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Old 8th April 2012, 08:42 AM   #1
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Default F5 problems

Hi

I build 2 F5 turbo V1 channels with low gain (R7-R10 = 100ohm) with a bias of 0.7a per mosfet (irfp240/irfp9240) 32v rails and they have been working nicely for the last 2 weeks until yesterday.

I think the jfets are dead in one channel, R7-R10 gets very hot/smokes. I choose some 7 ma fets.

Can a too high input signal cause this?

The amp gets the signal for a active XO, with some gain max output is +-10v. I was thinking about securing the input using some diodes or should I look for the problem somewhere else?
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Old 8th April 2012, 09:17 AM   #2
Zen Mod is offline Zen Mod  Serbia
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that's the culprit , very likely .

fix the amp and fix that xover
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Old 8th April 2012, 09:30 AM   #3
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The 2SJ108 has a Vgs breakdown voltage of 25V and the 2SK370 40V.

You could try adding 2 x 24V zeners back to back across the inputs as per the Alephs, in fact the Alephs use 9V zeners.

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Old 8th April 2012, 10:29 AM   #4
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I will recheck my XO.

How much voltage swing is to much for the input stage of the F5?

With the "normal" low gain 15.3 db, I will need close to +-6v pk or 4.2v rms to reach clipping with 32v rails.

If that is too much I can increase the gain of the F5 by changing R7-R10 to 200 ohm which should give me about 21 db gain.
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Old 8th April 2012, 10:38 AM   #5
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The 2sJ108 will take 25V as already stated.

That equates to 17V RMS so your 4V RMS is well within limits.

It could be that the inputs have sufferred static damage.
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Old 8th April 2012, 01:23 PM   #6
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Have you got long interconnects ?

Or a bad connection ?

Might be worth following my advice in Post #3
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Old 8th April 2012, 05:33 PM   #7
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I think I will do it all

Raise the gain in the F5 and put in some zeners (I think I have some 5-10v somewhere), redo my XO and lower with lower gain.

The interconnects are not that long, but the connection can be questionable.
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Old 8th April 2012, 06:40 PM   #8
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What are you using for Xovers if they are the miniDSP modules, I hear they put out quite a thump indicating dc when powered up.

Bill
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Old 8th April 2012, 06:43 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by wirewiggler View Post
for Xovers if they are the miniDSP modules, I hear they put out quite a thump indicating dc when powered up.
if this is the case then consider adding a mute on start up that unmutes after a short delay.
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Old 8th April 2012, 07:22 PM   #10
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No it is "normal" active XO with jeft followers and a opamp.
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