-
-
-
- Thread: Protecting Your Loudspeakers
-
- From: Blake A. Engel (Chicago, IL)
Subject: Re: speaker fuses...
-
- Hey all!
-
- Say I wanted to protect a speaker from being blown out--say
I thought a simple
- fuse would be a nice device to use to do this. How can
I determine the value of
- the fuse (based on peaks, etc)? Will the fuse limit my
frequency response at all?
- Is this an outdated or unused idea? I remember the church
I grew up in--they
- would need to replace the speaker fuses every once in
a while for some reason.
- (I think it happened mainly when the amps were turned
on first, and then the
- board--big pop...)
-
- ------------------------------------------------------------------------------
-
- From: Ray A. Rayburn (Boulder, CO)
Subject: Re: speaker fuses...
-
- Blake -
-
- I would ask my speaker vendor for advice on fuses. In
general you want to use
- fast blow fuses large enough that they don't blow in
everyday use, but small
- enough to offer SOME protection. Just realize that it
will not be perfect
- protection! A fuse can also add some distortion to the
sound - particularly if
- it is close to blowing.
-
- More speakers are blown from the use of too small amplifiers
than too large.
- If your amplifier never clips, then it is unlikely that
you will damage speakers
- in normal use. This assumes you are using speakers reasonably
close to powerful
- enough for your application. In systems I design I always
include a limiter
- before the power amp that is set to prevent clipping.
Where budget allows I
- always use an amplifier at least rated to deliver the
manufacturers rated power
- into a speaker - even when I know that much power will
never be needed.
-
- For example I have just bought 3 Crown K2 amplifiers
(2500 watts each into the
- 2 ohm loads they will be driving) for the new speaker
system in our 400 seat
- church. Our levels are not extreme (91 dB SPL "A"
weighted slow) but we are
- looking for very clean clear sound.
-
- ------------------------------------------------------------------------------
-
- From: Blake A. Engel (Chicago, IL)
Subject: Re: speaker fuses...
-
- Ray,
-
- Thanks for the information, I ask as I am dealing with
a church whose point-source
- speaker went out a year ago--they think it was because
a teen (or someone else)
- got in and wanted to see how loud they could get the
system... Just thought a
- fuse would be a nice cheap trick!
-
- >>"More speakers are blown from the use of
too small amplifiers than too large."
-
- I totally agree--the bigger the amp, the better--as long
as the person operating
- it knows the limits of the speaker(s)! ;-)
-
- ------------------------------------------------------------------------------
-
- From: Dave Hooker (St. Thomas, Ontario, Canada)
Subject: Re: speaker fuses...
-
- Blake et al:
-
- Joe deBuglio of JdB Acoustics in Toronto suggested to
me several months ago that
- quartz-halogen bulb and (assemblies) are better than
fuses for speaker protection.
- Excess power is simply consumed by the bulb.
-
- Apparently he has installed these units in several systems
with success. In one
- system, he installed the bulb assembly within a speaker
cabinet, and the speaker
- was reported to be "on fire" during a heavy
musical--turns out it was the light!!
- Contact Joe at jdbsound@echo-on.net. I would be interested
in any comments.
-
- I thoroughly enjoy reading the churchsoundcheck listserve!!
What a great ministry!
-
- ------------------------------------------------------------------------------
-
- From: Ray A. Rayburn (Boulder, CO)
Subject: Re: speaker fuses...
-
- Dave -
-
- Light bulbs have been used for many years as speaker
protective elements. One
- advantage is that they usually do not blow out. One disadvantage
is that they
- can add significant distortion.
-
- ------------------------------------------------------------------------------
-
- From: Andrew Smithies (Wellington, New Zealand)
Subject: Re: speaker fuses...
-
- mmm.......
- It seems that all is forgotten regarding the ideal amplifier
output impedances
- when we consider light bulbs for speaker protection.
First some comments on the
- idea amplifier for driving the ideal speaker.
-
- The ideal speaker is a device that accurately converts
supplied voltage into
- speaker cone displacement which translates directly in
to sound pressure waves.
