Emotiva XPA-5 and UMC-200 both have issues - AVS Forum
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post #1 of 30 Old 07-15-2013, 10:42 AM - Thread Starter
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Hello everyone,

I'm new to AVS and the forum, I actually found it when troubleshooting my gear and looking for information on Google. I found a thread on this forum: http://www.avsforum.com/t/1375259/emotiva-beware-of-emotiva-issue-resolved/270. I wanted to share my experience with everyone here with the Emotiva XPA-5 (amplifier) and UMC-200 (pre-amp) and hopefully receive some help or tips in return. So here we go:

Let me start of by mentioning that I live in the Netherlands. Last february I bought a XPA-5 and UMC-200 from Emotiva with some discount, they were on offer since it is factory renewed hardware. I was promised all units are like new but might have some cosmetic damage, furthermore they are tested before being sent out. The testing part is especially important in my case since I live in Europe and already realised any support might become troublesome.

I use the following speakers: front L/R - B&W 601 s2, center - B&W LCR60 s3, Surround L/R Monitor audio radius HD90. All speakers are somewhere around 88 dB spl sensitivity. When I received the XPA and UMC I hooked it all up and I noticed there was a buzz (100 Hz) coming from the speakers. I started troubleshooting and couldn't find what the problem was / is. I use decent interconnects (http://www.ruconnected.nl/18-73-productpagina/interlink-cinch-tulp-1m.jpg) and don't have any ground loops (I'll explain in more detail in a bit). I disconnected the UMC-200 and focussed my attention on the XPA. I found out that everything is fine when there are just speakers connected, but as soon as there is more than one input (RCA or XLR) in use the buzz becomes audible. I actually still need to verify with the oscilloscope if it is actually not present with only one input in use or not audible. I ruled out the buzz being caused by a ground loop by connecting my iPhone as a pre-amp using a jack to dual RCA cable. One RCA connected equals no buzz, add the second and the buzz is there. The iPhone bit was actually suggested to me by Emotiva and they have been helping me out up untill now (more on their support further on).

After the iPhone test the Emotiva techs themselves were also convinced the buzz is being produced by the XPA itself. This was probably somewhere near the end of february. I then got into a lengthy process with Emotiva tech to try and figure out what is wrong with the amp. A new input board was sent out to me twice. Both times it did not help to replace this board which contains the RCA connectors. It was probably near the end of may when I installed the second input board, so a lot of time went by waiting on Emo tech support and international shipping of the parts. After installing the second input board I was offered an option to return the XPA. This seems fair enough but it doesn't help if I have to pay the return shipping costs ($500! due to the weight of the unit). There was no possibility to get this arranged together with Emotiva in such a way that it would not mean me losing on $500 due to shipping. So I had to decline that offer, I basically had no choice.

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 (I can go into more detail for those interested, but I don't think it will add any useful information to this post). Furthermore there are some quite lengthy connections inside the unit for the ground connections, which seems "dangerous", or at least increases the chances that there could be issues. This is due to the placement of the boards inside the "cabinet", it's normally better to minimise the length and consequently the (low frequency) resistance of the ground connections in amp designs. 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 told me, and this makes perfect sense, that he had a harder time hearing the buzz because his speakers have a lower sensitivity of around 86 dB. 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.

So in the end it seems I managed (with help) to find a way to get rid of the annoying buzzing, but it would come at a price (I haven't installed any modified input board yet). The gain / volume wouldn't be as high anymore as it could be. So I'm left wondering if I am the only person to experience these issues with the XPA-5 and is there still a defect inside the unit? The power supply boards might have issues (too much ripple voltage on the power to the amp boards?), since it definitely seems linked to power supply due to the frequency (we use 230V 50 Hz here, if you rectify that using semiconductors you will produce 100 Hz). Or is it possible that one or more of the amp boards contain a defect which is feeding the 100 Hz to the inputs?

