Emu XL-7 & MP-7 User's Group

Here's a solution to an issue that has been mentioned here in this group and elsewhere: if the XL-7 is not securely mounted screwed together, you get hum and noise in the audio. This has sometimes been attributed to insufficient shielding, but the root cause is something completely different.

As it turns out, the bottom half of the XL-7, where the main board and the power suppply are mounted, is grounded via the main board. This is as it should be. However, as it turns out, the top half is grounded via the 12V lamp BNC connector on the top. Due to a small but still significant voltage drop along the internal power supply lines, there's a small voltage difference between the metal of the top and bottom halves. That means that when they are screwed together, quite a lot of current flows in the chassis, creating a ground loop. If the two halves are not in proper contact with each other, you get a lot of current flowing in the ground wire(s) of internal power supply lines between the top and bottom halves, in effect, forming a ground loop.

The solution is to isolate the BNC connector. For my part, I don't need the light anyway, so I just unsoldered the connector from the volume/headphone board where it is mounted, turned it 90� so it wouldn't make contact with the board, and put it all back together. Voila, no noise at all. I don't know if there are isolated BNC connectors out there which would fit, that would be the final solution to this problem.

There are still other reasons that the top and bottom halves should be securely mounted together, but at least this eliminates the noise one gets if the connection is not 100%.

/Ricard

Thanks Ricard, this is very useful info! I've always been annoyed by the hum from my Command Station. But I don't quite understand - do you disconnect the BNC from the top board completely? Is the problem is different ground between the two halves, wouldn't it make sense to connect the BNC connector to the lower chassis?

Sorry for the stupid questions, I'm not to knowledgeable in electricity matters...

--- In xl7@yahoogroups.com, "Ricard" <ricard2010@...> wrote:
>
> Here's a solution to an issue that has been mentioned here in this group and elsewhere: if the XL-7 is not securely mounted screwed together, you get hum and noise in the audio. This has sometimes been attributed to insufficient shielding, but the root cause is something completely different.
> 
> As it turns out, the bottom half of the XL-7, where the main board and the power suppply are mounted, is grounded via the main board. This is as it should be. However, as it turns out, the top half is grounded via the 12V lamp BNC connector on the top. Due to a small but still significant voltage drop along the internal power supply lines, there's a small voltage difference between the metal of the top and bottom halves. That means that when they are screwed together, quite a lot of current flows in the chassis, creating a ground loop. If the two halves are not in proper contact with each other, you get a lot of current flowing in the ground wire(s) of internal power supply lines between the top and bottom halves, in effect, forming a ground loop.
> 
> The solution is to isolate the BNC connector. For my part, I don't need the light anyway, so I just unsoldered the connector from the volume/headphone board where it is mounted, turned it 90� so it wouldn't make contact with the board, and put it all back together. Voila, no noise at all. I don't know if there are isolated BNC connectors out there which would fit, that would be the final solution to this problem.
> 
> There are still other reasons that the top and bottom halves should be securely mounted together, but at least this eliminates the noise one gets if the connection is not 100%.
> 
> /Ricard
>

Did you have hum before? because if the socket is causing a ground loop, 
it should hum regardless of the type of lamp or even no lamp at all......

The LED lamps are great though.

