A few questions on Specs and equations.

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Brian Murphy

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Jun 18, 2012
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I've been reading a few threads on subs and have some questions.

On one thread on subs Phil Graham was explaining V_rms=sq.rt of P_in*R_dc for the power input to a driver. I get that.

However some speakers like the UCS1 give "Program"(2400W) and "Peak"(1200W) so to use the above formula shouldn't I use .707*Peak in the equation?

So: Sq.Rt of [(1200W*.707)*8(ohms)]= 82.38 V_rms. If I use the equation for "Power": P= (E*E/R)*1.414 and you get 1200W peak give or take calculator rounding so this would seem logical.

OK so if you now look at Amp specs and they don't give a V_rms spec would one use (.707*Slew rate) as V-rms, since slew rate is the max change in output voltage? Or am I off base? If so, what spec would be the best to covert or arrive at V_rms if it's not given?

Should one match the V_rms output of the amp to the V_rms of the speaker or would it be better to convert both calculations to Peak?

Thanks,
Murph..............
 
Re: A few questions on Specs and equations.

RMS is continuous/thermal wattage
Peak is what it can physically handle

In most cases, peak is significantly more than 6dB higher than the continuous rated wattage, depending on passband and signal type. In most cases, you won't be able to calculate peak - it will be dependent on the exact box/driver configuration. You could measure distortion to determine this, or blow up a few woofers?

As far as continuous, there is obviously no way the UCS1 handles 1200 watts continuous. Since I know that driver is very similar to a LAB15, it probably is rated at 600. And I almost always take off 3dB for the continuous spec just to be safe; it usually doesn't affect peak output.

I hope my stream-of-consciousness post made sense. Like Ivan is going to say, "at what frequency" or "it's never so simple"...
 
Re: A few questions on Specs and equations.

Mark, thanks for the update! It makes a lot more sense.

Silas, this is the part I find confusing. If 600W=27.78db and I take off 3db leaving 24.78db, converting that back to watts=300W. So are you saying you would power a UCS1 with 300 Watts RMS power?

And how does this relate to Phil's suggestion of working with V_rms? 600W@8ohms = 69.28V_rms, and 300W@8ohms = 48.98V_rms that's a 27.8% reduction in V_rms but 50% reduction in power. If we want to keep the cabinet working close to peak efficiency (with respect to power in) that seems like a huge amount to give up.

I think I'm missing something here OR the two concepts are totally different in their approach.
 
Re: A few questions on Specs and equations.

Brian Murphy said:
Mark, thanks for the update! It makes a lot more sense.

And how does this relate to Phil's suggestion of working with V_rms? 600W@8ohms = 69.28V_rms, and 300W@8ohms = 48.98V_rms that's a 27.8% reduction in V_rms but 50% reduction in power. If we want to keep the cabinet working close to peak efficiency (with respect to power in) that seems like a huge amount to give up.

I think I'm missing something here OR the two concepts are totally different in their approach.
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Brian,

I'm not sure the thread you are referring too in regards to subwoofers, but hopefully this will clarify:

  1. Music, in the worst case of a hyperlimited CD will have an instantaneous peak to average ratio of about 3dB. Even with that, the long-term peak to average (level/voltage) ratio of said song will be about 6dB. A carefully crafted live mix, with plenty of compression, will double these two figures, or more.
  2. Modern high performance loudspeakers typically start exhibiting substantial deviation (>1dB) from "cold" performance with long term average input power levels 4-6dB below the AES rating. Just because a loudspeaker will survive at the AES rating doesn't mean you'd want to use it there. 6dB below the AES input power is still a substantial level of input.
  3. Loudspeakers mounted in most common LF enclosures exhibit highly reactive impedance, so the input power over a broad chunk of the low frequency is actually rather small. In the low frequencies where the average impedance is higher, the average cone movement is generally also higher, so there is some cooling aid from the cone movement.
  4. Most loudspeaker heating takes place in the midrange, where the loudspeaker Z is near Rdc and there is little cooling from air flux through the voice coil assembly.
  5. In the subwoofer range, there are (more or less) three places where the loudspeaker can trip up. The first is in simple over excursion. The second is in port compression, and the third is sub performance degrading from the shift in theile small parameters due to voice coil heating.

