Monitors hum [sound clip]

[wxn]

Recently I bought a pair of monitors (Event TR6). They sound nice and detailed, but have a faint but very annoying hum. It wouldn't be a problem if they were not close range monitors..

So anyway, the noises are quite varying, mostly affected by the distance of my hands to PCB, speaker etc. But it's not dependent on input, volume, distance from transformer etc, so I assume it must be in the final stage - LM3886TF (hence posting in this forum). The buzz decreases if I touch the ground but does not vanish completely.

Here's what it sounds like. Mic placed 2" from the woofer. Please ignore the background noises.
http://www.sendspace.com/file/mc2xm2

A quick FFT analysis showed harmonics of 50Hz and 60Hz. That's weird cause we have 50Hz mains. Where did that 60Hz come from?

The PCB seems to have some sort of star grounding. It's a pain to take a closer look because it's an pre/xo/amp-in-one dual layer SMD design, glued to a peace of foam. I also tried shielding the PCB by covering it completely with a peace of (grounded, naturally) aluminum foil but with no success.

I thought of making a thicker copper foil/mesh shielding but maybe there is something simpler I can do?

[AndrewT]

check your grounding (return circuits for all the currents).
Check your mains Safety Earth.
Check the hum (ripple) on the PSU.
Check for ground loops. Everything should return (ultimately) to one central ground point (the Audio Ground).

[wxn]

Power amp psu ripple 0.2V, preamp - 2mV at most.
There is no mains Safety Earth.
Star grounding is there, speakers return to one point near main capacitors. Ant power/line grounds/supply rails are separated. The only things that connect them are mute circuit (through a capacitor) and 100nF supply caps next to LM3886.

The weird thing is that the buzz (50Hz saw-like) is NOT stable. It's like 15min dead quiet no matter what I do, then an hour of buzz in both amps that gets weaker/stronger. Everything has influence to it, even position of the mains cable (!). In some configurations of mains/signal cables and the speaker itself, hum disappears. But may reappear the next day.

I suppose any form of ground loop or insufficient filtering would result in a stable hum, not this morphing buzz..

I have a schematic, it looks OK. Everything is bypassed, all inputs have RF filters etc. One thing that does look not perfect is that supply rails for LM3886 are not star-connected. Maybe it's a supply loop somewhere?

Of course I realize that everything can be solved by redesigning the PCB but the question is, can I do anything to get rid of this hum without redoing everything from scratch?

I can post the schematic and photos of the pcb if that would help.

[pacificblue]

Did you buy them new? If there still is guarantee return them.
Can you get support from the manufacturer?
Do they both hum or is it only one?

Check/resolder all solder joints, especially PSU capacitors.

Is any component in your audio chain earthed? If not, earthing the ground somewhere could help.

Shielding will not help much at 50 Hz.

[wxn]

I bought them with fried transformers, replaced them and they're working OK, it's just that buzz is quite annoying in the evening.

PSU caps are fine, I've added some capacitance to be sure.

Unfortunately I do not have access to earthing. So no.

The manufacturer sent me the schematic. I've heard of one guy once sending them these speakers with the same problem but they were unable to fix it.

[AndrewT]

Hi,
almost certainly a grounding problem.
A buzz is usually generated by the charging pulses between the transformer and the main smoothing capacitance. These are current spikes that repeat 100/second.

If your Audio Ground is attached to any wire in the transformer + rectifier + smoothing cap loops then you are likely to be at risk from this grounding error buzz.

This has absolutely nothing to do with the lack of a Safety Earth nor anything to do with Ground loops.

[pacificblue]

Originally posted by wxn
I bought them with fried transformers

If they were fried there must have been something severely wrong and it is quite possible that other components in the power supply are damaged as well. Check that all rectifier diodes are okay. How do you know that the PSU caps are fine? Replace them (all!) or take them out and run with the added capacitance only, if that is big enough.

Do both speakers show the same fault? If not, you could try to run the faulty one from the other's power supply and vice versa.

What about the SPiKe protection system. In some cases it can also sound like buzzing, e. g. while clipping at low frequency signals.

The coming and going could also be due to a small rupture somewhere. The material dilates or contracts according to temperature changes during operation. Ruptures that react like that are so small, they are nearly impossible to spot, so usually that means replacing the PCB.

[wxn]

Originally posted by pacificblue

If they were fried there must have been something severely wrong and it is quite possible that other components in the power supply are damaged as well. Check that all rectifier diodes are okay. How do you know that the PSU caps are fine? Replace them (all!) or take them out and run with the added capacitance only, if that is big enough.

The previous owner bought them in US, then brought back to Europe and well he was stupid enough to plug them into 220V mains. One went down quickly, the other managed to stay alive for a couple of minutes, as he described it. The reason I don't think anything else was damaged:
1. PSU caps are rated 50V. There's no sign of damage visually.
2. Preamp psu works flawlessly but the regulators only work adequately at up to 28V or so, any higher and they get very hot. So the original design must have used main supply of 25-28VDC or so.
3. The PSU itself works well with or without the added capacitance (checked with a scope)

Do both speakers show the same fault? If not, you could try to run the faulty one from the other's power supply and vice versa.

What about the SPiKe protection system. In some cases it can also sound like buzzing, e. g. while clipping at low frequency signals.

The coming and going could also be due to a small rupture somewhere. The material dilates or contracts according to temperature changes during operation. Ruptures that react like that are so small, they are nearly impossible to spot, so usually that means replacing the PCB.

Yes, both speakers show identical weird signs of buzzing. Sometimes one buzzes, sometimes the other, sometimes both and everything in between. I don't think I'm anywhere near SPiKe. Faulty PCB is a possibility, but then again, both PCBs must be damaged identically.

I played around by shorting this and that. Well the buzz doesn't disappear completely even with input shorted on the pcb. So I figure it must be noisy signal ground. It's all over the place, goes through some vias etc. This plus not-null impedance and we have an antenna

[sangram]

I would replace the chips.

I had an HK amp to which I did the same thing, it needed new driver transistors as well as transformers (the original transformers survived), and the main supply caps died a year later. Something like this usually has far-ranging effects.