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Yey another pa question

I've seen pa systems running four speakers while only using and running one power amp also i have been researching pa packages and some offer 4 speakers and only one amp presuming the amp only has the option of 3 speakon inputs how do they connect all four apeakers to one amp and what happens to the spread of overall power and per channel of the amp when four speakers are driven by this one amp. Is this approach called speaker daisy chaining if so/if not how does this work.?
Thanks people


JoeH Mon, 11/27/2006 - 09:19
There are several ways this can might be a stereo power amp, for starters. That would allow two 8 ohm speakers per amp, one per channel, all at 8 ohms. Double the speakers (assuming it's all parallel wiring) and you're now at 4 ohms per channel output, and in most cases, with today's modern amp designs, all's well.

The general rule of thumb is to half the resistance for every doubling of speaker. In other words, if you're putting two 8 ohm speakers in parallel on one amp channel, then the amp will now "See" a 4 ohm load. Double that, and you've created a 2 ohm load. Most modern power amps will handle this sort of thing fairly well, but there's of course a performance trade off; the amp will run hotter, the power available to each speaker will drop, (bass/low end droops); although it may sound OK in general, available power is cut by the same math, etc. etc.

If you're going to add more speakers to a poor, unsuspecting power amp, at least do it pairs so that it's looking at a balanced load on each channel. (Note: Some stereo power amps are really two separate mono blocks in one rack case, but most aren't and share the same power supply.) Best to make sure you've got 2, 4 or 8 ohms loading on BOTH channels, at least the draw from the power supply is balanced, regardless of the wisdom of heavier loads on the amp itself.

Savvy contractors & installers can run multiple speakers (all over a church or a hall, for example) and get around this sort of thing somewhat with a combination of series/parallel wiring as needed. In some cases; others convert the signal to 70 volt lines for long runs and multiple speakers (QSC has this option available on many of their contractor amps) but there's a performance cost on that as well.

Other folks (esp naive or lazy hotel AV people) love to daisy-chain speakers together, (I once saw as many as EIGHT on one channel!) in parallel all over a typical corporate ballroom meeting. I've seen it done many times, and it's appalling, but it usually works well enough to limp along for speech and powerpoint presentations. Of course, when one fails, usually they all fail, esp if you're running in mono. It's always best to have a multichannel amp to send dual lines out to the speakers in the house to prevent just this sort of catastrophe.

If you're at the entry level on this sort of thing, you probably should crack a book or two on ohms law/resistance theory, speakers, amplifier design, damping factor, etc. It sounds more complex than it really is; once youv'e gotten a handle on the way this part of a sound system works, it'll make sense for you for the rest of your career.

Hope that helps some.....

Pro Audio Guest Mon, 11/27/2006 - 10:35

Thanks man that was really helpfull i'm considering purchasing the mackie
Mackie SRM350 Active PA Speakers two of and then a smallish mixing desk with phantom power for around £200. Can i then update my pa in the future with another pair of the same mackie speakers linking them together is this recommended and can these speakers do this without compremissing power and sound qaulity etc as i'm really not to sure.

p.s Ive just purchased the live sound manual.

Thanks for your time again