Ground problem! Noise disappear when i touch the chassis

[zarana]

Hi,
Last week I finish 2 amps,

One Dumble style, and other 2 channel plexi.

In the two amps I have the same problem.

When I conect the guitar there are a low noise that change when I move the guitar, But when I touch some metal parts of the amp or guitar The low noise desappear. I test it with differents guitars and I have the same problem.

For wiring the ground I use differents bus
1 bus for all preamp grounds.
1 bus for all power suply grounds
1 bus for speaker outs grounds

Also Ihave other conections in the star ( Ct, the ground of the power chord, etc...)


What can I do? I have this problem in the 2 amps.

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subirimagenes

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An externally hosted image should be here but it was not working when we last tested it.

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thanks!

 

[Hyperman75]

Hi

@Zarana :

Is the metallic case conected to the ground inside the amps ?

If not you have to connect the case to the ground (0V), near the inputs plugs.

And, for safety reasons, the metallic case shall also be connected to the earth of the main supply.

Jacques

 

[DF96]

What can I do?

Post in the right forum area?

You appear to have a 'star ground' bolted to the chassis. This is asking for hum, especially as you say you have connected the secondary CT to it.

The ground wire from the mains lead should go to the chassis, with its own private bolt/tag etc.

Signal and PSU grounds should be connected to each other as appropriate for the circuit. At one point, possibly at the input as hyperman75 suggests, the signal ground should be connected to the chassis.

 

[Malcolm Irving]

Does this only happen when you have a guitar plugged in? If so, it may be the guitar that is causing the buzz. Most guitars will cause buzz if they are close to an interference source. They provide an unbalanced signal and are generally poorly screened. The effect is particularly noticeable if you have the chassis upside down on a workbench with the guitar nearby (usually above the open chassis). Touching a grounded part (e.g. a guitar string) means that your body acts as ground plane and reduces the interference.
When you have the chassis the right way up and the guitar 3 metres away is the buzz acceptably low?

 

[danFrank]

All guitar amps do this, some more than others depending how good the grounds are done in the amp. Do you have a grounded (three wire) plug installed in the amp?
Personally, I don't like connecting the audio ground and chassis ground at the input of the amp. This especially rings true when using Cliffs jacks which are isolated (insulated) from the chassis. I have had best luck making the main ground at the main filter supply capacitor, attaching this connection to the chassis.

 

[hooman]

fender use one cap between ac input line and amp chassis . so you can use it .

 

[Wavebourn]

Try to shield the bottom of the amp by some metal net.

 

[kevinkr]

Guitar amp threads in Instrument & Amplifiers please per forum rules. This thread will be moved to the proper forum.

 

[hooman]

Try to shield the bottom of the amp by some metal net.

this noise can not remove by bottom shield.

 

[CapnDenny1]

What you are describing is what happens when you have a guitar that is unshielded. You don't have your signal and shield reversed at the input jack do you? I did that once and I couldn't understand why it was so noisy? I misread the pinout on the input jack that had about 4 pins on it.