GX5 max voltage ouput? Setting limiters

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  • Hello all,

    Im dialing in my new setup. It consists of a set of BillFitzmaurice Subs I built. They are folded horns loaded with Emminence Kappalite 3012lf drivers. They are supposed to be limited at 50v and so I am trying to set the limiters on my DBX PA+ to do just that.. The only problem is that I'm getting huge voltage numbers coming out of my 2 GX5's and so Im wondering if this is normal. When I send a 60hz sine wave at normal levels (not clipping) out of my personus studio live 16.0.2 mixer and crank the amp gain all the way up it is reading above 100v. Everyone on the BFM forum says that this amp is rated to put out 60-65v max. So I'm checking here. What is the max voltage this amp can put out? is 100v an abnormal reading? When I plug my iPhone directly into the RCA inputs at the back of the amp and play the same test tone, Im getting 40v which seems closer to my desired voltage. I keep thinking this is a problem with the mixer being too hot, but it syncs well with the dbx pa+ at those levels and if I back it down to reasonable voltage it barely registers on the dbx. Obviously I'm not gunna run my amps wide open all the time, but I would like to set the limiter with them wide open to protect my system just in case it ever goes there. Can someone help me understand this better? Thanks!
    chrismccune
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  • The maximum gain of the GX5 is 34.4 dB, or about 52.5×. So at max gain, if you put 1 V rms in, you'd get about 52.5 V rms out. It would be informative to know what you measured with and how.

    It's unlikely that you were getting 100 V rms out of the GX5 with a sine wave unless the output was clipping severely. In such a case, though, the GuardRail circuitry would lower the gain to reduce the amount of clipping.

    Unless your limiter reads the outputs of the amp, you will have to take into account the amplifier gain in setting the limit thresholds.
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  • Hey Bob, I measured with a multimeter on the ACV 200 setting. No load attached to the amp and measuring with probes from the banana plugs output. Im def not clipping on the board, but am clipping the amps at about 10 o clock. What I did today to get the required 50v output with amp gains all the way up was to turn down the board output until it was reaching 50 volts at unity gain on the output with the signal just below clipping and limiting up from there on the dbx. That way i could turn my amps all the way up and only put out 50v. Some people say this is bad gain structure. What are your thoughts on this?
    chrismccune
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  • First, the board will not tell you if the amp is clipping.

    Unity gain, where the output voltage equals the input voltage, is not labeled on the amp because it's not a practical setting to operate at and because the gain controls would have to be turned down almost fully counterclockwise. It would actually be impossible to make the amp put out 50 V rms at this setting. I suspect that your multimeter might be defective or improperly set up.

    It's generally better for signal-to-noise performance to run power amps at gain settings that are well below maximum, and instead set up the mixer and processing gear to provide a good pro-type signal level. Keep in mind that the amp is just a voltage multiplier, so if you are concerned about its output and you have control over its input signal, you should be concerned with its gain. In a multi-way system, the gains of all amp channels in the system are important as well, because they will affect the overall spectral balance.

    You should check with Bill Fitzmaurice about whether you actually need limiting, and how to set it up if you do. Some types of limiting will help protect a subwoofer, and other types won't. Peak limiting, for one, is the most common type of limiting but is generally useless for protecting subwoofers, while limiting based on long-term average level is effective for protecting subs but is pretty rare and not easy to configure--if it can be done at all on a particular limiter.

    The GX5's maximum rms voltage, just at the onset of clipping, is about 63 V into an 8Ω load, so I'd be more inclined to just keep an eye on the clip LEDs and avoid anything more than an occasional flicker.
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  • Thanks Bob,

    The guys at Bill Fitzmaurice recommend limiting the drivers in the subs the 50v with a brick wall limiter. I have one in my DBX PA+. They do this with the amp gains set wide open to ensure protection even if the amps were to be turned all the way up in a real world situation just in case. The amps will not be clipping if I limit the input to 50v, but will they be efficient? This setup would make the input signal less than 1V rms, correct?

