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Just bougt a AI Contra - Wattage Question

Discussion in 'Amps, Mics & Pickups [DB]' started by elvinstheman, Sep 13, 2005.

  1. elvinstheman


    Jun 16, 2005
    Seattle, USA

    I just bought an Acoustic Image Contra 310BA amp. It was listed as a 300 watt amp but does anyone know if it's 300 watts without an extension cab and is more like 150 or 200 with just the built-in speaker? Or is it 300 watts with just the onboard speaker?

  2. drurb

    drurb Oracle, Ancient Order of Rass Hattur; Mem. #1, EPC

    Apr 17, 2004
    An amplifier wattage rating is independent of what is to be driven.
  3. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Just a quick look at the AI website... the Contra is rated at 200 Watts into 4 Ohms and 300 Watts into 2 Ohms. The website does not say whether the internal speaker is 4 or 2 Ohms, but 4 is a good guess. Also, if this is the same power amp as the Clarus, then it puts out roughly 120 Watts into 8 Ohms.
  4. Tbeers


    Mar 27, 2005
    Chicago, IL
    I'm pretty sure that the Contra type speakers are 4-ohm. Add one and you go down to 2-ohms, at which point the wattage of the amplifier is 300 watts.
  5. Jazzman


    Nov 26, 2002
    Raleigh, NC
    The maximum possible output without the extension speaker is 200W because the internal speaker has a 4 ohm nominal impedance. Drop the load to 2 ohms and it will be capable of outputting 300W.
  6. Not true.

    A solid state amplifier develops more power into lower impedance loads. So you could call it a 300W amp if that's its maximum output into a 2 ohm load, but it would mislead anyone using an 8 ohm load into which the amp can only develop say 100W.

    In other words, to market an amp you quote its maximum power output, but to be sure of getting that as a user you need to read the specs carefully. That may mean, as folks here have pointed out, adding a second speaker.
  7. I thought my Contra's internal speaker was 8 ohms or so I thought I read in the manual - I'll check it tonight.

    BTW, and I hesitate to meniton this in the face of all the experts around here, but as I remember it the impedance of a speaker varies with the frequency and gets lower with the note. If you get a driver from Eminence they quote these pararmeters. Which all goes to explain how a guy I met completely failed to understand why two 8 speakers wired in parralel to give a 4 ohm load to match his amps 4 ohm capability kept blowing the generic power amp module's fuses. (I had a plan to do this but got wise before the event for a change.)

    As many a thread on this subject has pointed out - headroom matters.
  8. Jazzman


    Nov 26, 2002
    Raleigh, NC
    Impedance does vary with frequency...that is why speakers are rated at a minimum nominal impedance.
  9. drurb

    drurb Oracle, Ancient Order of Rass Hattur; Mem. #1, EPC

    Apr 17, 2004
    No, what I said IS true. Here's what I said:

    "An amplifier wattage rating is independent of what is to be driven."

    I admit that my response may have been too terse, but it was not incorrect.

    Part of the wattage rating IS the impedance. If an amplifier is rated as 200 watts into 8 ohms or 400 watts into 4 ohms or whatever, then the particular 8-ohm or 4-ohm driver does not matter in terms of the rated power being developed into it. Of course, we are discussing nominal impedance here and, of course, drivers will differ in terms of what they each produce acoustically.

    As far as adding a second speaker goes, you might be interested in my post regarding stacking/combining cabinets.

    Finally, while the vast majority of solid state amps develop more power into lower impedances, it is not a function of their being solid state, per se, and not all of them do. It really comes about because the output impedance of most solid state circuits is quite low meaning that they never deliver what is called "maximum power transfer" with, say, 8-ohm, 4-ohm, or even 2-ohm loads. Maximum power transfer is achieved when the load impedance matches the effective internal impedance. You'd need a matching trannsformer to achieve that with real speakers and some solid-state amps actually have those (not many, though).

    For most solid-state amps, what is given up in terms of maximum power transfer is gained in damping factor (the ratio of the load impedance to the internal, or output, impedance). The low output impedance of the amp effectively "shorts" the back-EMFs (voltage) from the driver leading to more control of the cone. That's important for bass players.

