B&K EX442 blowing amp's speaker fuses

idadude

idadude

Super Member
I'd just purchased a used B&K EX442 power amp and have it hooked up to an AES AE-3 tubed preamp. When I turn on the amp, then turn on the preamp, it 'pops' the speakers, then blows the EX442's 6A 250V speaker fuses. For a while it only made the popping noise, but now it blows the fuses. What might be going on here that could be remedied? Where/what could I check?
When I hook up the preamp to a tube amp and turn the tube amp on first (let it warm up), then the preamp, I can't even get a small 'click' out of the speakers.....no noise, nothing.
Yes, I know, turn on the preamp first, then the amplifier, but it seems that something is going bad since it never used to blow fuses and now it does every time. Anyone have some suggestions on things to check? Thanks.

Larry
 
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idadude said:
I'd just purchased a used B&K EX442 power amp and have it hooked up to an AES AE-3 tubed preamp. When I turn on the amp, then turn on the preamp, it 'pops' the speakers, then blows the EX442's 6A 250V speaker fuses. For a while it only made the popping noise, but now it blows the fuses. What might be going on here that could be remedied? Where/what could I check?
When I hook up the preamp to a tube amp and turn the tube amp on first (let it warm up), then the preamp, I can't even get a small 'click' out of the speakers.....no noise, nothing.
Yes, I know, turn on the preamp first, then the amplifier, but it seems that something is going bad since it never used to blow fuses and now it does every time. Anyone have some suggestions on things to check? Thanks.

Larry
Click to expand...

First off you should NEVER do that. When the pre first "wakes up" a little DC or other weirdness happens that goes right to the power amp, and is amplified if it is turned on and this transient noise is amplified and fed to the speakers. Always turn on your pre first, if tubed give it a min, then flip on the power amp. Turning on anything up stream can make a snap if the power amp is on.

Turn off should be just the opposite. Turn off the power amp first, then when it is dead, turn off the pre. Again, turning off the pre first makes a snap, and if the power amp is still on, you get the snap to the speakers...or in this case their fuses...This is basic operating procedure for seperates. Lucky you've only smoked fuses! If not for them, you may have killed tweeters. If you see your woofers going out, then slowly settling back in, that's DC pushing them out, then as the pre stabilizes they float back in.
Depending on how much total dc is at the speaker out puts. Look in the Solid state forum stickies for the procedure to measure dc output...and maybe adjust it away...

russellc
 
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That's a powerful amp with no "soft start".

I own one.

I'd not turn the pre on after the amp- bad form with this SS gear.

Worth checking DC offset on it, as internal fusing probably shouldn't be blowing, wondering if it's the pre coming on that's doing the damage- at least to the speakers.
 
Yup...DC offset from the preamp and amp should be checked, IMO.


Oh, BTW, if you know you shouldn't turn on the amp first, then don't do it. ;)
 
OK.....How do I check these things?

Do I check the speaker terminals by selecting VDC on my multimeter and hooking to the + & - of each channel, then turning on the amp? I checked the amp that way and there was no DC coming through at all when I turned on the amp. No 'pop' either when I only turned on the amp. No DC at all.
I checked one RCA connector coming out of the preamp and when I turned on the preamp the multimeter went from -20 to +20VDC then it went to 0. Is there a problem with the preamp or just some type of incompatability? When I hook up a tube amplifier to the preamp, the tube amp never makes the speakers 'pop'. Also, I did not mention, both speaker fuses blew when I turned the preamp on, after I turned on the amp with the volume turned all the way down.
What is DC offset? If that's the problem, is it fixable or is that just the way it is? I don't have a whole lot of depth electrically, but I would like to understand what's going on and fix it if possible.

Larry
 
Seems like a problem in the preamp to me. The tube amp may not be affected the same as the solid state amp if the SS amp is direct coupled or has a significantly lower frequency response capability.
 
