Sizzle, smoke, stink.... black safety resistor

rabbitz · « **on:** 16 May 2005, 02:50 pm »

Did some wiring layout changes on a AKSA 55N+ and when fired up, sizzle, smoke, stink and 1 safety 100R resistor gone (+ve rail). Checked all the voltages, check for short to the heatsink and everything was fine, so started up again.... sizzle, smoke, stink.... grrrr.

This happened about 4 times and after much swearing got the DMM out and probed all over the place. The result was a rotten nylon washer on a speaker terminal had a crack allowing it to short to the chassis. So a 1 cent part threatened to destroy my beloved AKSA.

What I would like to know, what the benefit of the 100R safety resistor in conjuction with a fuse versus a fuse by itself. Being an electronic neanderthal I can only wrap my brain around the fact that when a fuse is blown, there is an open circuit and all is safe. I know the use of the safety resistor when testing but I have know idea of it's benefits when in normal use. That said, it looks like it saved my AKSA anyway, but why?

aurelius · « **Reply #1 on:** 16 May 2005, 09:21 pm »

Hi Rabbitz,

In normal operation there is no benefit at all at having the resistor in place. Its use is purely during testing when the fuse MUST be removed.

The issue this: in the race for self destruction between a fuse and an output transistor, the transistor often wins (that is, is damaged beyond repair before the fuse blows).

If you use a resistor instead of a fuse, the current is limmited by the resistor, preventing damage to the transistor.

Once the AKSA has been successfully tested and the fuses put in place, any subsequent fault that causes the fuse to blow may damage the transistors. The fuse in normal operation is more to prevent your house from burning down than to guarantee protection of your AKSA.

AKSA · « **Reply #2 on:** 16 May 2005, 10:03 pm »

Hi Peter,

Mark is right on the money. Transistors are extremely gallant, despite modern folklore, and always enthusiastically lay down their lives in foolish protection of absurdly cheap resistors. The use of the 100R resistor ensures that if there is a short in the amplifier, the current which flows is limited by this resistor. If we have half an amp flowing through the fuse resistor, it will drop 50V, and hopefully this reduction of rail voltage to near zero will safely permit the transistor, if it is good that is, to pass this current directly to earth without risking secondary breakdown, which usually, within a few microseconds, results in death.

This protects the output devices (the drivers almost never go!) while giving visible and malodorous indication of traumatic circuit conditions.

Cheers,

Hugh

rabbitz · « **Reply #3 on:** 17 May 2005, 04:14 am »

Thanks for the answers guys as takes a bit for this stuff to sink in and now know the benefit of this little resistor..... cheap insurance and an indication of something not right.

Cheers
Peter

lazyfly · « **Reply #4 on:** 19 May 2005, 01:49 am »

<subtle poke>
Peter makes beautiful speakers

</subtle poke>

:p

rabbitz · « **Reply #5 on:** 19 May 2005, 01:23 pm »

..... but I cook amps, blow up transformers and fry caps.

Me thinks I'll go back to speakers

Rom · « **Reply #6 on:** 19 May 2005, 03:51 pm »

Hi rabbitz,

Don't give up, it's part of the learning curve.

I would like to see your creations(speakers), can you give us a link?

cheers
Rom

lazyfly · « **Reply #7 on:** 20 May 2005, 10:10 am »

Hi Rom,

I can help with a couple of pics of Rabbitz speakers. No copyright issues here, they're mine! Peter built the boxes and couriered them up to me, all I did was screw in the drivers, xo and terminals

Peter built the cabinets out of sympathy... I think patience may have played a small role, too

PMt's Designed & Built by Peter Rabbitz :p photo's don't do them justice.

Cabinets on display.
http://home.iprimus.com.au/bearware/speakers/pmt/cabinets_stock.jpg

Cabinets inside and finished.
http://home.iprimus.com.au/bearware/speakers/pmt/cabinets_pop1.jpg

Natural light.
http://home.iprimus.com.au/bearware/speakers/pmt/cabinets_pop2.jpg

Cheers.

rabbitz · « **Reply #8 on:** 20 May 2005, 01:49 pm »

Hi Rom

Thanks for the encouragement. I'm a resonably experienced builder of power amps, up to 7 plus variations, and other electronics even though I'm not up on too much of the theory. What I don't know, I'll research and find answers. Have done one amp from scratch in association with lazyfly via emails... design and even Dremelled the circuit board.

What I had was a real run of bad luck, problems and disasters (my own stupid fault) such as:
An AKSA channel going dead after installing bleeding resistors

Killing a Mosfet in another amp within 24 hours of that

Wiring secondaries on a small EI transformer into the mains

Mind wandering when installing some caps.... sizzle

Trying to get a well known and respected amp to sound good

An omen to get back to speakers my boy as I have a good understanding of them.

Rom, here's some examples of my speakers and I can brag about them for days

2.5 way floorstander (0.5 woofer is on the back for Baffle Step Compensation).... SEAS 27TFFC + Peerless 850122 CSX.
http://img.photobucket.com/albums/v386/rabitzz/Speakers/PS8COMP.jpg

Centre speaker using a vertical TM arrangement instead of the usual sideways MTM..... Vifa D26NC + Peerless 850528 HDS.
http://img.photobucket.com/albums/v386/rabitzz/Speakers/Centre.jpg

Rear speaker with side firing woofers and front firing tweeter..... Peerless 811827 + Vifa TC11SG69.
http://img.photobucket.com/albums/v386/rabitzz/Speakers/RearPTw.jpg

A side firing sub..... Peerless 830452 XLS + Parts Express 300-793 sub amp.
http://img.photobucket.com/albums/v386/rabitzz/Speakers/xls10sub.jpg

Lazyfly has posted photos of the bookshelf which was designed to better the B&W DM303, Yamaha NS10M which were the benchmarks.

As the song goes, these are a few of my favourite things

AKSA · « **Reply #9 on:** 21 May 2005, 01:41 am »

Peter,

Those speakers are up there with $5K commercial products for fit, finish and presentation. Congratulations; there's some impressive skill there!

Cheers,

Hugh

Rom · « **Reply #10 on:** 21 May 2005, 12:55 pm »

Hi Rabbitz,

I will second Hugh's opinion they are quite impressive. The finishing seems top rate. These must sound very, very good.

thanks for the link.

When things get wrong I normally stop and have a good rest or sleep over it and you'll really not to push it as normally it will create another sizzle, but in the end lessons learned and things can only get better.

I think I have blown quite a few Power transistor as well but I learn from it not to do it again, resistor and even a PCB got toasted.
I even toasted some voice coils all not intentional of course and regret most of them as most of them are avoidable.

Cheers,
Rom

rabbitz · « **Reply #11 on:** 21 May 2005, 02:01 pm »

Thanks Hugh and Rom for the kind words. The floorstanders are really special and the result of 6 months of building, using, testing, modifications and tweaking. The only parts from the original design is 1 woofer and 1 cap plus the cabinet had several cut and shuts. Had to build something to be on the same level as the AKSA on sonics and compliment it..... very yummy.

These disasters are all a part of the DIY game... it's always the simple basic stuff I get caught on as you tend not to think about them and they leap out and bite you. The more complex it is, the less the problems as the mind is in gear.

BTW, I owe Hugh an apology. From the time I first started corresponding with Hugh to make a purchase and up to a week ago, I've called the amp and ASKA and not an AKSA.... doh. It's the you know when you start off doing something wrong syndrome and it sticks. I feel like an idiot but at least it sounds great what ever it's called