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Old 27th November 2012, 10:27 AM   #871
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Quote:
Originally Posted by kclusa@yahoo.co View Post
Hello,

I am having issues of burned out power transistor.

After adjusting the offset to near 0V (+/-5mv) and bias to 15mv (between TP1 and TP2, and wait for few minutes), I started the music listening on one channel.

Everything is fine for few minutes, the heat sink is not hot, output is no more than 3V RMS.

Suddently, the transformer fuse (4A) at 120V side blown and sound stop.

After debug, found out that only Q17's and Q19's Collector and Emitter shorted, thus cause fuse blow. Other 4 power transistors are fine.

Since the heat sink is not hot at all and only one bias controls 6 power transistor, I have to assume the bias setting is as good as it can be.

I am guessing that the three transistors in parallel may fight with each other.
Transistor with higher hfe may try to charge transistors with lower hfe ??

To allow hfe difference among transistors, my understanding is to use R37 - R42 to minimize the hfe difference.

While looking at other similar design with parallel transistor, they were using 100-470 ohms as the base resistor.

If my understanding is correct, my question is how is the value of 2.2 ohms be determined ? Is it too small ? Or it requires very tight hfe matching ?

I do not believe matching hfe can be done just by using a multi-meter. Hfe will shift based on bias and frequency (thus called beta instead of hfe).

If anyone do not have burned out issue yet, it may simply indicating that those three transistors are not fighting hard enough yet.

I also remember someone else has the similar issue immediately after building the amp.

Any thought ? Any input is appreciated.
Are you absolutely sure your output transistors are genuine? I ask because there are really a lot of fakes floating around.

Transistors fight each other? The emitter resistors are actually there to force the transistors to share the load regardless of hfe.

The official prototype has been playing happily for more than a year now, I even use it with my Numark Mitrack Pro for some DJ action.

The one you remember having busted outputs immediately after building the amp has been figured out already. It was the builder's fault not the circuit, he forgot to mirror the pcb when he cloned it, hence, most of the transistor leads were installed the other way around...
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Old 27th November 2012, 11:00 AM   #872
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Default Re:Burned out Transistor

Quote:
Originally Posted by JojoD818 View Post
Are you absolutely sure your output transistors are genuine? I ask because there are really a lot of fakes floating around.

Transistors fight each other? The emitter resistors are actually there to force the transistors to share the load regardless of hfe.

The official prototype has been playing happily for more than a year now, I even use it with my Numark Mitrack Pro for some DJ action.

The one you remember having busted outputs immediately after building the amp has been figured out already. It was the builder's fault not the circuit, he forgot to mirror the pcb when he cloned it, hence, most of the transistor leads were installed the other way around...
Hello again,

All of the transistors are from Mouser. It is as real as it can be.

I only purchase 6 power transistors per type and try to match as much as I can.

If we believe the circuit is as generic as possible, then I will go back and look for other issue.

Thanks
Lou
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Old 28th November 2012, 02:18 AM   #873
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Quote:
Originally Posted by kclusa@yahoo.co View Post
Hello again,

All of the transistors are from Mouser. It is as real as it can be.

I only purchase 6 power transistors per type and try to match as much as I can.

If we believe the circuit is as generic as possible, then I will go back and look for other issue.

Thanks
Lou
Hi,

Okay so we can at least remove that bit of worry in our mind about the authenticity of the transistors.

The output transistors do not need any matching in this application.

I suggest you go check, and re-check your parts placement, lead orientation, etc. With your described situation I am more leaning towards parasitic oscillation as cause of those busted output transistors.
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Old 28th November 2012, 03:08 AM   #874
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Default Re : Burned out transistor

Hi,

I think I find the issue and try to find the solution.

I may have thermal run away. I notice the transistor is too hot to touch while heat sink is not that hot.

This is the first time that I purchase heat sink and tap the screw by myself.

I am using thermal pad and using #4 screw to secure transistor :

a. After tapping, do I need to polish the aluminum surface ? Or just use fine grid sand paper ?

b. Will thermal grease with thermal pad help ?
I do know mica need thermal grease. But not sure for thermal pad.

c. Will washer for #4 screw apply pressure more evenly ?

d. How do I know whether the screw is not too lose nor too tight ?

e. Is there something that I need to watch out to lower thermal resistance ?

f. I am able to play music with about 30mA idle current.
I burned two pair so far. Only one pair is running for the first channel.