The average modern vented loudspeaker with good drivers will rarely be damaged due to mechanical over excursion. In similar fashion the same driver will rarely be damaged thermally with typical live sound performance material. The most common ill behavior from typical modern pro subwoofers is obnoxious noises and/or compromised performance due to port effects.

A second, longer term effect is the ongoing rise in steady state voice coil temperature over the duration of a session of extended output. This won't damage the loudspeaker system, but can hurt the tonality of the subwoofer system. This thermal change, as well as excessive excursion leading to IMD distortion components are two common tonality shift issues for subs.

One way that loudspeakers are damaged thermally is in the improper application of limiting! The well-meaning pa operator uses a compressor to "protect" their loudspeaker. Instead the compressor ends up increasing the average input power, and that destroys the speaker thermally. Removing "protective" limiting of this type can be a better way to protect the loudspeakers, especially if the amplifier has a good quality clip limiter.

Setting limiters (or unsetting them) is something that I do for clients. I wish I could say that there was a standard rubric I could post on how to approach this, but I've found it to be different in almost every setting due to the unique makeup of the gear in the system. It takes into consideration the details above, and usually combines a mixture of listening, measurement, and design simulation.
 
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Re: A few questions on Specs and equations.

Thanks guys! I see why some people blow the crap out of woofers and others don't!

OK...so...what specs on an amp data sheet are the most critical to good performance in the real world of live gigs? Obviously there can be a large difference in the price of two amps where "some" of the specs are similar. Reliability, construction materials, and the quality of components all come into play as well as brand recognition but what specs should I be looking at?
 
Re: A few questions on Specs and equations.

Brian Murphy said:
Thanks guys! I see why some people blow the crap out of woofers and others don't!

OK...so...what specs on an amp data sheet are the most critical to good performance in the real world of live gigs? Obviously there can be a large difference in the price of two amps where "some" of the specs are similar. Reliability, construction materials, and the quality of components all come into play as well as brand recognition but what specs should I be looking at?
Click to expand...

What can you afford? Can you afford a decent limiter if your amp does not provide one? My recommendation if you like DSP amps is the Crown Itech HD series, which has fantastic limiters. If you don't need amp DSP, the Crown Macrotech I series has similar limiters without any AD/DA stages. Both are relatively expensive, but offer lots of performance.

More usual amplifiers might be something like QSC PLX or Crown XTi...they are all OK. Not great.
 
Re: A few questions on Specs and equations.

Phil Graham said:
Setting limiters (or unsetting them) is something that I do for clients. I wish I could say that there was a standard rubric I could post on how to approach this, but I've found it to be different in almost every setting due to the unique makeup of the gear in the system. It takes into consideration the details above, and usually combines a mixture of listening, measurement, and design simulation.
Click to expand...

Silas-theres your "it depends".

It is not always simple and everybodies needs are different.

Kinda like setting up the tuning on a car. With a professional driver-it is quite different than with a beginner driver. Do you want perfromance or protection? Choose one-based on the particular operators skill level.
 
Re: A few questions on Specs and equations.

Silas, we all have to start somewhere and the iTech HD's are out of my range. Unless of course I find one or Macrotech I that needs repair, and if a schematic is available. For amps that didn't have a DSP I will most likely purchase a separate unit.

But in regard to other amps like QSC, Crown XTi, etc. what specs are the most critical after matching up for watts? Damping factor, Slew rate, THD, etc.
 
Re: A few questions on Specs and equations.

Ivan Beaver said:
Silas-theres your "it depends".

It is not always simple and everybodies needs are different.

