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Old 26th March 2007, 03:20 AM   #1
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Default What cable for outboard supply ? (pics)

I am building an outboard dual psu for a headphone amp.
I'm using a 6 pin Neutrik connector...so 3 wires per psu = 6wires.
I am not sure which cable is best to connect the two boxes, and why?
First pic are two Belden choices -- 24 awg per wire.
Current draw will be 300ma max, per supply.
4 foot cable.
(what worries me is the ground wire(s) being kinda skimpy..??)
Bottom pic is the unfinished dual supplies, + 15v each.
(Xformer, rectifiers, plus 1/2 of the filter caps)
Link to Headphone amp & case, showing regulator boards too.
http://i5.photobucket.com/albums/y17...t/DSCN1969.jpg
Click the image to open in full size.
Click the image to open in full size.


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Old 27th March 2007, 10:04 PM   #2
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Could there be interaction between the two seperate DC supply lines when all bundled in one cable?


Am I worrying too much ?

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Old 29th March 2007, 05:15 AM   #3
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Ok.
I chose the lower-right one, the mic cable.

Works , but I'll know better when I put the complete amp in it's proper place, and listen for hum, noise, etc.

One problem is that if I turn on the power supply with it not connected to a load (the amp case), the 2200uf caps in there charge up mightily.
How do I know this.?..

Any thoughts on preventing this? A bleeder resistor across the psu case's output ?

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Old 29th March 2007, 05:29 PM   #4
AndrewT is offline AndrewT  Scotland
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Default cap voltage rating

Quote:
Any thoughts on preventing this?
do the correct calculations before selecting the wrong/right voltage rating for the smoothing caps.

capacitor Vrating>=[1+transformer regulation]*[1+mains tolerance]*[sqroot(2)]*[transformer Vac]*[mains voltage/transformer rated input voltage]

an example for a 600mA 230Vac:20Vac+20Vac (24VA).
regulation=18%
mains tolerance 6%
cap Vrating>=1.18*1.06*1.414*20*240/230=36.9Vdc. Use 40Vdc or higher capacitors.
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Old 29th March 2007, 11:15 PM   #5
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Thanks Andrew.
I saved your explanation on my computer for future reference.

But I'm not sure I explained myself clearly....
The caps are indeed 50v rated.

I have two boxes:
1) (Dual PSU) transformers, rectifiers, 1/2 of the smoothing caps.
2) (Amp) 1/2 smoothing caps, regulator boards, amp boards.

I was trying to explain that if I turn on the outboard PSU, it will charge the smoothing caps (as it should). But if there is no load to drain those caps (perhaps if I forget to apply the cable to the Amp case), those caps will hold a charge for a while.
Granted, I've wired the cable for safety....female connectors on everything that is a power source, so....
I probably never get my fingers zapped, but I wonder if plugging it in to the amp's regulator boards (with a cable that is attatched to charged caps) will cause damage...or will they quietly dissapate making this a non-issue ?

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Old 30th March 2007, 06:01 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi Red,
do not apply a rapid discharge to your caps.
The PSU smoothing is up to voltage, the amp smoothing is at or near zero voltage.
Connect the two together and firstly, they will tell you they don't like it sparks at themoment of connection from each pole.
Secondly, there are special caps MADE to survive this duty, your's are almost certainly not and both the charged and the discharged caps WILL suffer damage resulting from the very rapid charging/discharge (probably =kAmperes).
Thirdly, your downstream circuit may dislike single polarity supply until the other pole makes contact.

Do you have a spare pole in the connector?
Use it to send a signal back to the mains PSU.
The signal could operate a relay that holds the secondary circuit open until the umbilical connection is established and then a pair of surge limiters initially run in circuit and a second relay shorts them out to make the low resistance cabling effective.

BTW,
a little resistance in the umbilical turns yours RC supply into an RCrC Supply and if there is a little inductance in the long cable then even better (now RCLrC) PROVIDED that there is sufficient smoothing at the amplifier to meet all the peak current demand.
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Old 31st March 2007, 09:33 PM   #7
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Now we are on the same page Andrew .
Thanks.

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