Hartke LX8500 cutting out

[Oddacious]

I bought this through reverb.com recently, and it sounds fine, but when i was slapping and plucking this evening, it started to cut out. I've made a video reproducing the symptom.



I'm currently using an Ibanez SR600 pushing an avatar SB112. The video was taken at a relatively at a low volume.

It gets worse when either:

-The brite is turned on
-The gain is passed 4
-the bass volume is all the way on
-the compressor is lower

Is this normal or should I send it back? I was using a backline 600 and never had this issue.

 

[agedhorse]

Is this a 4 or 8 ohm cabinet?

 

[Oddacious]

4

 

[agedhorse]

Ok, I think I have a reasonable idea of what may be happening.

First of all, that’s WAY too much power for that cabinet/driver. You are well beyond the mechanical power handling.

When a speaker is driven well beyond its mechanical limits (which is going to be around 300-400 watts RMS), the voice coil gets driven outside the magnetic field that’s contained mostly within the magnetic gap. The reactance from the coil inside the gap adds to the DC resistance of the coil outside the gap.

Now, when the coil is driven outside the gap the impedance falls to the DC resistance which is generally about 60% of the nominal impedance.

This means that the amp is now trying to drive about a 2.5 ohm load which is well below the amp’s allowable minimum load. This likely is triggering the over current protection circuit resulting in the amp shutdown.

this is one reason that most speaker manufacturers no longer offer single driver 4 ohm cabinets or even 210’s for that matter.

Todays popular amps, like yours, are too powerful. Your amp delivers 400 watts into 8 ohms, a much better match for any single 12” driver cabinet.

 

[6thplanet]

It's kinda difficult to really tell how loud it is, but it really doesn't seem like it's driving the cab that hard. I get what agedhorse is getting at, could be that amp is a bit more protective than your other one.

....or there's something wrong with it. Without other equipment to measure what's really going on it's a crap shoot.

 

[yonkrz]

Just asking as a curiosity,does it have anything to do with the compression being maxed or doesnt it work that way ? I have no experiance with a compressor.I am only a hobbyist.

 

[Oddacious]

When a speaker is driven well beyond its mechanical limits (which is going to be around 300-400 watts RMS), the voice coil gets driven outside the magnetic field that’s contained mostly within the magnetic gap. The reactance from the coil inside the gap adds to the DC resistance of the coil outside the gap.

Now, when the coil is driven outside the gap the impedance falls to the DC resistance which is generally about 60% of the nominal impedance.

This means that the amp is now trying to drive about a 2.5 ohm load which is well below the amp’s allowable minimum load. This likely is triggering the over current protection circuit resulting in the amp shutdown.

I think you might have nailed it. So, I would like to add that the symptom happens any any master volume level, even at 1. I have a couple 8ohm PA speakers, (full range), rated at 600w, and I plugged one in, couldn't reproduce the problem. I plug both in for a 4ohm load, couldn't reproduce the problem.

Another observation: When I was using the 4ohm cab, and I would pop the higher register strings, it was the worst when the treble was at 10 and the brite on, AND, evertime I would pop the string, even when the amp woudn't cut out, Heard the fan kickon for 1 second, then turn off, every pluck.

My real question for this thread is, is the amp ok? I am fine with upgraded the cab, I was going to build a couple of 12/6 cabs with eminence delta 12 LFA 500w 8ohm drivers anyway.

And thank you everyone for your input, this has been a learning experience, rookie mistake on my part thinking that power amp output is linear, from an impedance perspective

 

[agedhorse]

It’s possible that the 4 ohm driver is damaged from being overdriven also. This can result in additional causes for the amp to go into protect mode.

 

[6thplanet]

I'd be leaning toward that

 

[Oddacious]

Well, I just got the amp, was testing it, I haven't cranked it, and the cab still works fine with the backline 600 (300w RMS @ 4ohms), and I can crank it without any distortion.

Since I purchased the amp used through reverb.com, I was wanting to test it and ensure it was ok before the 14 day return window. I can try to put it on a multimeter to see how low the resistance drops on that cab, probably tomorrow.

 

[6thplanet]

So you can't get the Hartke as loud as the GK through that cab?

 

[Oddacious]

So you can't get the Hartke as loud as the GK through that cab?

With the treble maxed, that is correct. If I tone down the treble, than the hartke gets LOUD.

 

[Oddacious]

Im wondering if the crossover network is complicating the matter. the resistance at the jack is 5ohms, if I unplug the woofer, the speaker is at 4ohms. the crossover has some shunts in it.

Also, the issue i've been having I can get it do do it at gain @ 10, master volume at 1, but the brite is on and the treble has to be at max on both the amp and the bass. Obviously, I would never play in these conditions, just testing the amp for issues.

 

[agedhorse]

There may also be additional protection if the amp detects excessive high frequency energy. Many of these protection circuits can be interactive as well.

Since there is an iron core LPF choke present on the woofer, and with the cabinet rated for 1/2 of what the amp is capable of, it’s also possible that the choke is saturating. If there’s a shunt cap present and the choke saturates, the amp will then see an illegal capacitive load which some class D amps have protection against.

Put everything bait together and use the gear as it was intended. I think you are creating the problems through how you are using it.

 

[Oddacious]

I opened a ticket with hartke, we'll see how it goes. Agedhorse, I do appreciate your input, but I do want to say that the context of this is not griping because this is happening with a rig that I want to use, but testing an amp I just got with the only available cab I have to determine if I need to send it back within a 14 day return window or not. If I do keep it, I will be running dual 8ohm 12's rated at 500w rms a piece.

 

[agedhorse]

I opened a ticket with hartke, we'll see how it goes. Agedhorse, I do appreciate your input, but I do want to say that the context of this is not griping because this is happening with a rig that I want to use, but testing an amp I just got with the only available cab I have to determine if I need to send it back within a 14 day return window or not. If I do keep it, I will be running dual 8ohm 12's rated at 500w rms a piece.

Then that's what you should be testing it with.

My (extensive class D) experience suggests that there is something wrong with your current speaker.

 

[slyslam]

I bought this through reverb.com recently, and it sounds fine, but when i was slapping and plucking this evening, it started to cut out. I've made a video reproducing the symptom.



I'm currently using an Ibanez SR600 pushing an avatar SB112. The video was taken at a relatively at a low volume.

It gets worse when either:

-The brite is turned on
-The gain is passed 4
-the bass volume is all the way on
-the compressor is lower

Is this normal or should I send it back? I was using a backline 600 and never had this issue.

Is your compressor full tilt??

 

[Oddacious]

Yes, but at max, its a fixed compression ratio of 2:1. Interestingly, the documentation says the brite switch has no effect when gain is at 10.

 

[agedhorse]

The bright switch is linked to the gain control, so it is position dependent.

 

[DWBass]

Why is your compressor knob ALL the way up? Could be choking the life out of your signal. Bring it down to about noon. Lower your gain to about noon and turn the volume up.