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'[EE]: Hum problem- draw it again'
2002\06\23@201014 by William A Brown

I think you'r in for some flak on this.
Read Olin's post again..he can sound curt at times but he is right here..
PSU output Caps are an important part of the Unit (they stop/reduce ripple
on the supplt line).


----- Original Message -----
From: "Tal Bejerano - AMC" <spam_OUTkooterTakeThisOuTspamZAHAV.NET.IL>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Monday, June 24, 2002 12:48 AM
Subject: Re: [EE]: Hum problem- draw it again

> I draw again the PSU section. can you tell if all grounds ok?
> I didn't put all components (OPamp and it components) in place, cause I
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> {Original Message removed}

2002\06\23@221716 by Dwayne Reid

At 01:48 AM 6/24/02 +0200, Tal Bejerano - AMC wrote:
>I draw again the PSU section. can you tell if all grounds ok?

Not good, Tal.

Look at the problem this way.  The transformer charges up the capacitors
during a a very brief time period near the peak of each half-cycle.  The
peak current is MUCH higher than the average current.

What you have shown is one of the worst ways to lay out a power
supply.  When the diodes start to conduct, there will be a significant
voltage drop on that trace between the center tap of the transformer and
the main filter capacitors.  That voltage drop adds to and subtracts from
the reference point on the regulators (the ground terminals).  Voila!  You
have just added ripple to the supply rails.

What I would do is make the trace between the transformer CT and filter
caps as thick as you possibly can.  Then make sure you take the ground
point for the voltage regulators from the filter cap, NOT from the
transformer CT.

It sounds as if you already have a board made up.  Try this.  Run some
heavy wire between the leads on the filter caps and the transformer center
tap.  Use 14 AWG wire or larger and just lay it down on the underside of
the PCB.  Solder it into place.  Cut the trace that connects the voltage
regulator grounds to the CT and jumper it to one of the filter caps instead.

See if it makes a difference.  Is it worse?  Better?  Try thickening up
that wire between the filter caps and the transformer CT - lay another 14
AWG wire down beside the existing wire and solder the whole mess into one
large conductor.

Use your existing board to play with the grounding until you get the best
result.  Then make the changes to the artwork.


Dwayne Reid   <>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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2002\06\24@020936 by sambuddy

From where I stand it looks like you have your -9 volt regulator connected incorrectly. You have it connected as 1 in, 2 gnd and 3 out. It should be 1 gnd, 2 in and 3 out. It's
pinout is different to a positive voltage regulator.

24/06/02 9:48:14 AM, Tal Bejerano - AMC <.....kooter.KILLspamspam.....ZAHAV.NET.IL> wrote:

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>{Original Message removed}

2002\06\24@065129 by ISO-8859-1?Q?Ruben_J=F6nsson?=

Are you sure that the pinout of the rectifier bridge is
correct? Most has the AC and DC pins in a diagonal orientation.
Unless it is an DB???G, which by the look of it, it might be.

Oh well, just a thought,


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> {Original Message removed}

2002\06\24@075550 by Rick C.

Was wondering if you received my last post. Where is your star point ground? The
pcb drawing you just sent shows the worst case senario for grounding. The two
1000uf cap grounds are as far apart as you could make it. Why? Your ground
return(s) on the regulator(s) go to the caps and share a common ground back to
the transformer. All your grounds are sharing other grounds making it back to
the star point ground. Think of your heavy ground traces as being 10 ohm
resistors back to the transformer and you will see what I mean.

To improve your layout, rotate your 1000uf caps with the hot sides pointing
toward your opamp. Run your cap grounds together and make the center of that
point your star point. Run the transformer common through the bridge ac pins to
the star point. With as much current as you are consuming, your heavy traces
don't have to be as wide as they are now. You don't really need the two 10U
(10uf) caps near your regulators. Put those as close to your opamp power leads
along with your U10 caps (.1uf) as possible. The common ground from these caps
MUST go directly back to the star point between your bridge +/- without touching
any other ground return. All your signal grounds can then be tied to the common
ground of the opamp caps.

Your hum will go away.

Tal Bejerano - AMC wrote:

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> {Original Message removed}

2002\06\24@142011 by Tal Bejerano - AMC

Thanks Rick for your detailed answer! And to Ruben, the rectifier is

I think I finally got the idea
I'll try to do new pcb and post it.

I'm sure that others who didn't do a project like this can learn a lot like
I did.


Tal Bejerano

{Original Message removed}

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