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Another NCC build on the way

Bias was 34mA. Tested each channel for about 20 mins or so, so was pretty confident with that. Clearly misplaced. I should have monitored things more closely once it was playing. I'm fairly lucky in that I left it playing while I went out for an hour. It was only when I got back that it blew. As you say Alan, my previous build didn't get warm at all. I'll take care when it's rebuilt.
 
Bummer James,

I use these MF30-1F-75 heatsinks for my amps that are similar size to the the NCC200. (0.37 for 2 channels)

http://www.conradheatsinks.com/products/single_f.html

To be honest, I have always been surprised that there isn't more problems with NAP type amps because of the lack of real heatsinks especially as there doesn't seem to be any thermal feedback for Vbias. Normally this means you need much larger heatsinks than neccessary. The NAIM cases must be working as very good heatsinks...good engineering.

I know its too late now, but I follow this procedure for testing amps before they get a chance to play on my real system.

On the workbench, monitor output transistor temperature, be careful not to burn finger.
Measure DC offset.
Connect to an old speaker just to hear some music.
Monitor transistor and heatsink temperature.
After you are confident thermal runaway isn't going to occur, leave amp playing on old speaker for many hours (days) monitoring transistors and heatsinks every so often.
After this the amp gets to move into my secondary system. The secret here is to have speakers like **** which you want to blow up, so of course they never do. :)
Days later connect to real system.

Always measure DC offset before connecting to a new system, just to be sure.

And somewhere along the line I would stress test with signal generator and dummy load.

EDIT: I should have mentioned ensure you have fast blow fuses on power rails that are sized as small as posible.

regards
 
Cheers Greg. Yes, should have been more rigorous. I was keen to get it running as I wanted to take it to a bake off tomorrow. Bloody good job it blew when it did. Could have been an expensive bake off. Just wish the wharfedales had gone instead of the main speakers. For the cost of a few transistors (and heatsinks...) the amp will be up and running. Small beer as they say. Speaking of which....
 
I hate to say "told you so" but doesn't anyone understand how the thermal f/b works in a Naim amp???

http://www.pinkfishmedia.net/forum/showpost.php?p=836851&postcount=71

The pnly way that the temperature of the o/p transistors can be (vaguely, in the Naim example) tracked by the Vbe transistor is if the case is used as a heatsink and the box is closed, so that the ambient temperature in the box is fairly consistant.

BTW this also means the caps don't last as long as other amps because they are kept warm. It also explains why Naim amps take ages to warm up.

Depite the above, sorry to hear you released the smoke genie, James.
 
You live and learn, all part of the diy experience. Some more solder fumes and all will be well with the amp again. Just need to hunt down some new drivers.
 
I made a small psu for my squeezebox in a closed aluminium box. That was a total disaster. All i had to do was drill loads of holes in the bottom of the case and also along the top edge. I sorted the problem by bolting the hot stuff to the case to get conduction from the hot bits to moving air outside of the box. The closed naim case is marginal if you want to draw any power, the 135 has a fan. A closed box with still air is a great insulator and the amps will get hot and run away.

The hotter the inside of the box the shorter time that the electrolytic capacitors will last.
 
Very sorry to ehar about your spkrs James. The NCC really don't get very hot when driven hard. I would thyink fans etc are not needed. But i do think they need a metal box (steel will suffice imo and e) with vents (I use Pesante and it gets warm at worst when loud.
 
Right then, we're back in business :D

Now got velleman speaker protection just in case something goes haywire again. Front end VBEs are in place as well. Photos below are a couple of days out of date as the vellemans are now wired in and mmks in feedback. Sounds rather good at the moment!

Some obligatory pron:

ncc20.jpg


ncc19.jpg


No switch on thump. Music to my ears :) Lots of listening to be done.
 
Did you put thermal paste between the heat spreaders and the heat sinks?

Also, check with Les about the right type of paste. I think it has to non-electrically conductive.
 
Fair play James, that's impressive. Hoping to do something similar with my NCC amp. A couple of questions:

Could you post a link for the speaker protection modules.

And are you up for doing a layout for the VBE circuits?
 
Chris Hayes said:
Did you put thermal paste between the heat spreaders and the heat sinks?

Also, check with Les about the right type of paste. I think it has to non-electrically conductive.
Click to expand...

Hi Chris, I used the thermal paste/glue that was "people who bought this also bought this" on farnell. Cost more than the heatsink for a tiny combo of paste and hardener. Has a 20 year guarantee, so it may be a while before I change it....
 
LoBo said:
Fair play James, that's impressive. Hoping to do something similar with my NCC amp. A couple of questions:

Could you post a link for the speaker protection modules.

And are you up for doing a layout for the VBE circuits?
Click to expand...

Hi LoBo, the kits are these ones, from this supplier:

http://www.esr.co.uk/velleman/k4700.htm

It's the same VBE circuit as fatmarley is selling off pcbs for in his "everything for sale" thread in the classifieds, so prob best just get some of those. Otherwise, easy enough to put together on stripboard, with no track cutting required.

James
 
Those k4700 speaker protection kits are worth their weight in gold...

Nice build btw James.
 
Do the Valemann kits have any detrimental effect to the SQ? I ask because removing the SOA protection from Naim amps is supposed to improve SQ so do the Valemann speaker protection circuits have a similar adverse effect on sound quality?

If not then I might just pop one into my amp too.
 
James Evans said:
Hi LoBo, the kits are these ones, from this supplier:

http://www.esr.co.uk/velleman/k4700.htm

It's the same VBE circuit as fatmarley is selling off pcbs for in his "everything for sale" thread in the classifieds, so prob best just get some of those. Otherwise, easy enough to put together on stripboard, with no track cutting required.

James
Click to expand...

Absolutely bloody ideal James!

Cheers buddy. I didn't realise that the gyrator boards were VBE's. Nice one

Do I need two K4700 modules for a dual mono amp?

Rich
 
mikesnowdon said:
Do the Valemann kits have any detrimental effect to the SQ? I ask because removing the SOA protection from Naim amps is supposed to improve SQ so do the Valemann speaker protection circuits have a similar adverse effect on sound quality?

If not then I might just pop one into my amp too.
Click to expand...

Mike, they are not like the SOA protection which , I think, is some form of current limiter on the amp board. The KT4700 use a relay to turn the music on and off. Les recommends them, he uses them in his £3k Voyager amps so they can't be all bad.

The good thing about the KT4700 is that not only do they delay the output of the amp by 6 sec so no turn on thump, but they also protect against DC offset and short cct of the speaker terminals. For me these features are essential esp if your drivers are either too expensive or no longer manufactured.
 
LoBo said:
Do I need two K4700 modules for a dual mono amp?
Click to expand...

I think the purist would say yes, as the 0v on the protection module would be shared between the 2 channels otherwise, so to keep everything perfectly dual mono use one per side, but obviously only use one channel on each of the boards (as can be seen on the voyager photo on the Avondale site)

James
 
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