The OEM Power Supply & “Hum”: Inherent Design Issues

  • The Sparks OEM power supply I received is a SWITCHING Power Supply. I’m assuming it’s the same power supply everyone received. There are several great things about switching power supplies BUT they have some issues particularly if they are built inexpensively.

    Wikipedia has a great piece about switching power supplies if you’re interested. For me it came down to three comments that have a direct bearing on “the Hum“ These are inherent in the design of a switching power supply. These are direct quotes from that article. The comments are mine.
    *1) ...“EMI/RFI (is) produced due to the current being switched on and off sharply. Therefore, EMI filters and RF shielding are needed to reduce the disruptive interference” COMMENT: We always suspected that RFI was the issue, The filters and shielding in the stock power supply is therefore critical. Inexpensive components could either fail or fail to due the job properly.
    *2) ... (They are) Noisier due to the switching frequency (rate) of the SMPS. An unfiltered output may cause glitches in digital circuits or noise in audio circuits. COMMENT: Now that’s telling. So not only can they create noise in audio circuits but they can cause digital circuitry to have issues. Like the input jack circuitry perhaps? or simply cause the amp to fail?
    *3) Very low cost SMPS may couple electrical switching noise back onto the mains power line, causing interference with A/V equipment connected to the same phase. COMMENT: Again interference with audio equipment.

  • I don’t know some of the facts about PG’s Replacement Power supplies as it relates to my original post above. . .Could those of you that received a replacement power supply from PG please reply and answer each of the following simple yes or no questions.

    *1) Did the PG replacement PS use a grounded AC wall plug? Y/N

    *2) Was the PG replacement PS described as a Switching Power Supply (printed on the back)? Y/N

    *3) Did the PG Replacement PS fix the hum Issues on your amp? Y/N

    *4) Did the PG replacement PS fix any other issue you were having with the amp. Y/N

    *5) If the answer to #4 was “yes“ what other problem was “fixed”?


  • I know I have posted in another thread about this and I am pretty sure you were in. So I will build on that. Also, My name isn't "Sara". I'm a dude. Unfortunate initials.
    Good work researching this.
    Some notable thoughts:
    Its not uncommon to use a switching power supply in such an application. These are usually much cheaper to manufacture and in most cases if not the best choice, an acceptable choice. They are not bad because they are switching. Filtering plays a big factor in how they operate in a noise sensitive circuit. Although in this case, I would assume PG designed the amp with some filtering around the pre and post amp circuits as well (the most sensitive areas). The other issue you mentioned is shielding. Shielding is simply the prevention or limitation of noise. Noise is simple to understand what it is, but very difficult to understand why, when, and where it occurs and how it will effect the circuit. It is defined as any alteration of a signal that is unintended. Conversely, RF (Radio Frequency) is also a variation in a signal that is usually considered intended. RF can be noise and not depending on how its used. Shielding is any method used to isolate unwanted noise in a circuit by use or preventing it or limiting its effect. This is usually accomplished using bonding (connecting all metal case components together to ground (sinking to chassis or sinking to earth) or by shielding (coaxial cables for example, instrument cables are this way). Filtering is changing a signal through the use of some device that will smooth out the signal. Clean it. There are many ways to do this, but the most common is through the use of capacitors. These will charge with noise voltage and discharge cleaner voltage. This may require multiple stage in series to completely nominalize a DC voltage.
    The interesting one is point #3
    "*3) Very low cost SMPS may couple electrical switching noise back onto the mains power line, causing interference with A/V equipment connected to the same phase. COMMENT: Again interference with audio equipment."
    This is not always a bad thing. A well calibrated and accurate power supply will use this method to filter even more than capacitor filtering can achieve. This works by feeding noise back into the driving or driven side of the circuit slightly out of phase to the original signal to cancel out that signal variation. Noise cancelling headphones work this way. Done improperly, this would cause a lot of noise, however I doubt the PS in question utilizes such technology. The down side of this, is possibility of a feedback loop where feeding that harmonic signal back into an amplifier circuit (in the PS not the guitar amp) can increase in magnitude every time it is looped. Microphone feedback is an example of this. The signal is amplified in phase and builds in magnitude with each cycle.
    I apologize for rambling. However, its very rare I get to combine two things I love together.
    To sum up, An ungrounded power supply isn't the problem. The problem is either inadequate shielding or lack of filtering in either the Sparks amplifier circuit or in the sourced power supply they provided. A grounded power supply would be the simplest solution to the noise issue if it is not being generated in the amp. The method is right, just not the reason. I say this because a grounded power supply might not fix every noise problem people are experiencing. Especially if they are getting noise from the amp and not from an outside source, like the power supply, the natural to the environment, or from the source power.