Kinda like setting up the tuning on a car. With a professional driver-it is quite different than with a beginner driver. Do you want perfromance or protection? Choose one-based on the particular operators skill level.
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I should mention that a significant advantage of re-entrant horn designs is reduced problems with airflow-speed related issues. Tom, Ivan, and crew have quite the head start on designing good performing cabinets of this type, and I see it as a solid reason to consider Danley products for low frequency reproduction.
 
Re: A few questions on Specs and equations.

Brian Murphy said:
But in regard to other amps like QSC, Crown XTi, etc. what specs are the most critical after matching up for watts? Damping factor, Slew rate, THD, etc.
Click to expand...

20hz-20k watts are the only thing you should be worried about. The rest is basically marketing BS. Buy the most powerful amp you can afford and call it a day. Use your ears as a primary limiter, and whatever electronic piece you want as a backup limiter.




Evan
 
Re: A few questions on Specs and equations.

Brian Murphy said:
. Unless of course I find one or Macrotech I that needs repair, and if a schematic is available. .
Click to expand...
Schematics are easily available for most of the Crown products.

HOWEVER-Making a broken unit work is often a different story.

My experience is that when they go bad-it is not always an easy fix (schematic or not). There is a lot going on-and lots of various "control"-so it is often not a straight forward repair.

Just go to their website and download a schematic and you will see what I mean.

Some amps are very easy to fix (old QSC's and Peaveys come to mind). Pretty straightforward. I never enjoyed working on the Crowns.
 
Re: A few questions on Specs and equations.

Ivan Beaver said:
HOWEVER-Making a broken unit work is often a different story.

My experience is that when they go bad-it is not always an easy fix (schematic or not). There is a lot going on-and lots of various "control"-so it is often not a straight forward repair...

Some amps are very easy to fix (old QSC's and Peaveys come to mind). Pretty straightforward.
Click to expand...

I just purchased a Crest 8001 that was supposedly in full working condition. Turns out the B channel would go into protect with moderate input levels even with no load connected. It took a very experienced Crest bench tech 9 hours to figure out that it was a leaky cap on the protection PCB. The old Crests are usually pretty easy to fix, but even relatively simple amps can be a headache on occasion. Like you said Ivan, schematics are only so helpful. Knowledge of electronic circuit basics and experience are the real tools. As on old Sony 1" VTR manual from 80s once said in plain english for one of those alert balloons on a critical transport adjustment, "The key to making this adjustment is knowing what you are doing." Gotta love that one!
 
Re: A few questions on Specs and equations.

Ivan Beaver said:
Schematics are easily available for most of the Crown products.

HOWEVER-Making a broken unit work is often a different story.

My experience is that when they go bad-it is not always an easy fix (schematic or not). There is a lot going on-and lots of various "control"-so it is often not a straight forward repair.

Just go to their website and download a schematic and you will see what I mean.

Some amps are very easy to fix (old QSC's and Peaveys come to mind). Pretty straightforward. I never enjoyed working on the Crowns.
Click to expand...

Crown, AFAIK, does not release schematics of their class I amps due to IP concerns. However, they sent me one. How do you say it lightly....it's really, really complicated. I think if I blew up the 9 pages to wall size the components would still be barely visible...
 
Re: A few questions on Specs and equations.

I have a background in electronics at component level repairs, not as a board swapper. I have a bench with a load, scope, audio gen, frequency cntr, AC meter, multimeter, distortion anal, variac, etc.

I am aware that diagnosing a defect is not a walk in the park. I once worked on a "sample" amp from Japan. It took me quite a while to realize that there were minor differences in the supplied schematic v the sample unit. It turned out that a diode in a feedback circuit was in backwards....BUT...it was correctly oriented according to the board layout. Yeah, I've been around a few schematics. I'm not an expert on Crown, QSC, Crest etc. but I have worked on them.

If I can pick up a nice amp for a song, after asking a few questions and checking the schematic, it might be worth a few hours of my time.
 
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