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Old 25th April 2010, 02:45 PM   #1
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Default strange behaviuor of F5

sorry for starting new discusion, I wasnt able to find solution in curent threads.
my f5 works fine,all voltages OK, it may work weeks without any issues. until i conects some DAC's and put up some juice at the input. then sk170 just blow and speakers read solid 24V. it was first time, and second and third. i can even force amp blow-just put on some complex music with bass and crank up volume and after 4-6sec. sk170 blows. amp did not clip (at least audible clip) before that, it put clean undiscorted output until j-fet dies.

I believe input is no more than 3-4v, while other amps do not complain when i am feeding them with balanced dac with output 6V rms(they just reach max output level and starts clip) I do not use any preamp.

any ideas, may I overdrive fragile F5 input fets ,or there is some issues with input which I should know? or some mods can be done? with ineficient speakers it would be good idea to swing amp for its max. I still have plenty sk170 but its not solution to replace them twice a day..
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Old 25th April 2010, 03:10 PM   #2
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Check for oscillation, the F5 has a huge bandwidth. You might need a filter on the input.
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Old 25th April 2010, 03:22 PM   #3
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thanks Bill, unfortunataly I dont have scope on hands , so there basicaly no technical reason is posible-I am not overdriven input jfets? could it be DAC fault then? dac's is OPA IV stage with both digital and analog filters in place, output is 3V rms/ 6V xlr I would say dac may be mailfunction but how it comes that damage occurs only on high input only? and it have never ever happend with any other amps (including pass labs) with this DAC.
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Old 25th April 2010, 05:30 PM   #4
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Put a resistor to ground about 100-200k, then a cap, maybe 1.0ufd film or larger, up to say 20ufd, then keep the same resistor to ground that is at the input of the amp now.

This will keep DC out.

Now, add a 200pf +/- 100pf to ground across the resistor to ground at the gates (input) of Jfets. This will keep HF/RF out of the amp.

If this cures the problem, then check the output of your DAC with a DC voltmeter while it is not playing, and then while it is playing. See if there is any DC reading.

IF this does nto clear it up:

You may have built the F5 so that it is experiencing a parasitic - self oscillation. This may only get out of control at high levels, which explains perhaps the relationship between higher level signals and something blowing it up.

Maybe there is a clue in the way it is built, so perhaps you can post some clear jpegs of your build?

AND, does this happen to BOTH channels or just one??

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Old 25th April 2010, 06:16 PM   #5
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Try higher output mosfets gate resistors 100 to 200R...
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Old 25th April 2010, 08:03 PM   #6
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thanks fellows, i will try. it happens to both chanels, sometimes one blow earlier in few seconds than other -that depends on music material, all I wanna now is that this mismatch between this DAC(parasound dac 1600HD) and F5, but not at ALL high output voltage DAC (3-5V) I still need to drive F5 to full output. idealy this would be without any modifications as I want to have genue fruit of Papa creation..
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Old 25th April 2010, 11:57 PM   #7
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Higher value gate resistors will probably aggravate the problem.

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Old 26th April 2010, 01:09 AM   #8
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Ooops! -never mind then.
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Old 26th April 2010, 02:54 AM   #9
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This is a troubleshooting problem.

The first step is to isolate the problem - it is either in the amp OR it is due to the source.

The purpose of what i posted is to take two common problems out of the equation coming from the source. Another option is to use another source.

If there is a screaming HF parasitic coming out of the DAC then the use of an even larger shunting cap at the input will take that way down in amplitude. It won't sound good, but the amp won't fry either. It's a TEST.

Please do post a jpeg of your amp's construction - even if it is clip leads on a wood board (i've built amps like that) it may give a clue as to the source of your problem...

_-_-bear

PS. is the amp biasing up correctly, and has no DC offset without anything playing??
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Old 26th April 2010, 08:52 PM   #10
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Thanks , I still remeber my first ZEN V4 and how hlpsful DIYaudo comunite was then and now. I will measure DAC and will try higher gate resistors. while I am invalid in amp creation , I'am old enought to know that there is nothing for free with higher gate resitors I will gain xxx and loose yyyy.. ataching completed amp. I folowed original design except erased thermistor and 2.2kohm resistor.
BTW F5 is very fine amp. I still not belive how good it sounds with only few parts (almost like tube amp)

if I had enought knowledge and courage I would redesign input and add 3-5 pairs pairs of output fets and pump up the power thant would be damn good amp for most less eficient speakers. even stock tiny F5 does good job to drive 82db 40hm speakers better than 100w xa
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