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F5 Turbo Builders Thread
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Old 3rd February 2020, 11:50 PM   #5931
Maciej is offline Maciej  Poland
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F5 Turbo Builders Thread
Default Fried MOSFET and interior redesign...

May bit lengthy, but I want to share my experience and hopefully someone will learn from my mistakes...

I have finished chassis assembly of my F5T cascoded v2. Pictures will be provided later as I have to redesign interior... in particular move transformer to the front to fight against AC mains noise.

Configuration: stereo F5Tv2, DIYA PSU, Speaker protection and soft start. 800VA 2x24AC transformer. 2x4 33,000uF PSU caps. 5Ux400 modushop chassis.

Bias set to 0.75 per device (0V370 across source .5R), amp was running in stable manner for a couple of days. The right channel had audible noise hum, though. I had to be really close to loudspeaker to hear it, but the left one was deaf silent. Another two days of scratching my head brought solution - the "active" loudspeaker wire of "problematic" channel was too close to transformer (located near the back panel). Moreover the wire was let alone without its ground brother twisted, what would help cancelling current induction. It's my first build... I learnt a lot so far and gained the respect as well.

I did some measurements with sound card (pretty decent Focusrite Claret) + FFT SW (RightMarkAudio and REW). THD was quite ok ~0.007, but overall noise poor - 50Hz and its harmonics at -60dB. It's unacceptable and has to be fixed, but I need to fix the amp first.

Yes, it broke
It happened when I was replugging RCA inputs (amp on):
1. The speaker protection engaged and its led kept on blinking.
2. Waited couple of secs and turn amp off.
3. To check which channel is causing this I disconnected DC power rails from left channel and turned it on again. This time speaker protection went off after couple of seconds. OK - the right channel was healthy.
4. Turned the amp off and reconnected left channel DC and disconnected the right one.
5. After turning amp on I had sparks and smoke from IRFP240 Mosfet.

Now problem debug and damage evaluation.
- both IRFP9240 are gone - GDS perfectly shortened, visually not damaged
- fried one 240.
- JFETs and rest of the stuff OK

Here is what I think what happened:
1. Replug of RCA input damaged (GS shortened) both 9240. Why????
2. Speaker protection detected DC and disconnected load from ground and whole -V current started flowing thru 240, but they survived.
3. When I disconnected other channel the PSU doubled it's capability focusing on one channel with shortened 9240 and this must have been too much for one of 240 which ended it's life in a "sparcstacular" way.
4. AC fuse didn't blow - 4A (230VAC) is definitely too high. BTW: dual mono designs are better in this aspect as AC fuse value can be halved giving faster response for overloaded single channel.

DIYAudioStore is out of stock of matched MOSFETs so I did matching myself. I matched devices mounted on small heatsink @0.8A and 45oC. Found 2 P and N pairs with 1-3 mV VGS variation. I hope it's good enough - please comment.

I'd like to understand why RCA replug (Focusrite disconnect and connection of my preamp - that os also DC style) could've caused immediate damage?
Any clues?

Also my respect to the 200W heating technology powered up by 800VA transformer has risen significantly and I want to take all security measures to prevent from (or at least limit) unexpected disastrous effects. I studied this thread carefully and that's what I fished:
1. I'll increase 47 mosfet gate resistors to 200.
2. 3n9 polypropylene caps across R7/8 feedback resistors
3. 100 ohm Jfet drain resistors before cascode. That's not pretty as requires "aerial" mount.
4. Fuse DC rails conservatively. 4 individual fuses to be able to use the lowest possible fuse value. Any downsides of this solution - in particular when either + or - V is disconnected by blown fuse loosing the V symmetry?

To be clear: I don't think I had oscillation issue, just being a paranoid.

Did I miss something?
And again- what could be the reason of the damage, that definitely happed during RCA input connection?

To be continued...

Last edited by Maciej; 4th February 2020 at 12:09 AM.
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Old 4th February 2020, 12:16 AM   #5932
Zen Mod is online now Zen Mod  Serbia
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F5 Turbo Builders Thread
that's simply no-no ....... plugging and unplugging with amp ON

even with much less sensitive (not fragile, sensitive in meaning of responsive) amps , you simply don''t do that

it's the same as you trying to seat in your car while your lovely wifie is driving it , at least 30mph

go figure

from same reasons , you have powering up sequence - source , preamp , amp ......... and powering off sequence - amp , preamp , source

broken signal chain (and RCA is first breaking GND connection, leaving floating hot) is often resulting either in burst or oscillation or both , it doesn't matter did you heard it or not
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Last edited by Zen Mod; 4th February 2020 at 12:19 AM.
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Old 4th February 2020, 08:43 PM   #5933
Maciej is offline Maciej  Poland
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F5 Turbo Builders Thread
It makes a perfect sense especially your note about connecting hot end of RCA first.
I'm wondering whether protection transistors Q5, Q6 from F5 would help to prevent from such failures.

What do you think about fusing DC rails. I searched this thread and it looks like nobody is doing this... Why?
(a) beacuse it's useless as even the fastest fuse is too slow to prevent mosfets
(b) it has some drawbacks
(c) both (a) and (b)
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