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Old 23rd February 2006, 03:33 PM   #1
senderj is offline senderj  Hong Kong
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Default ground and earth

I've just DIY a GG pre-amp, and burnt my power amp and speakers. I found that there is a 0.1V AC all over the chassis.

I've check everything in the GG but still couldn't find the cause of the AC. In the GG, I use star grounding: all components' ground connect to a single point and from this point to the chassis. I was told not to connect the earth of the AC supply to the chassis to have better S/N. But than I got this AC problem and have to connect the earth to the chassis to, sort of, resolve it.

How do you experts handle the grounding and earth?
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Old 23rd February 2006, 08:49 PM   #2
SHiFTY is offline SHiFTY  New Zealand
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What a shame about your power amp and speakers! I find that hard to believe, are you sure the schematic was correct?

In my opinion, the chassis of the amplifier or preamp should ALWAYS be connected to mains safety ground/earth. It makes no difference to the S/N ratio in my experience.

My grounding is dual or triple star- I have one star ground for the filaments to chassis, another star ground for the signal/input grounds to the chassis, and then another to the cathodes/output tubes then to the chassis.

All I can suggest is making sure that you have something plugged into the preamp, otherwise you might get hum- and also that your heaters are earthed at one end...
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Old 23rd February 2006, 09:00 PM   #3
SY is offline SY  United States
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It never fails, whenever someone starts a sentence with, "I was told..." or "A friend of mine told me..", what follows is certain to be wrong.

As Shifty says, ALWAYS connect your chassis to earth (safety) ground. No exceptions.

Your signal ground can connect to the chassis at one point (usually the input socket); if there's a ground loop, remove the SIGNAL ground connection from the chassis, not the safety ground. You can reference the signal ground to safety ground through a small (say, 47 ohm) resistor.

There are quite a few ways to do signal/power supply grounding; I use a combination of star and ground bus. Morgan Jones has a nice explanation in "Building Valve Amplifiers." Aiken Amps also has a good write-up, though more appropriate for guitar amps than hifi.
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Old 24th February 2006, 06:28 AM   #4
senderj is offline senderj  Hong Kong
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Thank you for all the advises. Ground to earth is a must in my future project. But with this, is it safe to use the preamp in my case? My power amp and speakers are gone and I am about to use the preamp on their replacement.

The project is a kit with proven records. So it was just my mistake that cause the problem. But the point is, I am not able to find out why there is a 0.1V AC on the chassis. I thing grounding to earth is just a work around rather than a solution since there is still AC flowing on the chassis to earth. It may still damage other components.

What do you think is the likely cause of the AC?
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Old 24th February 2006, 07:35 AM   #5
ThomasC is offline ThomasC  Hong Kong
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senderj,

As SY said, """It never fails, whenever someone starts a sentence with, "I was told..." or "A friend of mine told me..", what follows is certain to be wrong."""

This is especially true in this place we are from. The 0.1V AC can be some sort of current "leaked out" from the power transformer. It should not be the cause of the death of your power amp and speakers. Perhaps you can try to reverse the connection of the primary secion of the power transformer and see if it will decrease (or increase). Some people say if you connect the inner end of the primary winding of the power transformer to the "L" point of the main AC, you will get more of this current leaked out from the power transformer.

by the way, what multi meter did you used to get that 0.1V A/C? Do you think it is good enough to give such a small reading accurately?
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Old 24th February 2006, 02:46 PM   #6
senderj is offline senderj  Hong Kong
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I use an ordinary non-digital Sanwa multimeter. The accuracy of 0.1V doesn't matter. What matter is there is some AC on the chassis and it goes away when switching the preamp off. In fact, it was measured with one leg on the chassis and another one on the window frame, which I use as the "earth". So the absolute voltage may not be 0.1V. But as said, the figure doesn't matter.

Thanks for the suggestion on the transformer. I will try to see if it does behave as said. Thanks again.
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Old 9th March 2006, 02:03 AM   #7
senderj is offline senderj  Hong Kong
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Default DC at power on and off

I finally found out what killed my speaker. I posted here on measuring nil to minimal DC voltage at the output of the pre amp before. It was measured when the pre amp was stable, i.e. long time after power on. Now I found that the DC shoots up a short while after power on and off. It is like this:

Power on: DC at pre amp output (gnd + and signal -) rised from 0V to 16V in 6-7 seconds, then down to -V and back to 0V (or close to zero).

Power off: DC (same polarity) rised from 0V to 12V in 12-13 seconds, then again down to -V and back to 0V.

I know the order of powering on should be pre then power, and off should be power then pre, and I did this all the time. But I didn't know I should wait for such a long time before switching the next one. My power amp has a power on delay for just about 4 seconds, which is not enough for my DIY pre.

Is there anything I can do in the pre to shorten the time of the DC or lowering its voltage?
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Old 9th March 2006, 03:04 AM   #8
lndm is offline lndm  Australia
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The value of the output coupling capacitor. Reduce it to the minimum acceptable value. I might try the value whose reactance equals the input impedance of the power amp at 2Hz, and go from there.
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Old 9th March 2006, 10:14 AM   #9
senderj is offline senderj  Hong Kong
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The output coupling cap is 3.3uF. I will lower it to 1uF. But will this affect the low freq. response of the equipment?

Did you mean the DC is from the output cap? Can you explain further?
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Old 9th March 2006, 10:44 AM   #10
lndm is offline lndm  Australia
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DC that is varying is in fact AC, or more accurately in this case, DC with an AC component. The change in a slow to rise voltage on the preamp side will be seen through too large a value of cap.

What is the input impedance of your power amp? If it is as high as 1Meg, a good cap size would be 80nF (0.08uF) but could go as small as 8nF. If your input impedance is as low as 10k, a good size would be 8uF but you could go as low as 0.8uF
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