Another A40 Ground Problem

[gpsmithii]

Hello All,

Got a call from a fellow diy’er today regarding his A40.
It seems that when he connects both inputs to any source he gets a significant amount of hum. I have reviewed my understanding of how the A40 is to be grounded and based on the way mine is configured, and works his sounds to be configured the same. If he leaves one ground on the inputs open the device is very quiet. And, if I understand him correctly the source i.e. signal generator or preamp doesn’t even have to be plugged in or turned on. If both inputs are open the unit is quite. I also asked him to short the inputs and it is again quiet… I’m stumped …

Thanks in advance fro your help!

 

[AndrewT]

Hi,
sounds like one or both RCA Grounds are shorted to chassis.

Is the Safety Ground connected direct to Audio Ground (Central Star Ground) or connected via a disconnecting network?

 

[gpsmithii]

Hi,
sounds like one or both RCA Grounds are shorted to chassis.

Is the Safety Ground connected direct to Audio Ground (Central Star Ground) or connected via a disconnecting network?

Nope one of the first things we checked...

His chassis layout is very different from mine. I hope it's not the location of the transformer (avel lindberg toroidal) i.e. it's inducing hum via the ground wires. The unit is quieter with out the connection to psupply center, but still has some noise. Any other suggestions?

 

[Babowana]

It seems that when he connects both inputs to any source he gets a significant amount of hum.

It seems that there is hum voltage between the amp ground and the source ground. Or, the amp ground is arranged to pick up the hum voltage, probably due to improper gounding path arrangement. In this case, giving any help without seeing the arrangement is hardly possible. Could you get the sketch showing the actual grounding diagram from your friend . . . ?

 

[AndrewT]

Hi,

Is the Safety Ground connected direct to Audio Ground (Central Star Ground) or connected via a disconnecting network?

which?

 

[gpsmithii]

Hi, which?

I think we may be getting closer. If I understand him (he has no digital camera) the EMI filter he is using has earth ground isolated from the chassis (good). The earth connection goes from there to a terminal strip where it makes connection to the chassis, power supply center (i.e. transformer windings) and circuit ground. If this is the case there is a ground loop between the chassis and the input jack via the 10 ohm resistor. When I asked him to make sure that one or both of his input connectors was not shorted to chassis I did not have him check to make sure there was 10 ohms between the chassis and ground.

 

[AndrewT]

Hi,
it reads like your friend has not attached a permanent safety earth from mains incomer to chassis.

DO THIS FIRST.

 

[Nelson Pass]

It seems that when he connects both inputs to any source he gets a significant amount of hum.

If the hum occurs when both inputs are connected but not
when just one is connected, then it is a classic ground loop
from the input grounds back to the source (where they are
connected) with hum induced by the transformer.

Moving the inputs far away from the transformer is a good
start. Also, you can place the inputs as close together as
possible and connect the grounds together on the RCA
connectors.

After that you have to look at the ground layout of the
amplifier to make shure that both channels ground back to
exactly one point.

 

[gpsmithii]

Re: Re: Another A40 Ground Problem

If the hum occurs when both inputs are connected but not
when just one is connected, then it is a classic ground loop
from the input grounds back to the source (where they are
connected) with hum induced by the transformer.

Moving the inputs far away from the transformer is a good
start. Also, you can place the inputs as close together as
possible and connect the grounds together on the RCA
connectors.

After that you have to look at the ground layout of the
amplifier to make shure that both channels ground back to
exactly one point.

Thanks we are getting closer. I suggested that he implement one of the more modern grounding options i.e. removing the 10 ohm to chassis resistors (replacing them with the thermistor between signal ground and chassis ground) and running all of the grounds to one point on his power supply common. He likes things to look very organized I had to get him to understand that the screws in the H (buss) formed by this CRC power supply's center point are all at different electrical potentials, and that one point means one point period. Thanks to all for your suggestions!

 

[gpsmithii]

Thanks for all of the suggestions etc ... It's working just dandy. He is happy. I'm happy! We are all happy. He is still fixated on the need to connect all the grounds to the same exact point, but it works now and didn't before.