View Full Version : Speaker protection fuse sizing?


Lefty
02-09-2006, 10:09 PM
Hi gang;

Well I've been putting this off for too long. With all the receiver and amp switching I do around here, I've always meant to get around to adding inline fuses for my main speakers. They are vintage Celestion Ditton 44s (12" 3 ways, they are very cool!) and I would hate to have them ruined because one of my several receivers and amps decided to commit suicide and take the Ditton's with them. :no:

So how does one go about sizing the fuses. Speaker power ratings are kind of a joke so I don't think it's a straight forward power calculation. Should I just start at some low rating say 1 amp fast blow and see if it blows under normal rocking levels and it it does up the size a step? Willing to listen to others ideas...

Lefty

Gibsonian
02-10-2006, 06:10 AM
Lefty,

For fast blow fuses this formula is the one you need:

Take the max. wattage you want to protect for, and divide that by four times the speaker's impedance. Take the square root of that and that is your fuse size.

For instance, to protect for 200 watts max. on an 8 ohm speaker you would need a 2.5 amp fuse. Sqaure root of 200/(4 x 8)= 2.5.

EchoWars
02-10-2006, 06:37 AM
Understand that fuses are poor protection (but sometimes better than nothing). Most fast-acting fuses take 4 hours or more at the rated current to blow, 1 hour at 135%, and from 1 second to 30 seconds at 200% of rated current.

I'd use a Littlefuse Pico II very fast-acting fuse if I was going to try anything at all.

Nakdoc
02-10-2006, 09:53 AM
Good subject. Small fuses, like 1/2 amp, ruin tweeter sound. Any fuse reduces damping factor. What I use on my test bench is the following:
For large speakers, I use a MDL 2 2 amp slow blow. This will take gobs of music power, but protect very nicely. I have sufficient amp power to not distort and destroy tweeters. For small speakers, like my ADS 200s I add a 400uf Non-polar cap in series with the hot lead. This low pass filters deep bass, which the speaker can't reproduce anyway, and block DC if the amp has a problem.
If you must fuse a tweeter, find plated nickel fuse holders, or use "polycaps" (used by Polk). 24v light bulbs also make interesting protection devices, but compress dynamics. The advantage to lamps and Polycaps is that they solder in. I don't know how polycaps work or how they sound.

cabinover
02-10-2006, 02:56 PM
Nakdoc (or anyone else for that matter), wouldn't the cap protect all drivers from DC, which is what the question at hand is?
What am I missing?

The Rebel
02-10-2006, 10:01 PM
This is a good thread. I just wish I knew what I am reading. Lol.
On a serious note, Nakdoc mentioned damping factor, what will the effects be in my normal listening after installing protective fuses? Will this cause more harm to my tweeters and mids than if I didn't use anything at all?

EchoWars states fuses are poor protection. EW, what type of speaker protection is suitable ? Are you using the LittleFuse Pico IIs on your set-up? How , where do you wire this in to your circuit? :scratch2:

EchoWars
02-10-2006, 10:24 PM
This is a good thread. I just wish I knew what I am reading. Lol.
On a serious note, Nakdoc mentioned damping factor, what will the effects be in my normal listening after installing protective fuses? Will this cause more harm to my tweeters and mids than if I didn't use anything at all?

EchoWars states fuses are poor protection. EW, what type of speaker protection is suitable ? Are you using the LittleFuse Pico IIs on your set-up? How , where do you wire this in to your circuit? :scratch2:I depend on a properly functioning protection circuit in the amp to do the job...my speakers are unfused.

Lefty
02-10-2006, 11:39 PM
I depend on a properly functioning protection circuit in the amp to do the job...my speakers are unfused.


As are most of us are.... It's also why I've put it off for so long, but still think it wouldn't hurt the sound and might help in a double failure event such as a shorted output device AND a stuck closed relay contact.

It's not like speaker fuses are a new idea. Many older speakers build them built into their termination plate. Many old receivers had speaker fuses on the rear plates. Now it did seem that most of the Japanese manufacturs kind of standarized on the DC voltage sensing, relay contact isolation for the speaker outputs by the mid 70s.

As far as which drivers would benefit from fuse protection for DC faults, I think it is only the bass and possibly midrange drivers that have DC continuity as the tweeters are usually hi-passed through a capacitor which would protect them from a DC source. This really is determined by the specific crossover design used in a given speaker.

