Members MrJoshua Posted July 13, 2009 Members Share Posted July 13, 2009 Hello again. I've been changing some things around about my setup, including switching to a dual-mono setup instead of running both mains off one side of the amp. This of course means I need to address my limiter settings. I'm pretty confident about the math, but I don't work with dB a whole lot so I'm hoping I can get y'all to take a quick glance at my numbers and make sure I'm not messing anything up, ok? I don't expect y'all to work equations for me, of course, but just a quick yes or no about whether I'm at least using the right equations would be great. Mains are Peavey Impulse 1012 (500W RMS, 8 ohms). Amp is a QSC PL236 (800W/side at 8 ohms using the EIA 1% THD number, or 725W/side at 8 ohms using the FTC 0.03% THD number). Limiter is a DBX 1046 4-channel compressor/limiter set for limiting only. The power amp shows an input sensitivity of 1.23V @ 8 ohms to get full rated power. Specs show a 36dB gain, which agrees with my calculations. To limit the output to 500W, I need to limit the output voltage to 63.25V, which corresponds to a .972V input. Now, here's where I get a little fuzzy. I don't normally operate the amp wide open. I tend to dial the input attenuator back a bit in order to keep the noise floor down. But I want to be sure I'm understanding the math here properly. If I set the input attenuator of the amp at -20dB, then the input voltage should be getting cut by 20*log(x)=20dB, so x would be 10. The input voltage gets cut by a factor of 10? Is that right? If so, then if I set the input attenuator at -20dB, would I need to set my limiter to 9.72V in order to get 500W of output from the amp? I appreciate the help. I don't even remember the last time I worked an equation with dB involved, and I'd much rather ask a silly question than assume I know something and have it come back to bite me later. Thanks in advance. Link to comment Share on other sites More sharing options...
Members agedhorse Posted July 13, 2009 Members Share Posted July 13, 2009 Correct (numerically). BUT, if you were to cut the input sensitivity by 20dB, you wouldn't be able to drivethe amp to 9.72 volts (RMS). To do so would require 14 volt rails plus about 2 volts, but most DSPs use +/- 12 volt and some use +/- 15 volt rails. Most analof limiters use +/- 15 volts. So, your example highlights exactly my comments in that you will be clipping your drive electronics well before you get 63 volts out of your amp. There's nothing left for the limiter to limit down from. You need additional swing on the drive electronics (and the front end of the amp too) in order for the limiters to limit down from. Now here's my take on your delemma. IF you choose to high pass the cabinets at say 100Hz (used with a sub) I wouldn't use the outboard limiter but would just use the amp's onboard limiter. You are around 1.5x RMS and if you use even a little bit of care, you will be ok because you have shifted the mechanical limitations of the speaker up when using the HPF. Now you are dealing with purely thermal considerations which are less demanding in most applications. If you were not to use a HPF, then I would suggest limiting because mechanical limits come into play, especially below about 40Hz. Link to comment Share on other sites More sharing options...
Members wades_keys Posted July 13, 2009 Members Share Posted July 13, 2009 ^ This post makes me feel a whole lot better about how I'm running my setup. Still don't understand all of the technicalities, but my amp rating and speaker rating are comparable to the poster (speakers: 250 watts RMS @ 8 Ohms, power amp 500 watts rms @ 8 ohms) I've been using the onboard HPF as well as the onboard limiters.... Link to comment Share on other sites More sharing options...
Members Dookietwo Posted July 13, 2009 Members Share Posted July 13, 2009 http://www.peavey.com/assets/literature/manuals/80304759.pdf http://www.onlineconversion.com/ohms_law.htm Things to consider. The min impedance of the box is 6.5 0hms at 200 hz aprox. This would draw more current. I entered 6.5 ohms and 500 watts into the above calculator for 57 volts. You could consider this or go with 8 0hms nominal. Just something to think about. If you have a Fluke type meter and have a sine generator set to 200 hz you could measure the voltage from your amp and set your limiter that way. Speakers UNPLUGGED. Most flukes are rms so be sure to account for that. You want your meter set to AC and it must be a sine wave not pink noise or music. Edit: Found this. http://www.slsloudspeakers.com/apnotes/Setting_System_Limiters.pdf Dookietwo Link to comment Share on other sites More sharing options...
