Thanks for all your help! You've set my mind at rest!

Regards,
Chris
--
'Therefore, if anyone is in Christ, he is a new creation;
the old has gone, the new has come!' - 2 Corinthians 5v17


"Trevor Wilson" wrote in message
...

"Christopher Hill" wrote in message
...
Hi!

(Sorry about the previous post, clicked Send too early. Please read this
one).

I'm a complete PA system novice, but I've been trying to work out how to
upgrade my church's sound system to include some volume controls. Done a
lot
of reading/researching on Google, and I've got the hang of some of the
concepts. I'm hoping you guys/gals can help me with the rest!

Currently, we have what seems to be a 100V constant voltage sound system
that's set up like this:

**It's not really "constant Voltage".


The amplifier (no model, no make at all, helpfully) is directly connected
to:

--It is helpful to know how much power output the amp is. Setting up 100
Volt systems is easy. You just add up all the power settings of all the
speakers and ensure that the figure is lower than the maximum power output
of you amp. (within reason, of course. After all, a 10% overload won't
matter much)


1) 2x 'Bouyer Type RC15' speakers.
At the back panel of these is the specifications which says that for a
100V
system:
Tap 1: 20W, 500 ohm
Tap 2: 7W, 1400 ohm
Tap 3: 2.5W, 4000 ohm
I'm not sure which tap is actually in use, but I'll take a look next time
I
see it.

**If you only have two speakers, use the 20 Watt tap, provided the speaker
can cope and the amp can deliver at least 80 Watts (most 100 Volt amps
can).


2) A box which contains a 100V 'Line Matching Transformer', which is in
turn connected to 2 fairly ordinary hifi style speakers. This transformer
has a number of taps, and a kind of 'volume control' selector switch, but
the volume control isn't wired at all. Instead, the wires from the amp
are hard wired onto one particular tap (can't remember which one). The
speaker side is connected to the 4 ohm connector. I also don't know what
impedance the speakers are, but I would guess that they should be 8 ohm
speakers... as they are connected in parallel, and it's on the 4 ohm
connector. Forgive me if my understanding of impedance is flawed!

3) An individual speaker which has another 100V 'Line Matching
Transformer' inside it, with a volume control which connects the speaker
to the different taps on the 100V side depending on where it's set. It's
an 8 ohm speaker.

Here is my question.

I would like to make it possible to have volume controls on all of these
speakers so that if necessary I can just have one set running at the
volume that I choose.

**No problems. Just use a switch to move the connections from tap to tap.
Lower power taps will provide lower sound levels. Alternatively, you can
buy a 100 Volts volume control, which is, in reality, a switch connected
to a couple of transformers.


However, I'm a little worried about various statements that I've found on
the net about not overloading your amplifier. Specifically, that the
impedance of the speakers must be higher than the minimum impedance of
the amps, and that the load mustn't exceed the maximum load of the amp.

**Don't sweat it. Just add up the power figures on the transformers and
ensure that the total is within the maximum power rating of the amp (give
or take a fudge factor).


In particular, I'm worried that the volume control for 2) was never wired
in in the first place. Was it just an oversight or was it because if you
turned down 2) and 3) at the same time, would you blow something up? Is
that why it was never wired in?

**We have no way of knowing. Maybe it was unreliable (many are), or maybe
they decided it was a waste of time (most do).


If I were to put volume controls on all of these three by rewiring the
transformer in 2) to use the selector switch, and using something similar
to this:
http://www.portset.co.uk/zone.htm
for 1), and I turned *down* all of the speakers, would I blow up the amp?

**Nope.

As
far as I can tell there's no problem running the speakers at full volume,
because that works at the moment, so the load at that end must be ok. I'm
just worried about messing something up if I were to turn things *down*.

**Don't sweat it. Just aqdd up the MXIMUM power figures of each speaker
and ensure it lies below the maximum power output of the amp.


My understanding of impedance is the higher the watts (which means higher
volume...?), the lower the impedance, so surely if I were to turn them
all *down* I would be *increasing* the impedance and so this just isn't a
problem?

**You're over-analysing it. Just add up the power figures.


Is it possible to *underload* an amplifier as well as *overload* it and
how can I find out if I'm going to do that?

**No.


Thanks for your help. I hope it's all fairly clear!

**Yep.


--
Trevor Wilson
www.rageaudio.com.au