Getting rid of transformer hum

Discussion in 'DIY Corner - Tips & Techniques' started by Newbie!, Aug 22, 2004.

  1. Newbie! Guest

    Newbie!
    Dear all,

    I have a problem that I hope someone can help me with. I have a power amp that uses a toroidal transformer and is humming. Don't say it's because it doesn't know the words hence why the transformer hums :) The hum is definitely coming from the transformer as it is noticeable from the moment it is switched on and is also audible thru the speakers. I suspect it is due to one of two main problems:

    1. There is a significant amount of DC in the power supply hence why it is humming.

    2. The windings on the transformer may not be as fixed as it should be so it is causing minor movements that induces the hum.

    I don't think there is a ground loop problem at all, although it is possible there is a small contribution the main problem is the transformer hum. As the power amp is old, I would not want to spend a lot of money to rectify this problem and ideally want to DIY a solution. I suppose I can dismantle the toroidal transformer and re-epoxy the windings to decrease any movement or even superglue each winding individually, is this likely to result in an improvement?

    Secondly, does anyone know where or how I can DIY myself a DC filter for the incoming AC power line before it connects to the transformer? That way I figured it would eliminate the DC current before it gets anywhere near the toroidal transformer. What components do I need for this and how do I do this? Thanks in advance for any answers.........
  2. Polkfan Audioholic

    Polkfan
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    I assume you have thoroughly ruled out the possibilty of the hum coming from a source ( ie you have disconnected all sources and still hear the hum). Is it limited to one or both channels? Or is it a buzz?

    Other possibilites:

    1. Intermittent interconnects that have a broken or cracked ground path.

    2. Faulty input jacks on the amp, or on your source or preamp.

    3. Plugging your source and or preamp in a different outlet than your amplifier.

    4. Using two to three prong adapters to lift the ground.

    5. Running interconnects parallel with power cords.


    Odds are high it is a connection issue or an outlet/ground issue.

    Some more information that may help:

    "The distinction between a 'hum' and a 'buzz' is extremely important! If you describe a noise as a hum, then the expectation of anyone knowledgable in the field will think "low frequency, no (or few) harmonics". This describes the noise made by an earth loop - a situation where two or more pieces of circuitry are joined by the mains safety earth lead and the shield of an interconnect (for example), forming a loop. This can inject a very low voltage (but sometimes surprisingly high current) into the loop, and the signal is picked up by the inputs. You hear hum - a single low frequency tone.

    'Buzz' has a sharp edge to it - there is usually a low frequency component, but it has a hard sound that may even be audible in tweeters at times. Buzz is caused by any number of things - input leads close to mains wiring, power transformer or bridge rectifier (and associated wiring), bad or no earth connection, loops (they can cause buzz as well as hum), the list is almost endless."

    Hums tend to be ground induced, while buzzes are more electronic. Since you say hum it seems to be a grounding issue somewhere.
    Last edited: Aug 22, 2004
  3. Unregistered Guest

    Unregistered
    Hi,

    Thanks for the reply. The sound is definitely coming from the transformer, you can hear it as soon as it is switched on without anything else connected to it. If the rest of the stuff is connected, the same sound is also reproduced and present thru the speakers.

    It is definitely something with the toroidal, maybe it is old and that's why the windings may have worken loose? Or just that there is more DC current leakage thru the mains? Hence the request for anyone who knows how to DIY a DC current filter for the AC supply before it reaches the transformer.
  4. Polkfan Audioholic

    Polkfan
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    So do you hear a hum without speakers connected? If that's the case
    then it's definitely something electronic. What type of amp is it?
  5. Unregistered Guest

    Unregistered
    Hi,

    As soon as it is switched on, the noise appears if you out your ear to the transformer. Actually it is noticeable from a couple of feet away. However, if no signal is connected to the input, no noise appears in the speakers but as soon as you connect the inputs to the pre-amp, the noise appears in the speakers. It goes away with an isolation transformer, but I don't want to spend much money on it as it is a spare power amp. That's why I am quite sure the noise is due to either DC current in the AC mains or loose windings of the toroidal coil transformer. If only I can get access to a DC noise suppressor circuit for the AC line...............

