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Old 4th September 2004, 01:03 AM   #11
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Well, some good news. I've got some positive results and managed to stop the transformer noise. Well, not quite stop completely but reduced to a very, very low level of less than 40dB (below the measureable limits of my SPL meter). Achieved this by wiring two caps in series, each with an opposing diode paralleled up. That way DC is completely blocked, better than the other way of one cap but two opposing direction diodes in parallel.

Now that the problem is overcome, I have another question. How can you be sre the capacitors will be enough for the current the power amp uses? The AC voltage is 240V and power amp fuse is rated at 4amps. So what value of capacitors do I need? I am using two capacitors in series at the moment - only 10,000 microFarads and 50V, will try to get higher value ones when they are in stock. Any suggestions?
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Old 4th September 2004, 02:40 AM   #12
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A transformer would produce hum noise for three reasons,1) is loose core... 2)is core saturation...3)overload...

Yuval.
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Old 4th September 2004, 04:12 AM   #13
byteboy is offline byteboy  Netherlands
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You won't be needing verry large caps because the caps only pass smaller AC currents below the 0.7 Volts or so of treshhold voltage of the diodes.
Once the voltage is > 0.7 Volts, all current will flow thru the diodes.
If there is a DC component present in the AC, these are mostly smaller than 0.7 Volts, but can be enough to cause the transformer core to saturate short term (and cause the humming) it will be blocked by the caps.
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Old 15th October 2004, 12:25 AM   #14
Stabist is offline Stabist  Slovenia
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If I understand you correctly - you've did something like this:
Click the image to open in full size.
(I've found picture on diyparadise)
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Old 15th October 2004, 01:45 AM   #15
byteboy is offline byteboy  Netherlands
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More something like this.

Click the image to open in full size.

(edited the original picture to not confuse - the original is somewhere on DiYAudio)
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Old 15th October 2004, 10:55 AM   #16
AndrewT is offline AndrewT  Scotland
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Hi
the AC goes through the back to back caps. measure the volts drop using your dmm. At switch on the high current pulse will generate a larger volts drop & the diodes short this out to prevent damage to the caps.
You need parallel diodes to ensure that current can flow both ways if the voltage rises too high. You need back to back caps or bipolar caps because the AC has a plus & minus component.
I have seen some circuits using two diodes in series then paralleled to give 1.4 volts before shorting out this would better suit 240vac supplies. because the caps only see a maximum of the diode drop across them they only need to be low voltage types plus a generous factor of safety.
I have also seen the caps in the neutral line only. I believe this is to avoid the high voltages being exposed to prying fingers, there is a lot of circuit and components operating at line level if the circuit is in the live lead.
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Old 15th October 2004, 11:11 AM   #17
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Question Transformer is making a noise - how to cure?

Hi audionutter, What is the brand of the transformer?. Some hum because of bad design........
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Old 15th October 2004, 01:28 PM   #18
Eva is offline Eva  Spain
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I don't recommend very high value capacitors since mains line suffers DC fluctuations, DC component is not constant. Too big capacitors will produce also saturation when DC value changes.

Use something with low enough impedance at 100Hz to prevent the diodes from conducting due to the own ripple of the capacitors [dependent on actual peak current consumption]
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Old 15th October 2004, 05:23 PM   #19
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Unhappy Diagram of Audionutter's solution.

Hi,
I am following this discussion with interest as I am having similar troubles with a noisy transformer. Audionutter, as you seem to have cracked it now, can you please tell us which, if either, of the two diagrams above is the one which describes best what you have done to cure the problem.
If neither is correct would you please post the diagram of your solution.
Thanks, hopefully........................
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Old 15th October 2004, 07:26 PM   #20
byteboy is offline byteboy  Netherlands
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Default I wouldn't try the circuit found/posted by Stabist....

You will probably be sitting in the dark after a loud bang!

If you follow the flow of current you will see that it flows through one of the capacitors during the whole duration of each half cycle.
The diodes do not protect/shunt the capacitors correctly.

I didn't test myself the circuit posted by me, but if you search for the original thread you will read experiences of people who did it with the correct result.
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