Humming Noise in my amplifier???

[lock dogg]

Hi everyone.

I have been searching all over the forums to find a solution to my problem but nothin seems to work.

I think its my pre-amp.

I have tried using my sc-480 with a mp3 player and it all works fine but then when I use my pre-amp and have a signal come through there is a hum, and if the ground wire to my pre-amp touches the case it makes a scratchy sound aswell. I am using all sheilded cableing for the audio signal, but there is still a hum.

If you need ANY more information please tell me what you need to know.

ANY help would be much appreciated.

Thank you all very much in advance

Thanks
Stefan

 

[gmphadte]

how is ur mp3 to preamp cable made... the details
is the preamp and the amp silent when the audio cable to mp3 is disconnected?

 

[lock dogg]

Hi, thanks for your reply.


The cable from my mp3 is just a normal sheilded cable. and I had no problem with that. but once I hook up my pre-amp with even better quality sheilded cable it makes a buzzing sound.

The pre-amp use to all work when it was out of a case and there was no problem, but now when it is in the case near transformers and stuff it makes the sound.

I still get a audio signal through ( with hardly any bass ) but there is a very loud humm.
Also when I touch the knobs of my pre amp the hum increses but also when I touch the ground of my pre-amp, it goes away abit aswell ( by a fair bit ) but there is always a hum still.

Even attaching the ground to the case makes it worse..

Any ideas??

THANK YOU VERY MUCH

Stefan

 

[lock dogg]

Hello again.

Just to answer your question.

I just unplugged the pre-amps OUTPUT to the amplifier and it still makes a LIGHTER humm, and even still when
I touch the pre-amps knobs it still increases the humm even though it isnt connected via the audio output, soo it must be something to do with the power.
Its somehow interfeering.

Also when I lift the 240V cable to turn the amp off it makes a really scratchy noise like 240VAC cable is also interfeering somehow?????

Also when I connect and dissconect the cable from my mp3 player it doesnt affect it.

All up I have 2 tranformers and 2 power supplies, I am running one power supply with one amp and the other is running another amp INCLUDING a Pre-amp, Do you think the power supply is interfeering??

Any ideas???

THANK YOU very much

Stefan

 

[gmphadte]

Disconnecting ur preamp, the power amp still hums, then get rid of this problem first.
U have to come up with better details so that we could help, may be a photo of how things look like.
If u suspect the transformers, what type are u using, are they toroids?

 

[lock dogg]

Hi,

I will go get some pictures now of everything, no the tranny isnt a toroid.

I just dissconnected the pre amp and the power amp still is fine with no hum what so ever.

Thanks
Stefan

 

[gmphadte]

tranny=transformer or transistor

 

[gmphadte]

Transformer shielding

This is what I saw in a 3in1
A copper plate is wrapped around the winding from outside the core(beware) and magnetic shielding material(may be mu metal) is used around the rectangular core.

try it if u can find the materials.

mu metal is not available, try iron sheet, may be more rounds of it

Gajanan Phadte

 

[lock dogg]

Hello,


Well I uploaded a pic on a site.

I will upload some more pics for you guys

The link is:

http://www.upload2.com/galleryview/2595

please take a look at the pics at present and tell me if u need more specific photos.

Thank you VERY MUCH

Stefan

 

[Sangram]

It's probably a ground loop in the mains supplies. At least that's what it sounds like to me. Which is why the MP3 player has no problems and the preamp does.

Are the chassis ground connected to the input/output jacks of the preamp and amp? You need to use insulated jacks.

The amp star ground needs to be connected to the amp chassis via a small resistor (10 ohm, 3 watts is a popular value) and the chassis should be connected to mains earth. Sometimes a improper mains earth will also cause this problem. If the amp is correctly grounded but not getting a mains earth, it may hum when other equipment is connected on the same outlet and kept in the vicinity.

There is one more possibility. You say you are using two transformers, and one of them is running an amp and a pre-amp, the second is only running one amp.

There is a ground loop between the two channels.

You need to connect the two power grounds together at the power supply outlets.

 

[gmphadte]

U have very messy wiring , clean it up. Give them separate paths away from the pcbs,

 

[AndrewT]

Hi,
the metal chassis of mains powered equipment MUST be securely attached to the earth (safety) connection. This is not an option.

If possible, no audio connection should make direct connection to the chassis. Use a decoupling network (or simple resistor) to connect central star ground to chassis.

 

[lock dogg]

Hi,

Sorry for the messy wiring ever since my amp hasnt been working I have tried difffrent things and its all messy now i know.

I will attatch the MAINS ground to the chassis. brb.

I have NOTHING except mains ground attached to the chassis yet when I put the ground of the pre-amp on the chassis it makes a scratchy sound.

And sorry if I sound stupid but what is Central star ??

Thank you
Stefan

 

[Sangram]

Whoa, that's some funky wiring...

But, yep, the problem is definitely the grounding and wiring.

Star is the single ground point which is the basic reference to ground, and is the meeting point of signal, output and power grounds. For well-designed PCBs, a central point is normally identifiable.

If the PCB does not accomodate it, there should be a separate point offboard where all the above three grounds are referred to (if not directly connected). Do NOT make the mistake of connecting everything to the point between the two reservoir capacitors. There is a lot of noise here.

Then connect the star grounds of the two channels together. Since you have a CT transformer, you should not have any problems.

That should reove any gremlins from gorund loops on the channels.

But yep, you desperately need a cleanup.

 

[Sangram]

Don't connect any amplifier or preamplifier ground directly to chassis. Use a decoupling network (R/C parallel, 10ohm + 1 uF) or 10 ohm resistor for connection.

 

[lock dogg]

Hi.

I will just draw a pic in paint of what I am understanding here....


Will be done in a tick


Thanks ALOT EVERYONE your all very kind

Stefan

 

[lock dogg]

I hope this pic I drew up makes sense.

I hope this is what you mean by attaching eveything to mains Central point??

Thanks
Stefan

 

[lock dogg]

Hi.

How you said about the resistor. How shopuld I go about wiring it just put a resistor in with the GND wire ???? and same with the pre amp ???

Thanks Alot

Stefan

 

[AndrewT]

Hi,
you're sketch shows why it's called central star ground (CSG).

Disconnect the chassis from the CSG.
Connect a wire from chassis to a 10r resistor.
Connect the other end of the resistor to another wire.
Connect this second wire to CSG.

As an option you can parallel the 10r earth resistor with a 1nF ceramic capacitor. You could mount these earthing components on a short piece of tag strip bolted to the chassis at the main earthing point. A further option is a diode in parallel with the resistor. Finally you can add a ground lift switch in parallel with the resistor. Normally this switch is open. I do not advise you operate this switch while the amp is powered up, it may cause a severe pulse from the amp output.

You have not shown the transformer centre taps. Connect each of these to the respective PSU grounds. DO NOT connect the centre taps direct to the CSG or another wire running to the CSG.

 

[lock dogg]

Hi Andrew. thanks very much for your help.

When you say

"You have not shown the transformer centre taps. Connect each of these to the respective PSU grounds. DO NOT connect the centre taps direct to the CSG or another wire running to the CSG."

What is a PSU ground? and how do I go about grounding the transformers.

Thank you very much

Stefan