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Flemming J P 8th April 2012 07:42 AM

F5 problems

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?

Zen Mod 8th April 2012 08:17 AM

that's the culprit , very likely .

fix the amp and fix that xover

KatieandDad 8th April 2012 08:30 AM

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.


Flemming J P 8th April 2012 09:29 AM

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.

KatieandDad 8th April 2012 09:38 AM

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.

KatieandDad 8th April 2012 12:23 PM

Have you got long interconnects ?

Or a bad connection ?

Might be worth following my advice in Post #3

Flemming J P 8th April 2012 04:33 PM

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.

wirewiggler 8th April 2012 05:40 PM

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.


AndrewT 8th April 2012 05:43 PM


Originally Posted by wirewiggler (
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.

Flemming J P 8th April 2012 06:22 PM

No it is "normal" active XO with jeft followers and a opamp.

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