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Old 15th October 2004, 06:46 PM   #21
byteboy is offline byteboy  Netherlands
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Old 15th October 2004, 09:36 PM   #22
Stabist is offline Stabist  Slovenia
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Default Re: I wouldn't try the circuit found/posted by Stabist....

Quote:
Originally posted by audionutter
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.
Hmm - I know I have problems with my english - but - aren't on "my" picture two caps in series and each has one diode in paralel - and they are connected in opposite way?? And I think that's what it says in the text by audionutter ...

BUT OFCOURSE - I'M NOT THE AUTHOR of the drawing and have not try it out in praxis ...
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Old 19th October 2004, 10:28 PM   #23
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If a transformer hums it is due to that either some loops of the winding has come loose or worse that the core is not tightly wound. Since there must be a vibration caused by alternating currents, I have a little hard to understand why blocking DC can cure. The only cure to my knowledge is to vacuum impregnate thetransformer. Simply take the transformer out and go to a shop where they rewind electric motors take away the outer plastic wrapping and let the good guys treat it with their magic laquer.
The transformer will be as quiet as a sleeping rock. If you continue to let it hum, there is a risk of shorting a few turns this will lead to heat build up and further deterioration.
If you don't have shop nearby you can use any low viscosity epoxi and pot the transformer yourself. But that is messy but with slow hardening and low temperatures the result will be decent. Vacuum is neccessary to get into the underlaying windings. I vaccuumed some high voltage transformers and every cubicmillimetre was filled with epoxi.

Andreas W
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Old 19th October 2004, 11:34 PM   #24
AndrewT is offline AndrewT  Scotland
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The schematic was wrong.
You need a bipolar cap or two back to back polar caps. The pair of caps then have a bypass using a single or double ( same direction) diode to let excess voltage pass the caps & then you must bypass the caps in the other direction for reverse current flow. Do not connect the junction between the caps to the diodes!!!
The second harmonic content in the mains supply causes vibration in windings plates or even the whole transformer or what it is connected to. The second harmonic can be roughly compared to a true sine wave plus a small dc offset. The caps remove the dc offset and make the distorted sine wave look more like a pure sine wave. There's lots of other harmonics and spikes & every other crap in the mains supply but short of regenerating that with your own alternator you stuck with it.
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Old 20th October 2004, 01:20 AM   #25
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Hi everyone,

Thanks for all the input. I have tried both schematics that have been mentioned here by Stabist and Byteboy. The first one I tried was the same one as Byteboy had posted (which I found in the earlier thread link that he has also posted) but that didn't seem to make any difference to the noise. I wondered if somehow the diodes are defeating the object/purpose of the filter.

Next I had tried the second one that Stabist had posted (which I also found on DIYparadise) and this did work. I also had reservations about the caps not having a failsafe but I couldn't work out how to filter DC with a failsafe in place. What I also found was the effectiveness of the filter does vary, probably depending on the amount of DC current present within the AC lines. Cos when I plugged it into one of the ring circuit which also includes the fridge, computer, etc the filter's effect was almost nil whilst when I plugged it into another socket the effect was noticeable. By noticeable I mean it reduces it by a measurable level using an SPL meter.

The power amp that gave me the humming/buzzing noise is a spare power amp, so I have not used it much. What I found very effective was when I connected the amps up to an isolation transformer which I was using for something else, that killed the hum/buzz completely. What I may try next is to epoxy the toroidal transformer windings as has been suggested by Andreas W cos this will stop the buzz/hum even if any DC is present. Then that way I won't have to worry about a failsafe for the capacitor in my existing filter.
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Old 20th October 2004, 01:25 AM   #26
Stabist is offline Stabist  Slovenia
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Thanks Audionutter - finnaly someone with practical experiences!!
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Old 20th October 2004, 02:39 AM   #27
byteboy is offline byteboy  Netherlands
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Audionutter, could/did you measure the actual voltage drop over the condensators?

Take care, work safe, 230 Volts!!
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Old 20th October 2004, 04:32 AM   #28
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stabist, let us know how you get on, OK?

byteboy, believe me, I was worried about the 240V AC too! Unfortuantely I do not have the necessary stuff/voltmeter to measure the voltage drop (if any) across the caps. What I did do is to have a listen to the amps after I had connected the filter up. They sound the same to me, but I had my reservations as to whether the caps I used (10,000 microF, 63V) is sufficient or not for the purposes of a power amp that has a 5amp fuse rating.....anyone here that can work out what is the minimum capacitor value sufficient for this?
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Old 20th October 2004, 04:04 PM   #29
byteboy is offline byteboy  Netherlands
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Audionutter:

I was asking because I wanted you to verify that the voltage over the caps will be higher than the intended limmiting function by the diodes of max. 0.7 Volts.

I think the circuit you are using will block DC but is IMHO wrong and potentially dangerous.
The diodes can be considderd not present as far as their application for the protection of the caps is concerned.
This means you are probably using those caps at a higher voltage than their intended working voltage.
This can cause the caps to explode, so be aware!

I think the capacitance value is not so much a problem: at 5000uF (2x 10.000 uF electrolytes in series) at 50/60 Hz it would give an impedance of abt. 0.5 - 0.6 Ohms,[ Xc = 1/(2 pi*f*C) ] not too much but could be lower.
High ripple current capability would be the thing to look for.

But the working voltage (without the protection of the diodes) would have to be 240 X 1.41 = 340 Volts minimal, better use 400 Volts.

Two capacitors of 10.000 uF/400 Volts will be realy expensive!
That is why the diodes were introduced in the first place: To get away with using a (smaller and much cheaper) lower voltage capacitor!

Isolation transformer
The use of an isolation transformer is a good solution to get rid of a lot of interferences (like mentioned by Andrew T) from the powerline, not just DC.
Especially if you use one with an earthed screen between the primary and secondary windings or the type that uses two different coils on two different legs (U/I core) to prevent capacitive coupling of the primairy and secondary windings and so the passing thru of interferences from the power line.

In case of DC it is of course the isolation transformer that will be humming now! ;-)
But this can be stowed away in a closet away from the listening room/position!

Andreas W:

I think you will have gathered that we were not discussing hum introduced by mechanical defects/shortcommings of a transformer but because of a DC component present on the AC power line.
But good info on getting rid of these problems, especially the tip to go a motor shop to get transformer coils impregnated!
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Old 16th April 2010, 06:24 PM   #30
impsick is offline impsick  United States
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Quote:
Originally Posted by byteboy View Post
Audionutter:
Andreas W:

I think you will have gathered that we were not discussing hum introduced by mechanical defects/shortcommings of a transformer but because of a DC component present on the AC power line.
But good info on getting rid of these problems, especially the tip to go a motor shop to get transformer coils impregnated!
dang old thread but i was reading it and like Andreas W was actually under the impression that the problem was a mechanical or transformer defect. I thought the thread was misdirected to a DC issue. Which makes me have to ask... Audionutter, was the xformer making an audible sound with no speakers plugged in? The xformer was literally acting like a speaker itself correct? Did you find the problem?
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