• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Buzz with XLR connection of Adcom amp

M

mrm12000

New Member
Hi All,

I have a high-power Adcom GFA-7805 five-channel amp that has a weird problem. I'm wondering if anyone has encountered similar or has insight into it.

When the amp is connected to speakers without interconnects, the amp is quiet -- just some very slight hiss, which is normal.

When the amp is connected to speakers and with RCA input, the amp has a very small amount of low-level hum through the speakers. This seems normal and doesn't affect listening.

When the amp is connected to speakers and with XLR input, the amp has audible/intrusive buzz through both speakers that's identical for any/all channels. If I unplug the interconnects at the source or the amp, the buzz stops. If the source is disconnected from other components, is turned off or unplugged, the buzz continues. I've tried multiple preamps and XLR cables, and this issue persists.

I should mention that the amp works perfectly and has done so in the past. There are no thermal or distortion faults (the amp has LEDs on the front panel for both). It plays music even with the buzz, doesn't overheat, and doesn't buzz mechanically except for a few second after it's switched on.

I've tried lifting the ground of various components, including the amp and preamps, and the buzz persists.

Finally, I should mention that my audio system is two-channel only and has no PC or network components. Just a couple of disc players, phono stage, turntable, preamp, etc.

Does anyone have any ideas? Thanks.
 
Register to hide this ad
If the connections have never been cleaned, they could be oxidized. Deoxit D5 will clean the oxidation.

You can play around with isopropyl or denatured alcohol and a variety of soft swabs as another attempt to clean them. These can be conductive, so air blast it or let it dry before making the connection again.
mrm12000 said:
When the amp is connected to speakers and with XLR input, the amp has audible/intrusive buzz through both speakers that's identical for any/all channels. If I unplug the interconnects at the source or the amp, the buzz stops. If the source is disconnected from other components, is turned off or unplugged, the buzz continues. I've tried multiple preamps and XLR cables, and this issue persists.
Click to expand...
If you are connecting RCA out to XLR, the cable should match #17 in the Rane document.
 
If this is XLR out to XLR in, then I suggest you try cables that have the shield floating (no connection to pin 1) at the amp end of the cables. Both cables, not just one of them.
 
whoaru99 said:
If this is XLR out to XLR in, then I suggest you try cables that have the shield floating (no connection to pin 1) at the amp end of the cables. Both cables, not just one of them.
Click to expand...
I will track some down and try. I have tried floating the ground of the source and the problem persists.
 
You may have to modify or order custom. I'd think most commercial XLR cables will be pinned all the way through.

I'd think about ordering some inexpensive Hosa or similar cables then just snipping back the shield connection to pin 1 at the amp ends to test out.

Don't know what's local to you but if there is Guitar Center, Sam Ash or similar likely get the cables off the shelf for about the same price as ordering.
 
Last edited:
whoaru99 said:
You may have to modify or order custom. I'd think most commercial XLR cables will be pinned all the way through.

I'd think about ordering some inexpensive Hosa or similar cables then just snipping back the shield connection to pin 1 at the amp ends to test out.

Don't know what's local to you but if there is Guitar Center, Sam Ash or similar likely get the cables off the shelf for about the same price as ordering.
Click to expand...
Thanks for the suggestion. Hosa makes an XLR ground lifter that I assume with do the job.

 
I'd think so, but so many folks seem to be adverse to adapters I didn't mention it. And cables to experiment might be similar or less cost. Saw Amazon Basic XLRs for $8 each.
 
whoaru99 said:
I'd think so, but so many folks seem to be adverse to adapters I didn't mention it. And cables to experiment might be similar or less cost. Saw Amazon Basic XLRs for $8 each.
Click to expand...
I ordered and they will be here tomorrow, so I'll have an answer then (I will post it).

The one thing about this that confuses me is why there's buzz when the amp is connected to a source that is not plugged in, turned on, or connected to any other piece of equipment. When the amp is disconnected to that source, even with the XLR cables still connected to it, no buzz.
 
mrm12000 said:
I ordered and they will be here tomorrow, so I'll have an answer then (I will post it).

The one thing about this that confuses me is why there's buzz when the amp is connected to a source that is not plugged in, turned on, or connected to any other piece of equipment. When the amp is disconnected to that source, even with the XLR cables still connected to it, no buzz.
Click to expand...

In very basic form, you're connecting an antenna to an amplifier. It's picking up a signal.
 
loudnoises said:
In very basic form, you're connecting an antenna to an amplifier. It's picking up a signal.
Click to expand...
Well, the antenna wins, because even with the ground of the XLRs lifted, via the Hosa adapters, the buzz remains. The amp must be picking up some EMI or RFI balanced that it's not single-ended or when there is nothing terminating the XLR cables. I may try an isolation transformer, if I can find an inexpensive one, but for now I'll use the amp single ended. Thanks again for your input.
 
You must log in or register to reply here.
Back
Top Bottom