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JL Audio 500/1 low output and clipping
JL Audio 500/1 low output and clipping
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Old 13th October 2017, 05:40 PM   #1
KevinMangini is offline KevinMangini  United States
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Default JL Audio 500/1 low output and clipping

Hi all,
I have a JL 500/1 V1, board Rev2. It was powering an 8 ohm W1 with no trouble, however I got a brand new W6V3 running at 2 ohms and am having trouble getting the power I feel should be there. I hear over excursion from the subwoofer at medium to high volume, and overall the bass seems muddy. I borrowed a crappy mtx 600W peak amp and I have about the same volume from the sub. The amp was repaired by JL a long time ago, so naturally I opened the thing up and re-solder anything that looked like it had been replaced or had gotten hot. When I plugged the amp in, magic smoke comes out of one of the FETs. Ordered and replaced all 8, with the 47 ohm resistors and 4 drivers for them. After the repair there's a bit more clarity to the bass, and seems to be more consistent... But still clipping/over excursion. I have a DMM and a scope, but my knowledge with switching power supplies and amplifiers is mediocre at best

Please help,
Thanks
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Old 13th October 2017, 11:05 PM   #2
KevinMangini is offline KevinMangini  United States
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I've been continuing to search through the countless threads on this amp, testing everything I can find specs for. Here's a list of measurements so far

Driver center legs
Q608 and Q611 - 0V
Q609 and Q610 - 12V

All other driver legs at 2.9V (should be 5V?)

Rectifier center leg - 80V
Rectifier outer legs - 100V

Output transistors - nothing below 2k Ohms between any combination of legs


I have dirty square wave, I'll attatch the signal I see on COP3 for example, all square waves have the same sloppiness right after switching on or off.

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Old 15th October 2017, 03:37 AM   #3
Perry Babin is offline Perry Babin  United States
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These amps are very voltage sensitive. Drops in voltage too quick to be seen by your meter could be part of the problem.

5v PS drive is for amps with no rail regulation. This one is regulated to just over 80v (rectifier center leg).

The amp is a very old version. The noise may be normal. Even if not, I doubt that the noise is causing the malfunction. Have you confirmed that the speaker load and wiring are ok?
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Old 15th October 2017, 04:51 AM   #4
KevinMangini is offline KevinMangini  United States
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Thanks for the reply Perry,

Are you saying these amps are sensitive to a drop in the line voltage ?

When measured with my meter I have a solid 80.1 volts out of the center leg on the rectifier. Totally different story if I look at the same leg with the scope though.

Also the scope pattern I sent gets significantly more sloppy under load, I'll upload a scope pattern of that. I'm not implying that it's abnormal, I just don't have the experience to know if it's an acceptable wave or not
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Old 15th October 2017, 05:05 AM   #5
KevinMangini is offline KevinMangini  United States
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My latest theory here is all of this messyness on the square wave may be "leaking" into the output, causing the sub to push/pull past where it is comfortable. Which in turn does not allow me to get the amp up to full output because before I get there the cone is at it's physical limits.

My output wiring to the sub is in good condition, under no load at the amp I measure 1.9Ohms (which doesn't prove much, but its a pretty short run with low chance of failure) the sub is brand new (which does not eliminate a fault of course, but unlikely) and the box is the JL recommended .55cuFt, 3/4 MDF sealed. It should provide plenty of physical resistance for the cone.
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Old 16th October 2017, 09:21 AM   #6
KevinMangini is offline KevinMangini  United States
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It looks like Perry hit the nail on the head. Here's a scope pattern at the input legs on the amp.

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It gets down to 9.9V here, but I saw even less at times. Every time the subwoofer "clips" I see a sharp decrease in voltage below 10 volts. I have 4 gauge wire and a brand new battery, I did not expect to see that kind of dip. Could be my ground connection to the body, which I'm definitely going to check... but I'd really love to experiment and see if a 1 farad capacitor will smooth out this voltage at all first ��

P.S. I also have a 300/4 with a scratchy RF channel. I'll be diving into that amp next on a new thread. Been searching the existing threads but I haven't turned up much for a solution yet.
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Old 18th October 2017, 07:50 AM   #7
KevinMangini is offline KevinMangini  United States
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OK Perry, I've fixed my low input voltage issue. the 1 farad capacitor did very little as expected. turns out I had 1.6 volt drop in the 4 gauge Rockford wiring. I ran copper welding wire instead and have steady input voltage. the "presence" of the audio has increased, but still clipping at the same volume.

I just noticed somebody did a very nice job replacing the output FETs, but I can't find a datasheet to compare the replacement parts.

Are you aware of JL ever using 50012L8's here?
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Old 19th October 2017, 12:42 AM   #8
Perry Babin is offline Perry Babin  United States
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The output FETs are generally IRF540s.
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Old 19th October 2017, 02:18 AM   #9
KevinMangini is offline KevinMangini  United States
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Thanks Perry

I ordered IRF40PBF's along with 300/4 parts. Since I couldn't find a datsheet I'm going to assume they're not a good sub and replace them
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