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Gallien Krueger RBH cabs - how is the tweeter protected?

Discussion in 'Amps and Cabs [BG]' started by Airfish, Mar 18, 2020.

  1. Hey everyone!
    I own a Gallien Krueger 410 RBH and a 115 RBH cab. Until now I have been using them with my GK 400RB head, i.e. in full-range mode only. In such configuration the tweeters in the cabs should be protected by some device in the crossover in the cabs, right? Can you please elaborate on that? Sorry, I´m not a technical type....
    I am also considering an upgrade to a GK 700RB head, which has the biamp mode - and unless I´m wrong, in biamp mode the internal crossovers in the cabs are bypassed, right? So if really so, how are the tweeters protected against overload, in the biamp mode?
    Thank you very much in advance for your replies!
  2. telecopy


    Dec 6, 2009
    I understand the bi-amp GK heads have a separate 50-watt amp for the tweeter. You control the volume of the tweeter from the amp head face.
  3. Kro

    Kro Supporting Member

    May 7, 2003
    New Jersey
    That is a weird way to put it, but I guess it technically isn't wrong... the crossover itself will only allow high frequencies to pass to the tweeter. It won't stop you from overpowering the tweeter, or high-frequency distortion from frying it, but it should prevent the tweeter from receiving lower frequencies than what it can handle.

    Right, there is an active crossover in the amp that does the same thing. It just splits the signal before it's amplified: only the high frequencies go to the 50w tweeter amp, and that is the signal that is sent to the tweeter on your cabs (when utilizing the appropriate 4-conducter SpeakOn cable, of course).

    Check out the top right-ish hand corner or so.
    Screenshot 2020-03-18 20.45.30.png
    Last edited: Mar 18, 2020
    BasturdBlaster likes this.
  4. Guys, thank you for your replies so far. I think I know how the crossover in the RBH cabinet functions, in general, but I thought there would be some extra protective feature, like a circuit breaker, or a limiter, or something like that. Once I really cranked my amp, with pretty extreme tone setting and I saw some kind of light flashing on the cab, but didn´t exactly notice where the light came from, cos it scared me, I stopped playing immediately and turned the amp down....
    BasturdBlaster likes this.
  5. Wasnex


    Dec 25, 2011
    The cab has a couple fuse lamps that provides protection when you have the cab set to Full Range. If you hit the speaker with enough high frequency power, the fuse lamps will light up. This cause the lamp's resistance to increase which limits power to the tweeter

    FUSE -Lamp (15w,12v) - gallien-krueger.com

    I believe in biamp mode the fuse lamps are bypassed but I am not 100% certain. I do know the cab's passive high pass filter is bypassed, as the RB series amps always apply a 5khz HPF to the tweeter amp. Also I believe the tweeters are rated at 50W, and the tweeters are way more efficient than the cone drivers. IMHO would be unusual to overdrive the tweeter unless you are really abusing it.

    You can see the two fuse lamps on the crossover assembly below:
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  6. Wow, thanks for your detailed reply, sir!
    ....so if I understand it correctly, in biamp mode the cab is not protected by those fuse lamps, since they are bypassed? And the only "protection" is the "safety margin" of the tweeter load capacity, in combination with me "not abusing" the cab too much?

  7. Kro

    Kro Supporting Member

    May 7, 2003
    New Jersey
    Oh yeah, I don't know how I forgot about that.

    Comparing the tracings on the top, and the tracings on the bottom, it seems to me like the fuses are indeed bypassed when in bi-amp mode. @Wasnex , I'd be curious if you see something different though.

    To my eye, it seems like the two tracings that go from the SpeakOn right to the full-range/bi-amp switch on the back (I'm assuming those are from the horn amp), shoot straight up to the horn terminals on the top in the shot you posted.
    BasturdBlaster and Wasnex like this.
  8. Wasnex


    Dec 25, 2011
    I believe this is correct. Can't find a schematic to prove it, but I agree with @Kro 's circuit analysis. You could confirm by running the cab in biamp mode, and pull one of the fuses to see if the tweeter goes dead. I don't think it will have any impact.