As far as effecting the sound of the speaker I will certainly install it in one speaker first and using the stereo reverse and mono functions, I should be able to determine if I can hear a difference or not. I'm not expecting to be able to hear a fuse. Certainly the current being supplied to the speakers throught the output device to the DC rails are passing through fuses in the DC power supply.

Glad many have found this topic interesting. I'm certainly not recommending to others to do this. Electronic componets, including speaker voice coils, are know to burn out first such as to protect the fuse anyway :yes:

Lefty

soundmotor
02-11-2006, 08:54 AM
Small fuses, like 1/2 amp, ruin tweeter sound. Any fuse reduces damping factor.

For small speakers, like my ADS 200s I add a 400uf Non-polar cap in series with the hot lead. This low pass filters deep bass, which the speaker can't reproduce anyway, and block DC if the amp has a problem.



How exactly would a fuse reduce the damping factor of an amplifier?

(BTW, your L200 filter is a high pass, not a low pass. ADS used the same technique on their Delta Lab M1 nearfield control monitors.)

soundmotor
02-11-2006, 09:01 AM
Nakdoc (or anyone else for that matter), wouldn't the cap protect all drivers from DC, which is what the question at hand is?
What am I missing?

Yes, large enough to pass bass frequencies and up, but block DC.

OvenMaster
02-11-2006, 09:09 AM
For small speakers, like my ADS 200s I add a 400uf Non-polar cap in series with the hot lead. This low pass filters deep bass, which the speaker can't reproduce anyway, and block DC if the amp has a problem.
Very interesting! What low frequency are we talking about here? I want to filter out everything below, say, 30Hz, to stop my woogers from flapping around from turntable and LP noise; my KEFs move like mad, but I can only hear how muddy the bass gets because of it, and my Yamaha's subsonic filter doesn't kick in until 15Hz.
Tom

Kegger
02-11-2006, 01:06 PM
I've often thought that fuses could/would effect sonics and since I don't use a lot
of power. the speakers I have that do use fuses I was thinking of omiting them from
the curcuit.

Gibsonian
02-12-2006, 08:15 AM
I've never had a fuse blown because of DC output but for protecting speakers against power (unclipping power) greater than they are rated for standard fuses work great. I cannot say how much they might degrade the sound but I don't think it is much (a test here would be good). I have in my past been a bit liberal with the volume control and have taken a few drivers out, but zero drivers taken out while fused. Many manufacturers do include them in their speaker designs.

The Rebel
02-12-2006, 04:19 PM
As far as which drivers would benefit from fuse protection for DC faults, I think it is only the bass and possibly midrange drivers that have DC continuity as the tweeters are usually hi-passed through a capacitor which would protect them from a DC source. This really is determined by the specific crossover design used in a given speaker.

Well, now I'm completely confused. I bought a set of 4 way speakers (HPM-100) with all three of the high range speakers shot, in both speakers, VCs I suppose. No reading at all on my multimeter. My electronics buddies tell me it is caused by amplifier clipping , which they say leads to straight DC voltage applied to the speakers. Usually resulting in toasting the tweeters. Right now I don't care how it happened (although it would be helpful in my education) , but I don't want it to happen again. These speakers are getting harder and more expensive to buy. I tend to air my system out on a regular basis so I want to know if I need to add some sort of protection to my speakers and where it needs to be installed.

I've never had a fuse blown because of DC output but for protecting speakers against power (unclipping power) greater than they are rated for standard fuses work great. I cannot say how much they might degrade the sound but I don't think it is much (a test here would be good). I have in my past been a bit liberal with the volume control and have taken a few drivers out, but zero drivers taken out while fused. Many manufacturers do include them in their speaker designs.

Yeah, I like to crank it too Gibsonian, and I can say that I haven't toasted a speaker yet. I bought my CVs new in '83 and they are still kicking arse. Now, how did you figure out what you needed for your set-up with your formula? Do you rely on speaker manufacturers ratings, or fuse for where your amp is rated ? Some speaker ratings seem non-realistic , while vintage amplifier ratings seem somewhat conservative. Thanks. :scratch2:

I depend on a properly functioning protection circuit in the amp to do the job...my speakers are unfused.

