JL Audio 300/4 Melting Speakers

[madpsi]

I'm on my second 300/4 (first had blown power transistors), and it's having serious issues that's causing my rear speakers to melt randomly.

The front outputs work fine, and the rear speakers played fine for a minute or so, then one of the speakers pulled down like it was shorted and the voice coil melted before I could get it turned off. I tried to test the outputs of the amp by hooking the speaker that didn't melt to each of the rear outputs and ran it pretty hard for about 15 minutes per channel. Everything was fine. So I hook up the new speakers and within seconds, the same thing happens, one randomly gets pulled down and stays there, but this time I think I caught it in time, as the speaker returned to neutral when I turned it off.

Has anyone had experience with this type of occurrence? It's happening randomly and on both Right and Left channels, but only with the rear outputs.

- Thanks

[Perry Babin]

Does the fault seem to happen more often when the amp is subjected to vibration?

[madpsi]

Not noticeably. I will do a little more testing and report back.

[madpsi]

I have a couple things to report:

- I have isolated this problem to a single channel (Left)
- AC Volts show normal (needle bounces around) during operation on the right channel, but there is very little on the left (0.00 - 0.02 during operation)
- Here's the scary bit. Left channel is showing -29.0 VDC.
- Vibration does not affect the output.

[Perry Babin]

The outputs in that channel may have failed.

[head_unit]

29 volts DC will shove over 200 watts into a nominally 4 ohm speaker. Plus, it will shove the cone to an extreme position, where the coil usually can't sink heat well.

It does sound to me like an output device is shorted (at least intermittently) and dumping the power supply straight into the speakers.

[pixelpusher]

I've been experiencing a similar problem with the left rear channel of a JL Audio 300/4 amp. It seems to function fine for a while but then a loud pop occurs followed by a smaller pop. The speaker is destroyed. This has happened twice now and getting expensive! Could this be caused by an intermittent ground?

[pixelpusher]

Today I did some more troubleshooting on this amp. Measured no appreciable DC on any of the outputs with no speakers connected. Hooked up an old driver to the left rear channel with the volume set very low. It played OK for 30 seconds or so then popped again and smoked the driver. I then measured
-21.7 volts DC on the outputs of that channel. I suppose that means that the output transistors have shorted. I could replace them but what caused them to fail? If the problem goes deeper than that it may be beyond me.

[Perry Babin]

I've never had this problem with any of these amps but since 2 people are having the same problem, it could be due to one of the common problems associated with the driver boards.

For each of the large 47k resistors on the driver board, add new solder to each solder connection. Remove all of the solder and resolder them. Simply adding new solder often traps oxides and the connection isn't reliable.

To prevent damaging the solder connections on the main board (when removing the driver board), I recommend that you use a low temperature solder like ChipQuik (Digi-key sells it).

To protect the test speakers, insert a 100uf, 100v non-polarized capacitor in series with one of the speaker wires. It will block the DC and protect the speaker. You'll have to monitor the DC voltage with your multimeter.

[nightanole]

Perry maybe on to something. Odds are the INPUTS float at 2.5 volts and are capacitor coupled to prevent a dc offset. However if the cap shorts or something else shorts, the unit sees zereo volts, which interpets as the bottom of a full volume base note. So it sends its full negative rail voltage into the speaker.

When your outputs are showing -21 volts, i bet your input is showing zero volts after the capacitor.

Thats just a hunch, but its what my setup would do.