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Old 13th December 2013, 04:50 AM   #1
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Default Minimizing induced power transformer noise

Any ideas of how to minimize noise induced by a psu transformer into an output transformer?

My case is as follows:
I built a 2 channel pre-amp with one output transformer (trafo) close to a 24-0-24VAC (on secondaries) power trafo. The Power trafo is mounted inside the chassis with the output trafos (QEE CA127 cards). The distance between power trafo and the nearest output trafo is aprox. 2".
The noise is notable when the vol pot for the closest trafo is anywhere between min and max position (no noise at max position).

Someone told me a while ago that a piece of sheet steel between power- and output trafo should do the job. Would that be true?
Are there any other "tricks"?

Thanks in advance,
Phantombox
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Old 13th December 2013, 05:56 AM   #2
Minion is offline Minion  Canada
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Well , im no expert from but from experience sometimes you can change the orientation of the transformers to a sweet spot so the magnetic field is not pointed directly at the OPT ....
If that doesn't work you can try using some grounded metal plate between the 2 transformers and if that doesn't help you can get mu-metal shielding for transformers that will pretty much block the magnetic field , and if that doesn't work then it's probably a different problem causeing noise ...
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Old 13th December 2013, 06:50 AM   #3
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by PhantomBox View Post
The noise is notable when the vol pot for the closest trafo is anywhere between min and max position (no noise at max position).
This point to an electrostatic coupling rather than a magnetic one. With the pot midway, the impedance is maximal and the input collects stray hum. You need to shield your pot, wiring, input capacitor and bias components.
You can also ground every bit of metal in the vicinity.
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Old 14th December 2013, 06:29 AM   #4
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I relocated the power transformer and shielded (grounded) my volume pots. Got the magnetic and electrostatic coupling to a minimum. Thank you, Minion and Elvee. Wiring and chassis (="every bit of metal") are grounded, as well as every switch since I used shielded wire for every connection. But what exactly do you mean by "bias components"?
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Old 14th December 2013, 10:20 AM   #5
davym is offline davym  Scotland
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I also had that weird issue with noise until the pot reached max position. That was with a passive pre-amp, it was also cured by correctly grounding the pot to the chassis.

By bias components I assume he must mean any variable resistor/pot used to adjust tube bias, also being grounded.
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Old 15th December 2013, 07:30 AM   #6
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by PhantomBox View Post
But what exactly do you mean by "bias components"?
The resistor(s) setting the DC level of the grid
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Old 15th December 2013, 08:40 AM   #7
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Actually, I'm talking about an active discrete preamp based on transistors. No tubes involved.
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Old 16th December 2013, 07:11 AM   #8
Elvee is offline Elvee  Belgium
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Different devices, identical solutions: all the components related to the input have to be well shielded.
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Old 16th December 2013, 04:40 PM   #9
picbuck is offline picbuck  United States
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Just to jump in, are your pairs twisted? The rule of thumb is that all pairs (positive with negative) should be tightly twisted. To get the wires equally twisted, instead of just one twisted around the other:

Start the twist, then hold the wire pair at each end. Hold them with pliers, the ends separated by an inch or so.

Now use the pliers to do the twisting. Amazing how you get an equal twist along the whole length.

It's my firm opinion that a lot of noise (most-nearly-all) is caused by poor connections. Nuts holding the pots tight? Insulating paint scraped away where the pots are mounted? Like that.

Did you use flux when you soldered grounding wires to the pots? In my experience it's difficult to solder to pots, flux is always needed. Otherwise it's a cold joint = poor connection.
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Old 17th December 2013, 11:46 AM   #10
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I used Belden patch bay cable for my connections. Although sometimes a signal "into" and "out of" a vol pot is not equal to balanced.
Here's a hint regarding grounding wires to pots: I used a sharp knife to scrap the surface shiny before applying flux. This reduces the time that the pot is exposed to excessive heat.
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