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Old 26th January 2002, 06:39 AM   #11
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Default where?

I checked the lcaudio website and can't find the dc-filter. Whassup?
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Old 26th January 2002, 07:16 PM   #12
SvErD is offline SvErD  Norway
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LC-Audio 's filter have 2 diodes in parallell (A toK) and a capacitor in each line (Live and Neutral). I can scan the diagram and mail it to you if you want.
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Old 26th January 2002, 09:30 PM   #13
mlloyd1 is offline mlloyd1  United States
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Default one version of DC blocking circuit ....

I found the circuit I mentioned in an earlier post in this thread. It is very similar to what Nelson described. I have not built it, but I will....

.....eventually

Enjoy and if someone builds it before me, please report to the group. It includes some surge protection and HF line noise supression, which could be removed if you desired.

Michael
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Old 27th January 2002, 09:05 PM   #14
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thank you all,


I will build the filter. Just have to order the parts and do a layout.The only problem I have is that I must change the parts a bit to get a 1200VA rating or use two per amp.

I will let the forum know if the noise has gone and if there are positive or negative effects on sound quality.

william
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Old 28th January 2002, 05:11 PM   #15
vbd is offline vbd
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Default Re: one version of DC blocking circuit ....

Quote:
Originally posted by mlloyd1
I found the circuit I mentioned in an earlier post in this thread. It is very similar to what Nelson described. I have not built it, but I will....

.....eventually

Enjoy and if someone builds it before me, please report to the group. It includes some surge protection and HF line noise supression, which could be removed if you desired.

Michael
Hi,
I' m afraid this circuit has a DC path ACROSS the capacitors via the diodes and will block no DC at all... The surge & HF supression part will probably be the only cause of improvement you can expect from it !
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Old 28th January 2002, 05:42 PM   #16
mlloyd1 is offline mlloyd1  United States
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Quote:
I' m afraid this circuit has a DC path ACROSS the capacitors via the diodes and will block no DC at all... The surge & HF supression part will probably be the only cause of improvement you can expect from it !
I susepct the diodes are in the circuit to protect the caps from excessive reverse voltage. Under normal operating circumstances, the diodes do not conduct at all - the caps wouldn't allow enough forward voltage to bias the diodes.

I initially thought as you did; then I though some more. I also tried an experiment using much lower voltages than power line levels. The diodes did not turn on.

Michael
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Old 28th January 2002, 05:52 PM   #17
wayne is offline wayne  United States
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Default Filter

On the filter schematic by Mloyd1 if D1 and D3 are removed DC will be blocked and the capacitors still protected. The caps see very low DC because of the diodes. Make sure to use high ripple current capacitors. Digikey has some good for about 4 - 5 amps and you could use more of them. The P6421 is 33000 at 16 Volts and good for 5.2 amps at 85 degrees C. The diodes need 400 PIV rating.
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Old 28th January 2002, 05:55 PM   #18
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It's not just reverse voltage protection, its forward protection too (notice how two diodes are used). Care must be taken in choosing capacitors with enough current capacity, especially with larger amps. I would stay away from polarized electro's in this configuration... they can make a really big mess when reversed with enough current (even at low voltages).
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Old 28th January 2002, 07:11 PM   #19
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I´m just building a part of the circuit. Only the capacitor and the two diodes (this is all that´s in the LC-Audio filter). They us a 22000microF/25v cap. I´ll use two 6800/35V paralleled.
I´ll see if there´s any dc-blocking done by this.

Unfortunately (or typically) my power amp is very silent today so I probably can only measure the filter today.

william
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Old 29th January 2002, 12:37 PM   #20
vbd is offline vbd
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Quote:
Originally posted by mlloyd1


I susepct the diodes are in the circuit to protect the caps from excessive reverse voltage. Under normal operating circumstances, the diodes do not conduct at all - the caps wouldn't allow enough forward voltage to bias the diodes.

I initially thought as you did; then I though some more. I also tried an experiment using much lower voltages than power line levels. The diodes did not turn on.

Michael

Let me think twice :
First, let's us assume DC only :
- as soon as the DC voltage on the upper mains terminal is greater than two diode drops (say 2 V with big rectifiers), D1 and D4 will conduct and bypass the capacitors...
- If the DC is reverse, i.e. negative on the upper terminal, D2 and D4 will be direct polarized and then bypass again the capacitors...
Then AC only : If I consider the bottom terminal as a reference, the voltage at the top terminal will change from +325V to -325V peak, assuming a 230V mains (european practice...). The diodes will be polarized as soon as the voltage across them is greater than two diode drops, but now they are shunted by the impedance of the capacitor. If the capacitor is big enough, this low impedance will keep the diodes from conducting...
Now let's us consider DC + AC :
-The AC component will pass through the capacitors, if they are large enough, but the DC component will be blocked by them, as they have a very high impedance and will take the less impedent path at zero frequency, through the diodes, except the first 2 V...
Maybe I missed a point and I would be happy that someone shows me where... A spice simulation showed the same blindness as me...
I fully agree with you that the diodes can be non-conducting under normal circumstances, i.e. no DC, but DC seems not to be blocked !
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