QW4F over QW218 and Crown amps...

crownaudio · **Joined:** Tue Oct 23, 2012 4:34 am **Posts:** 5

Hello all,
I'm a moderator for on the Crown Audio forums (dakos), one of our users owns a rig with 2X QW4F over 2X QW218, the QW4F are biamped using XTI1000 for the high end and itech6000 for the low end, the 2XQW218 are wired to another itech6000, he has a problem of frying a couple of LF drivers on the 218 and one HF driver on the 4F, he is interested in better protecting his speakers and setting the internal amps DSPs limiters to the proper levels. You can look at the original thread here and see what I already wrote him.

What he is interested in besides the obvious RMS and peak voltage levels are also the recomended attack and release times.

Any other recommendations are welcome

Avi

SemperFiSound · **Posted:** Tue Oct 23, 2012 11:15 am

Avi,
Thanks for trying to help him out on the Crown site.
Your advice is sound (no pun intended) and consistent with the advice offered here:
viewtopic.php?f=26&t=28629

Your Crown Forum comments confused me a bit:
"For the QW218 a good staring point would be:
Also for the peak limiter set the attack to 4sec and the release to 6 sec.
Also for the peak limiter set the attack to .010sec and the release to 1 sec."

I am sure 'twas a typo or cut 'n paste issue, but if you could please fix it to avoid confusing Mark even more, I'm sure he would appreciate it.

Cheers,
Steve

Marty McCann · **Joined:** Mon Oct 27, 2008 3:50 pm **Posts:** 3491

It is not clear what DSP unit is used and whether or not each power amp is set for identical voltage gain. If it is a Peavey DSP processor then all programs are written for a 40X's (+32 dB) voltage gain.

For the Sub band pass, the RMS limiter Ratio should be 20:1 with an attack of 100 mS and a release fo 1000 mS, the RMS Threshold should be +9 dBu and the Peak Limiter threshold at +16 dBu (Don't knpow about Attack and Release here).

For the Mid bandpass, the RMS limiter should be 20:1 with an attack of 40 - 50 mS and a release of 400 - 500 mS, the RMS Threshold should be +7.5 dBm. and the Peak Limiter threshold at +13.5 dBu (again don't know about At/Rl).

For the compression driver the RMs limiter should be 20:1 with an attack of 10 - 20 mS and a release of 100 - 200 mS, the RMS threshold should be -1 dBu and the Peak threshold at +5 dB (no info on Peak).

Again these are based on all amplifiers matched at +32 dB (40X's) voltage gain.

Mark Paschal · **Joined:** Tue Mar 06, 2012 9:20 pm **Posts:** 9

Thanks guys!

I use the internal DSP of the Crown amplifiers.

crownaudio · **Joined:** Tue Oct 23, 2012 4:34 am **Posts:** 5

Thank you for the great advice and the typo

Like Mark said, the DSP is the internal amps DSP, and I'm not sure if all gains are matched, probably not, but since mark is happy with the way the system sounds and he is only interested in adding a bit of protection, I'm not inclined to recommend changes that are more then helping him achieve what he wants to do. Saying that, I enjoyed reading the relevant white papers you wrote on gain matching, and quite a few more (probably all of them) and I understand the concept, but... please correct me if I'm wrong here, this is just to clarify things, since the limiters are on board the amp and not in an external DSP, we are using actual output voltage levels and for this issue only whether the gains are matched or not doesn't have any effect on the settings.

Thank you again
Avi

ctclark1 · **Joined:** Fri Mar 06, 2009 1:13 pm **Posts:** 219

The voltage gain that Marty mentioned is actually used in the calculation for the limiter thresholds, so I believe even using the onboard DSP it would make a difference what the gain of each amp is.

crownaudio · **Joined:** Tue Oct 23, 2012 4:34 am **Posts:** 5

Chris, thank you for your comment,
Since we are calculating the threshold directly from the speaker specs (power and impedance) and because the limiter is built in the amp and not located in a remote DSP such as the VSX, I still think that for the application of the on board limiters, the voltage gain doesn't matter.

SemperFiSound · **Posted:** Wed Oct 24, 2012 1:22 pm

crownaudio wrote:

Since we are calculating the threshold directly from the speaker specs (power and impedance) and because the limiter is built in the amp and not located in a remote DSP such as the VSX, I still think that for the application of the on board limiters, the voltage gain doesn't matter.

Avi - when we calculate the setting for the compressors (ratio set 20:1 for Limiting) for inside the VSX-26, voltage gain does play a part in the equation.

Your formula (from the other thread): RMS Limiting voltage = SQ Root (1/4-1/2 Continuous speaker rating * speaker impedance)

VSX-26 formula: 20log (((SQRT(Watts*Resistance)) / Gain) / 0.775) which yields a dBu which the VSX-26 requires.