    They are Soundstream monoblocks home amplifiers.
  6. Rip Van Woofer Audioholic General

    Rip Van Woofer
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    I've read that transformer chassis hum (one of your reported symptoms) can also occur if the voltage or frequency of your mains deviates too much from the "acceptable" range. If you can test your mains with a DMM that will tell you if that's the problem. Or maybe part of the problem. I have an intermittent slight chassis hum on my amp (but it doesn't make it to the speakers) that is likely related to that. But first...

    You seem to have concluded that it's a DC problem. Are you sure you're not jumping to conclusions? Maybe your preamp is the culprit since the hum only occurs when it is connected to the amp? Or a ground loop formed by the preamp and amp? Rather than filter out the offending source of the hum, tracking down and eliminating the source makes more sense to me. I'd take a good look at the amp/preamp connection.

    Othewise, try to test for and eliminate the easy (and less expensive) causes first! Go thru Polkfan's list for starters - those are pretty logical suspects. Take a look too at Dan Banquer's article on grounding and noise here on Audioholics. Good luck.
    Last edited: Aug 26, 2004
  7. Unregistered Guest

    Unregistered
    Hi,

    It is definitely a transformer noise, not ground loop as a ground loop should not do this:

    I have come across a possible DC filter that I will try and see if it works.......
  8. Unregistered Guest

    Unregistered
    LC audio has a DC filter-
    http://www.diycable.com/main/default.php?cPath=32_88

    PS audio also has interesting info on their site regarding mechanical hum. They would like to sell you either a Power Plant or their new Humbuster, but they DO provide good general info as well.
  9. Unregistered Guest

    Unregistered
    A word of caution I would understand exactly what the cct is before you install it in your amp. From the picture it looks like a capacitor and another small device like a MOV or TPC - resettable fuse, hard to tell.

    If you are considering using this make sure that it is rated to pass the current required by your amp. Also get the schematic of the circuit. If you get the schematic and don't understand it post it here, I’m sure you will get help.

    While I was on the diycable.com web page, I looked at their component to reduce surge current when an amp is turned on - LC Audio Soft Start Unit.

    The features they list

    Limits the start up current of large power amplifiers.

    Saves blown fuses by the instant current draw of a large power transformer and capacitor bank

    These claims can be correct however the following (from the same page)...

    Limits the power-on surge that slams power supply caps with full power often taking hours for them to reach full sound potential.

    Is total hogwash. This is simply NOT TRUE.

    There seems to be a bit of tweeko stuff happening here. Make sure they give you the specs before you use these items. You are dealing with 115v up to 15 Amps. Understand what you are doing before you do it.

    Finally here is a link to a discussion on another forum about blocking DC into a transformer. The discussion is not complete but it may help you.

    http://www.htguide.com/forum/showthread.php4?t=3584&page=1&pp=35

    Go halfway down page two, there is a discussion on blocking DC at the input to a transformer.

    Hope this helps,

    Doug Fraser
  10. JoeE SP9 Senior Audioholic

    JoeE SP9
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    I may be wrong but I don't see how DC can cause hum. Hum is an artifact of AC. Try checking the rectifier. If part of the bridge is shot you will get hum. This will be because you are getting AC leakage through your power supply rails. Bear in mind this is after the transformer. The transformer accepts AC from the line and converts 110V AC to a different AC voltage. If you have DC coming through your AC line you should contact your local power company. If your transformer has an audible hum (not electronic) it is probably a physical defect. Note: Transformers ingnore DC voltages:cool:
    Last edited: Aug 28, 2004
  11. Unregistered Guest

    Unregistered
  12. Newbie! Guest

    Newbie!
    Hi,

    Thanks everyone for all the replies, especially to Doug Fraser for pointing me towards the thread on htguide. Will try to build the filter they specified.

