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'[EE]: Hum problem'
2002\06\22@195728 by Tal Bejerano - AMC

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part 1 646 bytes content-type:text/plain; (decoded 7bit)

Hi 2 All

Attach a quick schematic that I draw to explain my problem.
The problem is a hum I can't rid of.
I try to increase the 1000uf capacitors with larger ones (4700uf) but no
change, only the hum sound changed to lower. I try to increase the output
caps' from 100uf to 470uf but no change at all.
what I can do more? maybe the bulky transformer that on the pcb can cause
this hum problem?

if you think the component placement is important I can send the pcb
picture/drawing (even I put it in the way that unregulated / unfiltered
traces will not be near the audio circuit)

I really need your help!

Tal


part 2 17430 bytes content-type:image/jpeg; (decode)


part 3 144 bytes
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2002\06\22@202504 by Jinx

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> if you think the component placement is important I can send
> the pcb picture/drawing (even I put it in the way that unregulated
> / unfiltered traces will not be near the audio circuit)

Audio should be kept away from any AC waves or ripple. Big
fat ground and power traces may help, and you may need to
augment them with bare wire soldered to them. Also a common
0V point is desirable. If all 0V lines go back to this common
point that can help eliminate grounding imbalances. This includes
the frame of the transformer too. Some transformers are enclosed
in a mild steel shell, but I don't know if that stops a lot of hum. If
you continue to experience real problems, you might try the
opposite - enclose the high gain section of the audio in a grounded
metal box

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2002\06\22@203544 by Robert Shanks

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Hi Tai -

You've probably already checked the simple stuff, but sometimes thats what
will bite you.  Did you lay out the circuit board yourself?  Have you
checked the output of the power supply with a meter AC and DC...  have you
looked at it with a scope?

-  Remember the 7912 has a different pinout that the 7812 - Ground is not
on the middle on the 7912.
-  Have you checked the continuity and orientation of the diodes.
-  Is it a new transformer?  Have you checked it's output?
-  Have you checked the orientation of the caps?
-  Where is the audio portion of the circuit grounded?  If your op-amps use
+/- 12 volts is the circuit properly referenced to ground?

I hope this is helpful...   so please don't be offended.   Good luck.

-  Robert

{Quote hidden}

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2002\06\22@205316 by Dwayne Reid

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At 02:51 AM 6/23/02 +0200, Tal Bejerano - AMC wrote:
>Hi 2 All
>
>Attach a quick schematic that I draw to explain my problem.
>The problem is a hum I can't rid of.

Check your ground and power routing.  The center tap on the transformer
should go directly to the mid-point of the 2 large electrolytic
capacitors.  Make that trace (or wire) between the 2 caps large and
wide.  Pick a single point on that trace and run all your other grounds to
that point.

The outputs from the bridge should also go directly to the large
electrolytic caps, then from the caps to the regulators.

dwayne

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2002\06\22@212707 by Brian Aase

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It sounds as if the strayfield magnetic radiation from your power
transformer may be inducing hum into the signal traces of the PCB.
Try this experiment: remove the transformer from the PCB and
make the connections to it temporarily using long (~1 metre) wires.
If the hum goes away, there's your problem.  Let us know what
happens.

Brian Aase

{Original Message removed}

2002\06\22@213122 by Tal Bejerano - AMC

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that's what I will try to do in the morning (it's 4:30am now)
I want to send the pcb layout but bad luck! eagle wont load it..

Regards

Tal Bejerano
AMC - ISRAEL


{Original Message removed}

2002\06\22@214337 by rixy

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Hum can be an annoyance when designing high gain audio circuits. I assume this
because your previous posts discusses high quality audio circuits. There is
nothing wrong with your power supply schematic electrically. However, all
ground returns MUST return to only ONE point in your power supply circuit.
Each capacitor, regulator, transformer common must go to one point in the
supply. In your application circuit you must adhere to the same rule. All
grounds must return to one physical point. Even a ground plane isn't
considered good audio ground if you enter the ground plane as much as 1 inch
apart. Consider the ground trace as a continuous resistor. Then you may see
that a lead going to any point on the ground isn't really getting to a clean
audio ground. In the audio world we call this point a "star ground" or "star
point". Also, it is not the thickness of the trace that determines its
effectiveness of a clean ground, but rather its direct and untouched contact
with another circuit on its way to star ground. Component placement doesn't
necessarily aggravate the hum as much as a ground loop does.
Rick

Tal Bejerano - AMC wrote:

{Quote hidden}

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2002\06\22@220706 by Tal Bejerano - AMC

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when you are talking about "ground", do you mean physically connect to mains
ground?


Regards

Tal Bejerano
AMC - ISRAEL


{Original Message removed}

2002\06\22@222156 by Rick C.