May you please give us a quick tutorial on how the formula Crown uses and the Crown amp Limiter settings? That would help tremendously if you have the time. Always interested in learning different languages!

Thanks,
Steve

crownaudio · **Joined:** Tue Oct 23, 2012 4:34 am **Posts:** 5

Dear Steve,
I belive the answer or confusion is due to the fact the Crown i-Tech limiter takes simple volt units while the VSX takes dBu units, that's why our formulas are a bit different but the principle is the same. But not all Crown amps limiters work the same, for instance the XTI takes dBfs units so the formula is a simple dB ratio formula. Now the reason the itech doesn't use the voltage gain is because it's limiting the actual output voltage and not the signal going into the amp like the VSX does. So if you have a 2000W peak power speakers at 4 ohm the maximum voltage they can hadle is:
SQ Root(2000X4)=89.44V
That's what you enter the peak limiter, it's that simple. On the RMS limiter it's even simpler, you just enter the wattage you want the amp to be limited to (not the voltage like the peak limiter) the DSP measures the actual impedance of the speakers and does the voltage calculation by itself. Now you don't care what the voltage gain is, you know your speakers are protected.

Did I understand your question correctly?

Avi

Mark Paschal · **Joined:** Tue Mar 06, 2012 9:20 pm **Posts:** 9

This has helped me tremendously to set my system optimally and I appreciate.

But...

Through further investigation I discovered that the speakers are reading open at the output but reading normally at the point where the wires enter the back of the paper cone. Forgive me if my terminology is incorrect. I took my finger and wiggled the wires just slightly enough to move them and on 3 of the 4 "blown" speakers a wire popped loose. This seems like a defect for two year old speakers. I'm calling Peavey customer service to see what they have to say, but I'm both pleased (in that my Crown amps likely weren't the culprit) and disappointed (in that the Peavey speakers failed in this way after just two years of what I consider normal usage.)

Thanks again for your help in this matter. I think I have the limiter settings problem licked, I just want to figure out why the speakers failed in this way.

SemperFiSound · **Posted:** Wed Oct 24, 2012 3:07 pm

Avi,
Thank you. Yes. I have not played with any Crown amps with Limiters, so I did not know what units they used or how that sub-system worked.

Makes perfect sense!
Cheers,
Steve

crownaudio · **Joined:** Tue Oct 23, 2012 4:34 am **Posts:** 5

@Mark,
Please continue to update us...
What do you think happened to the forth driver?

@Steve,
You're welcome and sorry for my mistake on the RMS limiter units (voltage Vs wattage), I also corrected it on the original post.

Marty McCann · **Joined:** Mon Oct 27, 2008 3:50 pm **Posts:** 3491

When it comes to power amplifier DSP limiters, I prefer settings based on Voltage as opposed to Wattage. This is becasue the voltage is the same for a given power in Watts, whether the loudspeaker load is 8, 4, or 2 Ohms. Of course Wattage is more simple for the weekend warrior operator, but it requires that the load impedance be entered also.

On the issue of amplifier voltage gain . . . this must still be part of the equation for balancing the over all system frequency response wise. It is a little easier when the voltage gains are matched as then you do not have to compensate for the differences.

To properly set up the amplifier internal DSP, the configurer must know the 1 Watt at 1 Meter sensitivity of the loudspeaker transducers, and adjust the X-Over bandpasses output gains to arrive at settings that will provide a flat response (for starters at least).

If the 1W @ 1M specs show that Sub is rated at 104 dB, the Mid at 98 dB, and the Highs at 112 dB, then all bandpasses must be adjusted for a uniform response. You could reference every thing to the Mid level, which would give you more headroom for the needed +6 dB of high frequency boost above 4 kHz to provide the constant directivity horn equalization after dialing in the -14 dB pad. Howevere just adjusting the output drive levels without taking into account differences in amplifier voltage gain is a mistake.

I do not have the time today to delve further into this subject. There are certainly a great number of people that do indeed understand the details of the issues. But not everyone has the required skill set. If the subject and the math are beyond your experience, then you should have insisted that the person who sold you the system components provide the programming.

Once again a fairly good argument for why sound systems should be designed by knowledgeable audio engineers and not muso's or marketing/sales clerks.

SemperFiSound · **Posted:** Thu Oct 25, 2012 1:02 pm

Marty McCann wrote:

I do not have the time today to delve further into this subject.

And what has you so occupied today, good sir?

Need to drop a name and see if it registers in your 1978-1985 Peavey dealer memory.
Cheers,
Steve

QW4F over QW218 and Crown amps...

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