On 2/15/2012 1:35 PM, Bob S. wrote:
> Years ago I found an LED replacement for the bayonet bulb on the 
> gooseneck....no hum on my XL-7 !
>
> Bob
> El Segundo, CA
>
> Sent from my iPad
>
> On Feb 15, 2012, at 12:36 AM, "Boris" <bgutman@... 
> <mailto:bgutman@...>> wrote:
>
>>
>>
>> Thanks Ricard, this is very useful info! I've always been annoyed by 
>> the hum from my Command Station. But I don't quite understand - do 
>> you disconnect the BNC from the top board completely? Is the problem 
>> is different ground between the two halves, wouldn't it make sense to 
>> connect the BNC connector to the lower chassis?
>>
>> Sorry for the stupid questions, I'm not to knowledgeable in 
>> electricity matters...
>>
>> --- In xl7@yahoogroups.com <mailto:xl7@yahoogroups.com>, "Ricard" 
>> <ricard2010@...> wrote:
>> >
>> > Here's a solution to an issue that has been mentioned here in this 
>> group and elsewhere: if the XL-7 is not securely mounted screwed 
>> together, you get hum and noise in the audio. This has sometimes been 
>> attributed to insufficient shielding, but the root cause is something 
>> completely different.
>> >
>> > As it turns out, the bottom half of the XL-7, where the main board 
>> and the power suppply are mounted, is grounded via the main board. 
>> This is as it should be. However, as it turns out, the top half is 
>> grounded via the 12V lamp BNC connector on the top. Due to a small 
>> but still significant voltage drop along the internal power supply 
>> lines, there's a small voltage difference between the metal of the 
>> top and bottom halves. That means that when they are screwed 
>> together, quite a lot of current flows in the chassis, creating a 
>> ground loop. If the two halves are not in proper contact with each 
>> other, you get a lot of current flowing in the ground wire(s) of 
>> internal power supply lines between the top and bottom halves, in 
>> effect, forming a ground loop.
>> >
>> > The solution is to isolate the BNC connector. For my part, I don't 
>> need the light anyway, so I just unsoldered the connector from the 
>> volume/headphone board where it is mounted, turned it 90� so it 
>> wouldn't make contact with the board, and put it all back together. 
>> Voila, no noise at all. I don't know if there are isolated BNC 
>> connectors out there which would fit, that would be the final 
>> solution to this problem.
>> >
>> > There are still other reasons that the top and bottom halves should 
>> be securely mounted together, but at least this eliminates the noise 
>> one gets if the connection is not 100%.
>> >
>> > /Ricard
>> >
>>
>

--- In xl7@yahoogroups.com, "Boris" <bgutman@...> wrote:
>
> 
> 
> Thanks Ricard, this is very useful info! I've always been annoyed by the hum from my Command Station. But I don't quite understand - do you disconnect the BNC from the top board completely? Is the problem is different ground between the two halves, wouldn't it make sense to connect the BNC connector to the lower chassis?
> 
> Sorry for the stupid questions, I'm not to knowledgeable in electricity matters...

I disconnected the BNC completely because I have no use for it and it was the easiest thing to do. It should not really be connected to any part of the chassis, either lower or upper, but should be isolated completely. Whichever way you do it, you would need to replace the BNC connecter with an insulated version which I didn't bother to dive into.

As I mentioned, it's not usually a problem with hum it seems as long as the two halves are securely screwed together, it's when the screw connection becomes dodgy that you get the problem.

/Ricard


> 
> --- In xl7@yahoogroups.com, "Ricard" <ricard2010@> wrote:
> >
> > Here's a solution to an issue that has been mentioned here in this group and elsewhere: if the XL-7 is not securely mounted screwed together, you get hum and noise in the audio. This has sometimes been attributed to insufficient shielding, but the root cause is something completely different.
> > 
> > As it turns out, the bottom half of the XL-7, where the main board and the power suppply are mounted, is grounded via the main board. This is as it should be. However, as it turns out, the top half is grounded via the 12V lamp BNC connector on the top. Due to a small but still significant voltage drop along the internal power supply lines, there's a small voltage difference between the metal of the top and bottom halves. That means that when they are screwed together, quite a lot of current flows in the chassis, creating a ground loop. If the two halves are not in proper contact with each other, you get a lot of current flowing in the ground wire(s) of internal power supply lines between the top and bottom halves, in effect, forming a ground loop.
> > 
> > The solution is to isolate the BNC connector. For my part, I don't need the light anyway, so I just unsoldered the connector from the volume/headphone board where it is mounted, turned it 90� so it wouldn't make contact with the board, and put it all back together. Voila, no noise at all. I don't know if there are isolated BNC connectors out there which would fit, that would be the final solution to this problem.
> > 
> > There are still other reasons that the top and bottom halves should be securely mounted together, but at least this eliminates the noise one gets if the connection is not 100%.
> > 
> > /Ricard
> >
>