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Old 26th August 2006, 04:34 PM   #1
claudio is offline claudio  Italy
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Default Bryston - AM Audio PSU design question

Hello,
I was looking at 2 preamplifier from Bryston and AM Audio, where both uses after the rectification diodes a resistor (4.7 ohm for AM Audio and 15 for Bryston) in series, and then the electrolitic caps. The Bryston schematic is at this address: http://www.bryston.ca/BrystonSite05/...2005-01-17.pdf

Since it is uncommon to use a series resistor before the filter caps, I would like to ask which are the negative sides of doing so. The positive side should be the resulting RC filter.
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Old 26th August 2006, 05:10 PM   #2
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Quote:
and 15 for Bryston
It's not 15 Ohm, it's 0R15 or 0.15 Ohm and as you said it makes the rc filter. A downside would be the power dissipaded across the resistor. Any other member care to add?
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Old 26th August 2006, 06:52 PM   #3
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I have been told, that when you add a resistor in series with the caps, you reduce some small "click" sounds, that are caused by the diodes/capacitors, because the diode isn't feed unlimited current to the cap, but a current controlled by the resistor i series

Others ????
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Old 26th August 2006, 07:17 PM   #4
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I have been told, that when you add a resistor in series with the caps, you reduce some small "click" sounds, that are caused by the diodes/capacitors, because the diode isn't feed unlimited current to the cap, but a current controlled by the resistor i series

Others ????
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Old 27th August 2006, 08:32 PM   #5
claudio is offline claudio  Italy
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My reading mistake, ifrythings.

Besides the lost of power, using the series resistor, any other negative effect?
ACD gave another positive effect of using it: then why few schematics uses it? I hope the big guys enlight us!
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Old 27th August 2006, 10:26 PM   #6
anatech is offline anatech  Canada
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Hi Claudio,
The resistor does reduce the current spikes, and also the peak inrush current to the caps. This reduces the higher frequency noise on the supplies where three terminal regulators have trouble. It results in a quieter supply.

Consider these are low power supplies, the power lost is not great. I can't see many downsides to doing this.

-Chris
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Old 27th August 2006, 11:11 PM   #7
claudio is offline claudio  Italy
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Hi Chris,
and in which range the resistor should be and why?
Bryston uses 0.15 ohm, Am-Audio 4.9.
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Old 28th August 2006, 12:40 AM   #8
anatech is offline anatech  Canada
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Hi Claudio,
It depends on the current draw and what the designer is happy with. In other words, the designers whim.

The higher value will result in a little lower voltage (could be intentional) and lower supply noise. The transformer then lives an easy life.

These resistors may also become part of the overcurrent protection.

-Chris
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Old 28th August 2006, 01:35 AM   #9
clem_o is offline clem_o  Philippines
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It could also be that the transformer on one is more loosely-coupled, or has higher internal resistance, limiting the current spikes by itself...

Cheers!
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Old 28th August 2006, 09:48 AM   #10
claudio is offline claudio  Italy
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What about the kind of resistor to use: carbon, non-inductive, metal type?
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