Looking formation on whole-house audio -- specifically impedance matching.

Thread Starter

the.doc

Joined Jan 22, 2021
5
I've done lots of internet searches looking for information about how impedence-matching volume controls work. Never found a thing beyond the fact that some of these things use auto-transformers, and some are purely resistive (e.g. L-pads). The problem I have is when you have several rooms, each with its own volume control, and all this is wired in parallel to the amplifier outputs. The cheap solution seems to be to put a 4-ohm power resistor in series with the speakers to limit the load on the amp. The house I'm looking at has 18 speakers; each of them has a woofer and a tweeter and a crossover, which means that wiring some of them in series is out; so they are all in parallel, which is less than 1/2 ohm. The 4-ohm series resistor saves the amp but also dissipates something like 90% of the amps output power.

If somebody can point me to some places that I can read about how this stuff works, I'd greatly appreciate it.

thanks
 

crutschow

Joined Mar 14, 2008
31,122
Some of the auto-transformer type controls have jumper settings for the number of speakers (here for example).
It has settings for different amplifier and speaker impedance ratings, as well as the number of speakers.
For example, the maximum number of 8 ohm speakers for a 4 ohm amplifier is 16, but if you have a decent amplifier, it should readily tolerate the slightly lower impedance (≈3.56Ω) 18 speakers would make.
 

nsaspook

Joined Aug 27, 2009
10,680
Seems like overkill for just a house audio system. :rolleyes:
The guy has 18 speakers. Driving that from a single audio source in 2022 is like driving a model-a car. It works but is several gens behind even 2010 technology. I would maybe look at wireless if the OP wants modern whole house technology instead of systems tied to old limited 70 volt tapped transformer technology.
 
Last edited:

Audioguru again

Joined Oct 21, 2019
5,414
Woofers, tweeters and crossovers were stated so hifi is probably wanted.
The cheap autotransformer has absolutely no audio specs so its lows and highs are probably missing and it probably produces plenty of distortion. Boomy bass like a bongo drum due to poor damping?
 

Audioguru again

Joined Oct 21, 2019
5,414
I did not see the pretty good frequency response spec before.
Its unknown amount of distortion is hidden and there is no mention of losses or damping.

The output impedance of a modern amplifier is never matched, it is much lower than the load for good damping of speaker resonances. Antique toobs (valves) amplifiers had a higher output impedance that matched the speaker impedance.
 

crutschow

Joined Mar 14, 2008
31,122
there is no mention of losses or damping
The damping is just a function of the impedance transformation of the control.
So if it's set to match the impedance of sixteen 8-ohm speakers in parallel to a 4 ohm amplifier output, the transformation factor from amp to speaker is 1:8.
The damping factor seen by the speaker would thus be increased by that same value.
 

Thread Starter

the.doc

Joined Jan 22, 2021
5
The guy has 18 speakers. Driving that from a single audio source in 2022 is like driving a model-a car. It works but is several gens behind even 2010 technology.
Agreed. The house belongs to the son of some good friends of mine. It has Leviton SGC65 ceiling speakers in a number of rooms all driven by a Sonos Amp. All of the rooms have local volume controls; 2 are of the autotransformer type and 3 are Leviton SGSVT (no transformers, no impedance matching) connected to the amplifier through a Leviton SGAMP System Matching Module which has a "Transformer-Free impedance-matching design". Each room is fed by a single pair of conductors from the amplifier, so there is no stereo. There is also an Episode speaker selector with "Amplifier Protection". I haven't looked inside that yet, but I suspect that I'll find a couple of power resistors in series with the inputs when I do.

The Leviton volume controls are just power resistors and a 12-position slide switch. I can see making a variable L-pad from those ingredients. The System Matching Module has no switches or jumpers; no moving parts at all; just a bunch of BIG ceramic power resistors on a circuit board. I can't begin to think how that can do impedance matching for an unknown number of the volume controls.

I can see how an autotransformer and a 12-position switch leading to taps in the winding can be a volume control. I've looked around for an explanation with a wiring diagram that shows how the jumpers change the wiring to multiply the impedance seen by the amplifier. Can somebody tell me where to look for that?

I said that I'd try to help this guy straighten out the mess. I'm not in that field, so I'm trying to learn about how all these things work. Inquiring minds and all that. Now I've just realized that I have two separate things going on in my mind here: one for him and one for me. For him I'd take the Sonos somewhere else, hook it to some decent speakers, and replace it with a streaming receiver and a six-channel amp that I've seen for sale used. $200 or thereabouts.

For me, I've gotten interested in how these things work; what do the jumpers do? And how does that reflect a different load back to the amplifier?

Thanks for reading, and just for the record, I wouldn't mind having a Model A, but I hear they're not cheap..
 
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