Speaker overload (tweeter) protection using bulbs (repost)

Eeyore · Nov 14, 2008

This is a well established technique for preventing voice coils burning
out under conditions of 'overdrive'.

There is a stage monitor I'm having problems with that uses this method.

The HF sounds very distorted and almost cuts in and out.

I looked closely inside and found some damaged push-on terminals. Ah, I
thought, probably a poor contact causing the probelm, replaced them,
checked driver DC resistances etc, reassembled thinking I'd probably
fixed it.

But no, the low level HF and distortion continued.

I'd checked the DC resistance of the protection bulb but later it
occurred to me that it might have 'very nearly' burnt out and have a
weak spot that wouldn't show up on a DVM but passing signal would heat
it and cause this trouble. I'll be able to find out soon enough but I
wondered if anyone else had ever encountered this ?

In the meantime I brought the HF driver home to check it's not voice
coil rub.

Graham

jakdedert · Nov 15, 2008

Eeyore said:
This is a well established technique for preventing voice coils burning
out under conditions of 'overdrive'.

There is a stage monitor I'm having problems with that uses this method.

The HF sounds very distorted and almost cuts in and out.

I looked closely inside and found some damaged push-on terminals. Ah, I
thought, probably a poor contact causing the probelm, replaced them,
checked driver DC resistances etc, reassembled thinking I'd probably
fixed it.

But no, the low level HF and distortion continued.

I'd checked the DC resistance of the protection bulb but later it
occurred to me that it might have 'very nearly' burnt out and have a
weak spot that wouldn't show up on a DVM but passing signal would heat
it and cause this trouble. I'll be able to find out soon enough but I
wondered if anyone else had ever encountered this ?

In the meantime I brought the HF driver home to check it's not voice
coil rub.

Graham

I imagine that there are variations in lamps, given that they are
designed to provide 'light', not speaker protection. I suspect you'll
find an issue with the driver, though. Swap them around to check and/or
do a sweep test.

I just picked up a set of Bose (I know...they were cheap) 201s at a
thrift store. The only thing wrong with them--besides being Bose--were
the blown out protection lamps in series with the tweeters. Not bad for
$7.99, plus the cost of two lamps...altogether less than $10. They'll
make decent computer speakers for my 12 year old.

jak

jak

Eeyore · Nov 15, 2008

jakdedert said:
I imagine that there are variations in lamps, given that they are
designed to provide 'light', not speaker protection. I suspect you'll
find an issue with the driver, though. Swap them around to check and/or
do a sweep test.

A 'swap', or rather replacement, was done earlier with no good effect. Hence I
laid the suspicion on the damaged push-ons. I will still test the drivers
though. I have 2 drivers that measure ok plus a third blown one and a
replacement diaphragm.

I just picked up a set of Bose (I know...they were cheap) 201s at a
thrift store. The only thing wrong with them--besides being Bose--were
the blown out protection lamps in series with the tweeters. Not bad for
$7.99, plus the cost of two lamps...altogether less than $10. They'll
make decent computer speakers for my 12 year old.

Nice steal. For undemanding work they aren't *that* bad.

Graham

Eeyore · Nov 15, 2008

Soundhaspriority said:
I think it's unlikely, because any fracture causes a local hotspot, which
rapidly leads to failure. I'd look at the socket instead!

I will double check that. Thanks for the tip.

We've probably
all seen strange tin-lead phenomena on lightbulb button contacts. The
constant thermal cycling, the pressure, the dissimilar metals...

Interesting point.

Graham

Eeyore · Nov 15, 2008

Ron said:
Some Machs and RCFs use a 24 volt projector lamp.

24V truck lamp here.

I`ve seen these become
intermittent - they check fine with a meter, but introduce terrible
distortion and crackling under pressure - it sounds just like the driver
diaphragm breaking up.

That's EXACTLY what I'm getting. Thanks for the effective conformation.

They also easily break if the cabinet is roughly handled.

Well it has been but the bulb's nicely cushioned.

Graham

Eeyore · Nov 15, 2008

George's Pro Sound Company said:
here is a novel thought
DON"T OVERDRIVE THEM
simple and effective

it's what I do
the easiest way to overdrive a speaker is to use a amp larger than the
speakers rateing

The amps aren't but a blast of HF feedback will do it since it will whistle
through the HF filter. I am as it happens going to fit an 80W P-Audio driver
in the damn second JBL JRX112M that's blown its puny HF driver for the second
time, despite improving JBL's original ineffective bulb protection which was
100% ineffective. Jesus that design is CRAP. 1st order crossover !

