100Hz Hum diagnosis

[ColinMac]

Hi.

(Apologies in advance if there's a sticky article I should read on this - I guess it's a very frequently asked question.)

I have a Laney HCM30 amp (schematic) that hums at exactly 100Hz (checked with a guitar tuner app on a tablet). It does this when there is no input connected (and the input path is shorted to ground by the jack socket) and gain and master volume are both at zero. In all other respects the amp works fine.

I have a reasonable knowledge and practical experience of hobbyist-level electronics and so far as I can understand the 100Hz is likely to be related to the full wave rectifier (I'm in the UK, with 50Hz mains power). So one possibility is that the smoothing capacitors are not really doing their job any more and that too much AC ripple is making it to the power amp IC (TDA2030). Visually the capacitors seem fine, with no bulges or leaks apparent (though the bases of them are embedded in a big blob of glue so I can't see all of them). Also the hum disappears instantly when I switch off the power, though the amp continues to run (I can hear the hiss) for a second or so on the charge left in the smoothing capacitors.

So even though the smoothing capacitors have no apparent problems, I'm going to replace them because it's quick, cheap and painless. It may fix the problem or it may clarify that the problem is elsewhere.

My question, therefore, is: are there any other likely causes of the hum that I can investigate and either rule out or fix?

[52 Bill]

Welcome to the place.

You don't mention how loud the hum is, but I assume that it is loud enough to be objectionable. Visual checks of filter caps will not show anything but the absolute worst case failures.

Have you tried reading the amount of ac voltage that is present on the dc supply lines?

[ColinMac]

I would say 'noticeable' rather than 'objectionable' - it doesn't take much gain and volume to overpower the hum, but I may occasionally be in a position to play in public and if the amp is miked up then I'd want it to be basically silent when I'm not playing.

I haven't done any diagnostic tests, but I've been reading other threads here that are trying to deal with similar cases to I'll follow some of the techniques there (e.g. DC-blocking capacitor when trying to measure the AC ripple with my DMM). I will give that a go, maybe tonight, and will report back.

[ColinMac]

OK, so I got my digital multimeter out. First I confirmed the DC voltage to the TDA2030 was correct, 44.7V across +Vs and -Vs. I then measured the AC voltage by turning the meter's dial to the appropriate setting; it measured around 0.5V with a little fluctuation between 0.46V and 0.56V.

Then I attached a very small capacitor to one probe to ensure that the DC component was being filtered out and measured about 0.44V AC between the same two pins with similar fluctuations. Given that, I'd trust the meter's 0.5V more since it doesn't have a random capacitor wound around the probe tip to distort readings.

[TigerAmps]

I would look at the two large caps (C44 and C45). Power it down, bleed of the voltage and check both caps.
If one of them is bad (open - zero capacitance) it would cause the type of hum you are seeing. Also check
solder joints in the power supply and the 4 diodes D10 thru D13 as well. If your power is 50Hz and you are seeing
100 Hz - it is most likely in the supply or a ground provided you get output went applying signal to the amp.

Does the hum increase with the volume control?

[ColinMac]

OK, so I have removed C44 and C45 from the board (I have replacements coming for those anyway). The tests that I can do with the multimeter suggest that they are OK, but since I can't measure the ESR then there's no certainty either way.

Solder joints elsewhere on the board look OK on an initial inspection but I'll check them better tomorrow when I have daylight to work with.

The hum doesn't increase with either volume or gain, in fact nothing I do with any of the controls makes it change. To me this suggests that it's not coming from the signal path at all, and therefore must be power related (though being a 100Hz hum is fairly conclusive in that respect already).

[Enzo]

At 100Hz it is clearly power supply ripple RELATED, but not necessarily due to lack of filtration. I don't think a half volt or ripple on the outputs is much of a problem, if it is even, on both sides, then it tends to balance out anyway. But each power cycle peak charges those caps, and if that charging current shares say a ground path with other circuits, it can modulate your signal. It is possible the thing has always hummed this much.

[lowell]

Second that motion. It's quite rare to have a "silent when not playing amp." There are some, I've designed one. I would guess it is poor ground layout. Simply grounding the power tubes cathode circuit to the PT CT ground, then grounding each stages filter cap to that stages cathode ground point can really help. Then, the 1st stage/filter should be grounded closest to input jack, and then everything should run physically towards the PT CT in order the stages are in. And this the next best thing to a dedicated ground bus. I guess it's called star grounding.

[Enzo]

I had an Ampeg GU-something in a while back with a hum complaint. A nagging low level 120Hz hum. Ultimately I found I needed to move one ground wire from the circuitry over to another ground point four inches away. The wiring was all factory, so clearly this amp had hummed like that all its life.

I have a 1988 service bulletin from Fender regarding Champ. They explain that a certain amount of hum in that amp is normal, and we as service centers are not to "repair" the "problem". Oh, we might well be able to eliminate the hum, but it is not a defect.

[ColinMac]

Yup, it's possible that this is just the way the amp always was, and I have no way of knowing if the hum should be considered normal. I have a Marshall AS50R that is, as near as makes no difference, silent when idle, while the Laney hum is instantly noticeable at power on. The Marshall may well be a better design and build, in which case maybe I'm unrealistic in hoping the Laney can be de-hummed. But the HCM30 reviews that I can find don't mention any hum and it's annoying enough that I'm sure someone would have commented on it (though one person did in a comment here: HCM30 Review | Laney | Guitar Amplifiers | Reviews @ Ultimate-Guitar.Com).

[ColinMac]

Final chapter: I replaced the smoothing capacitors, checked a few other things, fired up the amp and... no change. The hum is still there, no better or worse than before. I think it's just the way it is, a basic design done to a low budget. It's unlikely that I'll use it in a situation where the hum matters so I'll live with it.