Meditations on multi-zone speaker impedance – Oooohmmm!
Quite often we meet people who want to connect 4,6,8 or more speakers to their amplifiers. Only a few days ago, we were asked to help someone who had bought a house that had 8 speakers built in the ceiling. The customer wanted to have 6 of those speakers (spread across the kitchen and dining area) to be connected in such a way that the 6 speakers would play the same music, but separate from the other 2 speakers (that would be used for the audio coming from the TV).
To tackle this, we’ll have take you back to your high school days…Don’t worry, you can do it.
Who remembers the basic laws of electricity? Does the name Ohm ring a bell? Remember how you played with electrical resistors in series and in parallel? Not to mention the mathematical equations that came with them…. They should have told you then that these basics are essential in the world of audio, speakers and amplifiers. Let us explain.
What we are really talking about is a typical “2 zone” setup, which requires a “2 zone” AV receiver. So far so good, right?
We’ll start with connecting the 2 speakers to play the audio from the TV and call this ‘zone 1’.
Doing this is simple: connect one speaker to the “front left” output of the AV receiver and one speaker to the “front right” and connect the TV to the AV receiver with an HDMI cable.
First little challenge here is that usually the TV is connected to the HDMI output of the AV receiver, so the AV receiver is sending audio to the TV, not the other way around. The solution to this challenge is called “ARC” or “audio return channel”. With ARC the TV can send its audio to the AV receiver even though the HDMI cable is connected to the HDMI output (not input) of the AV receiver. This is only possible when both the TV and the AV receiver are “ARC” enabled. Most recent TVs have an “ARC” HDMI output, and the better AV receivers have ARC output as well. So with that under control, all should be well in ‘zone 1’.
The bigger challenge is to connect those 6 speakers in the dining area and kitchen to the “zone 2” output of the AV receiver. Our real aim is to be able to play music from a different source than the TV in “zone 1”. The reason for our troubles is that all AV receivers only have stereo (2 channel) outputs for their Zone 2. That means they have 1 left channel (2 wires per channel, one red wire and one black wire) and 1 right channel. Those 2 channels can connect 2 physical speakers. Not 6 speakers…
And that is where Ohm’s law comes to the rescue. Let’s refresh our memories.
In an earlier blog we explained a bit more the detail about how speakers (drivers) are built (remember the coils, the magnets, etc.?). In this blog, we will oversimplify the electrical nature of a speaker. For our purpose of solving this problem we will call the electrical resistance or “impedance” the ‘electrical key’ to what translates the electrical audio signal into moving air causing sound.
The measure for impedance (the property of a speaker that restricts the flow of electrical current through it) is called “Ohm”. Typical speakers have impedance ratings of 4 Ohms, 6 Ohms or 8 Ohms.
Why are Ohms important?
Two reasons:
(1) If you connect your amplifier to the wrong speaker impedance, you risk damaging the amp. In an ordinary amp, if the speaker impedance is too low, the amplifier will tend to overheat and more power is required from the amplifier than it is designed for. Too many speakers at tool low impedance on an amplifier can burn up the power output section.
(2) The amplifier will deliver maximum power (volume) to the speaker when the speaker impedance matches (is equal to) the internal impedance (called the ‘output impedance’) of the amplifier. Too high a speaker impedance will result in weak output and poor tone.
This brings us to Ohm’s law: Ohm’s Law states that in an electrical circuit, current flow is directly proportional to voltage (pressure) and inversely proportional to impedance (restriction).
For those lucky ones amongst you who were able to play with resistors at high school, you may (or may not) remember that if put resistors in series, Ohm’s law applies. The law says that you have to add the impedance of all resistors in series.
Translating that to our speakers assuming that you have six 8 Ohm speakers in series, you add 8 Ohm + 8 Ohm + 8 Ohm + 8 Ohm + 8 Ohm + 8 Ohm= 48 Ohm as the total impedance of the 6 speakers combined.