    Keep in mind the HPF in the RB series amps is unusually high at 5khz. A more common tweeter HPF in bass cabs is 3.5hkz. The high HPF will reduce the stress on the horn driver.

    If you use the cab for guitar the tweeter is more at risk, especially if you run heavy distortion. Extremely heavy distortion with bass may overload the tweeter as well, but most people turn tend to turn the tweeter off, or at least way down when they are running distortion.

    FYI, the woofers are run full range, regardless of whether the cab is in biamp or full range mode. The cabinet does not actually have a complete crossover, it's only an HPF section and switching to convert from full range to biamp mode.

    The intent of GK's biamp scheme with the later RB series is to allow the woofer to become compressed and distorted while the tweeter stays clean. The Boost circuit has a special FET that is designed to emulate tube amp drive. The FET is in the woofer's signal path, but not the tweeter. It is possible to over drive the preamp before the Boost circuit, and this will cause some harsh and nasty distortion that is passed the tweeter.

    One way I like to run the tweeter is more to broaden dispersion than change tone. I have a 700RB/112 combo. The dispersion of a 12" speaker tends to narrow and then get beamy towards the top of it's usable frequency range. What I do is adjust the horn so I can barely hear it on axis. Off axis the horn fills out the highs nicely. The idea is so I can move around the speaker a bit more without loosing the highs.
    BasturdBlaster and Kro like this.
  9. This is very important from your reply:

    "The intent of GK's biamp scheme with the later RB series is to allow the woofer to become compressed and distorted while the tweeter stays clean. The Boost circuit has a special FET that is designed to emulate tube amp drive. The FET is in the woofer's signal path, but not the tweeter. It is possible to over drive the preamp before the Boost circuit, and this will cause some harsh and nasty distortion that is passed the tweeter."

    ....and since there is no separate woofer/tweeter paths in the 400RB, I must have overloaded the cab´s tweeter during experiments with extreme volume and brutal distortion, with the Boost almost dimed.... ;) But a few flashes of the "warning light" were enough to tame me. :)
    Wasnex likes this.
  10. Kro

    Kro Supporting Member

    May 7, 2003
    New Jersey
    I'm not quite as good at reading block diagrams as some, in the diagram posted above, "boost" appears to be before the crossover, while the "boost gain" appears to be outside of the crossover path. I never really gave it too much thought, but in the past, I assumed that boost was before the crossover in the amp, and was just designed in such a way that it didn't create much, if any HF distortion.

    The boost stage not creating high frequency distortion is something that I've actually seen first-hand when using a spectrum analyzer on a post-di signal. Just curious.


    As @Wasnex noted, in those situations, usually users just turn off the tweeter. HF distortion from a tweeter isn't usually a very pleasant sound anyways... so usually players self correct on that front pretty quickly. ;) Otherwise, continue to go nuts (within reason). :bassist:

    Keep in mind that if you do go to a 700RB or any of the other bi-amp capable amps, any HF distortion that is injected before the amp (like from pedals) will still be passed to the horn... as long as it's on!
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  11. Wasnex


    Dec 25, 2011
    I believe the FET is labeled "Boost Gain" on the block diagram. The Block diagram shows the Boost Gain circuit can be fed either a full range signal or a (5khz) low passed signal. I.E. the FET is always in the woofer signal path, and also the post DI signal path.

    The Boost control is just a regular potentiometer and it is before the crossover, so it adjusts the level of both the woofer and tweeter channels. This control is basically an attenuator that allows you to vary how much of the preamp signal passes to the next stages.

    The signal path is a bit confusing to trace and varies a bit depending on how the switches are set. I have attached the schematic for the series I 700RB. If you like, open it up to page 1, and rotate it 90 degrees CW. The signal path for the series I 1001RB is the same. I believe the signal path for the series II amps is also very similar.

    I will use approximate grid coordinates to help you find components and labeled points on the schematic.

    The signal exits the Boost control at B2. This is labeled "A" and "39mV". The Boost control feeds the Crossover circuit at BA32. Note the labels "A" and "39mV". The tweeter signal is routed across the top of the circuit to the 50W tweeter amp at BA6.

    The woofer signal has two tracks depending on the position of the Woofer Xover switch. With the Xover bypassed, the signal goes through R7 at A3 to the switch at A6. The signal enters the switch on pin 1 and exits on pin 2 which is labeled "C".

