really stubborn mains hum

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Hi And thanks for reading this thread.
I am getting a lot of mains noise hum coming through into my speakers, have been searching all day and applied every solution i can find but nothing works. Switched everything in the house off, no change.
No noise from the power supply itself, just mains borne interference. I noticed a small choke inductor and a few caps before the bridge rectifier in my amplifier, these are possibly of a general value, what can i change them to to make them more specific to british 240v 50-60hz mains supply.
I have also added a few smoothing caps after the secondary, the smaller values made no difference, once i got up to 2000uf then the hum reduced. At the moment i have added 2000uf + 7500uf +15000uf. The 2000uf made the most difference with the 15000uf the least.
Do you think adding a regulator after the caps may help. What would you use for a +40v / -40v 1000va supply
Or any other possible solutions.

Thanks.
 
Hi And thanks for reading this thread.
I am getting a lot of mains noise hum coming through into my speakers, have been searching all day and applied every solution i can find but nothing works. Switched everything in the house off, no change.
No noise from the power supply itself, just mains borne interference. I noticed a small choke inductor and a few caps before the bridge rectifier in my amplifier, these are possibly of a general value, what can i change them to to make them more specific to british 240v 50-60hz mains supply.
I have also added a few smoothing caps after the secondary, the smaller values made no difference, once i got up to 2000uf then the hum reduced. At the moment i have added 2000uf + 7500uf +15000uf. The 2000uf made the most difference with the 15000uf the least.
Do you think adding a regulator after the caps may help. What would you use for a +40v / -40v 1000va supply
Or any other possible solutions.

Thanks.
What are the components in your system and how are they connected?

Do you still get the hum if you disconnect everything from the power amplifier or integrated?
 
+1

The only way to track down these sorts of issues is ruthless logic.

I'd start with just the speakers. I kid you not, unpowered. I'm not going to get all mystical, but what you think is hum from your amp is sometimes standing waves from other external sources. (In my case an air conditioner outside the building).

One by one, plug things in and turn them on until you find the culprit.

If it is as simple as your amp humming, short the inputs, turn all the gains down and work on that section by section.

Trying it all as a "big bang" has a good chance of running you in circles.
 
I have powered down everything single thing in the house, honestly nothing is on except the amplifier.
I contacted the amp seller and he said to short the rca signal input, if the noise disappeares then there is nothing wrong with the amp and it is outside interference. I did this and the noise disappeared.
Not sure why this worked but i am also concerned about this mains noise bothering the rest of my equipment so am hoping that fine tuning the choke and caps filter before the power supply or installing a regulator may be of some benefit
Many thanks for the much valued advice
What is my next step
 
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Mains hum noise can origin from the power supply but may as well arrive at the amplifier input, eventually as ground noise.
If your amplifier has no such noise when you turn it ON with short circuited inputs and you turn the volume up, then assume the amplifier to be OK for now.
Have you got a SmartPhone and a cable so you can connect the SmartPhone headphone output to the amplifier input? If you have, please try to play a bit of music (MP3) from the smartphone (not connected for charging). Do you then have hum noise?
What is the music source you use when you have hum noise?
 
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I have powered down everything single thing in the house, honestly nothing is on except the amplifier.


Not relevant. Only the audio components are at issue, and their connections to each other and to the mains.



We are talking about the chain from signal source, preamp, amp, speaker.


Start with amp + speaker only, physically disconnect what's before it, and proceed backwards adding stages until the hum appears. Then you know where is getting in.
 
As Mark says, when you have a (unwanted) signal at the output which is not present at the input of a signal chain, you work your way backwards to find where and why this signal enters the chain.
And, when you have a signal at the input of a signal chain, which is not arriving at the output or only in a heavily distorted form, you work your way forward to find out where the signal stops or becomes heavily distorted.
Last, it is normally easier to work from a system that functions and step-wise apply restrictions until the system starts malfunctioning than setting up a whole system that malfunctions and do diagnostics on a malfunctioning system. This is why I propose you to try with your SmartPhone as source because it is a very tolerant source with floating potential.
 
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1. I assume this is a Diy amp?
2. Do you get "hum" if you plug in only one of the Input rcas?
3. If not, I suspect you have used a single power supply for both channels (i. E. One secondary from the transformer). you will then get a ground loop when you plug in your stereo rcas.
There are other threads about this.
 
Hi. Thanks for the replies.


I shorted the left channel with a small piece of cable about two inches long and it has gone silent. For the right channel i rooted around and found a cable about a foot long, when using this cable the noise is back.

So probably is acting as an antenna.
Why is shorting the input getting rid of the noise
 
Hi. Thanks for the replies.

I shorted the left channel with a small piece of cable about two inches long and it has gone silent. For the right channel i rooted around and found a cable about a foot long, when using this cable the noise is back.

So probably is acting as an antenna.
Why is shorting the input getting rid of the noise


Because when you short the input, the impedance at the input becomes very low and the noise signal is short-circuited as well. A high impedance input will always pick up noise if there is no low impedance source connected to the input. A cable in itself is high impedance unless well connected to a source or short-circuited. There seems to be nothing wrong with your amplifier.
 
I wouldn't say "nothing wrong" when you have to short the input to eliminate hum. There definitely are some ways of degrading PSRR with non-zero source impedance.

OP, please find a battery-operated signal source. (Walkman, discman, MP3 player, mobile computing device, phone, ... with adapter as needed.)
1. Attach only right channel, short left channel. Hum or no hum?
2. Attach both left and right channels. Hum or no hum?

Next, connect the preceding stage of your normal playback setup, with nothing except power attached. (I.e. if it's a preamp, no sources attached.) Hum or no hum?

If you have no hum so far, add connections one by one until it reappears.

Anyway, in order to be able to help properly, we would need quite a lot of details about the amplifier in question:

* IEC Class I or Class II?
* Internal grounding scheme?
* Schematic? Or source / model / pictures of amplifier boards?
 
Thanks!


I don't think it is a ground loop, think it is this cheap smps i am using, or noisy mains.
I have a quality transformer lying around so will build a linear psu to compare aginst the smps.
I have put in a bank of caps from 220uf upwards to 15000uf after the bridge rectifier and this has reduced the noise by about 50%, but the shorting of the input rca has totally wiped the noise out.
So why don't all amps have their inputs shorted, assuming it is safe on a long term basis, why do people spend money on mains conditioners.


Thanks
 
............... snip ............................snip.........................So why don't all amps have their inputs shorted, assuming it is safe on a long term basis, why do people spend money on mains conditioners.

1. shorting the amp inputs is done for testing puposes ONLY just to assess whether the noise/hum may be fed to the amp from another electronic stage or is 'produced' by the amp itself 2. mains conditioners are bringing undisputed positive results... the only debatable aspect is how much one can/wishes to spend for a given result
 
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