So I had the XPA and UMC just sitting in a closet for months now, since I really get annoyed by the buzzing and I don't have another amp I can use with the UMC-200. After taking the XPA to the technician I decided to just hook it all up again and try to not be annoyed with the buzz. I then found out that when I turn on the XPA there is a popping sound in all the speakers and then again when turing on the UMC-200. The pop from the XPA is louder than the one from the UMC. I guess I could live with that if it would happen only at turning the stuff on. As it turns out, when I was playing a blu-ray through the playstation 3, there is a pop everytime the audio switches format.. So during all the intro's on blu-rays you get a whole bunch of pops after one another because they are usually in different audio formats. I went through all the connections and in the end had nothing but the subwoofer connected to the UMC-200 (also the XPA was disconnected) and it still popped. rolleyes.gif I'm not really sure what I should do now, I guess I will call Emotiva tech about the UMC-200 as well.



A little bit more on the XPA 5; the picture above shows the 5 amp boards at the top and in yellow I drew where the ground connections are that run from the front to the back of the unit, along the input board towards the back of the 5 amp boards. The "true ground" is in the bottom right on the board with the large capacitors. The ground is also connected on the other side of each amp board (I drew that in green for one board) where the power is connected as well to the amp board. Sorry for the Dutch text in the image.

I would appreciate any experiences that people are willing to share or ideas. I would actually also be very much interested in pictures you guys might have taken from the inside of your XPA-5, it would be interesting to see if there are any differences. I'm thinking mostly about the type of wiring used for ground connections that I drew in yellow and green.

To be continued...
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post #2 of 30 Old 07-15-2013, 01:50 PM
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Out of curiosity, did you try connecting a ground wire to the chassis of the XPA-5 and the UMC?

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post #3 of 30 Old 07-15-2013, 02:22 PM - Thread Starter
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I have had both chassis connected to each other with a wire. This did not help. Also the ground of the inputs on the XPA gets connected to the ground of the wall socket through the interconnects to the UMC. Because the UMC uses a 3 prong plug where the ground pin is internally connected to signal ground and the chassis. In the XPA the ground pin is lifted.
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post #4 of 30 Old 07-16-2013, 01:14 PM - Thread Starter
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No pictures of XPA insides? No similar experiences or tips? frown.gif
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post #5 of 30 Old 07-16-2013, 05:42 PM
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Quote:
Originally Posted by MegaJ View Post

No pictures of XPA insides? No similar experiences or tips? frown.gif

Have you posted these questions on the Emotiva forum? If not try there. Just choose your wording carefully, they have been known to ban members whom question them on some occasions.biggrin.gif
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post #6 of 30 Old 07-16-2013, 10:22 PM - Thread Starter
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I started a thread there months ago but never got anything useful. It wasn't as detailed as my post here however.

Just had a look on the Emo website at the gen 2 XPA-5. The input board (with the RCA's) looks different and so does the power supply board. Also in the gen 1 there is an additional transformer screwed to the casing of the toroidal amp, whereas in the gen 2 I now see a board placed there. I think I will call Emotiva and ask them about those parts, I was planning to ask them for a new power supply board anyway since it might be causing the issues I'm having.
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post #7 of 30 Old 07-17-2013, 06:18 AM
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it's a shame that emotiva still hasn't learnt from previous european customer issues. It's almost as if they are deliberately sending faulty products to far away european customers, hoping that expensive shipping will eventually scare away the customer, so that emotiva has got rid of its trash. They have admitted already that they only physically test 1 in a few hundred of a certain model, but still advertise on their web that they test each and every model before shipping, wich they do not ! At the very least they could do a quick test of the products they are sending out to other continents to avoid loss of reputation and a whole lotta trouble to the end customer, shame on them, they offer some good products but are completely ruining their brand by letting customers squirm and having to look for own solutions.