The issue with this is as we mentioned before that the the amplifier will deliver maximum power (volume) to the speaker when the speaker impedance is the impedance as specified by the amplifier’s manufacturer (most common is 8 Ohm). Since we are dealing with 48 Ohm speaker impedance while the AV receiver expects to see 4, 6 or 8 Ohm we are clearly not in an ideal situation.
It would mean that the speakers would only get 1/6th of the power they would normally get from the amp at 8 Ohm impedance. In other words, the speakers would play music extremely quietly, and you would push your amplifier’s volume so high it would go into “distortion” (see also our earlier blogs).
Not good….
A second scenario, is where the speakers are not in series (daisy chained) but in “parallel”. This means that all cables of the six speakers are coming together in 1 place like a star.
(In speaker world this would mean that you connect 6 “red” wires from 6 speakers to the left channel output of “zone 2” and 6 “black” wires from 6 speakers to the right channel output of “zone 2”.) NO!
You see, back to Ohm’s law again, it tells us that we have to apply a mathematical formula to our 6 speaker parallel series setup.
1/total impedance = 1/impedance speaker1 (8 Ohm) + 1/impedance speaker 2 (8 Ohm) + 1/impedance speaker 3 (8 Ohm) +1/impedance speaker 4 (8 Ohm) + 1/impedance speaker 5 (8 Ohm) + 1/impedance speaker 6 (8Ohm)
So, in the case of 6 speakers in parallel with each speaker at 8 Ohm, the AV receiver will see a resulting impedance of 6/8 Ohm or 0.75 Ohm. Again, not good!
Why? Remember how the AV receiver expects to see 8, 6 or 4 Ohm? If we present a total impedance of 0.75 Ohm we will “draw” so much current flow (in Ampere, remember there is a connection between current flow and Impedance) from the AV receiver that we will “suck the life” out of the amplifier and shorten its life span considerably. You basically kill the amp!
The worrying issue is that it happens more often than one would like to know for people to connect speakers in a multi-zone series or parallel set-up with an impedance mismatch between speakers and amplifier. In doing this they are unknowingly damaging the amplifier and/or resulting in poor audio performance.
So what is the solution?
There are a few.
First there are the likes of ‘Russound’ who build multi-zone amplifiers designed for multi zones. These are amplifiers that are dedicated to powering 4, 6 or 8 pairs of speakers in 4,6, or 8 different zones for 4,6, or 8 different inputs. In our example, we would have to use zone 2 on the AVR and then an extra 2 output pairs of a multi-zone amplifier to feed the 6 speakers.
A much cheaper option is to connect the 6 speakers to a “speaker selector”. This is a device that allow you to connect the 6 speakers to the speaker selector, and the speaker selector itself would then be connected to the “zone 2” output (left and right channel) of the AV receiver. The speaker selector has a built in “protection” to make sure that the impedance it presents to the AVR is not lower than 4 Ohm.
As a bit of an additional warning, we would like mention that contrary to what you would expect, most AV receivers have limitations as to what source (or input) you can play in “zone 2” depending on what you are concurrently playing in “zone 1”?
Often it is not possible to stream Spotify in zone 1 and a different Spotify song in zone 2 (or even a different source than Spotify). Check with Rapallo because it really depends on each brand and even different models within the same brand.
Last but not least…be careful to read the fine print about zones on AV receivers and the number of surround channels.
With certain AV receivers when an AV receiver states that it is a “7.1 channel” AV receiver as well as allowing for ‘2 zones’, you are “sacrificing” 2 channels in the surround zone. In other words, the surround zone automatically goes from 7 channels down to 5 channels, if you are using 2 channels for “zone 2”.
The smarter AV receivers (For example the Yamaha RX-V679 and higher models) allow to dynamically use the 7 amplified channels in the amplifier: if you are not playing music on “zone 2”, you can use all 7 channels in the surround zone.
So now you hopefully understand why it is not that simple…..until you understand the detail, and then it really is not that hard.
Let us know if you need more info for your personal audio-visual set-up.
Sources: Russound, Yamaha, Audioholics, AVSforum, Crutchfield