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[Tim McCulloch]

( Be vewwy vewwy quiet)

"We're hunting Wabbit!" - E. Fudd

[Art Welter]

I had assumed the bass amp or speaker cabs themselves would be designed in such a way to include an HPF protection for this reason so I had never even thought about adding one TBH.

Most "bass amp" speakers have stiff enough suspensions to not get torn up from excursion below Fb (box tuning) and high excursion tends to cool the voice coil, and the second harmonic distortion makes us believe that the fundamental is there even when it is almost non-existant.

Check that the amp is not putting out a bit of DC voltage, it takes surprisingly little to roast a voice coil, or heat it to the point that the former deforms or blisters.

[Debbie Dunkley]

Most "bass amp" speakers have stiff enough suspensions to not get torn up from excursion below Fb (box tuning) and high excursion tends to cool the voice coil, and the second harmonic distortion makes us believe that the fundamental is there even when it is almost non-existant.

Check that the amp is not putting out a bit of DC voltage, it takes surprisingly little to roast a voice coil, or heat it to the point that the former deforms or blisters.

Interesting. Not sure how I'd check that Art but of course a possibility.
The amps he has used with that cab is a GK 700RB, a GK MB500 and a TC Electronic BH550. He only owns the TC now.

He had this speaker cab and the GK MB500 at a friends home where he rehearsed in a small office a few times and it performed flawlessly - even at the last rehearsal they had which was over a year ago. He then brought it home and hadn't used it since.
We pulled it out at the weekend and one woofer rubs.

We just checked the woofer out to see if the heat helped and it is definitely better than before but we can still hear some subtle noise. I might try again to see if I can clear it.

[Debbie Dunkley]

Most "bass amp" speakers have stiff enough suspensions to not get torn up from excursion below Fb (box tuning) and high excursion tends to cool the voice coil, and the second harmonic distortion makes us believe that the fundamental is there even when it is almost non-existant.

Check that the amp is not putting out a bit of DC voltage, it takes surprisingly little to roast a voice coil, or heat it to the point that the former deforms or blisters.

I just remembered something and went looking for this thread from the A&H QU forum from a year ago:

https://community.allen-heath.com/forums/topic/ab168-pp-light

DC leak is mentioned here and I wonder if the 2 are related - as I said before he has used the TC Electronic amp with the GK cab.

[Mal Brown]

Today I ordered the Broughton HPF pedal. Thanks for the advice Patrick and everyone else too.

I'll get Chris to try out the woofer later today to see if there is any evidence of coil rub after the heat application...

I’ve been playing with the Broughton high and Low pass filter pedal. In my case I’m using to tighten up the lows and preserve the 50hz range for the kick and for a slope off at 1k...

Aiming at a Motown / Stacks ‘as recorded’ vibe.  Nice pedal, super clean / clear.

[Mike Caldwell]

I just remembered something and went looking for this thread from the A&H QU forum from a year ago:

https://community.allen-heath.com/forums/topic/ab168-pp-light

DC leak is mentioned here and I wonder if the 2 are related - as I said before he has used the TC Electronic amp with the GK cab.

I remember that topic.

Art brought up a good suggestion.

To check the amp for DC just put a VOM on the DC setting, if it has a range setting put it on one of the lower ranges, hopefully you do not need to move it to a higher voltage range.

Open up the speakon connector and measure across 1+ and 1-.
You will most likely something in the sub one volt reading .0XXX maybe .XXX. If you get into the solid two volts and higher I would say something is going on with the amp.

Check it with the amp disconnected from the speaker and connected to the speaker to see you get a different reading.

Also just watch the speaker cones and see if they move when you connect the amp. If they move at all and stay in a forward or backward position to any degree that is not a good sign!

[Stephen Kirby]

Seems like there are two different problems going on here.

One, a truly "blown" speaker where the coil gets fried and is electrically open.

Two, a warped cone causing voice coil rub.  That can be from any matter of storage the cab has been subjected to.  That you were able to "warp" it back in front of a heater suggests this happened.  There are some reconer people who can cleverly wet a cone and hit it with a heat gun and get rid of coil rub.  Not that I'd trust that driver for long, but for a starving musician...

GK has been making some really great lightweight cabinets recently.  I was helping a guy load recently who had these 2-12 boxes and they felt like they were empty cardboard boxes to pick up.  Sounded awesome as well.  Bass amplification has undergone such a technology upheaval in the last few years that anything 10 years old is best sold to someone who hasn't caught up.

[Debbie Dunkley]

Seems like there are two different problems going on here.

One, a truly "blown" speaker where the coil gets fried and is electrically open.

Two, a warped cone causing voice coil rub.  That can be from any matter of storage the cab has been subjected to.  That you were able to "warp" it back in front of a heater suggests this happened.  There are some reconer people who can cleverly wet a cone and hit it with a heat gun and get rid of coil rub.  Not that I'd trust that driver for long, but for a starving musician...

GK has been making some really great lightweight cabinets recently.  I was helping a guy load recently who had these 2-12 boxes and they felt like they were empty cardboard boxes to pick up.  Sounded awesome as well.  Bass amplification has undergone such a technology upheaval in the last few years that anything 10 years old is best sold to someone who hasn't caught up.

Yes I shouldn't use the expression blown really and should be more specific.

The GK brand...Chris has always owned nice equipment, guitars,  amps, cabs, pedals etc and that has often come with hefty price tag. However, when he came across the GK stuff about 9-10 tears ago, he absolutely loved the tone. This GK cab ( when working), his neo 410 and the even less expensive Backline series he owned in the past has all sounded really good. We haven't had any issues with any of the other cabs he has owned - just this one.

[Debbie Dunkley]

The driver still rubs a bit.
Question due to my ignorance regarding these things....

Why does the woofer exhibit scratching noise when applying pressure ONLY if connected? When I disconnect the wires and remove woofer there is no rubbing.

[Chris Grimshaw]

Could be that the frame is warping a little as you tighten the screws.

Just to be 100% clear, are you reporting this:
- Driver in cab with wires connected => rubs
- Driver out of cab, disconnected => fine

If so, I'm almost certain that you're warping the frame a little when tightening the screws up. Not sure how many there are, but here's how I do it for eight:

- Put all screws into holes
- Tighten on a low torque setting (usually just 1-2/10 on the electric screwdriver), to get the gasket to compress a little. When doing this, do screws in order: 1, 5, 3, 7, 2, 6, 4, 8 - with the screws numbered by going around the circle.
- Repeat same pattern as you go to full torque.

That applies pressure reasonably evenly, and should keep the frame from warping. If you want to double check, you can run a low-frequency sine tone through the woofer as you tighten the screws. If you suddenly hear scraping, loosen that screw and try a different one first.

Chris

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