Or, I may be misinterpreting your question. Do you have another set of PA/stage monitor speakers that you want to switch to? If that's the case, the RA100 may be enough to hear yourselves sing, if you don't get all the other stuff too loud. And that depends on the efficiency of the speakers and their placement, and what you are running into them. If it's just to monitor vocals, you may squeeze by...depending. If you are pumping bass and other stuff through them, probably not. Just ain't enough headroom in those...although I can run music through a set of ancient old Altecs (15" & horn) with one pretty loud, but those old Altecs were very efficient. They were made back in the tube PA days, where they didn't have 1000W power amps.

    If you are going to have a full band with amps and drums practicing, you'll probably need a bit more to hear the vocals clearly, and a lot more if you want to run everything through floor monitors or PA speakers or something.

    So, you may want to consider that you will have to compromise in one application, or the other. A decent QSC or Crown, as mentioned, with more power would be good for the live stuff, and you may just be able to turn the volume down for mixing...if it has level controls. You just don't want to have to mix with the level controls on the amp way down, though.

    On the other hand, if it's just fine for mixing, but too short of power for live stuff...you just ain't got no more control. Once you get it high enough, and you start pushing channel, bus and master faders up to add more...it's gonna get ugly, fast.

    If your PA/stage monitor speakers can be bi-amped, the RA100 may be enough for the mid/hi at not too loud a volume, but you'd probably want an active crossover and a more powerful amp for the bottom. It may eventually come in handy for something like that, again depending on the efficiency of the speakers...it just ain't gonna get very loud or clear pumping a lot of stuff through full range PA speakers.

    One other thing. Look carefully at the specs. It MAY say something like 100W/channel, 4ohms; 75W/channel, 8ohms. Again, depends on your speakers, and their efficiency.

    Just some things to think about.

    Kapt.Krunch
     
  6. perik

    perik Guest

    thanks for your replies!!!

    but how shall I match amp and speakers. Im thinking about the ohms.

    the specs on the Alesis RA-100 are:
    what does this exactly mean? (as I said I dont know much about electronics, sorry) are some speakers on 4Ω and some on 8Ω and if they are on 8Ω the volume and the power will be less??? or do I misunderstand something?

    yes my plan is to switch between PA speakers while reharsing and studio monitors while mixing and using the same amp for both. just switching the cables. so for example event 20/20 while mixing and some cheap PA-speakers while reharsing. the soundquality is not that important during reharisng. just to amp everything over the accordion.

    but you dont think it will be powerful enough?

    and last question. what is the QSC or Crown shortening for? what does it mean?

    thanks

    perik
     
  7. dementedchord

    dementedchord Well-Known Member

    ohms is a mesure of resistence to a signal... and yes you're getting it right the higher the resitence to flow the lower the power from any given amp and as a result lower relative volume... crown and qsc are amplifier manufacturers... two of the more respected in particular for pa use... having spent years as a repair tech on such things i cose to own amps from both companies... while in the extreme audiophile world they might not be considered sonicly the "purest" in terms of day in day out use they are amongst the best...
     
  8. Kapt.Krunch

    Kapt.Krunch Well-Known Member

    Not necessarily, but probably. It depends on the speakers' sensitivity....(I wrongly previously referred to it as efficiency...but that's the wrong term, though related).

    Sensitivity is measured in a speaker by running a test signal of 1 watt through it with a microphone placed 1 meter away. The amplitude (or SPL - "Sound Pressure Level") that is measured in dB (decibels) is its rated sensitivity.

    You have to understand decibels in terms of sound levels to understand what this means. Google up "decibel", or dB", and look for sound pressure, loudness or volume related expanations. (There's also electronics-related things, which use the similar theories, but will confuse you when you start reading about dBm, dBv, dBu...etc.)

    A simple explanation from the Crutchfield website: http://www.crutchfieldadvisor.com/ISEO-rgbtcspd/learningcenter/car/speakers_glossary.html
    Sensitivity
    A sensitivity rating tells you how effectively a speaker converts power (watts) into volume (decibels). The higher the rating, the louder your speakers will play with a given amount of amplifier power. Sensitivity is often measured by driving a speaker with one watt and measuring the loudness in decibels at one meter.

    The chart below illustrates that a few dB in sensitivity can make a big difference:

    Speaker Sensitivity
    rating Power needed
    to produce a given volume
    Speaker A 85 dB 100 watts
    Speaker B 88 dB 50 watts
    Speaker C 91 dB 25 watts

    A speaker with a sensitivity rating that's 3 dB higher than another speaker's
    only needs half as much power to deliver the same amount of sound.


    Basically, assuming one speaker, it takes about 3dB louder to usually notice the difference much. It generally takes TWICE as much power to get that 3dB. (A 100W Marshall is NOT twice as loud as a 50W Marshall).If you want something twice as LOUD, you generally have to provide TEN TIMES the power. That means going from 100W to 1000W just to get twice as loud. (A 1000W Marshall WOULD be twice as loud as a 100W Marshall....but they don't make them. And even if they did, the speakers they used would probably be different...so that skews things).

    Now to the ohms. If an 8 ohm speaker has a HIGHER sensitivity rating than a 4 ohm one, it's possible that even though the amp is now outputting only 75W (8 ohms) instead of 100W (4 ohms), that the 8 ohm speaker could actually be louder. The amp is using that 75W more efficiently. You'd have to do the math and consider all the variables to figure that out. We can't tell you without knowing the impedence of the speakers and their sensitivity ratings. And that assumes that they used the same test tones. If a different test tone/method were used for each, that could skew the specs. You could get in the ballpark if both speaker's specs read something like "measured 1Watt/1Meter/1KHz".

