HEELP RCA cables makes my amplifier hum!!

[kimschips]

Hello all.

The problem I am having is that my stereo power amp are quiet when there is no input connected, or if you short circuit the input.

But with RCA cables connected to the input I get loud hum.

Both input RCAs are insulated from the chassis.

I use starground.
Groundwires from each pcb is split up in output Zobel network, decoupling caps, feedback+ input ground, loudspeaker ground has it´s own ground wire to the starground.
The starground is connected with a 3cm wire, from the middle off the smothing capacitors.
I have tried many ways off ruting the groundwires
Therefore I show a picture off the input off my amplifier so you can see the basic setup.
It also looks like the groundwires goes into the middle off the capacitors, but it´s not, the connection is like a 6cm wire from the midpoint in the smoothing capacitors (old setup)

Please help me I have tried so many things now

Please rightclick->copy and paste the link into your browser adress field, to get it working.

http://5577.freewebpage.org/EPSN0004.jpg

I must mail you my schematic, got to find another upload server

All the best
Kim

 

[AndrewT]

Hi,
have you read Leach and others about separating clean and dirty grounds?
Also you must ensure that the PSU common is returned to the transformer centre tap and not included as part of the central star ground.
The input leads may benefit from screened cable and connect the chassis end of the screen to central star ground.
The mains cable feeding the input terminal board has single insulated cords lying exposed. These should be double insulated to increase safety. Where is your mains safety earth? It should be permanaently bolted or welded to the chassis. The connection from chassis to central star earth can be direct or capacitor//resistor//snubber//diode//switch (choose from any or all of these).
Finally the transformer washer plate and bolt are uninsulated. These MUST NOT touch the chassis/lid.

 

[jleaman]

Have you tried another pair iof input rca cables ? I once had this and the poblem was that the screw that fastens the rca connector to the chasss was loose maybe it is loose ?

J'

 

[kimschips]

Hi,
have you read Leach and others about separating clean and dirty grounds?
Also you must ensure that the PSU common is returned to the transformer centre tap and not included as part of the central star ground.
The input leads may benefit from screened cable and connect the chassis end of the screen to central star ground.
The mains cable feeding the input terminal board has single insulated cords lying exposed. These should be double insulated to increase safety. Where is your mains safety earth? It should be permanaently bolted or welded to the chassis. The connection from chassis to central star earth can be direct or capacitor//resistor//snubber//diode//switch (choose from any or all of these).
Finally the transformer washer plate and bolt are uninsulated. These MUST NOT touch the chassis/lid.

Hello.
First thank you both for your inputs.
Yes I have read and tried the leach grounding with a 82R resistor between power ground and small signal ground
It made no difference.
Got to try with the screened cable yet.

I have thought about grounding the chassis to mains earth-> but I don´t have mains earth in the power cord from the wall

Could it also be the amplifier itself, that oscilates, when it´s only the RCA cable are connected to the amplifier?

Please come with more suggestions for the ground wiring.

If it helps, I have attached the powersupply schematic:
The "hjælpeforsyning" are the small extra supply for the inputstage and voltageamplifier stage.
The other are the main supply.
All the Best
Kim

 

[Zimo]

Hey!

Here is my solution. My stereoamp groundings

I have used this one in my stereoamp and it works perfectly! Ok, you have only one transformer but it doesn´t make it a lot different. If I am wrong please correct. But it seems that your biggest problem there is that you have only one ground cable connected from RCA connectors.

You have to also make sure that there are one insulated star ground point to each channel. So the whole powersupply is only connected to ground via your signalcable which goes from RCA-connector to your PCB. There are maybe thousands of different grounding methods but I´ve so satisfied to this that I wouldn´t chance it.

But remember grounding is also art (and sorry about this picture.. I´m not familiar with PC graphic programs)

Simo

 

[kimschips]

NEWS NEWS NEWS

I managed to avoid the hum with another RCA cable connected!!!

Normally I have used a green RCA cable from Taralabs, with a cost in denmark for about 5years ago at 90$

The inscription on the "hummm" cable says:
Quantum CD interconnect cable by Taralabs
US.Pat nr 5033091->other us and patents nr.

I have tried with these cables with succes:
Ultra cheap standard phonocable
A 25$ stereo double screened phonocable from Audiostream A400

Can anybody tell me how an RCA cable can make an amplifier hum while others don´t??

Thanks
Best regards
Kim

 

[guido]

Yep, i have the same with my tube pre and audioanalyse amp.
It is the ground connection of the cable. Think it is not completely 0 ohms (ok, they all are not 0, but you get the picture).

In my case a vd hull mk1 which has some carbon conductor (not shure what it is but it is black and putting a new connector on a cable is not possible. Which is cause one of them came off.

GuidoB

 

[kimschips]

Hello guido

Maybe we can start a new post where people tells about RCA cable problems and information?

It´s a shame that such a cable issue makes a good cable unusuable

But thanks all

Kim

 

[AndrewT]

Hi Kimschips,
are you running mains voltage into the metal chassis?

If you are then you MUST run a 3core mains cable with earthing wire into the chassis as well. The metal chassis must be permanently connected to the earth wire and any removable panels should also be wired back to the earthing if some maintenance (inspection and testing) can be done with the product live.

You are only allowed 2core mains cables when the internal mains components (e.g. transformer) are double insulated (the double square symbol on commercial products).

Your single insulated mains wiring running exposed inside the unearthed chassis is LETHAL if a fault develops.

 

[kimschips]

Hi Kimschips,
are you running mains voltage into the metal chassis?

If you are then you MUST run a 3core mains cable with earthing wire into the chassis as well. The metal chassis must be permanently connected to the earth wire and any removable panels should also be wired back to the earthing if some maintenance (inspection and testing) can be done with the product live.

You are only allowed 2core mains cables when the internal mains components (e.g. transformer) are double insulated (the double square symbol on commercial products).

Your single insulated mains wiring running exposed inside the unearthed chassis is LETHAL if a fault develops.

Hello AndrewT.
I don´t have an earth connection from the 230V mains in my house.

But you´re right about the uninsulated, main input are dangerous. I will try to find a solution so it can be insulated.

But thanks for your good inputs

Best regards
Kim