That just sounds way too easy. :D

Rebel

Nakdoc
02-12-2006, 06:42 PM
How exactly would a fuse reduce the damping factor of an amplifier?
Damping factor is the ratio of load impedance to amplifier output impedance. If an 8 ohm spaeker is driven by an amp with .08 ohm output impedance, damping factor would be 100. Unfortunately, speaker terminals, speaker wire resistance, speaker input terminals, speaker fuses, and crossover losses all increase the amplifier output impedance, so if we simplify the path, we get better bass. Fuses less than 1 amp have significant resistance. A .02 ohm fuse reduces our example's DF to 80. A .05 ohm fuse holder drops it to 53.
Non-OTL tube amps rarely have DFs higher than 20, so the fuse issue matters much more than with solid state. Fuses above 1 amp are better, but I have no typical resistance values. Dahlquist DQ10s are a great lab speaker for hearing the fuse effect. Try different values in the tweeter fuse (stock was AGC 1/2), and compare to a alligator clip across the holder!
BTW, fat tube "sound" is 50% DF related.

(BTW, your L200 filter is a high pass, not a low pass. ADS used the same technique on their Delta Lab M1 nearfield control monitors.)

Oops..yes Hi pass. Thanks for the correction. I have L200s at home, but I'm using 200Cs at work. The ADS L badge meant the drivers came from Braun.

The value chosen has to keep phase shift minimized in the mini speaker's passband. The 200c has a 4" woofer. For a 5" woofer I would use about 800 uF. For a 6" I would use 1200 uF minimum.
To address a previous post, the caps in the crossover block DC from mids and tweeters, so this added cap protects the woofer. All electronic subs that have integral hi level crossovers put caps in series with the satellites.

Nakdoc
02-12-2006, 06:51 PM
I said this was a good topic.
First, plain and simple. Too little power blows tweets and mids. Too much power blows woofers. Clipping distortion does not produce DC. Clipping make gobs of hi power transients that persist long enought to heat the tweeter past the break point. Power ratings on any speaaker not branded "EV" are bs...well, let's say "for guidelines only". I can blow ANY non planar tweeter with a 15 watt marantz, and I can blow most woofers with a behemoth 6000 amp. A fuse is a nice way to prevent costly repairs when you accidently change audio cables with the system on, or come home to find the teenager decided to listen to Floyd loud.

Gibsonian
02-13-2006, 05:15 AM
Well, now I'm completely confused. I bought a set of 4 way speakers (HPM-100) with all three of the high range speakers shot, in both speakers, VCs I suppose.

Yep, I'm sure this is caused by what Nakdoc explained and is due to low power amplification and the resulting clipping. No DC output is at fault here.

Now, how did you figure out what you needed for your set-up with your formula? Do you rely on speaker manufacturers ratings, or fuse for where your amp is rated ?

I go with manufacturer ratings and then fuse accordingly. I fuse for maximum RMS wattage specified. I have a couple of Phase Linear 700's so when used I am also attempting to protect in the case of an internal failure and some DC output from the amp. I also have a pair of CV's (D9's) and HPM 100's (2 pair). Fuses are a cheap insurance policy - some folks don't test the output waters as thoroughly or at length and fuses might not be needed at all - more sensible types. In the case of amps like the old Phase Linears, they're required.

I will be trying this week to see if I can hear the difference in bass output from a pair of the HPM's fused and unfused - damping factor effect experiment. I can do this as I'm running the woofers with a non-Phase Linear amp at the moment.

OvenMaster
02-13-2006, 06:24 AM
Lefty,

For fast blow fuses this formula is the one you need:

Take the max. wattage you want to protect for, and divide that by four times the speaker's impedance. Take the square root of that and that is your fuse size.

For instance, to protect for 200 watts max. on an 8 ohm speaker you would need a 2.5 amp fuse. Sqaure root of 200/(4 x 8)= 2.5.
So if I want to protect my 6-Ohm KEFs to 50 Watts, I'd need to do this?:
(Sq. Rt. of 50) / (4 x 6) = 7.071 / 24 = 0.29 Amperes?? A lousy quarter Amp?
Tom

soundmotor
02-13-2006, 07:50 AM
Damping factor is the ratio of load impedance to amplifier output impedance. If an 8 ohm spaeker is driven by an amp with .08 ohm output impedance, damping factor would be 100. Unfortunately, speaker terminals, speaker wire resistance, speaker input terminals, speaker fuses, and crossover losses all increase the amplifier output impedance, so if we simplify the path, we get better bass. Fuses less than 1 amp have significant resistance. A .02 ohm fuse reduces our example's DF to 80. A .05 ohm fuse holder drops it to 53..