    When the Xover is engaged, the signal goes through the LPF section at A54, before entering the switch on pin 3 at A6. Again the signal exits on pin 2 which is labeled "C". Depending upon the position of the Woofer Xover switch, C is either a full range signal or has a 5KHZ low pass applied.

    Next find the input to the FET which can be found at B2. This is also labeled "C" The output of the FET is picked off at B32 to drive the woofer amp, and also proceeds over to the Pre/Post switch for the DI at B4.

    I have seen your scans that suggest the Boost Gain circuit does not create a lot of harmonic content in the tweeters frequency range. This would seem to take away from one of GK's stated reasons for biamping. Keep in mind the Boost Gain circuit will also compress, so maybe the intent is to keep compression out of the tweeter.

    I actually think limiting the harmonics the FET generates is probably a good thing since the output of the FET drives the post DI. Hopefully this would keep the DI from sounding too fizzy. I generally run my 700RB clean, so it doesn't matter to me ;).

    Note: I will be the first to admit that I don't see how an HPF section is formed on page 1...but I believe the signal path I have describe is for the most part correct. Perhaps the HPF is actually built into the tweeter amp, which can be found at B2 on page 3 of the document.

    Attached Files:

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  12. Kro

    Kro Supporting Member

    May 7, 2003
    New Jersey
    This makes a ton of sense. Before your post, one of the reasons why I assumed that "boost" was before the crossover in totality, is because of the level adjustment aspect. I had assumed that as a gain stage, the FET was an integral part. It makes less sense to me now, but previously that was the logic I had used.

    However, your post now makes the block diagram make much more sense given that the "boost" knob is just a pot that controls the level of signal going to both the crossover and the FET! I've looked at that diagram dozens of times and it finally makes sense! "BOOST" is the knob/pot "BOOST GAIN" is the FET. That's the whole reason why they're labeled separately!

    I don't think so. Coming from a history of lower-powered amps that were frequently run in a way that utilized power-section distortion, I believe the whole HMS system was developed purely to keep high frequency power-amp distortion from being sent to the horns - hence why the duplicate system is at the very end in the power-amp stage.

    Otherwise, they probably could have found an easier way to blend clean highs with distorted lows prior to hitting a single power amp. At least that's my belief.

    I'm curious about this as well, but I think the tweeter amp is a standard style integrated chip thing of some sort, of the type that's used across many different audio applications. My gut is that the crossover must come before it, but I have no evidence to support it. Maybe if I stare at the schematic enough it'll appear like a Magic Eye picture. ;)

    In some literature, GK references a "Triple pole constant voltage crossover" I'm sure you're already aware of that... I was able to follow your path through the schematic, but at the same time... I really have no idea what I'm looking at. :p
    Last edited: Mar 19, 2020
  13. Wasnex


    Dec 25, 2011
    The term "Boost Gain" used on the Block diagram confused me as well :confused:.

    GK speaks of driving the power amp to the rails, which will cause clipping in the power amp that is independent of the distortion caused by the FET stage. Especially when using this logic, it makes sense to have a separate tweeter amp that will remain clean after the woofer amp starts to transition into drive.

    I believe triple pole just means 18dB slope filters. Constant voltage relates to how the signal should theoretically sum through the crossover region.

    The crossover part of the schematic does appear to have components that make up an active LPF. The capacitors and the way the TL072 are connected is the key.

    An active LPF might look something like this:

    Basically some of the highs are sent back to the inverting input so they cancel out.

    You basically see three of these in the crossover circuit (thus three pole).

    I don't see any HPF circuitry, as the only capacitors on the top part of the crossover schematic appear to form an LPF for the tweeter cut switch. The rest looks like pure amplification to me.

    An active HPF might look something like this:
    There are no circuits like this.

    A three pole active hi pass with one stage of amplification might look like this
    The closet thing I see to this is in the actual tweeter amp.

    There is some interplay between the top and bottom of the crossover circuit that involves capacitors, and perhaps this interplay forms the tweeter's HPF, but I don't think so. Over my head if that is the way it works :dead:.
    Kro likes this.
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