The EX442 is a very high current amp with very low frequency capability (75A and DC for the Sonata version). The thing could be used as a welder. It doesn't surprise me that even slightly careless handling could lead to blown fuses. I have blown fuses in mine, but it's been years and I don't recall the exact circumstances. Be careful.
 
Well, no....no other working preamp.

I have an old Harman-Kardon Citation I, but it needs rebuilding. The AE-3 preamp works fine except on start-up with this amp. All the other amps that I'm using are tube amps and there are no problems with any of them using this preamp, only when I use the B&K, and only at start-up. All works fine after that.
I'll just have to remember the sequence and turn the pre on first.
My previous preamp (Cary SLP-88) had a 30 (or so) second delay after turn-on before the circuit was completed to the amp. Maybe that delay is what this preamp needs.

L.D.
 
Sending a -20vdc/+20vdc turn on bump to the inputs of a high current amp?

I suggest the OP observe the proper power up sequence, and NOT power the amplifier until the preamp has had however much time it needs to stabilize.

The fact that it still works tells me that's a damn good amplifier.
 
I'll say it again. Try turning on your pre first!....Always with a seperate set up.
You measured the problem...When the pre first starts up, it isnt stable just yet and a little DC slips through, and like has been said, when that transient hits the amps input, it is greatly amplified. Why dont you just turn the pre ON first, and OFF last and you wont have problems.

There is nothing wrong with anything other than operator error.

Try it, you'll like it. The only pre you'll likely be able to get away with this would be a tube rectified unit with a slow ramp up like 12x4 in the Dynaco pass.
It takes about 15-20 seconds for the B+ to start coming up, so it could be turned off and on w/o snaps ( or at least my Audio by Van Alstein super Pas III can be) but if you watch the woofers, you'll see them stickk out, then slowly suck back in to normal. This reaction is due to a small amount of DCleaking through...

russellc
 
bowtie427ss said:
Sending a -20vdc/+20vdc turn on bump to the inputs of a high current amp?

I suggest the OP observe the proper power up sequence, and NOT power the amplifier until the preamp has had however much time it needs to stabilize.

The fact that it still works tells me that's a damn good amplifier.
Click to expand...

Amen brother! I thought this was common knowledge for seperates.:yes:

Russellc
 
The OP did acknowledge he should turn the pre on first. Why he does not only he can say.

I do know that I hear it quite a bit from many different situations, "I know I shouldn't do that, but..."(I did it anyway now bail my ass out). :D
 
It's a matter of habit.......

I never had the 442 fuse problem when used with my previous tubed preamp, so when this preamp started 'popping' my speakers it had me puzzled as to what was going wrong. The pre's manual does not have any information on a turn-on sequence for the pre and amp. I'd been using the pre with tube amps without any hint of a speaker pop, so this something new to me. Never heard of turning on the preamp first.
Its a matter of habit.....when I come to my system, I come to the amp first, then the preamp is another step away. Time to change habits. All I wanted to know was if there was a problem with my amp or preamp. Now I know. Strange that the preamp does not make any noise at all when hooked up to a tube amplifier. Thanks for the input.

L.D.
 
Don't sweat it too much.

Try it out with pre on first, then amp (and remember to reverse this situation when shutting down!), and let us know how things turn out.


If you are still experiencing issues, next step is check DC offset. There's an excellent sticky (permanent thread) written by a well respected tech that will give you excellent instructions.


Best wishes with the amp- it's an amazing amp with a warm smooth delivery but absolutely brutal force behind it.


Best wishes,

Paul
 
I'd had a ST-202+ previously and sold it when I needed the money. Now I wanted to hear my Magnepans again to decide whether to keep them or not, so I bought the 442 ($250) to listen to them again. Yeah, I'll keep my rebuilt MG-1 Improveds. I'll keep the 442 too. I like the B&K sound.
There is a little 'pop' when I turn on the amp, either by itself or when the preamp has been on for a while. The ST-202+ did the same thing, so it does not seem to be anything abnormal.
I will look up the DC offset info and check into it. Thanks again for the input.

L.D.
 
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