Way back they once made a mistake in the pricing for my order, they called me back that they wer 500 euro's (!) off price, but were willing to let 250 euro's drop so i had to pay an extra 250 to validate the order. I said to matt hey, a mistake has been made and i will pay u 500 euro's as long as u would extensively test the components i ordered before shipping so that at least there would be no issues. he said well we do that with all components, but we will make extra sure u will receive faultless product. I said ok deal, so imagine my dissapointment when i got faulty speakers sent over, a clear evidence that they did not test it before shipping. I never got that 250 back either...

on your issue, i have no idea whatsover what could be causing your problem, but i do feel after more then 6 months u should've been offered a valid solution by know. I hope posting here will help u out with solving your issue, it sure did help me out back then.

good luck with it
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post #8 of 30 Old 07-17-2013, 06:29 AM
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Quote:
Originally Posted by MegaJ View Post

I started a thread there months ago but never got anything useful. It wasn't as detailed as my post here however.

Just had a look on the Emo website at the gen 2 XPA-5. The input board (with the RCA's) looks different and so does the power supply board. Also in the gen 1 there is an additional transformer screwed to the casing of the toroidal amp, whereas in the gen 2 I now see a board placed there. I think I will call Emotiva and ask them about those parts, I was planning to ask them for a new power supply board anyway since it might be causing the issues I'm having.

You may ask them to exchange for the Gen 2 to make up for the charges for shipping their bad amp to you. Maybe they will do that. Can't hurt. Good luck and hope everything works out for you.
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post #9 of 30 Old 07-17-2013, 12:41 PM - Thread Starter
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Do you mean ask them for gen2 versions of the boards I mentioned?
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post #10 of 30 Old 07-17-2013, 02:09 PM
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Quote:
Originally Posted by MegaJ View Post

Do you mean ask them for gen2 versions of the boards I mentioned?

No, I would ask for the Gen2 amp. It's their problem to correct.
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post #11 of 30 Old 07-17-2013, 10:21 PM - Thread Starter
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I see what you mean. It'll cost me around $500 to send the XPA-5 back though. I would need to get a very good deal from them...
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post #12 of 30 Old 07-18-2013, 10:10 AM
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Unfortunately the savings on Emotiva gear is negated for those outside of the US if returns or service are needed.

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post #13 of 30 Old 08-06-2013, 11:31 AM - Thread Starter
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Been in contact a lot again recently with Emotiva tech support about the XPA-5, I still need to go into the UMC-200 popping. I'm happy with the support I got in terms of effort that went into it, but the problem with the XPA still isn't clear. I'll keep you guys updated.
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post #14 of 30 Old 08-12-2013, 11:19 AM
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Quote:
Originally Posted by MegaJ View Post

Been in contact a lot again recently with Emotiva tech support about the XPA-5, I still need to go into the UMC-200 popping. I'm happy with the support I got in terms of effort that went into it, but the problem with the XPA still isn't clear. I'll keep you guys updated.

Hi,
Have you learned anything more about the XPA issue?

I may have run into something similar, I have a (first gen) XPA 5 that produces a slight hiss from speakers when only the speakers or speakers + one RCA / XLR is connected, but starts to buzz (audible to listening position) when more than one inputs are connected.
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post #15 of 30 Old 08-12-2013, 12:31 PM
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Quote:
Originally Posted by MegaJ View Post

I'm happy with the support I got in terms of effort that went into it, but the problem with the XPA still isn't clear. I'll keep you guys updated.

I have a buddy that is having issues with his XPA-2 and Emotiva has been very helpful with suggesting troubleshooting ideas. He is very happy with the effort Emotiva has made to help correct the problem smile.gif.

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post #16 of 30 Old 08-12-2013, 04:39 PM
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I could only suggest that until some point in the future people outside America buy more local............Bills point about savings being the reason. That's a pretty big gamble hoping nothing ever goes wrong with electronics. Best of Luck to ya.
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post #17 of 30 Old 08-12-2013, 09:27 PM
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I'm sure you must have pro shops there. If so, they may have this or a similar product, http://www.ebtechaudio.com/hedes.html. If thei pro shop has a generous return policy, you may want to give it a try. It'll sit between the UMC200 and amp. It acts in such a way as to break the ground between the two units and may take care of your problem.

