CXA80 amplifier Randomly going into mute

appo2396

Standard Member
My CXA80 amplifier is randomly going into mute . Details: The amplifier is 4 months old. I play at very low volume with the base at zero level. The amplifier hardly gets warm or hot to touch. I googled the problem and noticed that this is a very common problem with Cambridge audio amplifiers after a few months of use. No where on the internet I could find a solution other than that the manufacturer responding by asking them to contact tech support by calling the toll free number. One reply to the problem from " avforums" suggested that it may be the amplifier going into protection mode if it senses a decrease in the speaker impedense or speaker short circuit. I have to admit that I am running 2 sets of speakers in parallel (4 ohms) connected to Speaker output (A). One set of speakers 8 ohms connected to Speaker output (B). I play the amplifier with both speaker outputs A and B " ON".
Question!!! Will the speaker impedense go below 4 ohms irrespective of the volume levels of the music playing and the level of the Bass knob position. ? I mean is there a correlation between impedense and volume+Bass levels. I forgot to mention that I use the Bluetooth option to play my amplifier.
 

noiseboy72

Distinguished Member
If I read this correctly, you have a 4 Ohm load and an 8 Ohm load connected at the same time so you are presenting somewhere around a 3 Ohm load to the amplifier?

Impedance is not a static value but is not related to the volume you play the amplifier at. The speakers will present a different impedance at different frequencies and for an 8 Ohm speaker this could easily dip as low as 4 Ohms. Your 3 Ohm load could therefore be dropping to less than 1 Ohm at some frequencies, hence the amplifier cutting out.

The setting of the bass control is of little relevance to the load presented to the amplifier, but increased bass puts more demands on the amplifier as the voltage and current demand will increase. Increasing the treble control does the same thing but as there's less HF content the results are more subtle.

I doubt there's anything wrong with the amplifier but try removing one of the sets of speakers so that the impedance is within specification.

Worth also checking for any stray wiring that could cause an intermittent short while you are at it.
 

appo2396

Standard Member
Excellent information that I never knew before. Which is 3 ohms . Thanks for your suggestions. I will check the speaker connections. However I have another question. The 2 sets of speakers (4 ohms ) are now producing great sound. Because from what a forum member said, 4 ohms is like a 4 lane wide road vs 8 ohms a 2 lane road. 4 ohms let's more sound to pass vs 12 ohms. Mind you, the amplifier is rated 4 ohms capable. Back to what I was saying, since I was impressed with the 4 ohms setup comprising of 4 speakers. I decided to buy 2 more speakers and run them parallel to the speaker output connection ( A/B). Question, now I have 4 ohms running on both sides. Speakers outputs A and B. You are aware of this but still I want to mention that I have an amplifier and not a receiver. An amplifier has Speaker A, Speaker B, and Speakers A and B options. I run my amplifier under A +B option. Each channel running 4 speakers connected in parallel @ 4 ohms . As you mentioned currently you say the impedense is cumulative 3 ohms, when I am driving speaker output "A" with only 2 speakers per channel @ 8 ohms. And speaker output "B" with 4 speakers (2 per channel) connected in parallel @ 4 ohms. Per your theory cumulative impedense is 3 ohms. Question, as mentioned earlier if I buy 2 more speakers and connect them in parallel to Speaker output A @ 4 ohms, will the cumulative impedense be 4 ohms on both " A " and " B " speaker outputs. Please excuse me repeating myself too many times. Appreciate your response
 

noiseboy72

Distinguished Member
OK, so a few basics are needed here as you have some significant gaps in your knowledge which will end up breaking things!

Fist off, you have a stereo - 2 channel amplifier. Each channel will drive a load of 4 Ohms or more. The A&B buttons just add the speakers to the channel, they do not increase the power handling ability. In simple terms, you are trying to connect too many speakers to the amplifier and that's why it is cutting out.

Impedance and Power: Amplifiers will have a stated power output into a stated number of Ohms. IE: 100W into 4 Ohms and 50W into 8 Ohms. This is because of the fundamentals of how electricity works. The 2 simple equations that give you an idea of what's going on are as follows: Watts = Volts x Amps and Amps = Volts divided by Resistance - which in this case we are using as the impedance figure. (For any pedants reading this I am treating impedance as resistance and assuming it is a static figure, but the basic theory is the same as for complex impedance) Power amplifiers multiply current more than voltage. It's usually about 10-20x increase in voltage and closer to 200x for the current.

Therefore, in the case of a 100W / 4Ohm, 50W 8Ohm amplifier with power rails of 50V, the current into 4 Ohms is 2 Amps and 1 A into 8 Ohms. This means that you have double the current available for a 4 Ohm load. So why not add more speakers and make the load 2 Ohms, because the maths would suggest that it would make the amplifier play even louder? The amplifier has a maximum current that its power supply and output stages can supply, so by halving the impedance, you have doubled the current required. This extra current generates heat and the output devices will eventually overheat and either destroy themselves or will start to conduct less and high levels of distortion will result.

It's also important to point out that as sound levels of logarithmic, 100W does not sound twice as loud as 50W. Twice the speakers is not twice the sound pressure level.

You must therefore ensure that you keep the impedance of the connected speakers ABOVE the minimum value that the amplifier can handle. Your highway analogy works for voltage - where a lower impedance will allow more voltage to flow, but where current is concerned, think of it more as a pipe with a pump as the amplifier. If you make that pipe bigger - lower the impedance and increase the (current) flow, if the pump stays the same size it will reach a point where it cannot keep up with increased flow rates and will fail.

You are connecting all your speakers in parallel - lots of pipes all side by side. There's a formula for working out the impedance for parallel connections -https://www.speakerimpedance.co.uk/?act=three_parallel&page=calculator , but for 2 speakers you just divide by 2, so a pair of 8 Ohm speakers becomes 4 Ohms and 4 8 Ohm speakers will become 2 Ohms.

If you want to attach loads of speakers to your amplifier, you can connect them in series - one after the other rather than in parallel. The calculator I've linked to can help you to work out how to connect them to give you the best combination, but if they are all 8 Ohm speakers, connect each pair in series so you end up with 2 x 16 Ohm loads, making 8 Ohms in total.

The next questions is why the hell would you want to??? Multiple speakers just sound horrible. The HFs will comb so that the "system" will sound different as you move around the room, the bass will be very lumpy for the same reason and stereo imaging will be vastly reduced.

If this is all about ultimate level and not quality, you bought the wrong amplifier. You would do better with a big class D PA amplifier that can tolerate loads of 2 Ohms and simply stack the boxes up and revel in all the waffly, inaccurate glory...

If you want a surround sound experience, buy an AVR with multiple amplifier channels and then you can safely attach all the speakers and use the DSP on the AVR to add surround effects. Using a stereo amplifier with lots of speakers is hardly ever accepted practice.
 

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