- The departure from the ideal is that the speaker cone
and coil assemblies have
- mass and are mounted on an elastic mount. It is therefore
a resonant system.
- Speaker manufacturers attempt to remove this resonance
by damping the cones
- movement and therefore making the speaker less efficient.
(There are some grand
- old speakers such as those made by Langsen that are incredibly
efficient. The
- shame is that they have only average amounts of frequency
bandwidth.) Another
- way of helping the speaker problem is to rely on the
EMF generated in the speaker
- voice coil as the cone moves in the magnetic field. By
making the speaker a low
- impedance device. i.e. a few ohms. And then by using
an even lower impedance
- driver such as an idea amplifier, the amplifier has some
control of the velocity
- of the speaker cone. A quality amplifier will have an
output impedance of a few
- milliohms. (plus speaker leads) Ideally this would be
zero.
-
- Now to the problem of speaker protection. I've described
the speaker above. If
- lots of volts are pumped into a speaker (e.g. an amplifier
when an output device
- goes short circuit) there WILL be lots of current IMMEDIATELY.
The first
- 0.1 milliseconds will go directly to the tweeter through
your faithful crossover
- unit. If your unlucky the voice coil on the tweeter will
either evaporate or
- depart from is mounting! Within a millisecond the amps
of current will have been
- removed from the remains of the tweeter and will be applied
to the mid and then
- bass speakers. After several milliseconds of this the
fast blow cartridge type
- fuse will have melted the wire. But wait! there's more!
As these fuses rely on
- an air gap there will be a spark maintained across the
first bit of the fuse
- wire that melts away. Since the amp has failed with DC
on the output. The
- amplifier output voltage maintains the arc. Depending
on the crossover design
- when the fuse finally blows the input to the crossover
network will go to the
- opposite polarity of the DC delivered by the amp. All
energy stored in the
- crossover's inductors will be delivered to the tweeter
and mid range speakers.
- This ought to finish them off!
-
- This is very sad. This will be more spectacular the bigger
the amp is relative
- to the speaker size. Bear in mind though that a bigger
amp is less likely to
- overload and therefore die. But don't be to sure of yourself
if your system has
- a very big amp compared to the speakers. Overload is
not the only mechanism that
- can make an amp go ballistic. It is however, surprising
how much a speaker can
- survive.
-
- OK then. So a fuse is no good. What about the light bulb
you ask? A light bulb
- is a simple piece of resistive wire with a positive temperature
coefficient. It
- goes up in resistance with an increase in temperature.
A filament operates at
- white hot temperatures. 3000 odd degrees Celsius. As
a speaker protector during
- normal operation it remains cold and has a low impedance.
During an overload there
- is lots of current. The filament gets hot its resistance
goes up (by a factor of 10)
- and gracefully reduces the power to the speakers preventing
any nasty back EMFs.
-
- It is not (a good) idea though for two reasons.
-
- 1/ The filament has thermal mass. It will do absolutely
nothing to stop the first
- 0.1 milliseconds of power sent to the tweeter.
-
- 2/ The bulb has some resistance even when cold. The reduces
the ability of the
- amplifier to control the speaker cone velocity. (This
is referred to the amplifiers
- 'damping factor'. Many speakers cope ok with a low damping
factor but I would like
- to hear comments on what difference can be heard on Bose
and other speaker systems
- that use precorrection on the amplifier input. (PS Valve
amps have a low damping
- factor anyway because they use small amounts on negative
feedback. One main reason
- for this is that valve amps are hard to apply negative
feedback to due to limitations
- in the transformers required in a valve amp)
-
- The only real way to protect a speaker system is to monitor
the amount of energy
- applied to each driver in the speaker system and ensure
this is not exceeded.
- (A speaker system will have a tweeter that can handle
only a fraction of the
- system rating) In order to protect the tweeter the protector
must remove the applied
- signal very fast. Most big relays take 10 ms or so to
operate. This is far to slow
- for a tweeter. Some relays are fast but I challenge anyone
to find one big enough
- to operate in less than a millisecond. It is ok for a
bass speaker though. This
- leaves only electronic devices to do the current breaking.