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Yes and No. Mains ground can be tied to your chassis as ground for shielding,
but your power supply ground and application circuit ground should be floating
and not be connected to the chassis ANYWHERE except a single lead from mains
ground to the star point. This will eliminate any ground loop induced hum in
your application circuit which can be a real problem to fix. To test this,
remove this mains ground lead to the star point and measure the resistance from
the chassis to the application circuit ground. It should be infinite. (If you
get a reading of less than about 1 meg ohms, then you have a stray ground
getting into your circuit that needs to be removed). Then reconnect the lead.
Rick

Tal Bejerano - AMC wrote:

> when you are talking about "ground", do you mean physically connect to mains
> ground?
>
> Regards
>
> Tal Bejerano
> AMC - ISRAEL
>
> {Original Message removed}

2002\06\22@223902 by Rick C.

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Tal, here's a good example:
www.williamson-labs.com/emc/glencoe-emc-11.htm
Go a little more than half way down the page to the Power Supply section and see
how they give an example of grounding. Notice that there is only one point where
ground is going to be ground and no other ground return goes to the chassis.
Good luck - Rick

Tal Bejerano - AMC wrote:

> when you are talking about "ground", do you mean physically connect to mains
> ground?
>
> Regards
>
> Tal Bejerano
> AMC - ISRAEL
>
> {Original Message removed}

2002\06\23@025802 by Joe Farr

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When building PSU's (especially for audio/radio use) I always place 4 x 0.1uf caps across the each of the rectifier diodes in the bridge.
Open up a good quality audio amp and have a look - there usually there especially if it has a radio tuner input.

The switching action of the diodes modulates any RF present in the mains supply and can produce a 100Hz hum in certain conditions.

Joe



{Original Message removed}

2002\06\23@082307 by Olin Lathrop

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> Attach a quick schematic that I draw to explain my problem.
> The problem is a hum I can't rid of.

This is only the power supply.  It looks good enough assming that the
current drain doesn't cause the regulator input caps to droop below the
minimum regulator input voltage between power cycles.  You can easily see if
the power supply is at fault by looking at the regulated +12 and -12 lines
with a scope.  If they are flat, then the problem is elsewhere.  We can't
tell what the problem is without the rest of the circuit and some
particulars, like how much current is being drawn.


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2002\06\23@083111 by Tal Bejerano - AMC

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As I said I am drawing again the pcb as it is now, and please tell me if you
see something strange.

Regards

Tal Bejerano
AMC - ISRAEL


{Original Message removed}

2002\06\23@113925 by Olin Lathrop

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> > This is only the power supply.  It looks good enough assming that the
> > current drain doesn't cause the regulator input caps to droop below the
> > minimum regulator input voltage between power cycles.  You can easily
see if
> > the power supply is at fault by looking at the regulated +12 and -12
lines
> > with a scope.  If they are flat, then the problem is elsewhere.  We
can't
> > tell what the problem is without the rest of the circuit and some
> > particulars, like how much current is being drawn.
>
> As I said I am drawing again the pcb as it is now, and please tell me if
you
> see something strange.

Huh?  Your comment suggests you didn't bother reading my response above, so
providing any more responses would be pointless.


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2002\06\23@134225 by Tal Bejerano - AMC

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Olin

I don't have a scope so I cant see what is going on, don't be offended I
have to work "blind in the dark".


Regards

Tal Bejerano
AMC - ISRAEL


{Original Message removed}

2002\06\23@135028 by Alexandre Domingos F. Souza

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>I don't have a scope so I cant see what is going on, don't be offended I
>have to work "blind in the dark".

       Tal, it looks like you are just like me, "in development" ;o)

       I have a tip, that may or may not work for your problem

       Use a common speaker, with a resistor in series, in the output of your power supply

       If you have ripple in your PSU, it can (or not) be determined with this simple hack

       In my pinball repair times, when there wasn't a scope avaiable (always), this was something that could find a shorted diode or a dried cap on the psu. Dunno if it will be of use to you, but here it is anyways ;o)


---8<---Corte aqui---8<----

Alexandre Souza
.....taitoKILLspamspam@spam@terra.com.br
http://planeta.terra.com.br/lazer/pinball/

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2002\06\23@135649 by Roman Black

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Tal Bejerano - AMC wrote:
>
> Olin
>
> I don't have a scope so I cant see what is going on, don't be offended I
> have to work "blind in the dark".


For mains PSU frequencies you can just use a multimeter,
check your DC and AC voltages on your PSU filter caps.
The AC volts will give you a pretty good idea of the
"ripple" there. :o)
-Roman

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2002\06\23@151023 by Herbert Graf

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> >I don't have a scope so I cant see what is going on, don't be offended I
> >have to work "blind in the dark".
>
>         Tal, it looks like you are just like me, "in development" ;o)
>
>         I have a tip, that may or may not work for your problem
>
>         Use a common speaker, with a resistor in series, in the
> output of your power supply
>
>         If you have ripple in your PSU, it can (or not) be
> determined with this simple hack
>
>         In my pinball repair times, when there wasn't a scope
> avaiable (always), this was something that could find a shorted
> diode or a dried cap on the psu. Dunno if it will be of use to
> you, but here it is anyways ;o)

       Interesting trick. A trick I have used is to hook up a true RMS meter to
the DC line, if there is ripple it shows up on the meter. It's not super
accurate but it gives a qualitative result (ie. there is more ripple then
before). TTYL

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