AR LST Fuse operation

[Mexicomike]

Today I managed to blow the fuses in my LSTs due to attention deficit - I had been fooling around earlier and had plugged the computer audio output directly into the 300WPC Moscode 600. Later I had turned off the system, unplugged the output from the computer to the amp but for some reason left the computer plug partially inserted into the jack. A few minutes ago, I turned on the power amp. It has a 45 second delay and walked out of the room. When the relay kicked on there was a HUGE BURST OF NOISE out of the speakers and I raced in and shut off the amp.

As I was between cursing and crying, I plugged the cable into the preamp, turned on some sounds and both speakers played at very low level. I was convinced I had destroyed my LSTs. For some reason, not really thinking it would matter, I pulled the fuses while they were playing and there was no change in the volume. I then jumped the fuse terminals with alligator clips and the speakers sounded absolutely normal. WHEW!!!!

So here's my question...uh, how does that work? How do the Xover circuits allow the speakers to play at a greatly reduced level with the fuses out/blown? I would assume that with no fuse there would be NO sound at all.

Of course, I have no replacement fuses so I'm torn between not playing them until I can get some or HOPING I don't screw up again.

Hmmm -hoping for that is not realistic!

[KlausDK]

I plugged the cable into the preamp, turned on some sounds and both speakers played at very low level. I was convinced I had destroyed my LSTs. For some reason, not really thinking it would matter, I pulled the fuses while they were playing and there was no change in the volume. I then jumped the fuse terminals with alligator clips and the speakers sounded absolutely normal. WHEW!!!!

So here's my question...uh, how does that work?

Of course, I have no replacement fuses

Hi Mike

I have experienced the same with my 10Pi, if you look at the schematics, the fuse is located after the transformer, so that must be the explanation why there is sound, as I remember it is not only low volume but also twisted frequency wise.

Remember the source for LST fuses at 1½ $ I gave you in another thread.

BRgds Klaus

[dynaco_dan]

Today I managed to blow the fuses in my LSTs due to attention deficit - I had been fooling around earlier and had plugged the computer audio output directly into the 300WPC Moscode 600. Later I had turned off the system, unplugged the output from the computer to the amp but for some reason left the computer plug partially inserted into the jack. A few minutes ago, I turned on the power amp. It has a 45 second delay and walked out of the room. When the relay kicked on there was a HUGE BURST OF NOISE out of the speakers and I raced in and shut off the amp.

As I was between cursing and crying, I plugged the cable into the preamp, turned on some sounds and both speakers played at very low level. I was convinced I had destroyed my LSTs. For some reason, not really thinking it would matter, I pulled the fuses while they were playing and there was no change in the volume. I then jumped the fuse terminals with alligator clips and the speakers sounded absolutely normal. WHEW!!!!

So here's my question...uh, how does that work? How do the Xover circuits allow the speakers to play at a greatly reduced level with the fuses out/blown? I would assume that with no fuse there would be NO sound at all.

Of course, I have no replacement fuses so I'm torn between not playing them until I can get some or HOPING I don't screw up again.

Hmmm -hoping for that is not realistic!

Hi there;

You may wish to go to, "other speaker forum and electronics" and scroll down to, "fast blow fuses".

If you read most, if not all of the topic, you will have a better understanding of speaker fuses.

There is some advice and at least one source suggestion as well.

Certainly not a definitive guide but better than nothing, in my opinion.

More technical feedback is needed regarding downsizing the FNM/FLM Bussmann/Littlefuse slow blow fuses and or adding or substituting with fast blow fuses.

We can thank Acoustic Research, Advent, and Dynaco, at least, for recommending fuses and their sizes and availability, back in the old day's.

Vern

[dynaco_dan]

Hi there;

You may wish to go to, "other speaker forum and electronics" and scroll down to, "fast blow fuses".

If you read most, if not all of the topic, you will have a better understanding of speaker fuses.

There is some advice and at least one source suggestion as well.

Certainly not a definitive guide but better than nothing, in my opinion.

More technical feedback is needed regarding downsizing the FNM/FLM Bussmann/Littlefuse slow blow fuses and or adding or substituting with fast blow fuses.

We can thank Acoustic Research, Advent, and Dynaco, at least, for recommending fuses and their sizes and availability, back in the old day's.

Vern

Hi again;

Sorry, I failed to mention that the AR-LST in particular is wired internally so that the sound level is down about 10 dB when the external fuse blows.

Failing to notice this in the beginning of production was expensive for AR.

With warantee claims for problems costing them shipping and other expenses until they noticed and mentioned that fact in their owners manual later on.

There may have been other speakers with similar problems, such as AR-LST/2, 10pi, etc., but I do not know that for a fact.

I have written previously of a local musical group that kept blowing out all of their AR-LST tweeters, under warantee of course, using master tapes at their home.

With this statement alone, it would have appeared that the FNM 2 amp fuse was not protecting the tweeters at least.

The woofers and mids did not blow with that fuse.

Their final answer was to replace them with EPI 1000 Towers, they couldn't blow them, apparently.

Perhaps AR did not want to turn off customers by adding additional fusing requirements.

Their thinking may have been this would appear to be a speaker weakness rather than protection.

One definitely needs spare fuses at home.

Vern

[Mexicomike]

Thanks Vern, I reviewed all the info - good stuff.

I can't get the FNM-2 fuses here in Mexico but I'm going to the States on the 17th for 6 weeks so I ordered some from the supplier Klaus posted about and am having them shipped to my US location. I'll bring them back with me when I return.

But I'm still not clear - does the LST fuse ONLY reduce the volume if blown or does it also change the circuitry re the crossover itself. In other words, does the frequency response change or would it be the same without a fuse if you just cranked up the volume?

[johnieo]

But I'm still not clear - does the LST fuse ONLY reduce the volume if blown or does it also change the circuitry re the crossover itself. In other words, does the frequency response change or would it be the same without a fuse if you just cranked up the volume?

Hi Mike:

Fuse needs to be replaced for proper operation. However, the drivers can see some weak signal with the fuse blown.

If you examine the crossover circuit, you will see that each [+] transformer tap is connected through crossover elements to its respective driver and then to a common ground point. That common ground point is connected to the fuse and thence to the negative transformer tap. Removing or blowing the fuse leaves each driver's common [-] terminal connected together. Since the [-] driver terminals are still connected, the [-] terminal of any driver sees the [+] terminal of other drivers through an unconventional path. This would result in some weak signal across each driver. e.g. the fuse is in the autotransformer's secondary common return.

Cheers,