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Old 5th June 2012, 05:50 PM   #1
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Default IMPOSSIBLE TO REMOVE HUM IN PHONO PREAMP

Friends. I built a phono preamp, based on circuits of Rod Elliott. I used good quality components and OPA2134 operational. The construction was carried out by placing the transformer pre cabinet, using a specific one for him. The interconnection notif them has been done with three wire mesh, where the mesh is connected to the network by a mass and on the other side to mass pre star. The issue is that there is always a hum auque is low. It is never silent. Post diagrams to help me that I was wrong. Greetings Jose.
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Last edited by ALPUY; 5th June 2012 at 07:37 PM.
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Old 5th June 2012, 05:58 PM   #2
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If your layout is good and your problem is not ground loop then try using heavier gauge wire for your 0V connections.
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Old 5th June 2012, 06:31 PM   #3
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OK. Thanks, I'll try what I advised. Greetings Jose.
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Old 5th June 2012, 06:35 PM   #4
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Which is the AC voltage at the secondary of the trafo?. It says 7VA, but it is power. Perhaps, the voltage at the 4700 micro is few to the 78/79 to regulate (no drop out or too low), and hum goes though power lines. You need almost 12/15V, so have 20VDC at C1/C2, and personally unlike this voltage doubler. Donīt you have a 12+12V /100mA and make a Graetz bridge instead of this doubler?
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Old 5th June 2012, 06:39 PM   #5
sesebe is offline sesebe  Romania
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Hi,

With input in short-circuit the hum remain?
What is the voltage across C1 and/or C2 in power supply?

Last edited by sesebe; 5th June 2012 at 06:41 PM.
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Old 5th June 2012, 06:59 PM   #6
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The PSU is appauling, it is only half-wave rectified. Try a decent centre-tapped transformer correctly rated at 12-0-12V and using a full bridge rectifier.

12-0-12 will give you the required headroom for the regulators. No-one in their right mind would use a voltage doubler at this voltage.

Last edited by KatieandDad; 5th June 2012 at 07:02 PM.
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Old 5th June 2012, 07:02 PM   #7
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Quote:
Originally Posted by KatieandDad View Post
The PSU is appauling, it is only half-wave rectified. Try a decent centre-tapped transformer correctly rated at 12-0-12V and using a full bridge rectifier.

12-0-12 will give you the required headroom for the regulators. No-one in their right mind would use a voltage doubler at this voltage.
This is I trying to say.
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Old 5th June 2012, 07:11 PM   #8
sesebe is offline sesebe  Romania
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I have the same type of voltage doubler in an amplifier without voltage stabilizer and I do not have any hum.
If it is well calculated a voltage stabilizer can work well.

What is the voltage on filtering capacitors from input of voltage stabilizers?

AND

With input in short-circuit the hum it is present?
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Old 5th June 2012, 07:13 PM   #9
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Default Modified PSU with CT transformer

Try a decent PSU.
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Old 5th June 2012, 07:34 PM   #10
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OK. I'm using a transformer with CT. I put a diode bridge for full wave rectification. The voltage on C1 and C2 is + - 20v. And as if you hum with the inputs shorted, not tried. I think I need to change the PCB that is not the original of Rod Elliott. PCB shipping diagram used. I appreciate your information. Jose.
Postscript: If you disconnect the wires from the in put (those coming from the turntable) only blows at max volume.
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