    And know, too, that ohms is a reference to both resistance AND impedance. Impedance is what a speaker is rated, and if you measure a speaker's impedance with an ohmmeter across the terminals, it will usually read a bit lower...say, an 8 ohm speaker may read something like 6.3 ohms.

    Resistance is generally a fixed opposition in a circuit. 8 ohms is generally 8 ohms.

    Impedance (to a speaker) is usually a reactive opposition that varies with frequencies. When a speaker sees a powerful bass note, the impedance may rise past its 8 ohms. That reduces the efficiency of the speaker at that particular moment. So, 8 ohms is not always just 8 ohms. The sensitivity may have a bearing on how the speaker reacts to its impedance.

    What I'm saying is you have to know all the factors to determine what a particular wattage of amp will deliver cleanly to a particular speaker at what impedance. And, even then, it's a ballpark estimate...a general guideline. You have to use your ears to really know.

    Know this, too. Tweeters and mid horns, especially, are more usually blown by UNDERpowered amps than overpowered ones. What happens is everyone starts jamming, the amps start getting nudged up...suddenly you have trouble hearing yourelves sing. What do you do? First you crank the amp all the way up. When that ain't enough, you start sliding the master fader of the mixer up. If that ain't enough, you start sliding channel faders up. By this time, the vocals can be heard, but they are distorted. Input distortion to horns and tweeters may heat things up so much that they finally give up the fight, and blow.

    An OVERpowered amp, on the other hand, usually has plenty of headroom to not start distorting when turned up. You just have to be careful how MUCH you turn it up, because if transient peaks exceed the rated maximum power rating of the speakers...even though the signal sounded good and clean UP to that point...it may fry the speakers. Still, you really don't want to use a 1500W amp on 100W speakers. You are NOT going to be able to turn the amp up high enough to use it to its potential. Generally, you could probably use an amp that's about twice the power of the speaker ratings and get decent results...if you are careful and don't let someone like Zuzu, my friend, mess with the knobs. :shock:

    My advice is that if you are buying the Alesis new, make sure they have a good return policy. Before the return date is up, connect it FIRST to the PA stuff and see if it works well enough for that application. I'm pretty sure everyone would agree that it's going to be fine for your mixing monitors, so test it in your questionable application first. If it doesn't work for that, take it back. Or keep it for monitoring, and buy a more powerful PA amp later.

    Hope that all helped more than it confused. I may have gotten a fact or theory a bit twisted at 5 a.m., so feel free to correct me. :wink:

    Kapt.Krunch
     
  9. audiotec

    audiotec Guest

    The Alesis amp will drive the 200W speakers, but your output power will only be 75W which can easily be distorted with high levels. The problem is, the Alesis is not a heavy duty amplifier. It is basically similar to using a hifi amplifier for PA use. It probably would do the job if you were only running vocals through it, but adding instruments will put a lot of stress on the amp. It is not designed to perform at sustained levels for long periods of time as sound reinforcement amplifiers are required to do. When using it for studio monitors, you are not running it as hard. This is why pro sound amps are built with heavy-duty components.
     
  10. perik

    perik Guest

    Ok, so you dont think this will work as I want it to? both for PA and studiomonitors?

    So what about Deton T4?
    More specs are here:
    http://www.deton.com/html/amp_T_S.htm

    what do you think about these? Do you think they might work better?
     
  11. Kapt.Krunch

    Kapt.Krunch Well-Known Member

    Deton?

    Never heard of those. I suspect they are a cheap Chinese brand aimed at undiscerning buyers. Their webpage leaves a bit to be desired in the English translation. You have to wonder if they can't take the time or money to translate SOMEwhat properly to another language....how much care are they taking in running those things through the child-labor assembly lines, and what quality of components do they use? How reliable and true are their specs?

    To me, they even look kind of cheap and suspect.

    I dunno about those.....

    There's a lot of more well-known, reasonably reliable and decent quality brands around that probably aren't far from the price of those.

    But...that's just my opinion.

    Kapt.Krunch
     
  12. dementedchord

    dementedchord Well-Known Member

    i'm with the kapt on the deton...
     
  13. perik

    perik Guest

    ok, thanks for your help. I will have a second thought a day or two.

    thanks a lot for your help!!!

    very much appreciated!
     
  14. audiotec

    audiotec Guest

    You could check out the Behringer EP1500 which would give you 280W @ 8 Ohms, which is more of a heavy-duty amp for around the same price as the Alesis. These are good amps if you don't want to spend a lot of money.
     
  15. dementedchord

    dementedchord Well-Known Member

    friends dont let friends buy "B"
     
  16. audiotec

    audiotec Guest

    The Behringer products are not a favorite of many, though the EP1500 does get good user reviews and they are inexpensive. I have used some of their products with good results. But, I did buy a PMH660M to use for practice, which stopped working after only a few weeks. They replaced the unit with a new one and I have had no problems. The QSC and Crown amps definitely have the best sound quality and are reliable, but at a higher price. I have used Peavey amps for a long time and they can take abuse and keep on working night after night. They give the best bang for the buck in terms of power and reliability.
     

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