  • @sarawson

    Man if I used Sara, apologies, I’m pretty cognizant of gender possibilities when it comes to Board handles but hey I’m a hasty idiot at times. I assumed your last name was rawson, first initials SA. Is there a blush emoji? so embarrassing.

    But to the points at hand about a switching power supply. Yes I rather gathered that there are definite advantages to “switchers“. I think I can say on behalf of those of us interested enough in understanding the underlying electronics your reply is the most cogent and informative explanations received to date. Keoki Kudos.

    I’ve always wanted to know “What is a Switching Power Supply“ and this was the perfect opportunity to investigate. Obviously I was going to key in on ANYTHING that remotely indicated signal issues, noise or hum. Lol, to your point about filtering and the use of capacitors, my power supply has a small led, which I assume is on most if not all the OEM units. I was laughing my ass off last night because that damned thing stayed lit For about 5 minutes even after it was unplugged from the wall. The first thing that popped to mind was Man there has to be some hefty capacitors in-line with the LEDs circuit, that’s some seriously long bleed out time! Which is actually a good example of why you need need to leave electronics unplugged for a certain period of time when doing power resets in things like modems and routers.

    Obviously there are a large portion of the Spark SWPS’s out there that AREN’T causing hum issues, but some portion of the OEM supplies that are, QA issue, but not enough of one to do a recall replacement. Sounds like PG made the decision to simply send a grounded PS as a replacement and put a bullet in the head of the problem a specific owner was having.

    On another note I think you also answered the question for a few people who experienced hum even with a ground plug PS. The issue is within the amp circuitry itself. Time to file that support ticket.

    I enjoy our exchanges and look forward to many more. By the way my given name is George, but I was blessed with the Hawaiian version, Keoki, very early on in my life. A pleasure to meet you.


  • @keoki As a follow-up to my response to you in the 'other' thread. I did some quick tests with the two replacement SMPS units that I tried along with some various other laptop power supplies I had on hand (HP branded). Apparently on the three-prong 'grounded' supply that did nothing to alleviate the hum I was having in my Spark amp there is no internal connection between the AC earth pin and the ring/negative portion of the DC output. Or, if there is, its resistance exceeds 200M Ohms and is therefore useless in this application. On the power supply that did correct my issue there is definitely an internal connection from the earth pin on the AC side and the ring/negative on the DC side that measures 1M Ohm of resistance. All of the HP laptop supplies I had on hand that I tested also had a resistance of between 1M and 2M Ohm between the AC earth pin and the DC ring/negative.

    Food for thought, but, in at least some of the instances of hum, it would appear that this interconnection from AC ground to the negative DC supply is enough to nullify the difference in ground potential and eliminate the hum.

    Chasing this stuff down is a hassle, but it isn't unique to the Spark amp. But, as others have said, it's a bit of a let down that something like this wasn't addressed during development/QC of the product and that a better power supply wasn't sourced for the amp from the start.

    In my case, if I didn't have the desire to use any of the high gain amp models I'd likely never have seen the issue in the first place. I have zero hum using the original power supply on any of the blues/pop/clean/rock/acoustic amp models even with volume and gain all the way up and no noise gate at all. So there may be many other users that just haven't noticed the issue because they don't use any of the high-gain models or the default noise gate settings are masking the issue.

    If anyone who doesn't typically use high-gain amp models is curious to see if they have the same issue then try out the 'Yngwie Malmsteen' amp model that was uploaded by Jun Young Song on the Tone Cloud. I believe it is the one with the largest number of downloads for any 'Yngwie' amp model. Connect your guitar to the Spark with cable (not wireless) load that model, turn off the noise gate and don't touch your strings. The hum from that model is quite bad on my Spark. If I ground the chassis or use a 'verified' grounded SMPS the hum is totally gone and all I hear is the normal background hiss of a high-gain amp.

    It would be interesting to see how many people don't have the hum issue change their minds after that test. Then again, it may just be a 'so what?' thing since those types of amp models are not their cup of tea anyway.

  • Integral bonding for the win!

  • @radu7

    A much appreciated reply and yet another couple of informative pieces in the puzzle.

    A question about the Malmsteen amp download and its inherent hum. I know tone cloud presets were found to have volume issues because the uploaded file carried the volume setting in a data field.

