Confusing noise issue

adam2434

Been trying to track down the root cause for the following somewhat confusing noise issue. This isn't a new issue in my system, just something I've been recently trying to tackle.

The L/R preouts of my ARV are connected to the "DVD" input on my 2ch preamp. When using the AVR, I set the 2-ch pre to a predetermined/calibrated level. Things are connected this way so that I can use my L/R amps for HT and in a separate 2-ch system/context, without the need to swap cables. I know this is way less than ideal and that 2-ch pre with a HT bypass would be the best way to handle this.

The issue is that I get some hum/noise in my L/R channels when the 2-ch preamp's DVD input and is receiving the L/R preout from the AVR for HT use. The noise goes away when I select a different input on the 2-ch pre. Because the noise goes away when I select a different input on the 2ch pre, I've concluded that the 2-ch pre and L/R amps do not have inherent noise problems.

So, my next step was to determine if the noise was inherent to AVR's L/R preout signal. I connected the the AVR L/R preouts directly to the L/R amps - no noise. So at this point, I've concluded that the noise is not inherent to the AVR, 2-ch preamp, and L/R amps.

The noise is only present when the AVR preouts are connected to the 2-ch preamp.

Here's the strange part - if I connect 1 channel (say, R ch) directly from the AVR to the amp (2-ch preamp not in the chain), but have the other AVR preout channel going to the 2-ch preamp, then amp, I still get the noise in the channel that's connected directly to the AVR, even if the 2-ch preamp is off. Somehow, having one or both of AVR's preouts connected to the 2-ch preamp casues the noise. It's like somehow the connection between the AVR preouts and 2-ch preamp is creating noise in AVR's L and R preouts, even if one of the channels is not even connected through the 2-ch preamp and even if the AVR is off (although the noise is louder when the AVR is on).

This makes me think that there's some type of impedance issue or ground potential issue that only exists between the AVR and 2-ch preamp. I tried putting the 2-ch preamp on a different circuit, but that didn't help. I also tried a cheater plug on the 2-ch preamp - I think that dropped the noise slightly.

Any thoughts?

Thanks.

adam2434

Well, I played around some more yesterday and found the source of the noise.

Somehow, my Music Hall DAC was the culprit. I'm still scratching my head though.

The noise was present with the DAC power off.

The most noise was when the AVR's L/R preouts were selected on the preamp, not when the DAC's outputs were selected on the preamp.

When I disconnected the DAC's analog outs from the preamp, the noise went away from all preamp inputs.

Then I reconnected the DAC's analog outs and used a cheater plug on the DAC's power cord...no noise!

So, I'm assuming that there must have be some type of ground loop issue/interaction between the AVR, 2-channel preamp, and DAC - even when the DAC is off. When the AVR to 2-channel preamp connection is broken or when the DAC to 2-channel preamp connection is broken, the noise is gone. So, it must be a 3-way interaction between the AVR, 2-channel preamp, and DAC.

One thing I did not try yesterday was to disconnect my Squeezebox digital coax from the DAC and to remove the cheater plug from the DAC's power cord to determine if the Squeezebox digital coax connection was the source of the issue. However, the Squeezebox was also powered off yesterday when I was experimenting. I think I'll try that today.

Overall lesson learned - when investigating noise gremlins, isolate and test every component in the system, even if they are powered off when the noise is present and even if they don't logically seem to follow the noise observations.

disneyjoe7

Could it be just that the volume of the AVR is too low of a setting, and the Pre amp volume setting is set too high to compensate.

adam2434

disneyjoe7 wrote: »

Could it be just that the volume of the AVR is too low of a setting, and the Pre amp volume setting is set too high to compensate.

Long ago, I calibrated the HT with a SPL meter and ended up with the 2-ch preamp's volume at 12 o'clock (mid position) and +6 on the AVR's R and L channels. I could reduce the 2-ch preamp's volume a bit, but would have to recalibrate and raise the AVR's R and L channels to compensate. I'm already at +6 on the AVR's L and R, so don't have much room there anyway.

Right now, with the cheater plug on the DAC, I'm noise-free.

inspiredsports

adam2434 wrote: »

Long ago, I calibrated the HT with a SPL meter and ended up with the 2-ch preamp's volume at 12 o'clock (mid position) and +6 on the AVR's R and L channels. I could reduce the 2-ch preamp's volume a bit, but would have to recalibrate and raise the AVR's R and L channels to compensate. I'm already at +6 on the AVR's L and R, so don't have much room there anyway.

Right now, with the cheater plug on the DAC, I'm noise-free.

You beat me to the punch. I was running into the same problem when running a USB cable from my laptop to my DAC with all kinds of noise.

A gray 3 to 2 prong adapter on my laptops power brick silenced the problem in seconds.

And as you learned, ground loops don't care if the grounded component is powered on or not.

disneyjoe7

+6 on L/R settings on the AVR... Hmmm.... Ok what I would do is remove the pre amp setup the AVR so everything is balanced and see what the L/R setting is. Then re add the pre amp to rebalance the L/R but only use the pre amp volume then since the AVR output should be balanced.


But on the other hand if you fixed it good for you.