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NAD S250 repair/modification?
NAD S250 repair/modification?
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Old 21st October 2019, 10:39 PM   #1
CCU is offline CCU  Denmark
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Default NAD S250 repair/modification?

Hi everybody.

I've bought a NAD silverline S250 5 channel amplifier. it is about 20 years old and time has taken its toll on the capacitors in the power-supply, so I will be replacing the two 33.000 ones(and probably also the other ones).

My question is. can I add two additional 33.000 caps without screwing anything up completely?
It would obviously get a higher inrush current, but besides that I'm not sure what would happen.

I would like to add them to add a bit of extra punch when driving all 5 channels. Because the amp is rated for about double the output power when only driving two channels.
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Old 21st October 2019, 10:47 PM   #2
rayma is offline rayma  United States
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You may get into trouble doing that. The rectifier (or its heatsinking) could be inadequate,
or the power transformer could have insufficient rating. There is a triac circuit in the
power transformer primary that appears to be some kind of soft start, which may not be
rated adequately. There is also an internal fuse in the power transformer that could blow
from a larger than normal surge and render it useless.

Last edited by rayma; 21st October 2019 at 11:00 PM.
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Old 21st October 2019, 10:55 PM   #3
CCU is offline CCU  Denmark
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The transformer is a beefy toroidal transformer with a 885VA rating.

The rectrifier is according to the service manual rated for 2540W and is currently without a heatsink. There is also plenty of space to add another in parallel(If that is something that would work)
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Old 21st October 2019, 11:06 PM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by CCU View Post
The transformer is a beefy toroidal transformer with a 885VA rating.

The rectrifier is according to the service manual rated for 2540W and is currently without a heatsink. There is also plenty of space to add another in parallel(If that is something that would work)
That rectifier power rating is going to be momentary, not continuous.
If I were doing this, I'd add a second power board, and a second power transformer.
That input triac isn't likely to be able to handle twice the design current.

Last edited by rayma; 21st October 2019 at 11:09 PM.
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Old 21st October 2019, 11:10 PM   #5
Mark Tillotson is online now Mark Tillotson
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You probably won't get much benefit, as any high power bass note that lasts more than a couple of mains cycles is limited by the transformer (and ultimately its fuse), the capacitors sustain output between mains half cycles and will have been sized appropriately. 5mF or thereabouts is often seen for a single power amp, 33mF would seem to be a generous helping for 5 amps.


Doubling the value is likely to strain the rectifiers beyond the design rating (especially for switch-on surge), and render the transformer less efficient by causing higher current charging pulses.


The power limit may also reflect a limited heat-sinking ability shared by all the amps.

Last edited by Mark Tillotson; 21st October 2019 at 11:12 PM.
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Old 21st October 2019, 11:26 PM   #6
CCU is offline CCU  Denmark
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Quote:
Originally Posted by Mark Tillotson View Post
You probably won't get much benefit, as any high power bass note that lasts more than a couple of mains cycles is limited by the transformer (and ultimately its fuse), the capacitors sustain output between mains half cycles and will have been sized appropriately. 5mF or thereabouts is often seen for a single power amp, 33mF would seem to be a generous helping for 5 amps.
.
Fair enough. I've just seen some other class A/B amps with way bigger capacitor bank. But yeah, the engineers ad NAD must know what they are doing.

Quote:
Originally Posted by Mark Tillotson View Post
The power limit may also reflect a limited heat-sinking ability shared by all the amps.
The 5 amps are 5 individual mono blocks with individual cooling. So that wouldn't be a problem.
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Old 21st October 2019, 11:27 PM   #7
CCU is offline CCU  Denmark
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Thanks for your input. I'll keep it stock
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Old 21st October 2019, 11:48 PM   #8
rayma is offline rayma  United States
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Quote:
Originally Posted by CCU View Post
Thanks for your input. I'll keep it stock
Gear from most successful audio companies won't be highly over designed,
due to the extra cost not being reflected in the specs.

Last edited by rayma; 21st October 2019 at 11:56 PM.
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