Impedance Matching Volume Control

Hello! I have searched the archive but can't find what I am looking for. Similar question but mine has a little extra that I can't figure out.

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. 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.

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.

Please see my installation here:

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It shows want I want to do. There are some questions too. All the help is welcome.



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


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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.

That is correct.

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.

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").

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.

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Robert L Bass

If you only run two wires, mono is all you get, transformers or not. It should be pretty obvious that it takes at least three wires (and two sets of transformers) to deliver two channels. It should also be obvious that 70-volt transmission systems can be used for two-channel sound systems -- and in fact are, regularly.

In fact, it is possible to buy 70-volt audio transformers that have pretty decent characteristics (i.e. better than the speakers you intend to drive with them)?

70 volt transmission systems solve a lot of other pesky problems,too; not least of which is the ability to get lots of volume from good speakers in a big room while not blowing out the small ones in a bathroom. IOW, the reason professionals use that method is NOT because it's low in quality or low in cost.


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True, but I believe the gentleman has run separate cables for stereo speakers.

They are rarely used for residential music systems. The audio quality is usually poor. Most home stereo amps don't support 25/70 Volts.

Having worked with both 70-Volt and 8-Ohm systems over the years, I find the former to be almost invariably of poorer qudio quality than even moderately priced conventional systems. Yes, you can find "decent" quality 70-Volt components. However, for similar cost you can get superior quality 8-Ohm systems.

Actually, the primary use of 70-Volt systems is to get cheap sound out of *lots* of speakers.

Professional home audio installers rarely select 70-Volt systems and for good reason. FWIW, I've designed and built custom home theaters and multi-zone entertainment systems for years. I also sell these systems online to DIYers. I have access to various

70-Volt systems as well. I choose not to offer them for residential installations.
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