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GK MB2-500 trouble with some types of cabs?

Mr. voodoobass

Mr. voodoobass

Oct 8, 2003
1,556
2,797
gothenburg, sweden
www.4trackdemons.com
#1
Hi
I have recently got myself a MB2 that works great with the cabinets I have.

Today i got the aportunity to try some custombuilt cabs as I was thinking about buing one.
It all went great until I connected a cab with 1x15" + 1x8". The 15" goes up to 800hz and the 8" takes over from 500hz. The cab was 8ohm.

What happened when I conected was that the amp started making a pulsating loud noice and the light started pulsating as of warning!?!
This happened as fast as the cab was conected and power put on. Tried it with different cables but the same kept happening.

I tried the cab with some other amps (SWR220, EBS Fafner)and it all sounded great.

The problem was only when conected to the MB2-500.
Have any of you heard of this before, about connecting cabs with multiple speakers and built in crossovers. There was no problem with tweeters just with this cab with crossovers for both the 15 and the 8 speaker.
(Note that I could be wrong here about the crossovers)
What he told me is that the design is similar to the "fearful 15/6 cab" that has been built here on TB. Only that he uses a 8" instead for smoother highs. So has anyone with that cab used it with a MB2-500 without having any problems (or with) I would really want to know.

Is there some sort of safety in the amp that is responsible for what happened; because it works just fine now when put through my other cabs?
 
#3
I've had two of these heads....and I'm done with them. I use a GK NEO 4x10 8ohm 800w cab and both heads have consistently, randomly shut down and powered back on on gigs. I am confident there is something seriously wrong with the design of these. I'm taking the second one back today and getting something else.
 
#5
Yeah, I´ve heard of the shutdown issues but my problem was more strange. It´s as if the amp just rejected the cab(it´s of the fearful15/6 design). The answer I´ve got from GK is that to not use the amp with that cab. Right, but the cab works perfect with other amps(Tech-Amp,EBS,Audex). It leads to the conclusion that...
1. The amp can not be used with "PA or High-tech-typed cabs with multiple speakers and crossovers.
2. There is something wrong under the hood.
 
#6
Mr. voodoobass said:
Yeah, I´ve heard of the shutdown issues but my problem was more strange. It´s as if the amp just rejected the cab(it´s of the fearful15/6 design). The answer I´ve got from GK is that to not use the amp with that cab. Right, but the cab works perfect with other amps(Tech-Amp,EBS,Audex). It leads to the conclusion that...
1. The amp can not be used with "PA or High-tech-typed cabs with multiple speakers and crossovers.
Click to expand...

Did the cab builder do an impedance sweep ? Which crossover design did he use?
 
#7
What you've described makes no sense at all. For noise to come from the speaker, signal has to reach the amp first(unless the amp is defective, but then it will do the same thing with any cab, as you've said).
Did you have an instrument plugged in, possibly feeding back? That's the only sensible answer.
 
#9
I tried it with speakon-speakon and then with another and then with a speakon-tele.
Tried it with and without the bass plugged in-same noicebumps.
It really makes no sence to me eather?
I will ask the builder about the crossover design.
The same cables works just fine with other cabs and the GK. And the cab worked great with the same cables and different amps.
 
#10
It´s as if the amp rejects the cab, loudly:confused:
How can this happen, what can cause this in a cab.
Can the crossovers mess up the impedance?
There would be a dip in the mids between the speakers and the amp should still see 8 ohms. He had done a sweep when he built it, he has built lots of basscabs and is very skilled. It doesn´t seem to be something wrong with eather the amp or the cab, but put together it just wont work:bawl:

As is the answer i´ve got from Brian/GK
Hello ,
I am sorry, I feel unqualified to comment on your issue because I have no knowledge of the cabinet that you are speaking about. I have tested the MB2-500 with many professional cabinets and countless combinations without issue. My simple recommendation is to not use that cabinet with your head.
 
#11
Mr. voodoobass said:
It´s as if the amp rejects the cab, loudly:confused:
How can this happen, what can cause this in a cab.
Can the crossovers mess up the impedance?
There would be a dip in the mids between the speakers and the amp should still see 8 ohms. He had done a sweep when he built it, he has built lots of basscabs and is very skilled. It doesn´t seem to be something wrong with eather the amp or the cab, but put together it just wont work:bawl:

As is the answer i´ve got from Brian/GK
Hello ,
I am sorry, I feel unqualified to comment on your issue because I have no knowledge of the cabinet that you are speaking about. I have tested the MB2-500 with many professional cabinets and countless combinations without issue. My simple recommendation is to not use that cabinet with your head.
Click to expand...

No, I'm familiar with that cab design, and there is nothing in there that could really cause the problem. I guess the way to find out is to round up a couple of other heads and try them with that cab. If everything is OK, then its some sort of strange design issue with the GK.

Per some of the above comments, that particular head has so many issues that I would stay away from it.
 
#13
Mr. voodoobass said:
It´s as if the amp rejects the cab, loudly:confused:
How can this happen, what can cause this in a cab.
Can the crossovers mess up the impedance?
Click to expand...

Yes, especially if the mid driver is blown or disconnected. But personally, if the cab works fine with several other amps I'd just go with one of those and call it a day.
 
#16
Passinwind said:
Did the cab builder do an impedance sweep ? Which crossover design did he use?
Click to expand...

2nd order Linkwitz-Riley filter @800 Hz, low pass for the 15" and high pass for the 8", keeping impedance at 8 ohms (both speakers are 8 ohm nominal impedance).

From the manual

Smart Protection Circuitry:
The MB² Series protection circuitry constantly monitors
for unsafe operating conditions such as short circuits,
miss-wired speaker cables, blown speakers, improper
ventilation, and incorrect speaker configurations. In
addition, protection is provided during power up/down. If
the MB² Series senses any unsafe operating conditions, the
output signal is immediately muted and the power light
changes from Blue to Red. The amplifier will remain
muted until the fault is removed. If the protection circuit
activates while playing, turn the amplifier off and check
the amplifier speaker output connections. Be sure that you
are not exceeding the maximum recommend load
described in this manual. If the problem persists, contact
GK directly for technical advice.

So it seems the amp sees the cab as unsafe...
 
#17
Some crossover designs may cause the poweramp section to oscillate and shut down.

There have been several such reports on bassic.ch - all problems have been solved by swapping out the crossover.
(GK MB2 and GB Shuttle 6.0 into House of Speakers)

As to what the exact problem was, I do not know, nothing has been reported, except that swapping the homemade crossover with an off-the-shelf commercial one solved the issues....


Edit: Re-reading those threads - there was the hypothesis that its the RF filter on the speaker output which "resonates" with some filter in the crossover...
 
#19
I would like to add a little bit of general information here, as an amp designer as well as a speaker designer...

There are indeed some speaker cabinets on the market that represent an unsafe load to many amps. They tend to be amps with complicated crossovers where unintentional combinations of R's, L's and C's result in resonant conditions and in some cases cause the output stage to oscillate. These speakers may work ok with some amps but not others, and generally increase the liklihood of damage to all power amps over time. They are known as amp killers.

I recommend that folks do not support such products because they serve the industry as a whole in a poor way. Hopefully those manufacturers will improve their approach and consider the big picture effect that their products have on the amps you folks are using.
 

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