    JoeE, DC can be present on AC lines, usually caused by other equipment on the same circuit like fridges, microwaves, fluorescent lights, etc. The small DC then causes a lower quality toroidal transformer to hum because of loose windings, the solution is either block the DC or epoxy the windings to stop them vibrating.

    Anyway will also try rotating the position of the transformer as per the htguide thread suggestions and build the filter later today. Will let you all know how I get on! Cheers!
  13. Newbie! Guest

    Newbie!
    Positive results! I've not tried rotating the transformers yet but I have managed to stop the transformer noise. Well, not quite stop completely but reduced to a very, very low level of less than 40dB (below the measureable limits of my SPL meter). All using the circuit suggested by Doug Fraser. Thanks again, man!

    Now that the problem is overcome, I have another question. How can you be sre the capacitors will be enough for the current the power amp uses? The AC voltage is 240V and power amp fuse is rated at 4amps. So what value of capacitors do I need? I am using two capacitors in series at the moment - only 10,000 microFarads and 50V, will try to get higher value ones when they are in stock. Any suggestions?
  14. JoeE SP9 Senior Audioholic

    JoeE SP9
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    You can increase the voltage rating on capacitors by connecting them in parallel This will also increase the total capacitance. Unlike resistors, connecting caps in parallel is additive in terms of value. :cool:
  15. TSBEngineering Audiophyte

    TSBEngineering
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    Another thought or two.

    Hi, Newbie. I hope you are making some progress with all this excellent advice.

    I have some questions that would make it much simpler for us to get a picture of some details that might help you to get the simplest (least expensive?) DIY solution to this "hum" problem you are having, if you can stand more questions! So, in random order, here they are.

    First, I have read all the posts in this thread. But I haven't seen a few items that would be really very helpful in helping you look for the source of the hum.

    1. First, is this a power amplifier for instrument use, such as a guitar, or is it a power amplifier that is used with an audio or audio/visual device, such as a CD player or radio or home theater?

    1.1 Do you know what the output rating of the amplifier is supposed to be (such as 35 Watts, or whatever)?

    2. Do you know if the amplifier is solid state (transistors and integrated circuits) or if it uses tubes? (Knowing this would help a lot, because the voltages and amperages in the active circuits will be very different, the power supply and the transformer would be very different (between solid state and tube amps), and these differences would tend to lead to different suggestions.)

    3. In your several, and very patient, responses in the thread, you have reiterated that the source of the "hum" is a toroidal transformer. Hmmmm. After lots of time in this field, I can't think of a case where a Tor. Trans. has done that. With several degrees, and lots of application time designing and building stuff that *has* to work, and that gets lots of bench and field (use) testing, I have never heard (or seen on a scope) low frequency oscillations that originated at the connections of such a transformer as you have described (unless, perhaps, the connections have corroded and have some added resistance to them, which affects the power supply in general). New solder should solve something so simple.

    4. Can you get to the electronics of this amp, to do a little testing?

    5. If you can, and IF THE AMP DOES NOT USE TUBES, you could do the following: measure the incoming voltage at the transformer's primary, and then measure the voltages on the transformer's secondary windings where they leave the transformer (or at a connection that is close by, and not covered by insulation, such as a solder joint to a printed circuit board, or another component).

    There are probably at least two "pairs" of wires on the transformer's secondary side, representing the four ends of two different secondary windings. Sometimes it is a little tricky to sort out which goes with which. With the amp not plugged into the wall, you can usually tell what two wires are the two end of a winding because they will have a small (a few ohms) resistance across them, as measured on a DMM or a VOM; if you test two ends and get an open circuit, then they are on different windings. But beware that if the windings are connected to a bridge rectifier, that could confound things a little. Don't try this resistance test with the amp plugged in, or you could blow your meter.

    (All this would give us a sense of what kind of voltages and amperages are present.)

    6. Can you give us a sense of what an acceptable DIY fix for this problem would be? In cost, or in time, or in effort/energy before you through the whole thing into the dustbin :) ?

