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Old 20th January 2005, 04:03 PM   #1
murat is offline murat  United States
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Default External PS for Leach amp.

Hi all,

Having just finished the circuit boards I am planning to make a power supply for Leach amp in a SEPERATE enclosure. The following is taken from Prof. Leach's construction details:

"The following wires connect to the central ground point: the green wire on the AC line cord, the ground wires for the filter caps, the transformer center tap, two ground wires from each circuit board, and the two loudspeaker ground wires."

Now,

1) Should I still take a cable from transformer center tap and ground wire on the AC cord? What is the purpose of connecting center tap of the trans. to the central ground as we already connect the ground of the caps? Does it reduce the noise??

2) Should I put the big caps into amp enclosure or is it better to leave them in the PS enclosure?

3) Any other trick to remember for such seperate enclosures?

Thanks a lot,

Murat
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Old 20th January 2005, 08:42 PM   #2
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Just a try to answer your questions

AC line ground wire is NOT necessarry!! In Europe we normally do not use the AC line Ground wire to other things than our washing machines etc. The center tap of the power supply caps are just fine The purpose of this AC Ground is, that if you by mistake gets AC on the cabinet, your AC protection (in Denmark called HFI) would disconnect the AC main within a few mS.

The big caps should be placed as close as possible to the load.... Here it would be in the amp cabinet... However you can place half in the PSU cabinet and the other half in the amp cabinet ;

And rememer to use big fat wires with appropiate DC-connectors between the power supply and the amp
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Old 20th January 2005, 08:55 PM   #3
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Caps should definitely go in the amp cabinet. One of their purposes is to supply large amount of current when demanded by the amplifier. You want, therefore, to keep them very close to the output devices.

Since you are building a power supply, I would use the AC ground wire. I agree it is not *needed*, but I think you would rather be safe.

-b
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Old 20th January 2005, 09:17 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
I would put most of the capacitance in the amp case.
BUT there is an advantage in
1. putting the first stage smoothing in the PSU case (keeps the ripple out of the amp case.
2. using smallish connecting cable introduces a RCRC time constant/filter to reduce the ripple even further.
regards Andrew T.
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Old 20th January 2005, 09:21 PM   #5
murat is offline murat  United States
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Thanks a lot guys.

Jan, i am still confused about center tap issue. Leach talks about two separate wires from common point of the caps AND from center tap of the the transformer. If I am not wrong you are also mentioning "The center tap of the power supply caps". I especially wonder the other wire connected to center tap of the transformer - not the caps center.

Thanks again,

Murat
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Old 20th January 2005, 09:30 PM   #6
murat is offline murat  United States
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Thanks Andrew,

Here is a little English and electronics problem of mine: What I understand from your 2nd point is to use "thinner" (or do you mean shorter??) cable is better for ripple rejection. Is this right? If a slighly higher resistance is better than you probably mean thinner but just wanted to make sure.

Murat
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Old 20th January 2005, 09:33 PM   #7
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The center tap of the transformer is the ground in reference to the two poles of the transformer. for example, 60/0/60. This is the ground for your circuit and should be conntected to the - terminal of your rectifier and the - terminals of the caps. Ground is always relative. In this case it is relative to the other leads from the transformer.

After the rectifier, you will have +VCC/GND/-VCC, as the recifier yeilds polar DC from the AC current of your transformer. This DC voltage is still relative to the ground or center tap of the transformer.

Don;t know if this is helping, actually. It's late, time to go home.

-b
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Old 20th January 2005, 09:39 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
sorry for not being clear, I usually forget that this forum is read by others whose first language is not Englsh.
I meant thinner wire. More risistance increases the time constant and attenuates the hum even more than brute force capacitance.
Some designs use resistors before the caps.
regards Andrew T.
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Old 21st January 2005, 12:16 PM   #9
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AndrewT:
I have with good result used 0.01R resistors on the supply to the power caps..... This very small resistor prevent some of the "clicks" you normally see on the scope but normally donīt hear.
However I would still go for some heavy wires
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Old 21st January 2005, 01:03 PM   #10
murat is offline murat  United States
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Hi,

Inside an old computer PSU I found two caps of 200V; 330uf each. Can I use them in my PSU as first stage smoothing caps? It is getting quite expensive to order every forgotton component again and again via mail.

Thanks,

Murat
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