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Old 19th June 2012, 01:31 PM   #921
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THANK YOU for your help guys.ll soon I'll make changes to my amp and publish results and measures
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Old 19th June 2012, 01:44 PM   #922
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D1 and D2 look to be the simplest way . The base - emitter connection wont do much I suppose .

DC off set is sometimes a little over stated . Even 250 mV wont do any harm and if negative might do some good . However it is an indication that all is not well so with some caution as to is degrading effects I would say < 50 mV is what one should hope for . If it was 63mv I wouldn't loose any sleep . Check out this special offer below . If experimenting it could be useful . I think their shipping rate is reasonable .

Velleman Loudspeaker DC Protection Kit
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Old 19th June 2012, 02:23 PM   #923
AndrewT is offline AndrewT  Scotland
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The two resistors have a nominal 39.4V across them when the supply rails are at +-40Vdc. These supply rail voltages will vary with mains voltage. A 254Vac supply will make the resistors run even hotter !!!!!!!!

When the output swings with the input signal, these two resistors vary in their voltage drop and this can reach ~77Vpk, if the amplifier approaches clipping.
The average dissipation is ~400mW and the peak dissipation is ~1.3Wpk. These resistors will run hot. They will eventually degrade and may fail.

I do not recommend disabling the protection. You could lose the amplifier.

Instead I would try to find a new combination of protection component values that allow continued protection and does not impose unreasonably high temperatures on components. I would also look very seriously at what the 10uF do, to allow transient output currents to pass without triggering the protection.
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Old 19th June 2012, 02:47 PM   #924
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Quote:
Originally Posted by nigel pearson View Post
D1 and D2 look to be the simplest way . The base - emitter connection wont do much I suppose .

DC off set is sometimes a little over stated . Even 250 mV wont do any harm and if negative might do some good . However it is an indication that all is not well so with some caution as to is degrading effects I would say < 50 mV is what one should hope for . If it was 63mv I wouldn't loose any sleep . Check out this special offer below . If experimenting it could be useful . I think their shipping rate is reasonable .

Velleman Loudspeaker DC Protection Kit
Don't use that protection kit it is too slow and still too much DC before it works

Last edited by meanman1964; 19th June 2012 at 02:50 PM.
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Old 19th June 2012, 03:00 PM   #925
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Hi Andrew . I think the 1 watt Vishay resistors will be useful and will fit the PCB OK . I have to say about protection I doubt the amp will mysteriously destroy it'self . Personally I would remove the transistors as I feel they do more harm than good . However it is prudent advice and can not argue that having them there is not useful if prone to over doing it with volume . I doubt it offers any short circuit protection . The Velleman kit is for experiments where something gross might happen , better than nothing and cheap . When the amplifier is established remove them . The better form of temporary protection is non polar capacitors . > 2000 uF > 40 V ( made form a number , one set per channel between speaker plus and amplifier plus ) . These will have less effect on the sound than people think and will save the speakers . I lost 2 pairs of Sennheiser headphones because I did not do that ( headphone amp I was designing ) . Andrew has a point . If the resistor do indicate what Andrew says then perhaps best renew the resistors and definitely keep the protection . Hard for me to believe it is for that reason . Must say I never ever saw it on a real Naim and my customers certainly were champions at abusing amplifiers . The Naim used SFR 25 resistors which were 06 W . The 1 watt Vishay resistors seem well worth fitting . There were 2 watt resistors on the NAP250 which seemed to over heat a bit despite careful use ( my ultra careful friend Malcolm for example ) . If you have an old NAP250 it is worth a look as these discolour , look at the big capacitors also for signs of the electrolyte leaking out .

Last edited by nigel pearson; 19th June 2012 at 03:26 PM.
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Old 19th June 2012, 05:21 PM   #926
brianuk is offline brianuk  United Kingdom
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The NAP 250 / 135 has the benefit of a regulator board - itself with over current protection doesn't it? So the voltage is constant -ish.

Many of the 140 clones will be run with parallel cap type PSUs and certainly as Andrew says my amps see more voltage - over 50 Volts. My boards are not clones though and have never been an issue. One day no doubt it might.

Two double value resistors in parallel might help and leave the protection in until all other debugging + 1 months smooth running is complete would be my vote.
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Old 19th June 2012, 05:23 PM   #927
AndrewT is offline AndrewT  Scotland
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Yes, one of the easiest modifications ever, a pair of double value resistors straight out of your stock to 600mW lying there "just in case".
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Old 19th June 2012, 06:12 PM   #928
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I recently designed a PSU with the type of over current protection the Naim uses . It was totally useless in a real situation . One of those was charging a large capacitor from empty with a calculate 190A of serge current ( set to 3 A ) . The transistors still smoked . I then fitted constant current sources and all was well ( as said about the NAP 250 PSU ) . I also lost less voltage into the bargain than I thought . As Lindsly Hood says class A amps like his are very good in that way as they will not give more current . Equally Douglas Self says not good because they can not supply transient power albeit changing into class AB . In my experience current limiting seldom works as well as thought . MOSFET's do it better and need no extra protection mostly . If anyone wants to design a CCS PSU I would love to help . I suspect it would be cheap and excellent .
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Old 19th June 2012, 06:21 PM   #929
brianuk is offline brianuk  United Kingdom
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Maybe the protection wasn't for the customers benefit - if it somewhat limits damage?

The amp can certainly be made unstable if not looked after EG low capacitance high inductance cables are required.
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Old 19th June 2012, 06:34 PM   #930
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Hi Ciariello,
I've just joined diyaudio for a good snoop around prior to some planned audio projects and saw your message. Did some simple sums 40V rail 4K7 resistor = 0.34W since the resistor looks like a 0.25W part I'm not supprised it's hot. perhaps you could replace with 0.5w or 1W parts guess that's just an oversite of the original designer. Also as the parts are being over stressed they will degrade anyway which will accelerate the problem.
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