Back to back mosfets or
- bipolar transistors would do but they too could die when
needed most, have some
- internal resistance and are nonlinear. A complete protector
would also take care
- of any energy stored in the cross over network.
-
- My thoughts for protection is that you cannot have everything.
Expect to pay
- either in dollars, sound quality or potential for smoke
(more $).
-
- To make something fully bullet proof an individual power
sensor / trip and
- isolator circuit for each driver that is powered separately
from the rest of
- the PA is about the only solution I can see working.
This would be quite complex.
- In all situations I suggest that a selected, trained
group of people do all the
- operating. This requires much dedication. Also choose
a system with plenty of
- headroom. Especially in the speaker department. This
way the voice coils will
- have more thermal mass and strength than the fuses.
-
- Good luck.
-
- ------------------------------------------------------------------------------
-
- From: Ray A. Rayburn (Boulder, CO)
Subject: Re: speaker fuses...
-
- Andrew -
-
- Thanks for the nice overview on speakers and their protection.
-
- <<"Langsen">>
-
- I am unfamiliar with them. Where are (were) they made
and when? "If lots of volts
- are pumped into a speaker (e.g. an amplifier when an
output device goes short
- circuit) there WILL be lots of current IMMEDIATELY."
You speak of the worst case
- condition of catastrophic failure of the amplifier output
devices. I would submit
- that the only practical protection for this type of failure
is to buy only
- amplifiers from manufacturers with a track record for
producing amplifiers with
- very low failure rates. In the US many consultants will
only specify Crown, QSC,
- and Crest, for this reason.
-
- For the more common case of overload and clipping, the
best protection is in the
- form of purpose designed speaker controllers which sense
the output voltage of
- the amplifier, know exactly what the speaker will take,
and limit the input to
- keep you safe.
-
- With a trained operator, a hard limiter set to prevent
clipping of the amplifiers
- is often all that is required. With speakers with enough
power rating, and bi or
- tri amp'd, such limiters can usually make the system
almost idiot resistant.
-
- <<"Many speakers cope ok with a low damping
factor but I would like to hear
- comments on what difference can be heard on Bose and
other speaker systems that
- use precorrection on the amplifier input.">>
-
- If you consider that "precorrection" is often
just EQ, and almost all quality
- systems are equalized, I don't think there is any correlation
between quality
- of sound under low damping factor conditions and "precorrection".
Almost all
- speakers sound better if the damping factor as delivered
to the speaker is at
- least 50. I can't say if higher damping factors AT THE
SPEAKER help. In practice
- I use an amplifier with a reasonably high damping factor,
keep the distance from
- amplifiers to speakers as short as possible, and use
heavy wire.
-
- <<"PS Valve amps have a low damping factor
anyway because they use small amounts
- on negative feedback. One main reason for this is that
valve amps are hard to
- apply negative feedback to due to limitations in the
transformers required in a
- valve amp.">>
-
- <<Another factor is the limited amount of open
loop gain available in most tube
- amplifier circuits. Without a high open loop gain there
is not much to use as
- feedback.>>
-
- None-the-less, some tube amplifiers had fairly high damping
factors. I am
- thinking in particular of the McIntosh tube amplifiers.
They also had very
- extended bandwidths compared to other amplifiers of the
era (and of today).
- They were great sounding amps!
-
- ------------------------------------------------------------------------------
-
- From: Bill Lockhart (Tampa, FL)
Subject: Re: speaker fuses...
-
- I use regular, single filament tail-light bulbs to protect
the HF driver and
- it seems to work real well.
-
- ------------------------------------------------------------------------------
-
- From: Arthur Skudra (Olympia, WA)
Subject: Re: speaker fuses/protection
-
- It seems to me that there are a variety of methods (fuses,
light bulbs, peak
- limiters, common sense) used to protect speakers, each
with its own advantages
- and disadvantages. If fuses or light bulbs are the choice,
then it should be
- obvious that their types & values be carefully selected
to match the type of
- transducer being protected--one size does not fit all
here. Peak limiting is a