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Old 7th December 2010, 04:12 PM   #1
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Default Amp failure modes, preventing driver cooking

I've read some about failure modes... and I know that some amps when they fail - can cook drivers because they pass the full rail voltage. I know some amps have DC protection and some don't.

I know that heat/voltage are factors in transistor failure, but if an output is going to fail open or shorted --- is that far more likely to fail when it's being pushed hard? In other words, if it's not being pushed very much....... are the transistors that much safer?

How often does an output transistor fail just in routine service, if it's not being stressed? From what I've read, open vs shorted failures seem to either be 50/50 or more likely shorted. Seems to depend on who you ask.

I've seen some DC protection circuits... I'm just trying to determine if I can use a couple amps with some more expensive drivers, or if I'm better looking for some new amps that have DC protection that might keep the drivers........ safer.

I realize my question may be a "duh, transistors can fail at any time... they are more likely to fail when being pushed hard..." - just curious if that was in fact reality or not.
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Old 7th December 2010, 05:18 PM   #2
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Generally it's when they are pushed hard, as they are hotter. But if you short the output through a stray strand of speaker wire for example, then it can happen at low volume.

You can always build a DC protection add-on to go between your amp and speakers. Velleman do a kit, simple passive thing.
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Old 7th December 2010, 05:44 PM   #3
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There is the "idiot mode". Read about it here...

The MONGREL (supersym II)


In short , what will happen with the ultimate disaster (the slipping screwdriver) , it blew into molten balls !!
Of course fuses will blow , they better or you and the family gets burned. On that board , it was interesting what semi's got burnt. The 2 closest to the fuses (njw0281/0302) and the positive flyback diode (5A). On the new one i'm about to make the fuses are right in the middle , absolute current sharing.... I learned from catastrophic failure mode. The amp has been absolutely abused for 3 months after and I am confident it would either take a lightning strike , screwdriver , or glass of water to bring it down. The MJE15032/33 drivers never broke a sweat , I doubt whether anything , including a OP short , could ever affect them. The loudspeakers were hooked to the amp at this point , besides a "thump" , no damage.

In normal to abusive operation, the drivers/OP's stay cool but the heatsinks become slightly hot with 2 pair 8R hooked in parallel (3R). Thermal transfer is optimal (no aluminum angle bracket for op's)

The secret is just thermal/electrical derating of all the components. For 75V rails , all caps are 100V , VAS/ drivers and outputs are at least 250 Vceo. Thermally , oversize .05C/w HS's for drivers/outputs and at least a small 10 - 20 sq. CM HS for the VAS. Everything should be just warm to the touch , even on a 37 C day. Something such as this would most likely last 20 - 30 years - even 24/7.
OS
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Old 7th December 2010, 05:54 PM   #4
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Breaking a DC current fault is not as simple as opening a relay.
DC currents can strike an arc across the opening contacts and current continues to flow.
A fuse rated to break your worst case failure current may be a better solution.
A crow-bar to ensure the fuse blows quickly may also work better and may have a lower resistance than the arc and contacts in the relay, thus diverting the current flow from the relay and allowing the arc to extinguish..
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Old 7th December 2010, 06:31 PM   #5
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I'm familiar with the Velleman kit. It just gets expensive/cumbersome when trying to protect 8-10 outputs. I know that protecting from DC outputs is a heavily discussed topic in terms of most effective / least distortion.

I was most curious about how to prevent circumstances that would cook a driver.

I don't really want to risk cooking a $500 pair of woofers... so just trying to understand how to minimize risk.

I can't seem to grasp how often an amplifier cooks itself in regular usage, versus abusive usage (or straight up abuse like shorting outputs)
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Old 7th December 2010, 09:36 PM   #6
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Default protection

Music instrument resale shops are full of speakers with mismatched woofers, and amps with new shiny power transistors on the heat sink. Happens a lot in the trade.
My favorite amp Dynaco Stereo 120...can be beautiful is capacitor coupled to the speakers, which provides pretty absolute protection for your speakers. It has wimpy heatsinks, so after a 3.5 hour rehearsal with the amp filling a church, it went up in flames (resistors to output trans) The speakers were fine.
Direct coupled amps are much scarier if you invested heavily in speakers. Before I trusted my new PA amp, a CS800S to touch my $600 (used) speaker pair, I found out it had 1. diac triac crowbar to short the output and blow fuses if the output gets too close to the rail (shorted transistor) 2. thermistor controlled fan on the heat sink. 3. current transformer around output to measure the AC current flowing (oscillation can kill a speaker, too) 4. microprocessor to measure the current going out and compare it to some limit 5. speaker relays to pull the output from the transistors if 4 finds anything wrong. I maintain item 4 can't be done by ordinary mortals at home. The australian kit has a microprocessor, but doesn't do that and doesn't hook up to a current transformer.
Peavey also has a budget line of amps without the protection, for musicians that believe it can't happen to them.
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Old 7th December 2010, 09:48 PM   #7
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So.... if I care about my speakers, am I safer to not use my old-school Crown Microtechs - and "upgrade" to some newer amps? I was thinking about some QSC amps.

It just seems like a number of newer amps, like QSC RMX, have DC protection standard.
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Old 7th December 2010, 10:00 PM   #8
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Crown, Peavey, QSC all make some amps with serious protection, others without. You'll have to look at the schematic and decide if it is good enough. The CS800S has DC and AC protection. Rather then build my own split supply amp, I bought one with serious protection and problems, and fixed the problems. Turned out to be a $.10 problem, two toasted input resistors.
CS800S was designed in 98 and CS800X was before that, so older amps can have serious protection. Older 80's peavey stuff only had the diac-triac DC crowbars and fuses.
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Last edited by indianajo; 7th December 2010 at 10:10 PM.
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Old 7th December 2010, 11:53 PM   #9
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Default no protection

Well, I looked at the crown microtech manual available on eserviceinfo.com. That model apparently doesn't have any protection but an input fuse. I'm not the world's linear expert if someone else wants to take a look. I didn't see any current transformer or microprocessor chip or speaker relays, or diac-triac crowbar.
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Old 8th December 2010, 03:04 AM   #10
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Quote:
Originally Posted by indianajo View Post
Well, I looked at the crown microtech manual available on eserviceinfo.com. That model apparently doesn't have any protection but an input fuse. I'm not the world's linear expert if someone else wants to take a look. I didn't see any current transformer or microprocessor chip or speaker relays, or diac-triac crowbar.
I think if I understood the schematics too - I didn't see any real protection either.

So, other than use an add-on DC protect circuit - does anyone have any thoughts on how to best *prevent* a catastrophic failure that would cook drivers? Preemptively replace the output transistors, since it seems like those are the most failure prone - and most likely to result in rail voltage to the speakers? I guess that's $60 an amp ($2.50 x 12 per channel x 2 channels.... plus or minus)

I know ultimately newer amps simply have better protection. Now, I'm just curious about these older amps and keeping them alive..

I guess if it lacks output protection - what's to say that a wiring short wouldn't cause one of the output transistors to short...... and then result in the driver getting full rail DC.

Last edited by Creative1; 8th December 2010 at 03:08 AM.
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