A/B Switch stops Sonos right channel working - HELP

  • 6 August 2007
  • 14 replies

Userlevel 2
Moved my ZP100 to now work in a 2 room setup with a Parallel A/B Switch and four 8 ohm speakers.

Everything seemed to work fine and the A/B switch was doing its job nicely but at some point I noticed the right channel was no longer working in either room. I swapped the connections at the back of the Sonos and the right hand speakers were OK so it must be the Sonos right channel had failed !!

Turning off and back on at the wall did not bring the right channel back to life. I thought I had blown it somehow.

I checked the A/B switch resistances seen by the Sonos with a cheap multi-meter. Set to A pair the resistance was about 5.5 ohm (prob not expecting to see 8 here), the B pair was the same and the A/B pair did reduce the resistance to about 3 something, so at least no shorts and the wiring looks OK.

Anyway, leaving the ZP100 off for about 6 hours and it was OK again.

I thought maybe I can't switch from an 8 ohm load to a 4 ohm load whilst it is switched on, maybe I have to pause the music first. I tried this and it seemed to be OK if I paused the music before switching over. But another time when I tried this method the right channel stopped again.

So, any ideas?

(1) Is it OK to change the load from 8 ohm to 4 ohm on the fly with an A/B switch?
(2) Could my A/B switch be causing a short as I switch from A -> to B -> A+B (surely not)
(3) Could the ZP100 be cutting a channel when it sees a brief open circuit between A/B changes?
(4) Should I be pausing the music before changing the load? (worked the once I think)
(5) Am I blowing a resettable fuse each time I try this, should I stop experimenting immediately before I perm blow it?

14 replies

From the Sonos FAQ:

Using a speaker selector switch that is not impendence matching causes the impendence on the ZonePlayer to change when you select different speaker combinations and if it drops below 4 Ohms it may cause an amplifier fault (and loss of sound) until the problem is corrected.

It sounds this is what's happening here. Normally you can connect 8 ohm speakers in parallel to get 4 ohm impedance which will work with Sonos, but if the original speakers are actually less than 8 ohms, then they will be less than 4 when switched in parallel. This is below what the ZP amp can handle, and it will stop the output.

Looks like an impedance-matching switch is what you need.


Userlevel 2
thanks for that.

but when I power the ZP100 up with them in parallel it plays OK ( I havn't tried an exhaustive play yet though)

maybe the impedance is a little low and it just cant handle it when the switch over occurs.

could I do something like put a 0.5 ohm resistor in series with the right channel?
or would that have to be a 100W reistor?

The switch was quite expensive and its all plastered in now, dont really want to write it off.
You could put a resistor in, but you need to make sure it's rated correctly, and balancing the sound between the stereo pairs could be challenging.

Note that if you do this, the resistor will probably generate a lot of heat and you will effectively be "wasting" some of the audio output power of the amplifier.

A better way to do this would be to put a transformer in, but these tend to be expensive and difficult to find.

The best way would be to change the control. If you're in the US, impedance matching controls are relatively easy to find, and aren't very expensive.

Here's an example

I wonder why your current AB switch was so expensive as non-impedance matching speaker switches are normally very cheap.


Userlevel 2
Seems a bit strange that only the right channel would exhibit this behavior. Anything below 4 ohms and there is a chance the amplifier will shut itself off as a preventative measure, but I would think it would turn off both channels. Are all four speakers the same make/model?

Userlevel 2
Hi Jason,

Yes it is only the right channel (left plays fine).

All 4 speakers are also the same make and model, 8 ohm.

Can I measure the reistance with a digital multi-meter or is this not possible because we are talking about impedance.

Can you confirm that the ZP should work OK swapping between 4 and 8 ohm load as the music is playing. Also that a brief open circuit as I move from A to B should also not be a problem?

I could try swapping left to right on the back of the ZP100. - If the fault follows the speakers then could that indicate there is a low impedance problem, but there again you said you would expect both channels to go off, is that right?
Userlevel 2
Being a networking guy with a minor in audio, let me check with my audio people here and get back to you on that.

Userlevel 2
Here is a quote from him:

"Fundamentally it’s fine but the question becomes one of how the switching is accomplished. If the switch is “make before break” the two loads will both be tied to the output for a short period. If it’s “break before make” the output will be temporarily unloaded.

Either case will not break the unit but the former may cause the current limit to trigger if the signal is large when the switching happens. In such a case he’ll/she’ll loose signal until reset. I’m not sure if under such an over current condition if a hard reset is needed or the unit resets itself. "
Userlevel 2
sorry to trouble you once more Jason but could you find out if you would expect both channels to be cut under an over current fault condition on one channel.

If the answer is yes, and you would expect both channels to stop, then as its only my right channel, could I have a faulty unit perhaps?

I checked the A/B switch resistances seen by the Sonos with a cheap multi-meter. Set to A pair the resistance was about 5.5 ohm (prob not expecting to see 8 here), the B pair was the same and the A/B pair did reduce the resistance to about 3 something, so at least no shorts and the wiring looks OK.

I'm more concerned if there is a difference between the two channels. I'm only mildly concerned about the absolute value of the number because cheap multimeters rarely give accurate low resistance readings.

I suggest that you disconnect all of the speaker connections and measure each speaker and speaker wire run indvidually. Also measure the resistance between each conductor of your speaker wire and the power line ground. (Use a metal water pipe instead of the power line ground if you don't know which power line pin is "ground" ["earth" in the UK]) You should have an open circuit between any combination of (+), (-), and ground or each other.

Check your speakers individually, they should be nearly identical. Connect the speaker wires and measure again.