  10. I checked my Contra's manual (mines a 410BA) and it doesn't say - I still think its only plonking out around 150 into 8 but can't remember where I read this, but as Drurb explains in his most informative post (thanks Drurb, this can have advantages. The definition out of the Contra is really quite hi fi. Bob Grohlier may know? or e-mail AI? Does it matter? It's either loud enough as it is or it isn't and you need an extension speaker. Watts doesn't mean volume. Also, if its running into 8 ohms that's really rather comforting since you know its got more to give?
  11. elvinstheman


    Jun 16, 2005
    Seattle, USA
    When it gets here we'll see how it is. (Should be here next Monday.) But I tried a Series II last week and was blown away by how accurately it reproduced the sound of my DB. It was unreal!! I just hope the one I bought will be as good and I think it will. What a difference from the cheaper I was considering - sheesh!!

    Thanks for everyone's very helpful replies! I'm sure it will work fine for my purposes.
  12. I've just come back from a gig in another cramped playing situation - the sort of place where you can never put a speaker where you want it to sound right and so the band can hear it. This sort of thing just ain't a problem with the AI that is less senistive to placement than anything I know and everyone else can hear it clearly. That is worth a hell of a lot - well to me it is.
  13. Hi DRURB, I'm not disputing your knowledge of amplifiers, which you've so eloquently demonstrated.

    However I still take issue, perhaps on semantic grounds, with the bald statement that "An amplifier wattage rating is independent of what is to be driven."

    This taken literally would lead someone to suppose that if an amp is described as 300W they could connect any speaker, even a 16 ohm one to the amp and it would develop its full 300W into it, which is clearly not the case. Sure, the amplifier doesn't care what the name of the speaker manufacturer is; it doesn't care whether the speaker box has a single 15" driver or 8 10" drivers; it doesn't care what the efficiency and hence the acoustic output is - but it surely does care (as far as its power output goes) what the total impedance of its load is.

    So could I suggest modifying your statement to "For a given load impedance, an amplifier wattage rating is independent of what is to be driven"?

    Your point about damping factor is well made, and of course, since we're talking about impedance rather than resistance, it's frequency dependent. This how a designer can tune an amp's sound. It was said that the best-selling hi-fi amp from British manufacturer NAD was actually tuned to have its most controlled response around kick drum frequencies, which is why people thought it was a more powerful amp than its rating suggested.

    I'm interested in your experiences with solid state amps using transformers. I thought they were usually 100 volt line systems that used these - the sort that drive tannoy systems in buildings and have very long cable runs to cope with?
  14. drurb

    drurb Oracle, Ancient Order of Rass Hattur; Mem. #1, EPC

    Apr 17, 2004

    Doug-- we are in violent agreement. ;)

    My statement was too terse and I agree with your modification which was only implied. I really thought that the question being asked was whether the driver, per se, mattered.

    Actually, McIntosh still makes solid-state power amps with output transformers. From their site:

    Output Autoformer

    Output Transformers, a feature of many of our amplifiers, assures that you get all the power you paid for. Other amplifiers, optimized for 8-ohm speakers, have warnings not to use with many speakers that are 4 or even 2-ohm impedance. Others are optimized to work with these low impedance speakers, but deliver only a fraction of their potential power with 8-ohm speakers. Output Transformers avoid this problem, with separate connections for all three popular speaker types, insuring full power, without stress, into any speaker regardless of impedance.

    Here's even more:


    What it seems McIntosh is doing is balancing power transfer against damping factor in a very clever manner.

    Speaking of Tannoy-- I am in my office sitting next to my 1951 15" Tannoy Monitor Silver coax which is driven by a Bell 2200C made in 1954. This is the equipment to which I literally grew up listening. It belonged to my dad and I restored the Bell over a five-year period.

    Oh wait, I have to get this post back on topic-- the Bell and the Tannoy produce great bass!!!
  15. Okay, let's agree to ... er... agree.

    Interesting stuff from McIntosh. They're old valve hands so they understand transformers!

    I bet that Bell/Tannoy combo sounds nice. I use Tannoys for my home hi-fi - they're D 500s and are about 10 years old. They've got an 8" dual concentric and another 8" bass driver in a trapezoidal cabinet about 3 feet high. I've got two monobloc power amps from B&W driving them. Don' need no steenkin' sub woofer!

    Oh, and they're veneered in rosewood, which...er...is somtimes used for double bass fingerboards (pathetic attempt to stay on topic...)