Archived from groups: rec.audio.pro (
More info?)
>
> Just so I understand this correctly, what happens here is that lets say I
> buy a 150W amp. I use the 70V transformer on the output of the amp (but
> before my distribution box) Each speaker would hav e a transform (lets say
> 10 W) so that it only gets 10 W regardless of if 1 speaker pair is active or
> all 8? Is that correct?
YES... assuming a SET CONSTANT LEVEL for what's going through the amp.
>
> I had a second thought (that might be crazy). Since this system is being
> controlled by a computer, I was thinking that if I had the same 150W amp,
> and a bunch of regular speakers (all same Watt) and then had my software
> alter the volume of the computer (which is the input to the AMP) to control
> the overall power? E.g. If 1 speaker pair is active computer volume =10% ,
> 2 speaker = 20%, etc... Is that a crazy / silly / stupid idea?
It's fun
it's interesting
it's WAY more complicated and accident-prone that what you initially said
you wanted.
AND most importantly it does NOTHING about the mismatch of the speaker-load
to the amp.
Say you have a typical 8ohm amp output...
with 8-ohm speakers, you can turn on ONE speaker and have a match
turn on 2 and you have a 4-ohm load... will the amp tolerate that? probably
turn on all 10 speakers and you have a less-than-1-ohm-load... looks like a
dead short to the amp
poof.
A 70v system runs (in essence) runs the AMP always at full output (not
clipping, merely at a set output level) -YOU- then decide IN THE DESIGN
PHASE how much power you NEED at each speaker in the room it's in. Since
there is a STANDARD here (the amp is built so that at full-power, it
averages 70v across the output) You place a special transformer AT THAT
SPEAKER which then is hard-wired to pull some percentage of the amp's
running power into that speaker. the transformer pulls XXwatts off a 70v
line and then matches it to that speaker. Usually these transformers have
several wire choices so you can feed the speaker, say, 1/2w 1w 2w 4w
10w depending on what wires you hook up to the amp line. That speaker will
be louder or softer relative to others depending on how much power it can
pull off the line.
It's a hell of a lot like lightbulbs for different rooms on a single
circuit, some you want brighter, some you want dimmer but they all work off
the same line voltage. You want to be able to turn any one or more of them
ON at any time and not affect the others or blow the breaker.
Say you have 10 speakers and 100 watts (in REALITY you will lose some small
amount of real power going through the transformers... so we'll assume you
have a 120 watt amp but are going to be USING 100 watts to keep the math
easy)
If you plan to use ALL speakers at once at some time, you need to remember
that you only have 100 watts to use amongst ALL of them. Each speaker could
have, say a 10 watt transformer on it so when they're all ON, they;re
together drawing 100 watts. If 5 rooms are bathrooms and the other 5 are
bedrooms, you might want to draw only 2watts for each bathroom, that's 10
watts for the baths and then leaving the rest (90watts) for the 5 bedrooms
(18watts each).
The simplicity here is that the amp is matched to EACH speaker by that
speaker's transformer and, as long as the TOTAL load can never be more than
the amp can deliver, everybody's fine and no matter which speakers are
switched on or off (since the amp is always at the same level and each
speaker is matched to teh amp) no volume change happens to any speaker when
other speakers are switched on or off.
Buy good xfrmrs (cheap ones start to look like shorts at lo frequencies)
You're describing classic mistakes-in-ignorance approaches in your posts and
thus really should heed the first reccomendations you got...
GET, READ, and STUDY either the YAMAHA book on this AND get the DAVIS Sound
Engineering book ... best coverage of it I know.
If you dont really understand this approach, you will, at best have a
confusingly bad-sounding system and more likely just spend much money and
turn it into smoke.
/www.jblpro.com/pages/tech_lib.htm
Facebook
Twitter