    7. I could launch into a whole series of ideas for fixes of different problems at this point, but it seems a little silly, because with one or two more interchanges, we would have a much better idea of what the likely cause(s) might be, and hence what the appropriate fixes might be. For example, your experiment with an isolation transformer indicated that the hum went away when you introduced it. Good experiment, and useful result. But depending on the answers to these questions, I would eliminate about half of the things that you could look for, and save a lot of useless reading for the thread.

    8. By the way, isolation transformers won't alter the problem with "DC in the mains", so a filter for that "problem" may or may not help. By the way, one of your earlier correspondents indicated that one does not see "DC" in the mains. That is true, where a country's mains are driven AC (against the earth as a central "0 V" point, as in Europe and North America. Inductive items such as refrigerator compressor motors can put current spikes on the mains, which a coil (as in a transformer) can transform into a momentary (usually less than a millisecond) voltage, but that is not what you are describing, as you have a continuing sound, not an intermittent one.

    9. As a final point before I await the answers to these questions, I will point out to one can get ground loops in unforeseen ways: the most common of these is to have one part of a device (the power amplifier) plugged into a wall socket, and another part of the device (a preamp from a CD player, for example) plugged into a different wall socket. Even should both sockets be on the same feed line in the house, the ground loop is there. When the sockets are feed from different lines in the house, the ground loop can be attrocious. The solution (or the "test experiment") is to buy/borrow a recent version of one of these "six outlet, surge protected (very important, very common), RF filtering (if you can find this) outlet expanders. They were introduced years ago for powering computer rigs. But if you plug the supply cord of one of these (a decent one) into one outlet, then the six expansion outlets all have filtered AC all at the same ground potential (or near enough, within a few hundred microvolts of each other).

    (If you are expecting 5-10 amps into this amplifier from a North American wall socket, such a line is only rated (and cabled) for 20 amps total for the whole length of the line through the house and everything plugged into it. Without saying more on this subject now, this was part of why I asked earlier about the power rating of the amp (and the impedance of the speaker/speaker rig/crossover you are using?) and the possibility of discovering the voltages on the various windings.

    10. One more thing that I just thought of, could you let us know if you have had this amplifier a while, when it *was* working properly, and now it is not? Or was it humming when you acquired it. Do you have any idea how old it is? One very cheap possibility (to fix) is that the electrolytic storage capacitors (sometimes called filter capacitors, not to be confused with decoupling capacitors) in the power supply simply need replacing. While many passive components and many silicon components have essentially very long use lifetimes, electrolytic capacitors are the exception. In power supplies, they often last only a few years. There is probably only one, or maybe four, in the power supply section, and if that is the problem, you could fix the hum AND fix a hidden power problem at the same time.

    11. And, by the way, most power supplies have a low frequency roll-off filter built into them. Older ones might not have one. The filter consists of a single capacitor and a single resistor (usually)(filters can get quite complex, but at the power supply one doesn't need the complexity). If, for example, a solder joint had cracked where these two filter components were connected, you could get instant low-frequency noise in the power lines. (And if anyone else on another post is recommending a more complicated filter so solve this, I would strongly recommend that you save your money and time and get back to me.)

    The real question for me, at this point, is how the amplifier is constructed. Modern (for the past 25 years or so) amplifiers (including power amplifiers up to at least 100 Watts) using solid state components employed operational amplifier ICs as amplifying units, before that the op-amp circuits were built out of transistors and other discrete components, before the 1970's op-amp circuits were built with tubes and discrete components (and some amplifiers using tubes are still in use today, but I do not think that they would have a toroidal transformer in them, unless they are or somewhat recent make). But op amps circuits are specially designed to "reject" power line noise, down to the point of 80 dBs or more. So if the noise can actually come out of the speaker, it makes me think quite strongly of several other causes that can be quite likely, probably more likely, than a problem with the transformer. The problem might still be in the power supply section, but not in the toroid.