    Is it possible that an uploaded amp file could similarly "carry" hum as part of any data field within the file? Esoteric, I know but thats my kid like mind at work. Keoki

  • @keoki I appreciate the detailed information about noise issues. I’ve written about this before and I ask again, does replacing the power supply also correct problems with sustain ?
    Also why have no YouTube reviewers reported noise problems that appear so common and widely discussed ?

  • @sarawson Grounded DTK power supply fixed my hum problem.

  • @rexlarsenphoto The sustain problem is a result of a poorly designed noise gate effect in the chain. Turn it on and with the threshold anything higher than the lowest setting the length of sustain gets cut. Even with the decay set at its highest there is a noticeably shortened sustain. Turn the threshold down to its lowest and the decay to the highest and you get the best results in terms of reducing the noise inherent in unshielded guitars and single coil pickups and the longest sustain. Or remove the noise gate from the chain and run into possible noise in the signal. I (and I bet many others) are hoping for a better noise gate (or multiple noise gates) to be introduced in a future version of either the firmware, the app, or both. But the sustain problem is separate from the hum problem caused by the power supply.

    With somewhere close to 90,000 Spark amps sold, it's hard to know how vast the hum problem really is. Most of those 90K Spark owners are not members of this group, so we may be hearing of a disproportionate number of hum issues, greater than the average among the whole population of Spark users.

    My Spark, using the provided ungrounded power supply, is working great with no hum issues.

    The only noise issues on my Spark amp occur with the louder sounds designed with the gain turned up very high, which is also true of real-world amps when the gain is turned up very high. Differently designed guitars have greater or lesser issues with that sort of noise, but that noise is completely unrelated to any power supply issue.

    I will venture to suggest that the earliest versions of the Spark amp which got shipped to the people who created those early "Positive Grid sent me the new Spark Amp to try out . . ." videos used a different power supply than was purchased for the general production run where in order to keep costs down a cheaper power supply was sourced.

    When you consider that all these Spark amps sold for between $220(US) and $242(US) depending on which early bird or last chance special we bought included free shipping, that means that at least $10-$20 of the price of each unit went into the shipping expenses (try shipping a 17lb item through UPS or FedEx from City of Industry, CA to New Hampshire and you will be shocked at the price!) Admittedly PG got very special rates since they were contracting for large numbers of items to ship, but still there was expense involved. So it's no wonder that PG went with the cheapest power supply provider.

    It would be very intersting to know how many power supplies PG is actually having to replace because of the hum issue.

  • @r_koby As I said in the post above. The grounded power supply everyone is talking about should fix most peoples problems, if their original problem was a noisy power supply. This is just my thoughts based on my years as an EE, so take them with a grain of salt, but grounding doesn't seem to be the root cause of the hum. It is however the best place to start. Or at least the most bang for your buck test/solution to figure out each persons problem. It's like how an IT guy asks if you tried turning it off and back on to fix your problem. Its the most effective solution.
    My intent in saying all this is to recommend everyone that has this issue to open a support ticket with Positive Grid in the case that the power supply doesn't solve the issue for everyone. Then they are aware of all the problems and we can all put pressure on them to solve them. Besides, we all paid for a working amp. That was supposed to include a workable power supply. Sure, buy something that will make the amp usable now, I did. Delivered to my house today. But, don't let Positive Grid off the hook just because you solved it yourself. I am making them send me a new power supply either way.

  • @dhbailey

    It would be very interesting to know how many power supplies PG is actually having to replace because of the hum issue.

    And there's cost associated with this too. The product, logistics, and shipping to alot of people.
    On top of that, its pretty common for electronics suppliers (the people making these power supplies) to lower the quality through the process of a production run. The first units to the customer are usually the highest quality to pass initial testing. Then the quality goes down from there. Then the same thing happens in each components manufacturing process for every thing in it. Its all part of the life cycle of a production product.

  • @rexlarsenphoto said

    @keoki I appreciate the detailed information about noise issues. I’ve written about this before and I ask again, does replacing the power supply also correct problems with sustain ?
    Also why have no YouTube reviewers reported noise problems that appear so common and widely discussed ?


    Rex, I've never been afraid to say I dont know when it comes to making a factual statement and I'm going to say that here. Can't begin conjecture about that. Its obvious from reading the replies from @radu7 and @sarawson 's that a power supply with a grounded (3 prong) isn't necessarily the cure all. The observation that some after market three prong power supplies don't even have the ground wire connected inside the actual PS box was telling. The other comment I see worthy of consideration is it may be worth buying one simply to see if it fixes the issue. For under $20 bucks I'd do it. If it dint fix the issues at hand then return it and wait for the PG replacement to show up (which might not fix the issue either!). One way or the other be sure to submit a support request and get the ticket.

