I fried someting on my F5...ugh

[cjkpkg]

So I had the double MOSFET going good this morning on the F5 and when I cranked the volume up I lost the right channel. I quickly turned the amp off and brought it back to the test bench...

Now, R5,6,7,8 get REALLY hot and there is no current going to the MOSFETS. Am I fearing the worst that I lost the JFETS?

Also, I am getting about 24VDC on the speaker outputs.

What else would be the likely candidate?

 

[Zen Mod]

do you know how to test jfets and mosfets ?

matching procedure is what you need to test them , and that's easiest way for not experienced DIYas .....

look at FW site for articles - then "mosfet testing" article

 

[cjkpkg]

I verified that the 2sk170 in one channel was bad.

I replaced with a spare and the same thing happened...at lower volumes everything is a-ok...when I cranked it up the 2sk170 blew agian...and passed full DC voltage to the speaker terminal...

Could a bad ZTX550 also be causing this? I will explore later but I cant keep losing these JFETS!

 

[Nelson Pass]

Probably not. How is your F5 different from stock?

Is the output DC positive or negative?

 

[cjkpkg]

I added another pair of mosfets along with the 47 ohm and 0.47 ohm resistors.

I was running at about 525mA on each of the 4 mosfets...

the DC at the speaker terminals is 22.5V positive...

I should mention that this happened both with an aikido pre and the jfet boz pre...

 

[chinsettawong]

What kind of speakers do you play your F5 with? Did you blow your speakers as well?

Wachara C.

 

[cjkpkg]

CHR-70's in a fonken cabinet...So far they seem to have come out OK...

 

[cjkpkg]

Well...all went well on the test bench in the basement...brought it back upstairs to the listening room and adjusted the bias/offset after about 2 hours of listening downstairs...

hooked everything up, the F5 is now in the aikido...turned the amp on, then turned the aikdio on...and...crackle crackle crackle...lost both CHR-70's....

This really really sucks...bad day...

 

[Rodeodave]

May sound cheesy, but learn from this. Get some cheap test speakers or dummy loads.

My condolences.

 

[a.wayne]

Isn't that speaker 3 ohms and only 84 db/1/m ............. maybe clipping the amplifier ?

 

[Brit01]

ouchhh

25 volts on the speaker. no explosions?? no smoke??

I use the cheapest 1 watt 8 ohm 3 inch speakers for testing.

You don't want any more than 0.5- 1.0 dc on the speaker terminals. magnets do not like DC. I fried 1 test speaker last week with 4 volts (testing an OTL build!). You could feel the hot magnet.
I get concerned when I have more than 15mV DC on my Kipsch speakers from my 380 watt SS amp.

 

[tinitus]

hooked everything up, the F5 is now in the aikido...turned the amp on, then turned the aikdio on...and...crackle crackle crackle...lost both CHR-70's....

when dealing with units without power-on relays and such, always turn on source first, and power amp last

Im not suggesting it would have made a difference

 

[cjkpkg]

I had this rig set up with test speakers for at least 2 hours.

when I went to hook them up to the CHR70's I checked the dc offset and it was ~150mA. I reduced it to ~0 and fired everything back up. That is when the crackle happened.

So please stop lecturing on test speakers...I did that. Sorry to be short but I did the basics and still got burned...

Tinitus...I always wondered if there was a best practice...pre or power on first...

 

[Rodeodave]

Switching equipment on is best done towards the speaker (source, pre, amp), switching off is done away from the speaker (amp, pre, source).

 

[Nelson Pass]

I had this rig set up with test speakers for at least 2 hours.

when I went to hook them up to the CHR70's I checked the dc offset and it was ~150mA. I reduced it to ~0 and fired everything back up. That is when the crackle happened.

Adjusting offset is the same as adjusting bias. Are you using
the thermistors?

 

[cjkpkg]

Yes,
This is how I hooked up both ends...

 

[frags]

Wrong connection of 0.47 ohm resistor. The protection part based on Q5 is not engaged at all.
You should correctly connect the resistor to the place where it should be (R11)

 

[Melon Head]

R11 looks good to me

If you are using current limiting circuitry then there should also be wires running from R17/18 to the source of the mosfets. If you are not using the current limiting circuitry, then I can't see anything wrong.

 

[frags]

thanh1973,

This is the problem. W/o current limiting the guy is getting the FET fried at higher volume level.

 

[Melon Head]

Yes, adding current limiting circuitry might help.
But what is the root cause of the problem? There are several others, as well as myself who are using the amp without the current limiting circuitry without problems.
So it seems strange that he is having problems.

He must be heavily voltage clipping the amp.

A picture of the amp might help.