    Also will max volts change if its into a 4ohm load?
    chrismccune
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  • Also is it possible that my mixer is feeding the amp a signal way above 1.2vrms or the input sensitivity. I've tried multiple multimeters probes into the banana plugs on the back of the amps and they give me plus 80v..
    chrismccune
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  • I'm very skeptical that a brick wall limiter at a particular would be effective for protecting a subwoofer. I would expect that the protection should involve limiting based on average levels with relatively lengthy time constants to prevent burning out.

    It's likely that unless your mixer is a consumer or low-grade variety, it is capable of putting out signals of more than 1.2 V rms. Pro-type mixers can generally put out maximum signal levels of about 10 to 15 V rms.

    I'm sorry, I don't understand what you're asking about regarding efficiency.
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  • Hey Bob,

    I guess I'm not asking the right questions.. It seems their is 2 schools of thought in gain structure and I'm navigating both realms.. The guys at BFM run their amps wide open and limit on processing just before the amps to get, in my case, 50v.. Seems to me that the signal would have to be limited pretty severely coming out of my presonus studiolive mixer to give me only 50v at max gain on the gx5. IS that a reasonable assumption? That is the only way Ive been able to do it up til now..

    The other school of thought which I'm getting at the driverack forum and I believe here as well, is to set amp gains to the maximum level before clipping and balance down from there if needed. This way works fine for me and actually produces a better tone out of my setup, I think because of the stronger signal at the input.

    Either way both at BFM forum and here Im getting told that 100v coming out of the gx5 (at max gain) is not likely, but that is the reading I keep getting.. The levels look good coming in and out of my driverack. I don't think the signal is clipping. Thoughts?
    chrismccune
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  • The school of thought on gain structure that I recommend most is neither of those; you would only use maximum gain on the amp if it is actually necessary because of a weak input signal. It is best for signal-to-noise ratio to have a strong signal level early in the signal chain and maintain it than to throttle it down closer to the noise floor and then have to boost it a lot, along with the noise floor, in the power amp. I generally advocate that after you optimize the gain structure upstream from the amps, bring the power amp gains up to just above the least that is necessary, as long as nothing upstream clips and there is allowance for some adjustment room.

    However, if it is your preference to run the amp at maximum gain, then that's what you have to work with.

    The amp's output voltage is a function of its input voltage times its gain, as long as the output is not clipping. It is simple multiplication. To get a desired output voltage, you can adjust input voltage, gain, or both. Don't overthink it.

    As I said before, if you measure 100 V rms from the output of a GX5, either you are clipping the signal heavily or your measurement is faulty.
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  • When u say clipping the signal heavily, would that be clipping on the amp? or the drive rack? or the mixer? Or any and all? Also I tried two multimeters so with the same results so I believe the measurement is good. Also tested the wall outlet and it was 120. Could I be using it wrong?
    chrismccune
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  • I'm referring to clipping the signal in the amp output. When you measure the voltage of the amp output, only the amp output will affect the reading. A clean, undistorted sine wave has an rms voltage that is 0.707 × its peak voltage, but one that has its peaks clipped off at ± x volts will have an rms voltage that is closer to the peak value. If you have a sine wave that is so clipped that it becomes a true square wave, its rms voltage would equal its peak voltage.

    The power supply rail voltages are not high enough to allow the amp to produce a clean sine wave of 100 V rms.
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  • Hey Bob. So I clocked the amp output today at just about 80v with 1 channel running and no load attached. The amp clip indicator light turns on if I gain any higher. Is this reasonable or would you still say its a faulty reading? Im 99% sure the signal was clean all the way to the amp. It was 0vu coming outta the board and just under clipping on the drivrack. The amp input attenuator gain knob was at about 12 o'clock..
    chrismccune
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  • 80 V rms is a reasonable point for the amp to start clipping on a sine wave signal, with no load attached.
    Bob Lee
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