    Now I will conjecture on your second point... why didn't the YouTube reviewers have these issues? Seriously, we don't know for a fact they didn't have some issues. However, more likely their amps were screened and built with better QA and care (parts and labor) than the "production" versions that are currently being cranked out. Or maybe back then, before the demand hit, that's how well PG was building them. PG simply may have used higher quality components and higher quality assembly procedures.

    Again, this is simply conjecture on my part, it's not like I get personal calls from the PG CEO! Although I did send a couple of emails to their Customer Service Department last week telling them to get a moderator on the board and start paying attention to the issues being raised. Funny, but the day after I sent that email, Mike our Board Moderator showed up asking for ticket #s. I'm glad he's here regardless of how that came to be.

    Truth? I know that here on the forum we tend to see people who have issues, how representative those posts are relative to the entire production run is hard to know. I mean, "how to" questions don't seem to comprise a lot of the posts here. If your amp is working perfectly and you didn't have questions about how to work it would you be likely to come here and say "Hey my amp is working perfectly!!!" Yeah I know we've had a couple here but it certainly isn't representative of most posts.

    Mahalo Rex,

  • @keoki I agree re: moderation. There are only 15-30 new messages across all of the forums so it's very easy for them to read each one and proactively look for customer service issues.

    I have seen @Mike responding more frequently which is great. Not sure who else from PG is active in the forums...

    However, I did NOT see the weekly update that the CEO promised this week. They've done a great job in catching up on the shipping issues, but I still see many complaints regarding missed orders, hum issues and the occasional DOA Spark unit.

  • @keoki Thanks a lot, Keoki ! Mahalo

  • @sarawson i am not too convinced yet if it is a pure grounding issue. I get this sound ONLY when I strum a note/chord. its like they have a noise gate in the software. I also noticed the hum is not always present. If it was the power supply, then the hum would be there all the time. Has anyone in here contacted positive grid about this? and have they responded with a fix or mitigation? this hum is a disappointment, since I hoped to use this for recording.

  • @davidscott95 I had a hum that got progressively worse as I advanced through the presets. It remained even when the guitar was not plugged in. Changing the power supply ELIMINATED the problem. PG promised a replacement power supply and they generated a tracking number two weeks ago but the carrier has not received the replacement yet so it hasn't moved. So It's been about three weeks with no solution from positive grid. They seem to be of the belief that not everyone is having the problem so it more cost effective to leave the problem as is than provide a solution on their end. Based on your assessment, I would suggest trying a grounded power supply and, if that doesn't work, troubleshooting further. There are a number of threads providing information on compatible replacements, and I just picked up another one at Wal Mart for $20.

  • @cmcwillieb They are right that not everybody is having that hum problem -- my amp arrived in early August and I do not have the hum problem, using the original power supply that came with the amp. There's nothing special about the wiring in my house. The room I use the amp in mostly has an airconditioner running on the same circuit and with or without that AC working there is no hum. With PG having sold over 102,000 Spark amps, you can't take the very small number of people who are complaining in this forum or on the Spark Amp Owners Club at Facebook as being indicative of anything other than a small number of people received amps with the hum problem. That there seem to be a lot of them is because they would naturally come looking at the forum to try to find a solution. People aren't in general going to join a forum just to say "I don't have a problem with my amp."
    @DavidScott95 -- have you updated the firmware to the latest version? That supposedly solved some noise issues which weren't related to the power supply.

  • @dhbailey I should have a second Amp arriving today so we'll see if I'm 2 for 2. I suspect there are many who either haven't reported the issue or have found other workarounds. One person on here told me about their setup and how it resulted in a ground via a more indirect route. Once they plugged the amp in directly, the hum was there. While I would agree the number in this forum is relatively small (and I don't Facebook), I suspect the number overall is quite large. Some will find a solution and others won't look for one. That is what makes it more cost effective for PG to just replace the power supply for those who complain rather than fixing the problem. And let's not forget that many of those 100,000+ were sold outside the US so they may be on different power systems that manifest different issues or none at all. Congratulations on getting a good one, but I wouldn't be too dismissive of a problem that clearly exists for many.

  • @cmcwillieb I requested a replacement a month ago. Nothing so far. Not a big deal to me because I leave the USB cable plugged in which grounds the spark through the PC, so I'm not that worried about it. There's probably a ton of others in the same situation.