XPA-2 Buzz

[stephen]

Greetings all, I've had a few problems in the pursuit of audio excellence lately! First, I sent my UMC-1 in for repair after HDMI video started dropping out. Emotiva found a few burnt up components on the power supply, just got it back and it works great! Slightly worried one of my triggers is sucking too much current or something is shorted out... Second, my XPA-2 amplifier is not behaving.

Problem: Buzz in Sierra-1 speakers powered by XPA-2 amplifier audible from listening position

Troubleshooting (updated): Actions and outcomes described in posts below

Audio captured with camera walking up to speaker: XPA2 Sierra-1 MP3 Audio Clip 

EDIT: Audio captured with speaker hooked up to XPA-5, then walking back to rack and turning on XPA-2 (nothing connected on input or output): XPA-5 Sierra-1 MP3 Audio Clip without and then with XPA-2 powered but not wired in rack

Sounds like ground loop?








Thanks!
Stephen

[Dan Laufman]

Does this noise occur with NOTHING plugged into the RCA jacks on the back of the amplifier?

Are the speakers bi-amplified? Could the output of the XPA-2 amplifier possibly be connected to another point?

[Dan Laufman]

This sounds like a nasty ground loop and is not normal under any circumstances.

[hemster]

G'day Stephen,

Thanks for posting the sound clip. It could very well be a ground loop issue. Can you try the following?

1. Unplug all other components, leaving only the amp plugged into the wall. Is the noise still present?
2. Try plugging the amp into another power outlet. Run an extension cord from another room to see if the issue is still present with the amp plugged into another circuit.
3. Test with a cheater plug (2 pin) to see if the noise goes away.

Cheater plug:

Hopefully this'll help identify the source of the noise. Remedial action can then follow. 
Let us know how you get on. 

[stephen]

Aug 22, 2014 17:34:50 GMT -5 Dan Laufman said:

Does this noise occur with NOTHING plugged into the RCA jacks on the back of the amplifier?

Are the speakers bi-amplified? Could the output of the XPA-2 amplifier possibly be connected to another point?

Nothing! (no RCA inputs, I also disconnected the trigger input). Speakers have their own crossover network, so not biamped.

I think I'll try some new temporary speaker cable tomorrow just to see along with XPA-5 from storage idea below... Current speaker cable is 12/2 w/o shield, I'm wondering if it got nicked during our move, but I doubt it's creating a new ground point somewhere...

The audio recording was done walking right up to the speakers with the camera, first held by the tweeter, then woofer; so although the sound is easily covered up with program material, I'm like one of those videophiles that's easily distracted by LCD TVs not showing true black, or in the case slight audio hum during otherwise quiet moments:)


(Dan thank you for personally weighing in! Must say I've enjoyed UMC-1 and XPA amps over the years)

[stephen]

Aug 22, 2014 17:38:57 GMT -5 hemster said:

G'day Stephen,

Thanks for posting the sound clip. It could very well be a ground loop issue. Can you try the following?

1. Unplug all other components, leaving only the amp plugged into the wall. Is the noise still present?
2. Try plugging the amp into another power outlet. Run an extension cord from another room to see if the issue is still present with the amp plugged into another circuit.
3. Test with a cheater plug (2 pin) to see if the noise goes away.

Cheater plug:

Hopefully this'll help identify the source of the noise. Remedial action can then follow. 
Let us know how you get on. 

#1 - reference reply to Dan's post above

#2 - Good suggestion! I'll try tomorrow.

#3 - I believe XPA-2 doesn't have a ground pin on its power connector? I wonder if I grab a wire and ground the chassis what would happen...

New idea: #4 - I have an XPA-5 in the closet that I may unpack tomorrow too to test/compare (have gone down to a 2.1 system since moving to a condo).

[hemster]

I forgot to add: make sure the speaker wires (and other signal wires too) are not aligned with the power cables. If they must cross, then cross them at 90^o. This'll avoid inductive interference. 

[stephen]

Troubleshooting in progress:

First experiment: Unpack XPA-5 from storage and plug into same power strip as XPA-2, wire to left speaker (to compare to right still wired to XPA-2), nothing connected to inputs. Outcomes:

1. No buzz
2. Hiss only audible with ear up to speaker(input switch: un-bal)
3. However, switching the input switch to bal dropped the hiss to barely audible! That's an awesomely quiet amp with great noise floor that I would expect from Emotiva!!!