Non-OTL tube amps rarely have DFs higher than 20, so the fuse issue matters much more than with solid state. Fuses above 1 amp are better, but I have no typical resistance values. Dahlquist DQ10s are a great lab speaker for hearing the fuse effect. Try different values in the tweeter fuse (stock was AGC 1/2), and compare to a alligator clip across the holder!
BTW, fat tube "sound" is 50% DF related..

Do you think the fuse has greater resistance than the short sections of lead-wire coming off the voice-coil where it is soldered to either tinsel leads or input terminals to the tweeter?


Oops..yes Hi pass. Thanks for the correction. I have L200s at home, but I'm using 200Cs at work. The ADS L badge meant the drivers came from Braun.

The value chosen has to keep phase shift minimized in the mini speaker's passband. The 200c has a 4" woofer. For a 5" woofer I would use about 800 uF. For a 6" I would use 1200 uF minimum.

To address a previous post, the caps in the crossover block DC from mids and tweeters, so this added cap protects the woofer. All electronic subs that have integral hi level crossovers put caps in series with the satellites.

Actually, only the first year ADS 200's had non-ADS woofers & tweeters in them. Indeed, ADS bought the speaker complete from Acron and rebadged it. It is easy to distinguish as it had hook-up wire coming out through a grommeted opening on the back. All 200's from ADS had speaker terminal posts & ADS made the woofers & tweeters from mid '77 on. Braun never supplied woofers or tweeters for these speakers & they made there own compact called the Output-C.

There is a minor downside w/ 6dB filters and that is full excursion at driver resonance. On a smaller speaker that could create an issue if the driver has limited Xmax. 12dB or greater eliminates that condition and you can lower the highpass another octave. You might try it and see if you like the result.

soundmotor
02-13-2006, 08:00 AM
I said this was a good topic.
First, plain and simple. Too little power blows tweets and mids. Too much power blows woofers. Clipping distortion does not produce DC. Clipping make gobs of hi power transients that persist long enought to heat the tweeter past the break point. Power ratings on any speaaker not branded "EV" are bs...well, let's say "for guidelines only". I can blow ANY non planar tweeter with a 15 watt marantz, and I can blow most woofers with a behemoth 6000 amp. A fuse is a nice way to prevent costly repairs when you accidently change audio cables with the system on, or come home to find the teenager decided to listen to Floyd loud.

Well, actually exceeding the thermal limit of the voice-coil is what blows them. It does not matter if the amplifier is clipping or not, high-power or low. If the driver has, say, a 30W RMS rating and you exceed that; Poof! It doesn't really matter what state the amplifier is in. Certainly the rise time of a squarewave can bring that on sooner but it is still a matter of going beyond what the voice-coil can handle thermally.

One area not mentioned is high-frequency oscillation either because the amplifier is defective or the source program has that information present within the signal. RTR decks could sometimes have this occur when fastwinding over the tapehead. The tweeter goes poof then too and a fuse will protect it (usually) under these conditions.

Nakdoc
02-13-2006, 09:06 AM
Hi frequency overheating using music is difficult. Strange circumstances such as you describe, hi frequency or rfi oscillation or tape head squeal can ruin a tweeter quickly, but the RMS value of music is pretty low above 5kHz.

As for questions about speaker fuse sizes for full range use, this is my approach. Put in a small fuse (slo-blo preferred), MDL1 or MDL1 1/2. Play music loud. If the fuse blows, try an MDL 2. 4 ohm speakers and electrostats might need an MDL 3. Don't worry about the math. If you need protection :)), use the lowest value that allows you to play music.

Lefty
02-13-2006, 09:26 AM
Hi frequency overheating using music is difficult. Strange circumstances such as you describe, hi frequency or rfi oscillation or tape head squeal can ruin a tweeter quickly, but the RMS value of music is pretty low above 5kHz.

As for questions about speaker fuse sizes for full range use, this is my approach. Put in a small fuse (slo-blo preferred), MDL1 or MDL1 1/2. Play music loud. If the fuse blows, try an MDL 2. 4 ohm speakers and electrostats might need an MDL 3. Don't worry about the math. If you need protection :)), use the lowest value that allows you to play music.