On a cheap meter "3" is close enough to half of "5.5" and I'm not concerned. However, if I was using my meter, I'd be concerned about the "3" -- that's getting too low. For a "3" I would take additional measurements that you are not equipped for and, based on the results, I would add some protection elements to the speaker network or use impedance matching volume controls or a transformer.

In spite of your care and previous checks, check your wiring again. (been there, done that, makes you feel stupid)

I have seen only one channel shut down on a ZP100.

Have you seen the yellow light on the ZP100? Is the ZP100 hot to the touch?


Normally, a ZP100 does not care about changing speaker loads or disconnecting the speakers completely on one or both channels.
Userlevel 2
sorry to trouble you once more Jason but could you find out if you would expect both channels to be cut under an over current fault condition on one channel.

If the answer is yes, and you would expect both channels to stop, then as its only my right channel, could I have a faulty unit perhaps?

I'm awaiting a second reply, but he initially answered that it is possible for one channel to turn off. So I wouldn't be worried about it being a faulty unit. I hope to get some more technical info from another colleague, but he is always very busy, so it may be a bit before I get a reply.

Userlevel 2
Ok, I have done some more testing and am really confused by what I have found.

I played some more music, to just one speaker pair (pair A), so should be 8 ohm and after a while (maybe when I turned the music up) the right channel turned off again.

Made some measurements on the wires at the back of the ZP:-

Left Channel Pair A Only 4.1 ohm
Left Channel Pair A+B Only 2.1 ohm
Left Channel Pair B Only 4.1 ohm

Right Channel Pair A Only 4.1 ohm
Right Channel Pair A+B Only 2.1 ohm
Right Channel Pair B Only 4.1 ohm

I thought eh, looks like they are 4 ohm speakers to me.

Disconected one of the speakers measure directly on the terminals = 3.8 ohm.

Looked on the back of the speaker, it says:-

"Nominal Impedance - 8 ohms compatible"

So, is that telling me it is an 8 ohm speaker?

These are Mission M30i speakers.

Just looked at the website and it says
"Nominal Impedance 8 Ohm (min. 4Ohms)"

How can they do that? it doesn't seem to be compatible with 8 ohm to me.

So to fix it then, is there anything I can get that will impedance match but still let me keep my A/B switch?
Something to increase the impedance back to 8 ohm with a single pair connected or 4 ohm with both pairs connected?

"Impedance" cannot be measured with the meter that you are using. Impedance varies with frequency and your meter measures "resistance" using a simple battery.

The number stamped on your speaker is the "nominal impedance" that can mean almost anything that the manufacturer wants. Although somewhat helpful, a Wikipedia article will impress you that the subject is complicated (be sure to follow the impedance link for maximum jumbo). I have no direct experience with your speakers, but another manufacturer might rate them at 6-Ohms.

Keep in mind that the speaker wire will add to the reading that you observe with your meter. It is normal for the reading taken directly at the speaker's terminals will be lower.

The 4.1 reading is not necessariliy a problem, but the 2.1 is a little low. Again, you can't make a proper measurement with your test equipment.

If I was faced with your situation, while I can make a proper impedance measurement, it is generally a waste of time because we already know that the amplifier is upset with the situation, and we need to take action. There is a remote possibility that the proper measurement would expose a problem with a speaker or the switch, but in my real world experience, this rare.

I don't like A/B switches from an operational convenience standpoint. I prefer to use impedance matching volume controls located near the speakers. This allows the operator to adjust the volume for each room and there is no need to travel to a central, typically not so convenient spot, to make the adjustment, then return to the room and verify that the adjustment is appropriate.

I'm assuming that your speaker wiring has passed all of the restistance tests that I suggested.

I've been through similar issues with other amplifier-speaker combinations and certain combinations are known to interoperate poorly. I've never had this sort of issue with a ZP100 -- even with speakers known to cause this sort of issue with some amplifiers.The ZP100 is a very nice amplifier.

Does the ZP100 shut down when the volume is above a certain point? Is it mostly loud bass notes that cause the shutdown?

I'll ask again -- have you seen the yellow light on your ZP100?


I recently was trouble shooting a SONOS system that had been operating for over a year with no problems. Suddenly, we had problems that from the description that I received over the phone, seemed like a wireless network issue and we were getting nowhere with wireless fixes. Finnally, I got my hands on the system and saw the yellow light that had not been described over the phone. The speaker wiring failed the simple tests that I suggested for you. My next question was "what has changed", answer, "nothing". After some digging, I found that one of the impedance matching controls had failed and there was an intermittent short circuit at another control location. It seems that in a bathroom location an electrician had done some work and moved the volume control too close to the enclosing metal box casusing a short circuit -- sometimes.
Userlevel 2
Hi Buzz/Jason,

I have some more information on my problem if you can spare me 2 mins to look at it I would be gratefull.

I connected a speaker from my living room setup to my ZP100, different one to the 4 on the A/B switch but of the same make and model, 8 ohm nominal impedance, 15-75 Watts. It has been working fine for 18 months powered from my TV.

So, 1 speaker, speaker measures 4.1 ohm with my cheap dvm and I connect it to my ZP100 right channel.

Music plays fine but when the volume gets up to about 75% the volume suddenly reduces to just about hear-able, exactly the same problem I had with the A/B switch.

I looked on the front panel for the LED color (correct place to look?) and it is still white.

I pause the music, swap the wires over to the left channel, press play and it is fine all the way to 100% volume (same speaker).

Is it time to consider a faulty ZP100 or could anything else be happening?

Based on your latest test, a faulty ZP100 is moving up on the list of possibilites. Make sure that there are no problems with the speaker wiring that could temporarily cure as the wires are moved to the other channel. (I've had wires damaged by a pet or rodent cause very strange problems because the damaged area is hard to spot.)

The amplifier is holding a diagnostic report that SONOS support can access. It would be good to give them a call.