NRG Control 400wpc Class A Monoblocks, What to do? Repair or upgrade? - diyAudio
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Old 5th October 2014, 07:47 AM   #1
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Default NRG Control 400wpc Class A Monoblocks, What to do? Repair or upgrade?

I recently got hold of a pair of these monster mono block amplifiers and documented the story on Audiokarma...

I got them from a scrap dealer and as expected they didn't work. The problem appears to be that the same pcb in both amplifiers has been messed around with by someone in the past. One has had parts removed, the other has had incorrect bits added.

The problem is no schematics or circuit diagrams exit for these amps, and every tech I have contacted has said they can't help.
I have tried to contact anyone related to NRG control to no avail.

At 55kg , with 48 output transistors and a transformer larger than a brick each , I REALLY want to get them working, even if it isn't practical to use 400w class A amps everyday, one day I might just have a use for them.

The original plan of hoping a tech could figure out the problem without a diagram didn't work.

Next I thought about reverse engineering the pcb and drawing a circuit diagram from that, but it would take ages AND only give me the diagram for that particular board, there are 2 other similar sized boards + all the outputs + the power supply section which would take far too much time to draw out...

The latest idea was to build a clone of a more popular amp, like a Krell KSA 250 based around the power supply and chassis of the NRG amps....

Can anyone advise...?
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Old 6th October 2014, 07:22 AM   #2
roger2 is offline roger2  United States
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Where are you located dazaa?
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Old 6th October 2014, 12:29 PM   #3
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It's a big box.

It might not have been terribly stable, thus the "mods".
A good tech could trace the PCBs and determine the schematic, unless someone finds one online or has one.

I'd first check the condition of the output devices. If enough of them are blown, then it becomes questionable as to what to do with the chassis.

Part of the issue is the long runs to the ouput devices... this tends to make for a layout that is difficult to run at a wide bandwidth.

For all I/we know this was a clone of a Krell...
You could clone one of the Krell amps, they have been discussed here, and PCBs made, iirc. Low down on my list of favorite amps, but some people love them.

Interesting find!
_-_-bear -- Btw, I don't actually know anything, FYI -- every once in a while I say something that makes sense... ]
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Old 26th October 2014, 08:52 AM   #4
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I'm in the UK, London.

I totally forgot about this thread. Anymore advice? More interested in building a clone amp around these rather than hold out for a schematic as I don't think one will ever turn up.
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Old 30th October 2014, 02:05 AM   #5
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According to Google:
"NRG A401M ClassA mono-block amplifiers are extremely rare, just a few of these units were manufactured by NRG Control."
"A mighty amp from the 1990's designed by Denton Clarke" You may want to give-a-go at contacting Mr. Denton Clarke for a schematic and suggestions.

"Ghosts in the machine"..... a warning in several SciFi movies. A ghost in a 400W amp will destroy any speaker in a few seconds. Your PCBs look full of ghosts A QUICK BLOW FUSE in series with the speaker is good insurance.

OPTIONS you probably already considered.
1) Historic restoration to add a "SELDOM USED" amplifier to your collection / trophy room.
2) Use chassis + parts to construct a ghost-free modern stereo amp with circuits you understand. High power supply voltage means two 400W, lower current amplifiers.
---what output transistors and driver transistors are used? Do the output transistors test good.
---mounting: all N on one heatsink.... all P on the adjacent sink (N-N sinks next to P-P sinks) or ?? ... mixed N & P on each heatsink??
3) Sell it on eBay Euro in "as-is" condition.
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Old 30th October 2014, 02:31 AM   #6
Bonsai is offline Bonsai  Europe
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They look a mess - clearly attacked with a soldering iron . . .

I'd use the parts and build a modern design.
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Old 1st May 2015, 03:32 PM   #7
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Hello dazaa,

I write this from Germany, as I have the schematics you are looking for.
Plesa contact me:

Best regards
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Old 13th January 2016, 05:30 AM   #8
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this are what these look like -

Click the image to open in full size.

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how are this going with these?

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internet photographer

Last edited by tomtt; 13th January 2016 at 05:33 AM. Reason: 1282
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Old 13th January 2016, 06:06 AM   #9
bwaslo is offline bwaslo  United States
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That is a big chassis, but doesn't look like 400W worth of class A unfanned heatsinking to me.
Max efficiency 50% means 800W min dissipation, which should look like about a kW space heater but without anything being allowed to approach glowing or be cooled by air, so much bigger. Methinks the Class A designation was using the loose definition sometimes used (class A to maybe 80W, AB the rest of the way up). Or maybe a sliding bias scheme.

Still a nice chassis to work from. Power supply, case, and heatsinks are most of an amp's cost, anyway. Throw away the boards and transistors, and you have a great development platform for a custom amp build (along with LTSPICE of course).
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Old 30th April 2016, 10:58 PM   #10
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Can anyone give me the contact info of Mr. Denton Clarke.
I cannot find any info nor his email adress.

I have 400 and A201s here, both are broken and noone seems to can help.
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