Crest power amp

[tonezoneonline]

I'm having a rough week here.Had a guy bring in a large Crest power amp the other day and said it was blowing speakers.18" JBL's and expensive.
He swore he was running it easy,tried different cables and checked the cables.
If it wasn't distortion I figured it was DC.Checked the amp out and found nothing wrong.He just called and said he blew 2 more speakers.Here's the trip.It was on the third gig with the amp.The first 2 gigs were fine.Eight hours is a bit long for even a heat related problem to surface.
Any ideas where to go next.
TIA

[Steve Conner]

How about hooking it up to a dummy load and signal generator in your shop and drivng the ass off it for eight hours to see what happens? It could be a problem getting a dummy load if it's like a 2,000 watt amp though.

[Rick Erickson]

Crest Amp

[QUOTE=tonezoneonline;3801][ large Crest power amp ]

A model # would be helpful here. The Molex interconnects sometimes lose connection. Some models tend to overheat the pc board around large ceramic resistors. This kind of problem could be especially aggravated after an extended period of operation. Usually there are DC sense circuits and output relays to guard against DC on the output line. A look at the damaged speakers could help you diagnose the problem. The voice coils can often give hints as to the type of damage incurred, as well as the cones themselves.

RE

[tonezoneonline]

Crest 8001.It just came in with the speakers.I didn't see any sings of excess heat on the board around the power resistors the first time.One speaker is fine the other has no cone problems .It's the voice coil.I'll pull the speaker and amp
apart again in the morning and maybe have some more info.
The sound engineeer did say the speaker cable was veryhot and arced when he disconnected it from the speaker.I didn't know this before.
Thank for the reply.

[Enzo]

Arced when he disconnected it? He pulled it out while the amp was running?

WHat does "blew the speakers" mean? Have you examined them? Or are you just going on his say so?

Cut open the bad speaker and examine the VC. If it is discolored or warped, then it took too much energy for a period of time. If it looks like new and is just open, then more likely some impulse hit it.

Blew two more speakers? What as the output of the amp and what is the rating of the 18s? The two channels of the amp are separate. Is it bridged? Or are the two used separately? I cannot imagine something wrong with both channels blowing speakers at the same time. I CAN imagine someone cooking his speakers with the 16Hz band of his graphic set to +12db.

Really doesn't sound like the amp.

CAbles don't blow speakers.

WHat crossover arrangements are in use?

[tonezoneonline]

I posted before the equipment was here.It just blew me away what the guy was telling me.I've pulled the speaker apart and got some more info on the amp which isn't easy.They have absolutely nothing as far as specs on the back of these amps.He is not running it bridged.Of course it arced when he dissconected it because the amp was still on.He did blow one speaker.The voice coil shows no sign of overheating.It was blown apart at the top of the magnet.It did show some signs of rub and I did have a hell of a time getting it out so it may have been a bit distorted.The Crest 8001 is rated at 1400W at 2 ohms and he is running 2 of these JBL's off each side rated for 600W at 4 ohms.He said he is not running the amp hard (3/4 on the knob) and no +db on the EQ.
He couldn't tell me anything about the crossovers and he brought the speakers in loose.
Speaker overload.

[Enzo]

Not running the amp hard because it is only set at 3/4 is like telling the policeman you couldn't be speeding because you were only in third gear. The knob doesn't set the power of the amp, it sets the sensitivity of the amp.

If I turn the power amp down, I can turn the mixer up and still be just as loud. And the fact that the EQ wasn't peaking still doesn't tell us how much power the amp was producing.

When an amp goes to DC you generally hear it - it makes the loud 120Hz noise, well unless it is a switcher.

If the VC is distorted, it was melting even if it was not discolored yet. Or possible the pole piece in the center of the magnet has shifted. Is teh gap even all the way around?

[Amp Kat]

If that speaker is toasted it should read OL on your ohm meter too. What is the speaker jack reading from tip to sleeve in ohms ? When it's powered up what does it read in voltage on the speaker out from tip to sleeve ?

[tonezoneonline]

Amp Kat the speaker is blown.I have taken the actual speaker apart and the voice coil is gone as stated in the previous post.

Enzo thanks for all the help and I understand what you are saying.I'm just relaying what the customer has told me.Trying to sort out why these speakers are blowing since I can find nothing wrong with the amp.
What do think is the bottom line on this?

[Andy]

it sounds to me like he's blowing speakers because he's got 1200W of speaker power capacity, and he's driving it with 1400W. with this configuration it will eventually fail. the fact that it blew the speakers on the third gig makes some sense. its not a big power difference, but enough to eventually fry the speakers.

[Steve Conner]

In all the time I've messed with sound equipment, I've never got a spark from a speaker connection. I think there would only be arcing if you pulled the cable while the speaker was playing at high volume, which is kind of a stupid thing to do since 1/4" jacks might short briefly as you pull them out. So I've never done it, nor seen anyone else do it.

So, if the engineer saw an arc even though the speakers weren't making any sound, that says DC. Otherwise I'd tend to agree with other posters, he is just driving the speakers harder than they can take and burning out the voice coils. A 1400W amp will put out a good deal more than 1400W if you drive it hard into clipping, and a speaker cabinet with 1200W RMS on its nameplate probably can't take anything like 1200W RMS continuously. (Even if it is a JBL.) It kind of seems plausible that the sort of doofus who would pull speaker leads out with 1000 watts blaring through them also wouldn't know how to drive bass bins properly.

[tonezoneonline]

That's my consensus as well.Especially after he told me how hot the speaker cable were.He's putting out more watts than he thinks he is. Now I just have to explain it to the duffus.
The numbers don't lie.
Thanks guys.It's been an interesting thread.

[Steve Conner]

Why not just rewire it for 240V line voltage and not tell him

[Amp Kat]

[QUOTE=tonezoneonline;3838]Amp Kat the speaker is blown.I have taken the actual speaker apart and the voice coil is gone as stated in the previous post.

Yeah I realized that but if the speaker was shorted it would read a resistance. If it was blown by a large amount of current it would read open. Seems the 1200 watt speakers would start distorting way before 1400 watts so it would have so much thd + N it would be hardly decernible at 1200 watts. I do agree that the amp is most likely outputing more than it's RMS rating and pulling them out at that wattage can't be good either but I'm not convinced there isn't something wrong with the amp either.

[Rick Erickson]

The sound engineeer did say the speaker cable was veryhot and arced when he disconnected it from the speaker.

CREST 8001 Specs:
750 watts per channel at 8 ohms
1225 watts per channel at 4 ohms
1400 watts per channel at 2 ohms
2450 watts bridged at 8 ohms
2800 watts bridged at 4 ohms
Don't even THINK about bridging it at 2 ohms!!!

What Gage & length of speaker cable is he running? At 4 ohms this amp will deliver 17.5A of current. At 2 Ohms it will deliver 26.5A of current, per Channel! It takes some hefty cables and connectors to handle this kind of power safely. !/4" Phone plugs are not up to the task. I would let this amp idle with no load for several days and monitor the outputs for DC & HF oscillation. The protect lights should come on if there's a problem but you can't always trust them. These amps are rather complex to service. There are surface mount IC's under the XLR input jacks, Add-On boards hanging off the power modules, my favorite feature is the fencing staples used to jumper the Octal connectors.

RE