Bear in mind these are the 15 year old monitors made of MDF and have been very
little trouble generally over the years.

Graham

Eeyore · Nov 16, 2008

Meat said:
"George's Pro Sound Company"wrote:

Who, me?

From what I've heard they're well considered and in the IAG group now.
http://www.internationalaudiogroup.com/

The Chang brothers don't believe in messing about AIUI.
http://www.internationalaudiogroup.com/executive_summary.php

Graham

Eeyore · Nov 16, 2008

Dave Plowman (News) said:
Easily said - but not so easy to do when setting up this sort of
equipment. Let alone when that equipment is being used by all sorts.

And it is. Another new house engineer recently for example.

Strangely that's not always so.

Well this is where it gets slightly complicated.

The best solution would be a decent limiter on the amp input - but these
cost if it's not going to sound horrid when it operates. A bulb is a very
cheap solution to help protect the speakers.

But a simple level limiter still won't stop the amp delivering full power say
@ 8kHz. Tweeters aren't meant to see that. It would have to be a frequency
sensitive limiter matched to the crossover. Which has given me a product idea
of course.

Graham

William Sommerwerck · Nov 16, 2008

the easiest way to overdrive a speaker is to use a[n] amp

larger than the speaker[']s rateing [sic]

Actually, the easiest way to overdrive a speaker is to play it at a level
where it produces audible distortion. But as most listeners have no idea
what distortion sounds like...

William Sommerwerck · Nov 16, 2008

George's Pro Sound Company said:
the easiest way to overdrive a speaker is to use a[n] amp
larger than the speaker[']s rateing [sic]

Click to expand...

Actually, the easiest way to overdrive a speaker is to play it at a level
where it produces audible distortion. But as most listeners have no idea
what distortion sounds like...

Click to expand...

OTOH distortion does not destroy speakers, speakers are as happy to play
distortion as clean signal only when you exceed the heat dissipating ability
of the motor do you burn out a speaker

Although what you say is true in terms of physics (the speaker's excursion
limits do not have a necessary relationship with its heat-dissipating
capability), in practice, audible distortion is a warning that you should
turn down the volume. William Michael Watson Dayton-Wright told me horror
stories of how he could not design a speaker with sufficient power-handling
capability to keep "deaf" listeners from overdriving and damaging it.

clipping does not damage speakers and distortion does not damage
speakers, it[']s overheating and over excursion that damages speakers
distortion is one method of achieving overheating, but you can overheat
with a spotlessly clean signal as well

The distortion I was talking about was the sort that comes from pushing it
into its excursion limits.

William Sommerwerck · Nov 16, 2008

I grant you all your points as valid, and bottoming (or farting)

a woofer is truely a horid sound.

Agreed. I've never heard it called that, but it's an apt description.

What gets you up so early on a Sunday? (I'm on the west coast, and have been
up since 2AM.)

William Sommerwerck · Nov 16, 2008

What gets you up so early on a Sunday? (I'm on the west coast,

went out had a couple of dogfish 90 minutes and that set me down
early(8pm) some days your just not tired at 4 am

As in...

http://www.dogfish.com/brewings/Seasonal_Beers/Punkin_Ale/3/index.htm

Tony · Nov 16, 2008

I like to protect gear from whatever might come, so I like lamps and polyswiches. I would
be almost as happy with a frequency-selective limiter that could independently account for
the thermal and displacement limits of the tweeter, mid and woofer (with appropriately
different time constants - an even greater challenge), but the reality is that a lamp in
the tweeter circuit handles 80% of abuse, so it's cheap insurance.

But an interesting twist is that with the simple 2nd order HPFs I have been trying (with a
very high Fc, eg 8 kHz, to flatten CD horn response and match sensitivities), a lamp or
polyswitch in the INPUT to the crossover has the nice benefit of also seeing more MF, and
so potentially account for both thermal and displacement limitations. Admittedly the time
constants cannot be independent as would really be required, but it's still better than
having the lamp in the crossover output circuit, and a LOT safer for the amp, which
becomes UNLOADED when the protection operates, instead of shorted.

Tony

liquidator · Nov 16, 2008

Dave Plowman (News) said:
But no speaker is designed to
handle DC for long - which is what you can get from a grossly overloaded
amp. To be certain that DC couldn't wreck the speakers would require a
*much* smaller amp than would otherwise make sense. Or, of course, use an
amp which can't pass DC.

Jeez...a little knowledge is a dangerous thing.

Phil Allison · Nov 16, 2008