JL 250/1 dc offset quirk - diyAudio
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Old 5th February 2012, 09:32 AM   #1
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Default JL 250/1 dc offset quirk

so, i got this amp with the symptom "3vdc to speaker output" and no other info. the side covers were already loose, so i popped it open to check things before powering it up. i have to say that the output setup is interesting, to say the least, and i would really like to know exactly how it works, but on to the problem. so, while i was in there, i noticed that all6 p/s fets seem to be replaced with matching irfz44n's, iirc, and the smd 47ohm gate resistors have been replaced, or r&r'd. the outputs however were 4 irf540's, but they appear to be from 3-4 different build lines. originally, it took quite a bit of adjustment to get the offset under.1v, then i shut it off and hooked a test sub to it. when i powered it back up, it popped some and then went to something like .6v, then climbed to well over 6vdc. i shut it down, and powered it up again without the sub, and the voltage wasn't that high, but i set it close, and noticed the pot was a little touchy to pressure, so i worked it back and forth and powered it up and started again from close to good position. once, i got it close, i was able to put the load back on and it liked .2v, with a little noise. now, i can get it to idle at -.08, but when i shut it off, the voltage goes to .4 for a second, and then falls, and when it powers up, it jumps to -.6 and then slowly tapers back down. when playing music, it will fluctuate to -.2, and with connection pops. at a higher voltage offset, it would stay pretty constant and sound a little cleaner playing, but have more start-up and idle noise. all i have on hand are irf640's, but i can order the 540's and just replace them next month if they wouldn't work well. next question- do you think that the miss-matched outputs are sole to blame, or is there more to it? i sure hope that would do the trick. i would hate to choose between better playing sound vs. turn-on/off and idle noise. that somewhat defeats all the over-tech jl crammed into it for better sound, lol. any help appreciated, thanks-
-Heath H-
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Old 5th February 2012, 06:00 PM   #2
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There are no parallel output transistors so mis-matching them won't be an issue.

The 640s won't work except possibly for testing.

In the future, please leave blank lines between individual statements/questions. Having one constant paragraph makes it more difficult to read.
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Old 6th February 2012, 07:30 AM   #3
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Okay, and thank you. I'm sorry about the grammer. I have to admit that it has become a bit "lax" over the past few years, from several different forums. I'm trying to work on it.

If the probable tolerance difference between the 4 outputs would not cause the offset, could I assume the same for the drivers?

Do you think the pot could be at fault? Ii am thinking about replacing it anyways, along with the outputs. (2 appear to have been replaced by trimming the legs and attaching to the originals)

Do you happen to have the schematic for this, or similar amp, so that I can see how the output circuits work and part numbers?

Is there any danger of putting the amp to work "as-is" for the time being. It functions fine, though I would like better sound, reliability and operation from it.
Again, thank you
-Heath H-
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Old 6th February 2012, 04:39 PM   #4
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It's unlikely that it's in the drive circuit. For class D amps, if there is a problem in the drive circuit, the amp generally blows output with no load. There is a feedback loop that monitors the offset and corrects for it. I don't know which op-amps are used for that in this amp (no schematic).

I don't think that the pot is at fault if you have to go to the end of its range to get the offset close to 0. If it's intermittent, then that would indicate that it's defective but I don't know if that fault could prevent it from setting the offset to 0.

There is a danger of using the amp if the offset pot is intermittent and allowing there to be significant DC on the output.
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Old 6th February 2012, 06:38 PM   #5
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Just to clarify, I do not need to go to the end to reach near 0v offset. the pot was intermittent during initial adjustment, and responsive to tapping/contact. Working it through the range a few times seems to have corrected that, and the pot will adjust smoothly. I think it is just a matter of time before the pot will become "finicky" again, so replacing it would make me more comfortable.

With the offset set as close to 0 as i can set it to maintain (-0.02 or less) there is the turn on click/pop, which reads to -0.6vdc+ and tapers down. Also, the turn off pop, which reads to -0.3v+.... and i believe fidelity may have minor suffering, by ear. With the offset adjusted to ~ -0.2v, these issues are decreased, and the offset remains steady and constant both at idle, and while playing music. My scope is down for the moment, so I am solely relying on my fluke87 to take readings.

thanks
-Heath H-
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Old 7th February 2012, 09:01 PM   #6
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Is the scope going to be operable soon?
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Old 7th February 2012, 11:26 PM   #7
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No, it will not. Still have to drop it off
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Old 7th February 2012, 11:32 PM   #8
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I don't think you'll be able to do much more troubleshooting until you can confirm that the signal is actually distorted. Then you'd have to see if the problem was on the positive or the negative output and then check the square wave for that section.
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Old 8th February 2012, 03:34 AM   #9
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Well, i rolled the dice and hooked it up to 2ohm load (JL 10w1v2 x2) set the gains with a dmm on a tone. it played clean, well, as clean as the signal through the head. It only lasted 10min. of high and low volume music from different sources while fine tuning the head. Then, it shut down for a few seconds at high volume and came back on playing with 14vdc on the output.
so, now can order all new outputs, which i was planning anyways, for this, and a couple rockfords. Hopefully, I should have my scope repaired by then. thank you.
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