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Topic: Speaker in-line fuse ? |
Doug Jones
From:
Oregon & Florida
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Posted 15 Jan 2005 7:03 pm |
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| Is there any need or benefit to having a fuse in line from the amp to the speaker? If so, should it be on the hot or ground wire and what amp fuse for a 225 Watt amp? Thanks. |
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Bob Metzger
From:
Waltham (Boston), MA, USA
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Posted 15 Jan 2005 7:25 pm
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| If you're blowing alot of speakers then there is a need. [This message was edited by Bob Metzger on 16 January 2005 at 01:09 AM.] |
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 16 Jan 2005 6:35 am
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| No, there's little need or benefit to using a fuse in the speaker line. Also, if you did use a fuse, you'd want it rated for the speaker, not the amp. |
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John Daugherty
From:
Rolla, Missouri, USA
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Posted 16 Jan 2005 6:53 am
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Fuse holders and circuit breakers in the speaker circuit are "problem makers". The electrical contacts in these devices are not dependable. I say don't add trouble to a good working system.
Some stereo speaker systems are equipped with circuit breakers. I have had to bypass the breakers with a jumper wire when they started to cause trouble........JD |
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David Spangler
From:
Kerrville, TX USA
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Posted 16 Jan 2005 9:03 am
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Here's something to consider. A while back our bass player was merrily playing along and suddenly his Peavey amp shorted a transistor and sent the full power supply DC to his BW speaker. The speaker caught on fire and required a new basket in addition to repairs to the amp. Seems like a fuse would have helped in this case. What value? Maybe a SLO-BLO in the 5-10 amp range. Maybe one of our more technical forumites would have a recommendation.
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David Spangler |
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John Daugherty
From:
Rolla, Missouri, USA
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Posted 16 Jan 2005 9:18 am
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The formula is: current=square root of (watts divided by resistance).
Example: 200 watts,4 ohm speaker.
200 divided by 4 = 50. Square root of 50= 7.07. In this case I would use a 10 amp fuse.
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Blake Hawkins
From:
Florida
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Posted 16 Jan 2005 9:35 am
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I have had troubles with speaker fuses in the past. All the troubles mentioned by Donny and others.
In general you want to use a fast acting fuse instead of a Slo-Blo.
Because the Slo-Blo is not fast enough to protect your speaker.
In the case of the catastrophe mentioned above, even a fast acting fuse could not have prevented destruction of the speaker voice coil. However, it may have opened before the fire started.
If you size the fuse too small, it will blow when you play loud. If the fuse is too large you will get no protection. The happy medium
is very difficult to find.
Blake |
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ajm
From:
Los Angeles
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Posted 16 Jan 2005 9:37 am
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Question:
You have a tube amp.
You have a fuse in series with the speaker.
You have no other load except for the speaker.
You blow the fuse.
Do you now have a second problem, namely a blown output transformer, since you now have an open load?
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Artie McEwan
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Ray Minich
From:
Bradford, Pa. Frozen Tundra
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Posted 16 Jan 2005 9:57 am
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Determining the appropriate fuse to use in a given situation is called "coordination". Amplifier to speaker circuits are a real pain to "coordinate" effectively. Typically the energy used to open the fuse is also enough to kill the speaker. Many different types of "speaker protection circuits" have been developed with varying degrees of success. Fusing the circuit is lowest on the "success" scale. Use a current limiting fuse if your gonna try it. Get the Bussman fuse manual & start reading. It gets real interesting when you start working with I squared T.
A current limiting fuse works by absorbing the fault energy instead of letting it pass thru to the "downstream" devices.
One technique Seneca Sound used was to put a 500 watt 1/2 ohm glass power resistor in series with the speaker to at least limit the current when the output circuit went to the rails.
[This message was edited by Ray Minich on 16 January 2005 at 10:00 AM.] |
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Dave Grafe
From:
Hudson River Valley NY
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Posted 16 Jan 2005 11:30 am
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Some PA speaker cab manufacturers have been known to wire an incandescent light bulb in series with the speaker rather than a fuse. It has minimal resitance at low power but as it begins to receive enough power to glow brightly the resistance rises exponentially and thus protects the speaker from being overdriven.
