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Old 18th December 2005, 06:40 PM   #1
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Default Zapulse mains hum since going dual mono

Hi, I have rebuilt what was previously a fully functioning stereo zap 2.2SE amp. This time I used dual powersupply boards instead of a single powersupply board. Wiring is balanced input and a fairly obvious wiring for the rest of the board with a star earth arrangement to the chassis earth point (and from there to safety earth in the mains socket).

Problem is that the new amp hums quite badly when fed from my passive pre-amp. The pre-amp is a passive attenuator in each signal wire (8 altogether), but each signal wire is individual and nothing is joined until you get to the amp.

Photos of the amp are here:
http://www.wildgooses.com/downloads/IMG_8175.JPG
http://www.wildgooses.com/downloads/IMG_8171.JPG

I have another very similar stereo amp powering my sub from the same pre-amp so I know it's not just due to the passive pre-amp. The main difference is the use of a dual powersupply instead of a single powersupply. The mains hum did disappear when the inputs were shorted intead of being wired to the passive pre (note that the other zap with a *single* PS was plugged into the same pre-amp and showed no problems)

So I next tried to float the powersupply by removing the wires from the center of the capacitors to star earth and also adding a wire from the ground pin on the XLR input socket to chassis earth (previously the XLR input was floating with respect to the chassis). This has made the hum much, much worse. Even with the inputs shorted there is still a gentle mains hum!

Can anyone please advice on a correct grounding arrangement for a dual mono zappulse stereo amp. Can you please also advise on the proper way to wire an XLR input socket for these modules, should pin 1 be taken to chassis earth immediately or is it better to leave it wired only to the modules? (The recent UCD thread recommended leaving the powersupply to float and taking the

Grateful for your help

Ed W
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Old 18th December 2005, 09:13 PM   #2
Bgt is offline Bgt  Netherlands
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where are the yellow/green wires connected on the PSU pcb? Are they on the 0V line between the caps? Are they connected in symmetry on the PSU pcb's otherwise humm..... Why do you earth the mains input ground? Asking for trouble if doing it this way.
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Old 18th December 2005, 10:14 PM   #3
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The green/yellow wires are my earth wires from the center point (0v) of the capacitors to the star earth point. No they are not symmetrical because I am using opposite sides of the PCB left and right. However, apart from using different holes in the boards they *are* connected to the 0V points on the boards

What do you mean about wiring the star to safety earth? How else could I wire it?

Completely removing the wire from 0V to chassis point (and also wiring input ground to chassis ground) caused MUCH more hum. Not sure which was the biggest culprit though.

In the UCD180 thread several folks suggested that leaving the 0V line floating was a good idea in general...?

Grateful for any debugging here. All was fine with a single powersupply

Oh, another thing I noticed was that the voltage to each capacitor was not entirely identical left and right... No doubt this means that the 0V on each powersupply will be slightly different - this was the reason I originally thought it might be worth removing the wires from 0V to the chassis point?

Ed W
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Old 19th December 2005, 03:14 PM   #4
Bgt is offline Bgt  Netherlands
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Ed, the 0V should be on exactly the same position on each power supply and that is in the middle of the 2 caps. on the same spot on each pcb. Don't forget that if you put it on a different position you get humm because of asymmetrical connection/voltage drop from power amp. pcb./interconnect to preamp. where both shields are coming together again. I meant........the mains earth should not be connected either to chassis. This will only create groundloops through preamp/any other grounded(radio) device. This last suggestion is 1 of the main problems for humm.
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Old 19th December 2005, 03:23 PM   #5
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Hi, it's simply not possible to connect the two PCB's totally identically. Also when I measured the PCB's unconnected there was definitely a difference in +ve and -ve lines between the two modules and so there must surely be a difference between the two 0V lines - perhaps this is contributing to my problems?

So how to wire the 0V lines given that they can't be identical? Also do they *need* to be wired to ground at all...?

Also I don't see that the it is "safe" not to take the chassis to safety earth? I am not happy that the construction is safe enough that some sort of failure could not make the case life (and I have a young child). Additionally the mains inlet socket has the outside of the socket wired to safety earth, so the chassis is grounded that way (and in fact I may have a ground loop by also connecting the star point as well as the chassis via the mains inlet point..?)

I stress that a similar construction Zappulse has zero mains hum, the difference here has been the dual powersupply. I wonder if there is any need to connect 0V to chassis at all though?

Ed W
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Old 19th December 2005, 03:37 PM   #6
Bgt is offline Bgt  Netherlands
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Ed, when you had 1 0V point you had no humm.......you'll HAVE to make the 0V connections from psu to starground and to the power amp. boards as identical as possible. Mains grounding a power amp. chassis is asking for trouble if there is a mains ground point somewhere else in the chain. I never do it anyway, for what? Safety is no issue if you do it properly with isolated mains terminals/cables/connections. Grounding the mains only adds to problems(not always). Just see to it it is grounded somewhere in the chain. I know you do ground everything in the UK, here in Holland it is only used for domestical devices but for HiFi it is not a necessaty here. Otherwise take the 0V psu ground away from the starpoint and ground it at the input socket so this will be the only place for the 2 channels to meet on the chassis.
BTW making a power amp internal connections not symmetrical also adds to crosstalk/humm. Try to mirror the channels in connecting. You'll always will be rewarded with a much better working amp.
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Old 19th December 2005, 03:42 PM   #7
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But *do* the 0v lines have to be connected to each other at all??? What is wrong with leaving them floating?
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Old 19th December 2005, 03:44 PM   #8
Bgt is offline Bgt  Netherlands
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no, you don't have to. But than your starpoint is only the mains ground. Connect each 0V from the psu's to each of the 0V's of the power amps. and from there the input wires meet eachother on the chassis input shielding.
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Old 19th December 2005, 03:50 PM   #9
Bgt is offline Bgt  Netherlands
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you cannot have the 0V's from the PSU's grounded AND grounding the zappulse input connector/XLR. This will give humm.
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Old 19th December 2005, 04:06 PM   #10
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Quote:
Originally posted by Bgt
you cannot have the 0V's from the PSU's grounded AND grounding the zappulse input connector/XLR. This will give humm.

I am not quite sure what you mean here? My *badly* humming amp has the toroids connected to the powersupply so that the "center" of the torroids is connected to the powersupply 0v (of course). Then the module is connected to the powersupplies only by +ve, 0v and -ve. There are no other connections - nothing goes to chassis ground

Then I took a wire from pin 1 on the XLR to the chassis ground tab on the XLR. This combination has very bad hum

Note the connection from mains safety earth to chassis earth still exists. I would be keen to keep this in place actually.

I think the issue may be because the two modules now "see" each other through the input ground connections. I wonder if the solution might be to leave the 0v wires unconnected from the chassis and take the XLR pin 1 to the 0v point?

Does this sound workable?

Ed W
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