    12. By the way, can you discover if this hum happens (when it is plugged in and operating) at your house, and at work (using a different input device, a computer speaker output, perhaps?), and at a friend's house that has a different input device (maybe a different manufacturer's CD player, or a different pickup/preamp on his/her guitar?)? Knowing if this is problem is present on different sets of mains would help.

    With my best intentions,

    Tom Bellows, Ph.D.
    President, TSB Engineering Corporation
  16. TLS Guy Audioholic Overlord

    TLS Guy
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    There seem to be a lot of assumptions here. You need to check your neutral voltage and look at your mains signal on a scope. It sounds to me as if there may be some significant neutral gauging somewhere. Those problems are serious and should be stopped at source. Your AC mains should no have significant DC offset and if it has the cause needs to be discovered and corrected.
  17. sparky77 Full Audioholic

    sparky77
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    I'll make this really simple for you, you have both loose windings, and a ground loop issue.

    You have audible hum from the amplifier itself with nothing else connected, that means loose windings or poorly damped constraints, which may be corrected by a good coating of high temp hot glue, but if the transformer gets too hot you may smell the glue heating up with usage.

    The hum you hear from the speakers could be a ground loop or more rarely an impedance mismatch from the preamp. What kind of preamp and what kind of connections, anything along the lines of balanced to unbalanced?
  18. Haoleb Audioholic Field Marshall

    Haoleb
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    I realize this is an old thread and this information probably wont be of much use at this point but lately I have been dealing with similar problems and ended up building a DC trap, And when that didnt work buying an Isolation Transformer which Did actually partially work.

    Here is the schematic I used for the DC trap. If I were going to build it again I would have used 35 or 50A rectifiers just because, But since I am only using this on a preamp these are fine. I also threw in a IEC jack with a filter on it just for the heck of it and a fuse for safety. It did lower physical transformer hum but did not fix the problem I was having with my preamp.

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    I ended up getting an Isolation transformer however and while it also eliminates DC on the line it would not be the best option for that exact problem because it also humms a little bit. Isolation transformers have other benefits however besides just eliminating DC.
  19. Dan Banquer Full Audioholic

    Dan Banquer
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    Transformer Hum

    I had this problem when I was manufacturing power amps which used large toroidal transformers. DC may be the problem or as I found it could also be a low frequency "oscillation" riding on the AC sine wave. There are two ways that I know of to attack this problem.
    The isolation transformer may help if this is DC, however the mechanical hum wil be transferred to the isolation transformer. The best solution that I came up with is potting the transformer. Potting, for the uninitiated, is just inserting the transformer in an enclosure with the potting compound. Plitron Transformers (www.plitron,com) have potting compound filling up the interior "hole" on the inside of the transformer. The potting compund will act as an absorber for the mechanical hum and it certainly appears to be rather effective. If using potting compound on the power transformer is not an option, see if you can find yourself an isolation transformer that is already inserted in potting compound. From what I understand the used market may be the way to go here, but expect to pay a hefty price for shipping as these things are pretty heavy.
    Good luck;
    d.b.
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  20. Richard Black Audioholic Intern

    Richard Black
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    All mains transformers hum. ALL of them. The extent varies, from 'barely audible even if you put your ear up close' to 'really annoying from the other side of the room', but they all do it, because the nature of electric-to-magnetic-to-electric conversion creates physical stress in the transformer as an inevitable byproduct.

    Several factors can contribute, often more than one at a time, to the noise being more than one might wish:

    1 A cheaply designed transformer with unduly high magnetic field levels
    2 DC on the mains, which has a vastly disproportionate effect in moving the magnetic circuit asymmetrically towards saturation
    3 Unduly high mains voltage
    4 Transformer designed for 60Hz being used on 50Hz
    5 Transformer inadequately (or barely adequately) rated for the load
    6 Transformer badly assembled with insufficient glue

    And probably others I've forgotten. Obviously the cure will depend on the cause(s) from case to case. Mounting the transformer on a resonant steel chassis can clearly make things a lot worse - sometimes a housebrick on the amplifier's case can make the critical difference!
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