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post #18 of 30 Old 08-12-2013, 11:03 PM - Thread Starter
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Quote:
Originally Posted by Chu Gai View Post

I'm sure you must have pro shops there. If so, they may have this or a similar product, http://www.ebtechaudio.com/hedes.html. If thei pro shop has a generous return policy, you may want to give it a try. It'll sit between the UMC200 and amp. It acts in such a way as to break the ground between the two units and may take care of your problem.

A ground loop is not the issue.
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post #19 of 30 Old 08-12-2013, 11:11 PM - Thread Starter
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Quote:
Originally Posted by Pkartti View Post

Hi,
Have you learned anything more about the XPA issue?

I may have run into something similar, I have a (first gen) XPA 5 that produces a slight hiss from speakers when only the speakers or speakers + one RCA / XLR is connected, but starts to buzz (audible to listening position) when more than one inputs are connected.

This sounds exactly like the issue my XPA-5 has.

I have a few questions for you if you don't mind me asking:

Are you in the US or are you also on 230V 50 Hz power?
Has your XPA-5 always had this issue or did it pop up at some point in time?
Are you able to take a measurement of the output to the speaker? In other words, do you own or can you borrow an oscilloscope and know how to operate it? I would really like to compare to measurements I took of the hum voltage on the speaker outputs. I can help you / walk you through it if you are interested. Just let me know in this thread or send me a PM.

Thanks for posting.
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post #20 of 30 Old 08-13-2013, 01:27 PM
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I was about to get an Emotiva amp for my HT. After reading MegaJ's and another similar incident on Amazon, I decided to buy an QSC DSC 1644 instead. It has one channel less than the XPA-5, but it is okay. It costs less than the XPA-5. It is a pro amp, THX approved and proven product that has been around since 2001 . I love my DSC 1644. I might get another one.
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post #21 of 30 Old 08-14-2013, 11:30 AM
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Quote:
Originally Posted by MegaJ View Post

This sounds exactly like the issue my XPA-5 has.

I have a few questions for you if you don't mind me asking:

Are you in the US or are you also on 230V 50 Hz power?
Has your XPA-5 always had this issue or did it pop up at some point in time?
Are you able to take a measurement of the output to the speaker? In other words, do you own or can you borrow an oscilloscope and know how to operate it? I would really like to compare to measurements I took of the hum voltage on the speaker outputs. I can help you / walk you through it if you are interested. Just let me know in this thread or send me a PM.

Thanks for posting.

Thanks for you for posting your experiences and the work you have done.

I am also in Europe so its 220/50 power.

I have had the issue from the beginning. I have currently a Yamaha Z11 that I use as AVR. I have been planning to move first to use it as pre/pro with separate amps (for this I then bought the XPA-5) and later to replace it with a newer model.

As I got the XPA I noticed the the buzz right away with the first connection. I trouble shooted this and that as I of course first thought is was a ground loop issue, but then noticed that the amp had the hum even if I used cheater plugs or disconnected else from Z11. Then I packed the XPA away and returned to using Z11 amps as well. I retried the connections this summer and then noticed your posting and after reading got to testing this one/several RCA/XLR connection thing and then noticed that the buzz goes away with only one line connected.

Unfortunately I do not have anywhere to borrow an oscilloscope.
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post #22 of 30 Old 08-14-2013, 01:11 PM - Thread Starter
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Quote:
Originally Posted by Pkartti View Post

Thanks for you for posting your experiences and the work you have done.

I am also in Europe so its 220/50 power.

I have had the issue from the beginning. I have currently a Yamaha Z11 that I use as AVR. I have been planning to move first to use it as pre/pro with separate amps (for this I then bought the XPA-5) and later to replace it with a newer model.

