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Old 19th December 2008, 05:09 PM   #3071
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by udailey
I have a DC on the mains problem. .....................
Both mains Line and Neutral now have a DC blocker circuit on them now. Just like This
http://sound.westhost.com/articles/xfmr-dc-f8.gif
Except that I used 1N4007 in triplet for each diode.
you don't need to block both the Live and Neutral.

I don't know what VA rating your transformer is but, 4700uF seems far too small for the blocking capacitance. This has a reactance to 60Hz of 1r13.
The diodes will pass @~1.2V. The maximum peak current that the blocker is able to pass is only 1.06Apk ~115watts on a 110/120Vac mains system.

The peak charging currents to your transformer are likely to be very much higher than this.
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Old 19th December 2008, 05:14 PM   #3072
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by udailey
The speaker ground and input ground both go to the board where ground was generated from the 0V combo of the 25V secondaries.
Earth ground goes straight to the chassis and NOWHERE else.
Safety Earth to chassis only does not sound right.
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Old 19th December 2008, 05:28 PM   #3073
udailey is offline udailey  United States
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Quote:
you don't need to block both the Live and Neutral.
I know but I was reading DC on both with two different multimeters. DC on the ground to.
Quote:
4700uF seems far too small
Sorry, each cap in mine is 10,000uf. Rod Elliott said to double the 4700uf if in the US.

Quote:
Safety Earth to chassis only does not sound right.
Didnt sound right to me either. However I asked Brian and he said Earth GND has no place on the amp. If you check the amp schematics you will see this as well, I think. Maybe I misread them though but I see nowhere that allows for earth to go to signal or output GND and there is nowhere on the board for earth either.

Transformer is 600VA 24/0/24 my little pic says 25V out but thats wrong its 24.
Uriah
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Old 19th December 2008, 05:42 PM   #3074
AndrewT is offline AndrewT  Scotland
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Hi,
all exposed conductive parts must be connected to Safety Earth.

Your Safety Earth must be permanently connected direct to chassis. Bolt the third earth wire direct to chassis.

Can anyone disconnect any of the cables coming out of your amp and touch the exposed ens while the other end is still connected to the power amp? A child might do just that, worse they may put the free end in their mouth.

You MUST connect all your Audio Grounds to Safety Earth, you have no choice in the matter. If your supplier is telling you otherwise, then he is incompetent.

Even 10mF is too small. That's just 230W at the current peak.
If the 10mF were in parallel then the peak current quadruples to >8Apk. and other than during a fault condition the caps will never have >1.5V across them.
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Old 19th December 2008, 06:14 PM   #3075
udailey is offline udailey  United States
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AndrewT
Thank you.

I took everything possible to earth ground and I have .002-.003VDC
Uriah
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Old 19th December 2008, 10:34 PM   #3076
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Quote:
You are swearing by it at every opportunity so I believe you and would like to test it in a circuit soon. Uriah
He, he, it is just that this is the Only DIY regulator that I know how to build (apart the same without monolithic preregulator: zener diode and gyrator). The fact is that it sounds so much better than LM317.

Dear Andrew, what do you think about adding a gyrator based PS to the 3886 from RevC? (like in the link I posted above)

Cheers,
M
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Old 20th December 2008, 02:29 PM   #3077
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Okay, let me clearify my position on the grounding for this amp...

The question was (in an email):
Quote:
Should I earth the input and output grounds and should I take the ground thats created by the transformer secondaries and bring that to the earth/chassis ground as well?
My answer was:
Quote:
Just connect Earth ground to the chassis, nothing else. If any voltage accidentally touches the chassis, it goes to the safety ground. That's all you need. Anything else you will just have ground loop issues.
...which was inaccurate and incomplete. For diagnosing noise that is most likely a ground loop, it is a good first step or remove excessive grounding (under safe workbench conditions, then reapply proper grounding).

What I left out of my reply was to indeed connect the center-tap ground to the chassis, so that the primaries and secondaries are working at the same potential. For this amp/layout all you need is this one ground connection, as all grounds are tied to. Grounding the connectors will yield a ground loop. This is not always the case, and depends on the layout in question.

If in diagnosing noise you find the Mains Ground to be the trigger, then you need a "ground breaker" to keep current from flowing from/to it unless there is actual voltage present.

In an ideal world, you would also fuse each of your secondaries, as a fuse will reach faster than a circuit breaker, which typically must wait for a certain point in the wave to trip (unlike a fuse) and will also have substantially higher current trip point (20A).

Incidentally, fusing the secondaries is how most consumer equipment works when there is no mains ground connection.
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Old 20th December 2008, 02:45 PM   #3078
udailey is offline udailey  United States
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So Brian,
Let me see if I understand.
I need to take the 0V from my transformer and create a little star ground with it and Earth GND. Then take a wire from that star to the GND on the board.
I DO NOT need to take any extra wires from input or output to this star or anything connected to that star since this is taken care of on the board.
Am I understanding this correctly? Right now I have Earth and 0V tied and a wire from there to GND on the board but I do have extra wires from signal and output to the star GND. There is nearly zero noise and almost zero DC but there is always room for improvement.
Also, if I fuse the secondaries what rating should the fuses be for each channel?
Thanks for your help,
Uriah
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Old 20th December 2008, 02:50 PM   #3079
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Correct. By wiring the connectors to the star ground, they have path to ground through the board and through the wire, which is a ground loop.

For secondary fuses, I would go with 4A on each.
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Old 20th December 2008, 03:20 PM   #3080
AndrewT is offline AndrewT  Scotland
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F4A fuses feeding the chip amplifier rails are far too high.

A 4A fuse will allow upto 250W into 4r0 or upto 500W into 8r0.
F3.1A <=150 into 4r0 or <=300W into 8r0.
F2.5A <=100W into 4r0 or <=200W into 8r0.
F2A <=60W into 4r0 or <=120W into 8r0.
F1.6A <=40W into 4r0 or <=80W into 8r0.

It appears that 4ohm speakers require ~F2A and 8ohm speakers require F1.6A fuses in the supply rails fitted after the main smoothing caps if one designs to the power limit of a 3886.
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