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Old 13th November 2002, 09:12 PM   #21
hifiZen is offline hifiZen  Canada
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Hmm... I would look at mains grounding as a loop culprit. I've seen this before, where both the preamp and power amp have their circuit grounds connected to the ground at the mains outlet. By connecting signal ground between the two pieces of equipment, you'll get a great big loop. I've seen a case where two power amps, each connected to mains ground, are then further connected through the signal grounds going back to the preamp. Lifting the mains ground in the preamp may help. When lifting mains ground connections, a good idea might be to use a thermistor instead of something like 10R fixed. Nelson Pass does this on some of his designs. It provides better safety if there is a ground fault as it's resistance will drop when loaded.

I don't know if splitting the PSU will solve the problem. I'd save that as a last-ditch effort. Focus on ground loops and high current wiring first. From your initial description, I think you're clearly picking up power supply rectification pulses as signal noise. Rectification pulses in the power supply are high current bursts which can produce high magnetic field strength. When dealing with magnetic fields, everything pretty much boils down to electrical loops, and their physical size, or area. Current flowing around any electrical loop will "transmit" magnetic flux, and any circuit which forms a loop will pick up a current when immersed in a changing magnetic flux. So if your power supply is producing pulses of high current which flow around a loop, then any ground loop or circuit which picks up this magnetic flux will show these pulses. I haven't seen the pictures, but one thing you might try is twisting power supply wires together in pairs, where the current will flow in equal and opposite amounts through each wire... leads from trafo to rectifier bridge can be twisted together,same with leads from bridge to PSU caps, and likewise leads from PSU to amp circuit. Keeping all these wires short will help too. I'd like to draw a diagram, but I'm at work right now... maybe this evening I'll whip up a sketch for you.
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Old 13th November 2002, 09:14 PM   #22
UrSv is offline UrSv  Sweden
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Chad,

Problems do not disappear with inputs disconnected from source even when they are shorted/grounded it seems.

/UrSv
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Old 13th November 2002, 09:17 PM   #23
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I'm using a portable cd player on batteries and a laptop on batteries as signal source, so it can't be a mains earth loop.

I guess EM fields could be causing trouble, but would that really result in such a high noise output? The ripple is VERY audible.
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Old 13th November 2002, 09:29 PM   #24
UrSv is offline UrSv  Sweden
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I think you mentioned the portable Cd player earlier but I just didn't pay attention I suppose..

The cable you are using, does it have the ground connected in the 3.5 mm plug to both left and right channel cables? That might be a problem since that is a long loop. However from your posts it seems the problems are there even when inputs are grounded.

/UrSv
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Old 13th November 2002, 09:32 PM   #25
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Yea, it has both channels grounded in the cable, but the noise is still there when the inputs are just grounded.

Fixed some pics of the waveforms at the speaker terminals:
Right chan, 50mV/div
http://daviruz.homelinux.org/pics/amp/right-50mVdiv.jpg

Right chan, 5mV/div
http://daviruz.homelinux.org/pics/amp/right-5mVdiv.jpg

Left chan, 5mV/div
http://daviruz.homelinux.org/pics/amp/left-5mVdiv.jpg
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Old 13th November 2002, 10:44 PM   #26
swede is offline swede  Switzerland
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What about bringing the thing up on a high bridge or an overpass, letting it go? ;=)

Just kidding. Kind of frustrating, even for us, though. Hope you sort it out soon.

I'm not sure if I'm not paying attention, but what happens when you completely disconnect one board? Tried both boards by themselves?

//magnus
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Old 14th November 2002, 10:46 AM   #27
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If your chassis is plastic that means your earth ground is already lifted, try connect the star point of PSU to Earth with a jump wire to see what it does to the hum.

What I meant is from the speaker terminal -ve to the star point of PSU and Earth to the star point of PSU and the input -ve to the Earth point. Use jump wire with clips so less soldering for now. Make sure you only jump grounds( clean and dirty grounds only).
Good Luck.
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Old 14th November 2002, 10:58 AM   #28
UrSv is offline UrSv  Sweden
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Have you tried another bridge?

/Urban
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Old 14th November 2002, 11:06 AM   #29
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One time I thought I had a ground loop problem, but it turned out I have a cold solder joint of one of the ground return points. Recheck your solder joints too....
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Old 14th November 2002, 11:08 AM   #30
swede is offline swede  Switzerland
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Default Bridge...

Hi,

By the way. IF (and I emphasize on IF) the humm comes from your diode bridge, you could probably smoothen it a bit by connecting a 100uF Cap across it. Give it a try, to reduce glitches.

//magnus
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