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Old 7th June 2005, 01:22 PM   #1
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Question Power Supply Upgrade

Just purchased a Cambridge Audio 540P phono stage which is surprisingly very good for the money but I think the weak link that I want to address initially is the wall-wart style power supply.

It's rated at the typical UK ac mains input voltage and the output voltage is 12v AC 500mA.

Can anyone help direct me to a kit or project that you, the DIY'er have already built that would offer a suitable upgrade?

I would also need details on power cable upgrade from the new PSU to the phono amp which at the moment is via bell-wire terminated with a DC style power jack pin.

Thanks.
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Old 7th June 2005, 01:23 PM   #2
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Hi,

Since the rectification and smoothing takes place inside the phono amp (and possibly regulation). I would stop worrying about improving the psu...as it pretty smart taking the transformer outside of the phono amp. If the transformer was onboard a tweak would probably have been to take it out and use a wallwart

The only way to improve it then would be to get a similar rated transformer that was quieter..and then you would have to put it in a box...a lot of extra expenditure for a tiny improvement if any.

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Bas
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Old 7th June 2005, 02:03 PM   #3
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well, plugging the wall-wart into an EMI filter would probably help -- you can "liberate" the filter components from an ATX supply or build one yourself. The usual circuit consists of a bifilar wound choke, and 3 capacitors -- look at the front end of this circuit:
Click the image to open in full size.

depends on the amount of noise and harmonics on your power line -- in my area this becomes a problem during the summer when air conditioners are cycling on and off, etc.
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Old 7th June 2005, 03:43 PM   #4
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Default 12 volt wall wart

Hi:
You might just try a higher current rated wall wart or a 12v ac transformer that puts out at least 2 or more amps. The internal impedance of the transformer will be lower which will have a beneficial effect on the voltage and current regulation in the pre.
Hope this helps
Ed
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Old 7th June 2005, 03:54 PM   #5
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Thanks for the replies but I have to say that I agree with other opinions on the wall-wart that changing this does make a difference.

My thoughts were to change the wall-wart to a toroidal transformer placed in a box and the projects I have seen on other sites since posting this thread indicate that this would cost no more than 30.00. That's no money at all for such a project.

I would then look at replacing the caps in the power stage of the phono-pre, maybe use Black Gate or something similar.

Was just hoping that maybe someone on here has a kit they recommend.
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Old 8th June 2005, 06:47 AM   #6
ghemink is offline ghemink  Netherlands
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Quote:
Originally posted by CraigLand
Thanks for the replies but I have to say that I agree with other opinions on the wall-wart that changing this does make a difference.

My thoughts were to change the wall-wart to a toroidal transformer placed in a box and the projects I have seen on other sites since posting this thread indicate that this would cost no more than 30.00. That's no money at all for such a project.

I would then look at replacing the caps in the power stage of the phono-pre, maybe use Black Gate or something similar.

Was just hoping that maybe someone on here has a kit they recommend.

Some people would advice against using a toroid as a toroid is "too good" as it will let through more high frequency disturb. In fact some high-end equipment makers avoid toroids because of that reason. In my TEAC Esoteric CD player they don`t use toroids, just several "conventional" transformers (of very high quality of course, at least I hope).

Changing the caps in the amp itself maybe a better idea.

Best regards

Gertjan
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Old 8th June 2005, 07:01 AM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
I would upgrade the transformer and combine it with rectification, hi freq filtering and first stage smoothing in one steel box. Then second stage smoothing in your pre box.
You will then be creating a CRC supply and can use PSUDII to optimise the values of each C and R. R is likely to be quite small 0r47 to 10r for a pre.

Bass power and dynamic improves with lots of second stage C, but some say you lose sweetness at the top end. Room for experimentation.
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Old 17th June 2005, 12:52 AM   #8
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Do you have a noise problem?

Why do you think you need an upgrade?
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Old 18th June 2005, 03:47 PM   #9
Eva is offline Eva  Spain
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I would just add a common mode filter by wrapping several turns of the wire that comes out of the wall wart around a high permeability ferrite toroid core. This actually reduces measured noise on small signal circuits since part of that noise is allways conducted EMI

Toroids should be avoided in small signal circuits since its huge primary-secondary capacitance acts as a bridge for EMI and 50/60Hz leakage. On the other hand, wall wars usually employ a split bobbin transformer only suitable for low currents but featuring very small primary-secondary capacitance and unmeasurable leakage
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Old 18th June 2005, 04:05 PM   #10
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Quote:
Originally posted by Eva
I would just add a common mode filter by wrapping several turns of the wire that comes out of the wall wart around a high permeability ferrite toroid core. This actually reduces measured noise on small signal circuits since part of that noise is allways conducted EMI

Toroids should be avoided in small signal circuits since its huge primary-secondary capacitance acts as a bridge for EMI and 50/60Hz leakage. On the other hand, wall wars usually employ a split bobbin transformer only suitable for low currents but featuring very small primary-secondary capacitance and unmeasurable leakage

just to visualize -- a perfect transformer, diode with their added imperfections:

Click the image to open in full size.
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