The buzzing from the transformer inside the amplifier is caused by a poorly made transformer.
Since the noisy audio gateway is silent when playing then it (not the amplifier) produces the noise when not playing.

The noisy audio gateway is a PoE gateway, if I plug a headphones to the 3.5mm audio line out there is no noise, but if I take the 3.5mm audio line out from the gateway to the aux in on the amp that's when I get a buzzing sound unless I play something from the gateway otherwise it buzzes. As soon as I play something it stops or if I disconnect the 3.5mm jack it stops. Any ideas what could be wrong?

 

Do you use a shielded audio cable between the gateway and the amplifier?
Try disconnecting the cable's ground wire only at the gateway but leave it connected at the amplifier.

 

Do you use a shielded audio cable between the gateway and the amplifier?
Try disconnecting the cable's ground wire only at the gateway but leave it connected at the amplifier.

I cut the ground wire off on the gateway end but didn't help. But I was using the speaker output terminals on the gateway. I didn't try the 3.5mm audio out with the ground cut. Not sure if that will make a difference? I even tried to put one of those magnetic loop things around the sound wire and that didn't help either

 

You should never connect the input of an amplifier to speaker terminals on a gateway because the speaker terminals might be bridged (each speaker wire is amplified) with no ground.
Sure, try the 3.5mm aux output or headphones output instead. Then maybe the ground wire does not need to be cut.

 

You should never connect the input of an amplifier to speaker terminals on a gateway because the speaker terminals might be bridged (each speaker wire is amplified) with no ground.
Sure, try the 3.5mm aux output or headphones output instead. Then maybe the ground wire does not need to be cut.

I was using the 3.5mm aux output from the gateway all the time. The only time I tried using the speaker output from the gatway was to try connecting it without ground, I basically cut the other end of a 3.5mm aux cable and exposed the 2 wires and ground, I then connected the two wires to the speaker output of the gateway without the ground cable but it gave a louder buzzing sound. Should I try the same thing with the 3.5mm aux gateway output but just cut off the ground from the aux cable ? Do I just expose the cable and cut the outer ground cable and just leave the two red and white cables intact?

 

The outer ground wire of the cable shields the inner signal wires from picking up interference. If there is buzzing when the source ground connects to the shielded cable and amplifier's ground then it could be caused by a "ground loop" which sometimes can be fixed by cutting the ground wire at the source.

I think the gateway produces buzzing when it is not playing, but you said it did not do that with headphones.

 

Ceiling speakers for ambience music is not typically done with a 70V line amplifier. See https://geoffthegreygeek.com/understanding-distributed-speaker-systems/

For home use the standard amplifiers are used. See: https://geoffthegreygeek.com/understanding-speaker-impedance/

There are "limits" imposed by the system.
1. Available voltage
2. Maximum current
3. Speaker impedance
4. Speaker wattage

Each one places a LIMIT on what the amplifier can deliver. So an amp rated at 100 W into 8 ohms may not be able to deliver that into 4 ohms because the current capability of the amplifier is too low.

Bridging inverts one of the outputs and drives one of the speakers 180 degrees out of phase allows an automobile to effectively use a +-12V supply rather than a 12V supply to drive the speakers. The amp cannot achieve 24 V p-p.

As was stated, two 8 ohm speakers in parallel is 4 ohms and it's always less than the smallest impedance. The formula is 1/Rt=1/R1+1/R2+....1/Rn
As it turns out if two are placed in parallel, it's half.

1/Rt=1/R+1/R; 1/Rt=2/R; Rt=R/2;

the types of amplifiers used for audio are divided into classes. The major ones are Class A, Class AB and class D. A friend of mine just bought a class B amplifier which is rare. Class D is the most efficient.

 

So I tried cutting the ground wire from the 3.5 aux cable at the gateway output end but then the buzzing becomes much louder if i cut the ground cable. If I disconnect the input wire on the amp, I noticed that there is also a hissing sound but not buzzing sound. The hissing is much quieter and you can only hear it if u go close to the speaker. But if nothing is playing on the amp, should it not be completely silent?

 

It is a cheap Chinese amplifier so of course it is not perfect and it produces some hiss.

 

OK so i've ditched that gateway and using a different gateway which is better. Now the only noise is the amplifer buzzing noise which isn't too bad and i'm planning to place it above suspended ceiling tiles so that noise will dampen out anyway. If I connect two 8 ohm speakers to one channel in parrallel that will give me a 4ohm impedance on that channel. However, if lets say I had a 4ohm speaker that I wanted to add in parallel to the 8ohm speaker, this will end up with impedance just over 2 ohms. Can I add a resistor or something to the 4ohm speaker to make the 4ohm speaker into 8 ohms and then connect in parallel with the other 8 ohm speaker and hence create a 4ohm output?

 

So let's look at two different pieces of equipment used in different frequency ranges to understand the different power outputs. Now I've done a lot of measurement in my time and I can understand the basics of a power meter in theory but it was still a small wonder to me to compare the two in practice. Now the measurement equipment was used in the same manner and in the same locations (Upper left corner of a room) but the reason I did this was to show the differences between these different equipment pieces and to get the rationale for the quoted peak figures of the products. Then once the theory was understood what seemed to be the logical explanation was like a mini episode in itself.

 

If you connect a 4 ohms resistor in series with a 4 ohms speaker then you are reducing the output level of the speaker -12dB at most frequencies and changing its frequency response.

Your low level problem is that the input source signal level is too low for the amplifier.

 

If you connect a 4 ohms resistor in series with a 4 ohms speaker then you are reducing the output level of the speaker -12dB at most frequencies and changing its frequency response.

Your low level problem is that the input source signal level is too low for the amplifier.

The spec of that amp says that each channel can support between 4-16ohms. So could I wire a 8ohm and 4ohm speaker in series giving a total of 12ohms. I could also technically wire 3x 4ohm speakers in series too right? I will mainly be using the 8ohm speakers but in a few locations I have to use wall hanging ones that I already have which are 4ohms

 

The total power into 12 ohms might be only 18W RMS. Then the 4 ohm speaker gets 13.5W and the 8 ohm speaker gets 4.5W.
Speakers in series sound "boomy" (one note bass like a bongo drum) because the extremely low output impedance of a modern amplifier is not damping their resonance.