This sounds like a practical method.

Also a lot of these posting refer to lower power amps and receivers clipping and causing speaker failures. While I'm sure clipping is to be avoided it doesn't seem to me a matter of what the power rating of the amp is rather that one shouldn't over drive the INPUT level any amp of any size. This would seem to me to be a problem with the source level Vs amp max input value rather then the amp/speaker interface. A 30 watt amp can certainly deliver 30 watts of clean power to any speaker even if that speaker is rated for a much higher power, heck looking at my watt meters I mostly listen at the 1-5 watt output level. Now if I was to hook a line level source into my phono input I would expect to create lots of clipping at almost any normal volume setting.

Lefty

Gibsonian
02-13-2006, 12:08 PM
So if I want to protect my 6-Ohm KEFs to 50 Watts, I'd need to do this?:
(Sq. Rt. of 50) / (4 x 6) = 7.071 / 24 = 0.29 Amperes?? A lousy quarter Amp?
Tom

No, a quarter amp is not correct. I screwed up my representation of the formula - I added parenteses below to clarify but basically it's the square root of the whole works.

Sqaure root of (200/(4 x 8))= 2.5.

So for 50 watts/6 ohms it would be as below

Sqaure root of (50/(4 x 6))= 1.44, so 1.5 amp would be your fuse size to use.

nashbap
02-13-2006, 08:53 PM
I like Nakdoc's practical approach of slowly stepping up the fuse value; but Nakdoc why do you recommend a slow blo fuse as opposed to a fast acting fuse which is what I believe Gibsonian is suggesting. Would your approach not work with a fast acting fuse and afford better protection?

Nakdoc
02-14-2006, 09:52 AM
I like Nakdoc's practical approach of slowly stepping up the fuse value; but Nakdoc why do you recommend a slow blo fuse as opposed to a fast acting fuse which is what I believe Gibsonian is suggesting. Would your approach not work with a fast acting fuse and afford better protection?

Fast blow AGC and GMA fuses are best for steady state protection, like DC power supplies. slo blo, like MDL ot T fuses ignore fast transients, in fact the open on a time delay basis. I believe 1/2 second at 500% over curent. Anyway, you can use a smaller value, and yet play it loud. If you look into the glass you can also see that slo blo fuses have a fatter element. Why use 1" diameter speaker cables made of gold only to pipe the audio through a wire as thin as a (fat) hair?
I agree with a previous poster that no fuse is the best fuse, but if you must, then use MDL or T. IF you replace an AGC 5 speaker fuse, use 1/2 the value of slo blo, which would be MDL 2.5. :music:

nashbap
02-14-2006, 10:19 AM
OK. My primary concern is to protect the speakers from an accidental or sometimes overexuberant turn of the volume knob. Even though I am very careful and turn down the knob before and after listening there have been times when there has been a burst of volume so loud as to blow the fuses on my B&W DM6 speakers. And I am sure glad that those fuses were there. These have a 2.5 amp fuse as the main system fuse and 0.5amp as the high frequency fuse. However, the manual does not specify whether they should be fastacting, regular or sloblow type.

Gibsonian
02-14-2006, 12:39 PM
nashbap,

What are your speaker rated for? That would be a clue as to whether the fuse at 2.5 amps is slo-blow or fast-blo. 2.5 amp fast blow is the correct value to use for 200 watts but 2.5 amp slo blow would be for 300 or more watts.

nashbap
02-14-2006, 01:14 PM
Says that the speakers are rated 25W-350W. Impedance 8 ohms.

packrat
02-14-2006, 02:42 PM
here's what AR said back in the 70's....

(taken from the classicspeakerspages.net AR area...)

OvenMaster
02-15-2006, 08:21 AM
No, a quarter amp is not correct. I screwed up my representation of the formula - I added parenteses below to clarify but basically it's the square root of the whole works.

Sqaure root of (200/(4 x 8))= 2.5.

So for 50 watts/6 ohms it would be as below

Sqaure root of (50/(4 x 6))= 1.44, so 1.5 amp would be your fuse size to use.
Whew! Thank you, sir! I kinda thought a 250mA fuse would be blown real fast!
Tom