The lamps that I have seen used were small in-line units - quite similar to a fuse in appearance - but at the moment I can't help you with the correct value, mebbe one of our talented electroid types can go there...
Of course, if it ain't broke, we can fix THAT, too....[This message was edited by Dave Grafe on 16 January 2005 at 11:33 AM.] |
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David Spangler
From:
Kerrville, TX USA
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Posted 16 Jan 2005 12:17 pm
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Eminence uses the light bulb in their crossover networks, presumably to protect the HF driver. Parts Express also sells in-line products to protect speakers but I have no experience with them.
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David Spangler |
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Eric West
From:
Portland, Oregon, USA, R.I.P.
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Posted 16 Jan 2005 12:50 pm
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Dave Spangler.
The Grestch 'Nashville' amps used to have that particular failing, and all it had to be was "on" for it to happen. Mine cooked outdoors while I was away from it in '78.
Hadn't heard of it happening to PVs, but I spose if it can, ot will.
EJL |
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John Daugherty
From:
Rolla, Missouri, USA
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Posted 16 Jan 2005 2:05 pm
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To quote AJM:
"Do you now have a second problem, namely a blown output transformer, since you now have an open load?"
I have heard this mentioned for many years and wondered how an open load could damage an output xfmr.
That question was raised to the Guru of "Nuts and Volts", Don Lancaster. Don agrees with me, that this WILL NOT damage a transformer. A short circuit across the output can damage it, but not an open circuit.
Some amplifiers do not have output transformers. The speakers are direct coupled to the output transistors and form the load impedance for the transistors. In this case, sizable dc currents,as well as the signal flow through the speaker voice coil. A shorted voice coil in this configuration can cause damage to the transistors. Some amps,however, have protection circuits to combat this.
But then........ there is Murphys Law.
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Blake Hawkins
From:
Florida
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Posted 17 Jan 2005 8:13 am
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John and Artie,
The destruction of an output transformer in an unloaded tube amp will not happen instantly as in the case of a transistor failure.
The destruction is caused by the output tubes going into oscillation which will cause the plate current to increase and the output transformer to overheat. The process will continue to build until something blows, either the tubes, the transformer or the power supply.
This is most likely to occur in a high power tube amplifier like a Fender Twin.
You will see the tube plates begin to glow red hot before the transformer goes.
Generally you will have time to shut it off before it is permanently damaged.
Will all tube amps do this? No. It depends on the design and on the amount of feedback
in the circuit.
On a small amp with 10 watts or less,of output power you could probably run it without a load with no problem.
I respect Don Lancaster's opinion and his writings. I've read most of his books and I also subscribe to "Nuts and Volts."
Blake
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ajm
From:
Los Angeles
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Posted 17 Jan 2005 12:59 pm
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Some thoughts on the previous comments from someone who is not an expert....
1) In general, tube amps have output transformers, transistor amps do not.
2) In general, tube amps do not like an open load, transistor amps do not like a shorted load. From what I know it just doesn't make sense to do either condition if it can be avoided, whether tube or solid state. (I personally think that it is just better to pay attention and be aware of what you are doing, but some people opt for another method.)
3) I do not understand technically EXACTLY why tube amps do not like an open load condition. However, every thing that I have ever read has said to NOT do this. That is enough for me to not try it with MY amps. I have also heard that if you turn a tube amp on and realize that you don't have a speaker plugged in, it is better to simply plug it in and to NOT turn off the amp before doing so. This would tend to back up the statement that damage does not occur instantaneously. I would think that if you have a speaker fuse on a tube amp and you blow the fuse, you are living on borrowed time.
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Artie McEwan
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Blake Hawkins
From:
Florida
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Posted 17 Jan 2005 7:17 pm
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Just an additional note: Fender tube amps which use a 1/4" jack for the speaker, have a switch on the jack which shorts out the secondary of the output transformer if you disconnect the speaker or forget to plug it in.
Evidently the Fender designers felt that the tube amps should not be run with out a load.
Blake |
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