As I got the XPA I noticed the the buzz right away with the first connection. I trouble shooted this and that as I of course first thought is was a ground loop issue, but then noticed that the amp had the hum even if I used cheater plugs or disconnected else from Z11. Then I packed the XPA away and returned to using Z11 amps as well. I retried the connections this summer and then noticed your posting and after reading got to testing this one/several RCA/XLR connection thing and then noticed that the buzz goes away with only one line connected.

Unfortunately I do not have anywhere to borrow an oscilloscope.

It's a shame you can't get hold of an oscilloscope. It really sounds exactly the same as my issue.

I have made some progress with Emo tech support to correct the problem. I will update here, but it needs a little bit more work first.
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post #23 of 30 Old 08-14-2013, 08:16 PM
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I recently built a custom subwoofer and hooked it up to my pioneer elite receiver and noticed a strong hum in the sub when I connected the LFE rca cable. Adding a cheater plug to the subwoofer power cord fixed it. Then I swapped the receiver for a umc-200 and sherbourn pa 7-150 and decided to try without the cheater plug and this time the sub AND speakers hummed. After extensive troubleshooting the LFE jack in the subwoofer amp was the root cause. Added a ground isolator loop from radio shack in the RCA cable between the sub LFR and umc-200 and voila. Problem solved.

So, I recommend trying a ground isolator loop at any unbalanced or xlr connections from an external amp connectedntonthe receiver or processor (one connection at a time). That worked for me, only needed one isolator to fix the hum.

Hope this helps.:beer:

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post #24 of 30 Old 08-14-2013, 09:46 PM
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I also have the XPA-5/UMC-200 combo. While I haven't had any problems with buzzing speakers I do have a popping issue with the UMC-200. Which is why I have swapped it out for a Yamaha HRT-7065 - an almost clone of the A820. No problems with the XPA-5 and the Yamaha.

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post #25 of 30 Old 08-15-2013, 04:46 AM
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Quote:
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I also have the XPA-5/UMC-200 combo. While I haven't had any problems with buzzing speakers I do have a popping issue with the UMC-200. Which is why I have swapped it out for a Yamaha HRT-7065 - an almost clone of the A820. No problems with the XPA-5 and the Yamaha.

KH,

Do you feel that you gave up anything in SQ by going form the UMC-200 to the 7065? I ask as I was just reading a post over at the Lounge where a UMC-200 owner commented that the UMC-200 is a " huge leap up in sound quality" over a Denon. I'm just curious as to what would give one a " huge leap up in sound quality" over a SSP in similar price ranges. I could see if one was not setup properly but I'm really interested in what would cause this.

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post #26 of 30 Old 08-19-2013, 06:19 PM
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MegaJ,

The fact that the noise is a multiple of your AC frequency leads me to believe this is a power supply issue. I have built amplifiers and ran into a 60 Hz noise (North America) that stubbornly would not go away. I obtained some mu-metal and planned to wrap it around the transformer and as I moved the mu-metal around inside the amplifier the noise suddenly decreased. It wasn't the transformer though - it simply was AC lines that were run to close to one of the amp circuit boards. Block the magnetic field emanating from the AC wire and the noise disappeared. Looking at the picture of the XPA-5 and lack of shielding perhaps some AC input wires are just a little too close to another wire or a circuit board. This would be a quality control problem where some manufactured units are okay, but other units manufactured with wires slightly differently placed would cause a hum.

Just a thought...

