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From: kludge@grissom.larc.nasa.gov ( Scott Dorsey)
Newsgroups: sci.electronics
Subject: Re: Design idea: PA and Music Amp questions
Message-ID: <1991May15.132559.8441@news.larc.nasa.gov>
Date: 15 May 91 13:25:59 GMT
References: <72314@microsoft.UUCP> <1991May14.164451.17@cmkrnl.uucp>
Sender: news@news.larc.nasa.gov (USENET Network News)
Reply-To: kludge@grissom.larc.nasa.gov ( Scott Dorsey)
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In article <1991May14.164451.17@cmkrnl.uucp> jeh@cmkrnl.uucp writes:
>In article <72314@microsoft.UUCP>, gordonl@microsoft.UUCP (Gordon LETWIN)
> writes:
>> I'm building a house which will have good quality ceiling speakers in 
>> most rooms.  They'll play music at low volumes and act as alarm/PA/Intercom
>> speakers at higher volumes.
>> 
>> Hooking say 20 pairs of speakers in parallel provides too low an impedance
>> for the amplifier.  
>
>Gordon, you need to talk to someone who knows about PA systems... NOT sound-
>reinforcement systems, and NOT home audio... and ask about 25-volt and 
>70-volt lines.  I don't know much about them myself... but I do know that 
>that is how this problem is addressed in, for example, school and office
>paging systems.  

  70V systems use an amp with a constant voltage output driving a large number
of high impedance speakers.  Usually the speakers are stock 8-ohm beasts with
a transformer whose primary is in the 1K-20K ohm range, and whose secondary is
8 ohms.  Speakers with high Z voice coils are also available.
  These systems sound godawful because of the distortion induced by the
transformers.  It would be possible to make good quality transformers with
reasonable power handling too; I might consider using output transformers from
tube amps.  But this will cost you more money than you want to pay.

  Paralelling all the speakers is a bad idea, because you'll wind up with an
impedance in the sub-ohm range and it's difficult to build an amp that will
drive low impedance loads properly.  It can be done, but it will cost you a
whole lot of money.

  Putting all the speakers in series is an improvement, because this way you'll
have a very high impedance, so your amp will need a very high voltage output
and a reasonable current level.  (In fact, you could probably build a tube amp
that didn't have an output transformer, which would be kind of interesting).
The problem here is that your speakers had better all be identical.  If the
efficiency varies between speakers, so will the sound level, and it'll be
astoundingly different.

  Your third alternative is to wire several banks in series, and then wire
those banks in parallel (or vice versa).  This way you can get an impedance
of your choosing (and I suggest you choose 8 ohms or so).  Not only that,
but you can pad down each bank individually and give yourself zone controls
for the levels in each part of the house.  Since your banks won't have 8 ohms
each, your pads will be somewhat nonstandard, but that's not an impossible
thing to build (especially if you banks consist of series speakers... high
resistance pots are easy to get).

  It'll still sound better if you use an individual amp for each speaker,
because you won't have each speaker influencing the amount of current
available for the other ones.  When you put speakers in series or parallel,
not only are you increasing or reducing the resistance across the line but
also the reactance.  Still, it'll be a lot less expensive and might be
satisfactory.
--scott