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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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 (ie, 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 peakers 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

Thanks a lot for your reply Robert!

I have modified my drawing. It can now be seen here:

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I have followed your advice to have one pair of speakers per IMVC. It also means that I need 2 A/B Switch (one per IMVC in that room).

Scenario 1: all speakers set to AMP 1 (assume IMVC set at 8X) AMP 1: 1/64 +1/64+1/64+1/64+1/64 = 5/64 ? 64/5 ? 12.8 Ohm AMP 2: 0 ????

Sceanrio 2: room 3 on Amp 2, others on AMP 1 AMP 1: 1/64+1/64+1/64= 3/64 ?64/3 ? 21.3 Ohm AMP 2: 1/64+1/64 = 1/32 ? 32 Ohm

Are my calculations correct? Aren't 12.8, 21.3 and 32 Ohm too high for the amp? What about AMP 2 in scenario 1, doesn't it see a short?)

From your previous replies to other posts, you mentionned that not all the IMVC needs to be set at the same X level. In my setup, room 1 and 2 will likely always be played louder than room 3. Maybe this changes the X level I should set them at.

Also, please note that the second set of speakers in room 3 will be installed at a later date IF 1 pair isn't enough/doesn't sound good enough in that room. Does that change anything?


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Your new drawing is the correct way to do it. As to figuring the IM settings, you are correct. Most speakers are nominal 8 Ohms. Most amplifiers are very happy feeding an 8-Ohm load. To keep it simple, I tell most people to just set the IM circuits to be equal to or greater than the number of speakers which may be fed from the same amp at any given time. In your case that is 5 pairs. Since the settings are typically in powers of 2 you have correctly selected 8X.

If you have one room for which you would like to provide more power you can safely set that room at 4X. The system will be about

10.5 Ohms. If you set that one room to 2X your system will present an 8-Ohm load to the amp. You already know the math so as long as you don't hit the amp with an inordinately low resistance (high amperage load) you won't hurt anything. Beginning with a low volume level, experiment a bit to see what sounds best.

Don't worry about a higher than spec resistance for Amp 2. That won't hurt anything although purists will point out that radically altered (from design spec) loads color the audio. If these rooms are not primary listening rooms (i.e., it's background music) it won't matter.

BTW, if you need additional parts (speakers, IMVC's or whatever) I sell them online.

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