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Preventing DC spikes to monitors
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ben_allison
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24th December 2012
Old 24th December 2012
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Preventing DC spikes to monitors

So obviously, we power everything on, and monitors last, and the reverse when shutting down. However, when brownouts happen, or an interface's FW/USB connection gets accidentally yanked, nasty pops can make it to the monitors... I've blown a tweeter this way.

Is there any way to prevent these aberrant spikes from reaching (and damaging) monitors?
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ben_allison
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24th December 2012
Old 24th December 2012
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Maybe some kind of inline filter?
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28th December 2012
Old 28th December 2012
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Bumpin dis.
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28th December 2012
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Speaker protection is always a difficult subject. Fast-blow fuses that have a low enough rating to protect a driver from a short-duration transient will blow at average levels that are too low for most applications. Most powered monitors have internal amplifiers that limit maximum power to safe levels, but a fast transient "pop" or "click" may still overdrive a delicate tweeter. Some active monitors have circuitry that monitors average levels and will reduce gain if the average level exceeds a calculated thermal load for the drivers, but that won't protect against high-level transients.

With passive monitors driven from external amplifiers there is usually no protection and often very high power amplifiers are used with speakers rated for much lower levels. That's fine at normal listening levels and prevents speaker-damaging clipping, but also is an invitation to transient "pop" damage.

Some speaker manufacturers have incorporated PPTC devices (commonly known as a resettable fuse, polyfuse or polyswitch) for speaker protection because they're faster acting than plain fuses, but those can affect the sound and can result in dynamic compression. One solution that does provide an absolute limit of peak power (on passive monitors) is to use a pair of back-to-back Zener diodes in parallel with the speaker to clamp the maximum voltage that can be applied across the speaker. If the Zener voltage is chosen correctly (a value that just exceeds the peak voltage of a typical audio waveform at maximum listening levels) some protection is provided to the tweeter, but still there is no guarantee that the maximum power capacity of the tweeter might be exceeded with a long-duration "pop".

Certainly, an active monitor circuit could be designed that could analyze a waveform and reduce amplifier gain if certain limits were exceeded, as are used on very high power commercial sound reinforcement systems, but such systems are probably too expensive to be practical for studio use.

Personally, I use an active crossover, multi-amplifier driven passive monitor system with expensive ribbon tweeters. I have separate fast-blow fuses installed between all the amplifiers and the drivers except the subs, and although I'll occasionally pop a fuse, I haven't blown the tweeters yet.
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ben_allison
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28th December 2012
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Wow what a great post! Thanks for Ning so thorough.

I'll be running a power amp and passives, so your solution sounds like a good one. I suppose I could also add the diodes for even more protection.

I just like knowing a $3000 investment isn't one small gaff away from complete destruction! lol
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28th December 2012
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Quote:
Originally Posted by ben_allison View Post
Wow what a great post! Thanks for Ning so thorough.

I'll be running a power amp and passives, so your solution sounds like a good one. I suppose I could also add the diodes for even more protection.

I just like knowing a $3000 investment isn't one small gaff away from complete destruction! lol
If you do use back-to back Zeners be sure to use high enough power ratings: a pair of 10 watt stud mounting pieces are usually enough and use a fast blow fuse before the diodes.

Power amp--> crossover-->fuse in series-->back to back Zeners in parallel with speaker driver. If you're willing to dig in to the speakers a little, connect the fuse/Zeners after the HF crossover. That allows use of a much smaller fuse than you would have use for the whole system. Average currents at mid and low frequencies are often 3X to 5X more than what the tweeter needs.

If you can stick a scope on a speaker you can get an idea of the peak to peak voltages you're seeing at your maximum normal listening levels. Obviously, you want to choose Zeners that are 5 or 10% higher voltage than any normal peaks.
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