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
