Re: Impedance Matching Volume Control

["Robert L Bass" <robertbass1@xxxxxxxxxxx>]

> Look into 70 volt (70.7 V, to be precise)
> audio distribution systems.

I disagree.  70-Volt (and 25-Volt) systems are primarily used for offices.  Most are mono -- not stereo.  Sound quality is what you
expect in an elevator.  The gentleman is installing a multi-room stereo system and is likely expecting far superior sound to what is
available in most 70-Volt systems.

Below I have inserted my reply to the same query in another HA related newsgroup.

> I want to distribute the sound to some
> speakers at various location in the
> house and have control of the volume
> locally.  Using Impedance Matching
> Volume Control (IMVC) seems to be
> the solution...

That is correct.

> However, If I want to add an A/B switch
> in one location so that I can use the
> speakers in that location for the central
> source or the local source, it changes the
> impedance of the central circuits.

Switching one set of speakers out of the central amplifier will not hurt anything.  It will lower the load on that amp but that is
not a bad thing.  If you're using auto-former (virtually all popular impedance matching VC's are auto-former)  volume controls,
changing the level or even switching out one room will not affect the volume in the others.

In short, go ahead.

> I've been told I need to use a speaker selector
> box (and leave them all ON if I want) but I am
> not sure I understand...

That's bullshit.  You don't need a selector box.  Just set the IM circuits on each VC at least as high as the number of speaker
pairs you're installing (i.e., if you have 6 pairs, set the VC's to "8X").

> Please see my installation here:
> http://img518.imageshack.us/img518/4228/imvclc1.jpg

Nice drawing.  Scratch the selector box.  Switching A/B won't adversely affect anything.  The only issue I see is you seem to be
planning to install 2 pairs of speakers per volume control.  It would be far better to install one VC for each pair of
speakers.  With 5 pairs of speakers, set each IM circuit to "8X" and you'll be fine.

BTW, I used to install these systems for a living.  Now I just sell them online.  Call if you need more help.

--

Regards,
Robert L Bass

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