Members MrJoshua Posted July 13, 2009 Author Members Share Posted July 13, 2009 Thanks, guys. Sounds like I'd be just as well off simply leaving the limiter engaged on the amp and watching my output. And since I'm just running a pair of Carvin 700W 1x18 powered subs under these, I never push the mains very hard - the subs wouldn't keep up if I was driving the mains amp up near clipping. Well, I feel better. I have a tendency to make this more complicated than I need to sometimes, I think. I think I'll just put the DBX on the patch bay and use it for any channels where I want compression. Link to comment Share on other sites More sharing options...
Members agedhorse Posted July 13, 2009 Members Share Posted July 13, 2009 ^ This post makes me feel a whole lot better about how I'm running my setup.Still don't understand all of the technicalities, but my amp rating and speaker rating are comparable to the poster (speakers: 250 watts RMS @ 8 Ohms, power amp 500 watts rms @ 8 ohms)I've been using the onboard HPF as well as the onboard limiters.... Not comparable. you are at 2x the RMS speaker rating... too high to be comfortable in most cases without additional processing. IMO. Link to comment Share on other sites More sharing options...
Members agedhorse Posted July 13, 2009 Members Share Posted July 13, 2009 http://www.peavey.com/assets/literature/manuals/80304759.pdfhttp://www.onlineconversion.com/ohms_law.htmThings to consider. The min impedance of the box is 6.5 0hms at 200 hz aprox. This would draw more current. I entered 6.5 ohms and 500 watts into the above calculator for 57 volts. You could consider this or go with 8 0hms nominal. Just something to think about. If you have a Fluke type meter and have a sine generator set to 200 hz you could measure the voltage from your amp and set your limiter that way. Speakers UNPLUGGED. Most flukes are rms so be sure to account for that. You want your meter set to AC and it must be a sine wave not pink noise or music. Edit: Found this.http://www.slsloudspeakers.com/apnotes/Setting_System_Limiters.pdfDookietwo The minimum impedance doesn't apply for RMS limiter settings because the whole point is to look at the average thermal performance over the entire driver's operational bandwidth. This means that the "nominal" value is the correct value to use as this is average impedance (more or less) over the driver's passband. If you wanted to get more "exact" (no reason to), you would average the impedance in the passband but this requires a lot of work. Using the minimum impedance would give a more conservative limiter setting which is ok in my book. Link to comment Share on other sites More sharing options...
Members wades_keys Posted July 13, 2009 Members Share Posted July 13, 2009 Not comparable. you are at 2x the RMS speaker rating... too high to be comfortable in most cases without additional processing. IMO. Okay. Club15 mains 250W RMS 8 ohms Amp is 500W/side 8 ohms Probably a stupid question - but all the ratings I've seen for my speakers list the "program" rating: 500W 8 ohms. Also, the QSC amp selector lists the RMX 2450 for these speakers. So I guess what you're saying is I'm right on the edge of what is "safe".... Well then I guess I don't understand the purpose and the intent of this "program" rating - I always thought that was done to provide a safe operating range for consumers as an alternative to the difficult to understand RMS rating.... Link to comment Share on other sites More sharing options...
Members agedhorse Posted July 13, 2009 Members Share Posted July 13, 2009 Program ratings are as much for marketing as for anything else. At least Yamaha is more reasonable than most. For your case, I would recommend around a 350-400 watt/ch at 8 ohm amp. 500 watts is ok as long as you use the 50Hz HPF and are very careful. QSC recommends big amps because in part it's more profitable to sell big amps. It's also an easier sell because people have been conditioned to "supersize". If you blow up other manufacturer's speakers due to too much power at their recommendation, do you think they will step up to the plate and pick up the cost of the repair? Naah. If you look at QSC's own powered speakers, they are powered generally around 1.5x the RMS rating of the system if they were passive. Link to comment Share on other sites More sharing options...
Members wades_keys Posted July 13, 2009 Members Share Posted July 13, 2009 Program ratings are as much for marketing as for anything else. At least Yamaha is more reasonable than most. For your case, I would recommend around a 350-400 watt/ch at 8 ohm amp. 500 watts is ok as long as you use the 50Hz HPF and are very careful. Ah, good. At least I'm doing something right. And yeah, I definitely know to be careful... I had the limiters lit up for the first time yesterday night - scared the crap out of me. Link to comment Share on other sites More sharing options...
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