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Old 2nd April 2007, 03:10 PM   #1
beppe61 is offline beppe61  Italy
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Default Switching power supply and preamp.

Dear Sirs,

I would like to know it anyone has experience with off-the-shelf switching power supplies used to power preamps .
I intend to use a 48V/1A SMPS to power my Brid of Zen preamp, down powered at 48V from the original 60V.
Is this a sensible idea ?
I like SMPSs because they are compact and come with a long umbilical power cord, so that it can be placed conveniently far from the circuits.

Thanks and regards,

beppe
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Old 3rd April 2007, 07:32 AM   #2
beppe61 is offline beppe61  Italy
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The selected PS should be the following:

http://www.mascot.no/pdf/9921.pdf

What intrigues me is the switching frequency of about 40kHz and a low enough ripple at 50mV p-p.
I wonder if it could be used with success also in a line preamp as the BOZ (that I intend to mod in order to use it with a 48V PS instead of the original 60V).

Thanks and regards,

beppe
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Old 3rd April 2007, 07:43 AM   #3
AndrewT is offline AndrewT  Scotland
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50mVpp sounds enormous.
Long connection leads defeats the primary advantage of regulated supplies - low supply impedance.
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Old 3rd April 2007, 08:13 AM   #4
beppe61 is offline beppe61  Italy
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Originally posted by AndrewT
50mVpp sounds enormous

Thanks for your valuable reply Mr Andrew.
Not a sensible choice I understand.

Long connection leads defeats the primary advantage of regulated supplies - low supply impedance
Is this requirement mandatory also with class A designs where the current draw is constant ?
Actually I see a lot of high quality preamps coming with umbilical power cord, in order to place transformer away from the circuits.

Thanks again and kind regards,

beppe

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Old 3rd April 2007, 09:59 AM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
class A designs where the current draw is constant
This is not true.

For push pull ClassA:
the rail currents rise and fall directly with the output current.
As the ClassA limit is reached the rails are flowing alternately from virtually zero current to twice Iq. i.e. Iq+-Iq=0 to 2*Iq

For single ended ClassA:
the supply rail current changes from Iq to Iq+-Iq as the hard limit for ClassA is reached, for the majority of topologies.
There is one topology, that I am aware of, that is rarely implemented that draws near constant current from the supply rail and sends near constant current into the ground return (the speaker is NOT connected to the ground return instead it is connected to the supply rail, this topology may not be applicable to pre-amp use since the ground return is also the reference for the next stage).

Don't repeat the mistakes of others, no matter how many times you read it.
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Old 3rd April 2007, 11:58 AM   #6
beppe61 is offline beppe61  Italy
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Originally posted by AndrewT
This is not true.
For push pull ClassA:
the rail currents rise and fall directly with the output current.
As the ClassA limit is reached the rails are flowing alternately from virtually zero current to twice Iq. i.e. Iq+-Iq=0 to 2*Iq

For single ended ClassA:
the supply rail current changes from Iq to Iq+-Iq as the hard limit for ClassA is reached, for the majority of topologies.

There is one topology, that I am aware of, that is rarely implemented that draws near constant current from the supply rail and sends near constant current into the ground return (the speaker is NOT connected to the ground return instead it is connected to the supply rail, this topology may not be applicable to pre-amp use since the ground return is also the reference for the next stage).
Don't repeat the mistakes of others, no matter how many times you read it.


Thank you very much Mr Andrew for your extremely helpful advice that changes completely my belief.
I thought that full class A were synonymous of constant current consumption, indipendently from the output.
This, as I say, changes things completely.
On this basis I understand that long umbilical power cords are bad by design, unless maybe they have a very big section ?

Thank you very much again.
Kind regards,

beppe
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Old 3rd April 2007, 03:06 PM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
unless maybe they have a very big section
this only minimises the resistance.
The problem is inductance and capacitance.
As the load current changes in sympathy with the reproduced frequencies, the voltage available at the amp will modulate.
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Old 3rd April 2007, 03:48 PM   #8
beppe61 is offline beppe61  Italy
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Originally posted by AndrewT [/i]
this only minimises the resistance.
The problem is inductance and capacitance.
As the load current changes in sympathy with the reproduced frequencies, the voltage available at the amp will modulate


Please excuse me again, but placing additional caps very close to the amplification circuits would act reducing these problems ?
or the power supply must be placed close to the circuits in any case ?

Thanks again for your very kind and appreciated help.
Kind regards,

beppe
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Old 3rd April 2007, 04:15 PM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
a good regulator will be offering an output impedance around 1milliohm.
Most/many regulators do not like big or low esr caps on their outputs.

How can small, medium to high esr caps ever approach 100milliohms, never mind getting back down to 0.001 or less?
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Old 4th April 2007, 06:45 AM   #10
beppe61 is offline beppe61  Italy
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Originally posted by AndrewT
Hi, a good regulator will be offering an output impedance around 1milliohm.
Most/many regulators do not like big or low esr caps on their outputs.
How can small, medium to high esr caps ever approach 100milliohms, never mind getting back down to 0.001 or less?


Thank you very much again for the interesting advice.
On the basis of your valuable advice I understand that a regulated linear power supply is indeed the best, or better, the only solution to power a preamp.

Thanks again and kind regards,

beppe
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