Second experiment: Move XPA-5 near equipment rack and connect up one channel of XPA-5 to UMC-1 to see how it behaves. Outcome:
1. No buzz! (regardless on bal / un-bal)
2. Hiss a bit louder when UMC-1 is connected to a particular RCA input, but not unbearable...

Third experiment: Completely swap out XPA-2 for XPA-5, wire up with original cables to speakers, etc. Outcome:
1. No buzz until both L and R RCA cables connected from UMC-1! aha, revisit previous experiments and/or re-hookup XPA-2?

Fourth experiment: Confirmed buzz when both RCA cables connected from Oppo BDP-93 (instead of UMC-1) to XPA-5, buzz disappears when either one is disconnected...

I am at a loss to explain why my original troubleshooting showed the buzz remained with nothing connected to the XPA-2, tomorrow I have have to wire it back up and experiment further. Meanwhile, I have an annoying ground loop situation on my hands!

[LCSeminole]

Just a thought, I notice that you have a Comcast/Motorola cable box. With everything hooked up as you've always had it before this problem, if you disconnect the cable tv connection from the wall, does your buzz/hum go away?

[stephen]

Aug 26, 2014 18:26:12 GMT -5 LCSeminole said:

Just a thought, I notice that you have a Comcast/Motorola cable box. With everything hooked up as you've always had it before this problem, if you disconnect the cable tv connection from the wall, does your buzz/hum go away?

Profile updated after our move! Downsized a little here to a 2.1 system and transitioned from Comcast to FIOS, Internet only, however. Taking your queue, I did disconnect the RF input to the UMC-1 (my antenna was split between TV and UMC-1 for FM reception) just to confirm that didn't have an impact...

[stephen]

I have had some success rerouting power cables, taking surge supressors out of anywhere in the signal chain, etc. (still w/ XPA-5, haven't re hooked up XPA-2 yet), but this is interesting, and at first glance similar to my issue (buzz with more than one channel connected), Post 30: www.avsforum.com/forum/90-receivers-amps-processors/1481773-emotiva-xpa-5-umc-200-both-have-issues.html

Several mitigation strategies:

1.

Emotiva supplied me with circuit schematics of the boards, which I took to a technician. The guy really knows a lot about amps and was not entirely sure a certain section of the amp boards is designed in the best way to reduce the risk of buzzing or other noise ... He found out that in some way a 100 Hz signal is being fed back to the input of the amp boards when more than one input is in use. ... He also tried to place a resistor (10 Ohm) in the ground connection of some interconnects between a pre-amp and the XPA, which made the XPA go into the safe mode. This I actually find strange. Funny thing was, when placing a pure resistive load on the inputs (more than one), there was no buzz at all and the XPA did not go into the safe mode. In the end the only suggestion he had left was to modify one of the input boards and add a voltage divider to each of the inputs. This means the buzz signal which is being fed back to the input is also divided and becomes inaudible, the pre-amp will need to be used with a higher volume setting to reach the same volume level as before.

2.

First off just a bit of extra information, which I found interesting to see but it didn't really help to solve the issue with the XPA-5; the buzz / hum on the speakers turns out to be exactly in phase with the ripple on the (positive part of the) power supply voltage of the amp boards. I concluded this from the measurements I took with an oscilloscope on the +65V of a certain channel and the voltage on the speaker output of this channel. See the scope picture.... With the help of the VP of product development at Emotiva I managed to reduce the buzz to a level I can accept. Let me explain how I reached this situation.

I got the suggestion to see what happens when all RCA shields of the inputs (signal ground) are connected and connected to the chassis via a 10 Ohm resistor. At first it seemed like a bit of a dead end, because I tried it on two channels and after raising the volume on the pre-amp the XPA went into protection. After trying again with all five input grounds connected to each other and to the chasiis via 10 Ohms the amp no longer went into protection. The buzz level is also significantly reduced. I ran these tests by taking pieces of electrical wire (solid core) and bending these around the RCA shields and soldering it all together with the resistor on one end and connecting that via a piece of wire under a screw in the side of the chassis. I'm mentioning this because this is the easiest way to try this if anyone should want to with their XPA. See the picture of my 2 channels attempt, which I expanded to 5 channels later on.
...
Seeing as the buzz volume is significantly reduced in this way, I decided, together with Emotiva, to modify an input board to make the change permanent. On the bottom of the PCB I connected all the GND together. This specific PCB also has an extra wire in all the GND connections for each channel which needs to be soldered to the AGND on each amp board (from an earlier modification suggested by Emotiva, which in itself did not solve the problem but is still needed now). This connection is made by the yellow wires that can be seen on the pictures below. On the side of channel 1 I soldered the 10 Ohm resistor to the GND on the bottom of the PCB and connected a piece of wire with an "eye" to it (blue wire on the picture below). The "eye" I placed under the screw under the AGND on the amp board for channel 1, this screw connects to the heatsink which is connected to the chassis with a whole bunch of screws. Seemed like a nice place to fix it. Especially since this channel normally has the longest ground connection and now it is connected to chassis ground with the shortest possible connection. See the pictures.