Thanks
Lou
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Old 28th November 2012, 03:50 AM   #875
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If you suspect thermal runaway then those transistors blowing might not be thermally bonded to your heatsink.

a. yes you need to make it smooth and level.

b. most thermal pads don't need the use of thermal grease, though I've seen some using both so I guess that won't hurt.

c. yes a washer can distribute the clamping force of your screw over a wider area.

d. there is a recommended torque for tightening bolts for power transistors.

e. the transistor and the heatsink must be smooth and clean, also check your thermal pad if they are really that good. I leave the chewing of specs to you.

f. fyi, the official Honey Badger amp that I have in front of me now runs with an idle of 250mA. this is too much but the reason I did this is to torture the amp and see what happens if a too high idle current was inadvertently chosen. the amp runs fine with 30mA to 50mA idle current.
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Old 2nd January 2013, 09:45 AM   #876
clm811 is offline clm811  United States
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Question 4 ohms with 63vdc rails?

(Also posted elsewhere)
Making monoblock amps with a similar circuit, but only 2 instead of 3 OPT's.

I have a pair of 300VA 45-0-45vac transformers(63vdc rails, measured), and will be using two each of the beefier ON Semi MJL4281-4302 transistors, mounted on large 350mmx150mmx50mm heatsinks (Conrad MF35-151.5).

My speakers are rated at 4ohms (Dynaudio Focus), and therefore present a lower impedance than most.

Any concerns or cautions?

Happy Holidays!

-Chas
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Old 2nd January 2013, 11:07 AM   #877
repeet is offline repeet  Australia
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I'm no expert but can say from experience that it is risky. I built the mongrel amp (which this design came from) with the 2 output transisters NJW03, NJW0281 mounted on a conrad 300 heasink for each channel and a single 300va 45v transformer 60v rails. This ran fine while driving 6 ohm speakers, however blew the outputs when I cranked up the volume on a set of diy mini statements which go down to 4 ohm.
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Old 2nd January 2013, 12:06 PM   #878
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Default Life is risky ..



First of all , Jojo's construction PDF is VERY well done ... and the boards are so nice I am considering a purchase.

I test all types of used speakers and sources at the thrift shop I work at. And guess what I use ? .... the original AX/(badger)! I blew the rail fuses twice with
PA speakers(cheap/4R) , one time just the negative rail fuse blew and the amp sounded distorted but had no offset.

I started using this amp because the OEM test amps would not survive too long.
The badger is also much better at auditioning all these used speakers (they sell quick after the customer hears all that headroom ) I repair many Sherwood/KLH/Sony discrete receivers/amps and they are inferior (sound/durability).

As far as instability , !! With 10pf miller compensation .. maybe ? (I detected minor ringing with a single 22pf silver mica) ,but the amp still was rock solid.

The DIYaudio badger PCB has a better layout than my "hacked" (recycled)
prototypes , so it should be even more bulletproof.

Really good work.. guys !!
BTW , my account was not hacked .. my brain was .


OS
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Old 2nd January 2013, 12:17 PM   #879
Bonsai is offline Bonsai  Taiwan
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Quote:
Originally Posted by ostripper View Post


First of all , Jojo's construction PDF is VERY well done ... and the boards are so nice I am considering a purchase.

I test all types of used speakers and sources at the thrift shop I work at. And guess what I use ? .... the original AX/(badger)! I blew the rail fuses twice with
PA speakers(cheap/4R) , one time just the negative rail fuse blew and the amp sounded distorted but had no offset.

I started using this amp because the OEM test amps would not survive too long.
The badger is also much better at auditioning all these used speakers (they sell quick after the customer hears all that headroom ) I repair many Sherwood/KLH/Sony discrete receivers/amps and they are inferior (sound/durability).

As far as instability , !! With 10pf miller compensation .. maybe ? (I detected minor ringing with a single 22pf silver mica) ,but the amp still was rock solid.

The DIYaudio badger PCB has a better layout than my "hacked" (recycled)
prototypes , so it should be even more bulletproof.

Really good work.. guys !!
BTW , my account was not hacked .. my brain was .


OS
Hey Ostripper, welcome back man!!!!

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Old 2nd January 2013, 12:36 PM   #880
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Welcome back!!!
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