DC Offset Question
In the past 2 weeks, I've fried (3) drivers. Each time, there was a distinct burning smell and no indication that the speaker was overextending. Seeing as how I haven't blown a speaker in 15 years, I'm assuming something is wrong with my amp. I'm using an Alembic F1X pre into a QSC CX power amp.
Last night, I checked the output of the QSC with a multimeter- about 1mv DC. However, the pre is putting out 159mv DC. This seems high to me, but I'm not very electronics savvy. Could this be what's frying my drivers? I didn't check the output from the power amp with the pre driving the power amp. I assume the DC from the pre will pass through and be amplified by the power amp. Is this correct, or is there a some sort of filter in the power amp to prevent DC from passing from upstream? Should I be doing other diagnostics?
I apologize if this is in the wrong forum. Thanks for your help!
What's your cabinet?
The first cabinet had an EV 15" driver. I can't remember if it was a 15L or not. The other cabinet was an Schroeder 1210L. I believe I've read that the early versions had Eminence Neos and later one Celestion Neo.
There are many ways you can kill a speaker. Over-extending and jumping the voice coil out of the g ap is one of them. Thermal failure is another. You can make a speaker fail with long term power without over-extending it. I don't know the output rating for your amp, but you have to be careful about driving a single EV too hard.
Are your voltage measurements taken power-on / idle, or at idle after some serious use? DC offsets will drift. In general:
0.001 VDC coming out of your amp is nothing to worry about.
0.159 VDC will bother some people and not others.
0.020 VDC is my personal threshold for safety.
Here is a thread on DIY audio that covers this topic. There are lots of opinions on what is "safe" DC offset.
I am not familiar with your QSC product. I don't know it's power output in relation to your speaker power ratings, and bear in mind that the speaker power ratings are never really what the numbers say they are. So you have to be careful. It's entirely possible to do thermal damage to your voice coils with long term power without over-extending them.
Any modern design power amp that is of any reasonable design quality is going to have a protection circuit that monitors for DC output and sends the amp into protection mode if it senses DC output. Again, I am not familier with your QSC product, but if it's a decent design then it should have DC output monitoring/protection so that it doesn't waste your speakers if a DC offset problem should develop. If the protection circuit is absent in an amp then you have no protection for your speakers should it develop a DC offset failure mode. If the protection circuit is present, but isn't working properly, then it's like you don't have a protection circuit at all.
I've seen a number of older amps with absent or failed protection circuits that ended up being speaker killers. The amps developed DC offset and the speakers went silent. The poor end user thought there was a cabinet problem and hooked up another cab for testing, which was immediately smoked by the malfunctioning amp. The only way that the user realized there was a problem was when they tried to hook up an alternate cabinet after one cabinet failed, and the amp immediately smoked every cabinet that was plugged into it. That's an expensive way to find out that you've got a malfunctioning head. It's good that you're taking DC measurements and looking at the problem from that perspective.
I guess DC offset could be a problem, but I'm wondering if you just killed the cabs with too much power. You haven't told us how loud you're playing, so it's hard to know.
Exactly what power amp do you have and what is it's power rating??
So, to clarify: if my pre is putting out DC, the power amp should sense that and engage the protection circuit? I'm wondering if the 159mv is below the QSC's threshold?
The (2) times I burned drivers were on gigs without PA support for the bass, so it very well could have been thermal failure even without the DC offset. Plus, they were smaller cabs than I sometimes use.
I believe the amp is a QSC CX302. I run it bridged, so 700w into 8 ohms (for the Schroeder). I can't remember the impedance for the EV offhand.
There should be DC blocking at many points in any amp. While you may have signal degradation because of the of the DC offset from your preamp it should (DC) be ignored (blocked) in the power amp. Did you check for offset from the amp with NO speakers (load) attached to the power amp (as it should be done with SS amps)?
Usually a little bit of DC offset will not harm a speaker since they are made to handle quite a bit more voltage. For example, if an 8Ω speaker can handle 400W it will get 50V at the power. Giving it a DC voltage of 0.16V is not likely to harm it.
As far as acceptable? Depends on amp topology to some extent... but if you're under 50mV, I wouldn't worry a bit about it.
If you have blown 3 drivers in the *same* cabinet, I would be very suspicious of the components in the crossover of your cab.
The CX amp has a high-pass filter at around 4 or 5 Hz and therefore won't pass DC.
We just lost an 18" Black Widow right after repairs were done to a Peavey head.
The timing was suspicious at best, as the first time out with the repaired head completely smoked the 18".
I was wondering if something got done wrong, left out, or ?? that allowed high voltage DC to get into the driver.
My thoughts were a cap that failed as Shorted, rather than Open.
Is the head working or not?
It went back to the tech.
We didn't dare try it with another cab, for fear of damage.
Once the tech has it, whatever he claims then becomes The Truth.
Since he is on the hook, I suspect the 18" BW will be the cause of the failure...:spit:
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