[stephen]

As told yesterday I figured out how to get the XPA-5 replacement relatively buzz free in the system.

So, today I went back to retest the XPA-2. To my surprise, just turning the XPA-2 on in the rack with nothing connected (input or output) caused the dreaded buzz to return in the already powered up and wired up XPA-5! I moved over the speakers and confirmed the buzz was also present on the XPA-2's outputs (with nothing connected on the inputs).

Audio captured with speaker hooked up to XPA-5, then walking back to rack and turning on XPA-2 (nothing connected on input or output): XPA-5 Sierra-1 MP3 Audio Clip without and then with XPA-2 powered but not wired in rack



I'm forced to conclude there's something wrong with my XPA-2! (and maybe some underlying XPA-5/XPA-2 design issue?)

So for now I'll leave the XPA-5 in place for my 2.1 system. No lack of power for sure...

[jackpine]

I have no idea if these will do anything but here's a couple suggestions. One try swapping in a different power cable to the XPA-2. Two take the cover off the XPA-2 and tighten any of screws that go to the shielding over the transformer.

[garbulky]

Did I read right that the buzz comes after you connect the RCA cables? Have you tried a different set of RCA cables?

[stephen]

Aug 27, 2014 16:44:27 GMT -5 jackpine said:

I have no idea if these will do anything but here's a couple suggestions. One try swapping in a different power cable to the XPA-2. Two take the cover off the XPA-2 and tighten any of screws that go to the shielding over the transformer.

- Took off cover, checked screws on top and bottom to make sure they were tight; I hesitate to take apart further...
- I will try another power cable!

Aug 27, 2014 17:28:56 GMT -5 garbulky said:

Did I read right that the buzz comes after you connect the RCA cables? Have you tried a different set of RCA cables?

- Good suggestion as well, I have a few sets of bluejeans RCA cables, I'll try swapping on the now hooked up XPA-5 just to see what happens. XPA-2 seems plagued even with nothing connected to input... I did by accident discover that switching one of input switching from unbal to bal appears to resolve this problem, further testing required to confirm I wasn't accidently switching the bridge switch or something...

[Gary Cook]

Aug 27, 2014 16:15:35 GMT -5 stephen said:

maybe some underlying XPA-5/XPA-2 design issue?

I think you can dismiss this possibility as there are plenty of guys with XPA amps that have no noise present. Personally I have had 4 x XPA amps and they are as close to dead silent as any power amp that I have ever heard, and I've heard hundreds, maybe thousands.

The connecting one RCA = no noise but connecting 2 RCA's = noise indicates some ground loop issues (refer below).
The no connections = noise is quite common in areas of high EMF/EMI/RFI. For example if my next door neighbour is welding I hear quite a buzz when nothing (input wise) is connected to the power amps. Which completely disappears when a shielded input is connected.
The fact that you notice lower noise with XLR connections (versus RCA connections) is an indicator of the presence of EMF/EMI/RFI.
Hence I'm not surprised that you have some noise with no input connections.


BTW, ground loop noise is commonly described as a "hum", in an attempt to replicate the frequency (50 hz, 60 hz, 100 hz, 120 hz etc).
While induced noise of higher frequencies can be described as a "buzz" or a "hiss", again in attempts to replicate their frequency.
I have frequently (sic) found that the frequency of the noise is a reasonably reliable indicator of the source. ie; ground loop noise isn't usually a "hiss".


Cheers
Gary

[stephen]

Installing a SurgeX to perform power filter/distribution has helped considerably (got rid of APC surge protector)! The XPA-5 still had a better noise floor for my 2.1 system (barely audible hiss/hum with ears up to speaker; with SurgeX, turning on the XPA-2 in the rack didn't add any hum as it had in prior experiment), so I kept it. XPA-2 may be just overkill for my efficient bookshelf speakers! I think it might be time to sell it!