Derek
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post #27 of 30 Old 08-20-2013, 07:27 AM
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A mistake has been made and i will pay u 500 euro's as long as u would extensively test the components i ordered before shipping so that at least there would be no issues. he said well we do that with all components, but we will make extra sure u will receive faultless product. I said ok deal, so imagine my dissapointment when i got faulty speakers sent over, a clear evidence that they did not test it before shipping.myzVeek.gif
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post #28 of 30 Old 08-27-2013, 08:30 AM - Thread Starter
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Quote:
Originally Posted by bDerek View Post

MegaJ,

The fact that the noise is a multiple of your AC frequency leads me to believe this is a power supply issue. I have built amplifiers and ran into a 60 Hz noise (North America) that stubbornly would not go away. I obtained some mu-metal and planned to wrap it around the transformer and as I moved the mu-metal around inside the amplifier the noise suddenly decreased. It wasn't the transformer though - it simply was AC lines that were run to close to one of the amp circuit boards. Block the magnetic field emanating from the AC wire and the noise disappeared. Looking at the picture of the XPA-5 and lack of shielding perhaps some AC input wires are just a little too close to another wire or a circuit board. This would be a quality control problem where some manufactured units are okay, but other units manufactured with wires slightly differently placed would cause a hum.

Just a thought...

Derek

Hi Derek,

thanks for your post. This could definately be the case. The separation between power lines and ground lines isn't very good in my opninion in the XPA. It is then a case of being lucky and receiving a unit that was built with (just) enough separation between crucial parts, or being unlucky like me. I agree with you this could very well be the case with XPA-5's. This is quite hard to diagnose however, I also don't have any mu-metal. I also wonder how effective this stuff is however against low frequencies, I'm assuming this is a type of foil? In my experience foils help against RFI and not so much against lower frequencies like 50, 60, 100 or 120 Hz. In your past experience did you wrap the mu-metal around AC lines?
Can you tell me a bit more about your experiences? After that I might consider investigating the potential for the XPA.

Furthermore there are long ground lines that run back to the single ground point on the PCB in the bottom right on the picture, these probably have small differences in impedance and thus differences are created in the potential that represents ground in different parts of the amp. Which leads to interference voltages on the ground.

All in all I believe the XPA amps could benefit a lot from a bit of research with respect to the aspects mentioned above.
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post #29 of 30 Old 08-27-2013, 09:14 AM
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Have you looked into any other amps? Goods deals on Outlaw(made by ATI). Also ATI from the site below.

http://www.classicaudioparts.com/index.php/amplifiers.html
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post #30 of 30 Old 08-27-2013, 09:25 AM - Thread Starter
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Ok here's the update I promised.

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.
Although now that I'm writing this it actually seems interesting again considering Derek's post about the mu-metal and supply voltage seperation, see his post and my reply above. I'll give it some more thought.

Scope Image (speaker voltage in blue, positive rail voltage in yellow):


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.

2-channel test:


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.

Chassis connection at amp board for channel 1:


End situation:


To sum up the changes to my XPA:
- GND of all the individual channels on the input board are interconnected
- GND of each input on the input board is connected with a piece of wire to AGND on the amp board for the channel (yellow wire on the picture). I believe this connection is also made via the connector between the input board and the amp board, but those wires have a smaller diameter.
- GND of the connected channels is connected to the chassis via 10 Ohm at the side of channel 1 on the input board and fixed to the heatsink screw on the amp board of channel 1 (blue wire in the picture)

As I said before this is now acceptable for me. But I do wonder if the buzz can still be heard from further away on more sensitive speakers. Also Derek provided me with some food for thought, but it something that has been in my mind ever since running into this issue, how is it possible that (quite) some XPA's seem to have buzzing issues? Is it a quality control issue? Or is it a separation / shielding issue (ground and power) which might turn out badly in some units due to the specific construction and well in others?

I still have the feeling this issue can be solved completely in the XPA line but it would require some research by Emotiva with respect to ground connection placement. I was even told that it is likely that small differences in ground line impedance are causing the issue, this seems like something which can be avoided by engineering / research and precise construction. Even cabling with a bigger diameter might solve it if you ask me.

Anyway I hope this information is helpful and useful for other people out there with XPA amps. If anyone would like more information just send me a message or reply in this thread.
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