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Author Topic: Power amp specs and ratings....  (Read 11599 times)

John Roberts {JR}

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Re: Power amp specs and ratings....
« Reply #30 on: August 07, 2015, 06:42:03 pm »

John, I think duty cycle is the missing variable, it goes hand in hand with crest factor.  The new PLD amps from QSC have a 20ms spec.  I find that useful.
Thats only part of a spec. Sounds like maybe burst power, back in the '70s the IHF had a similar transient power (dynamic headroom) spec. The 20mSec stands for how long the amp makes the burst power. The legacy headroom spec also quantified a rep rate or recharge time. So 20 mSec of full burst power and 480 mSec for the PS to recharge. Obviously the faster the rep rate the less burst power. Yes this gets a little complicated. 

FWIW the duty-cycle for that 20/480 mSec burst is something like 4%.  (Caveat I'm just guessing I looked and didn't find complete details on the QSC burst power, just 20mSec of 1kHz on time, no rep rate or off time. )
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I know older amps had more energy storage, if you unplugged them playing you could still listen for a minute at some volumes.  That is somewhat invalid as I am sure control circuitry when the +5 rail goes down mutes outputs.
An arbitrary data point that means nothing. Modern high power amps turn off before the PS collapses to prevent awkward situations like the output trying to pull up and down at the same time. Old school analog amps will play for several seconds until the PS discharges and fuzzes out the sound.
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I do like the Peavey Class D's.  The price is not too pad and they are very aesthetically pleasing. 
The recent PV class D are after my time... I am still trying to forget the old PV class Ds.  :o
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Lastly, your statement on subjective reviews is interesting.  I have had cases where someone reports that the xxx product blows away the yyy.  When I have both products side by side the delta is not that great.
Whenever somebody reports a night and day audible difference, put both hands over your wallet and slowly back away.  8) Back in the '70/80s my informal rule for audibly significant is can you hear the difference through a screen door out in the yard.
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I think quality equipment at various price points, none of it is going to be widely disparate in performance.  Case in point the QSC KW-181, the JBL PRX-718xlf, the EV ETX18p.  They can all be interchanged without any problem form a client perspective and very little change even with trained ears.  To get a better sub (or amp, processor, mixer etc) than any of these ~$1000 or so boxes requires you to at least double in price, maybe more.  It continues to rise logarythmically and places significant barriers for small operators to move up to the next level.  You have to either capatalize or finance the equipment and be ready for a lower ROI until you actually sell it. 

It gets worse because now that you have updated your amp as this thread is about, now you need to upgrade both tops and subs, more amps, then your board is no longer relevant at the new level, you may need new lifts, it never seems to stop. 

Thanks for distilling down my chaotic thoughts into something digestible.  Oh look squirrel...... ::)

Amplifiers are a mature technology these days, while I would stick to names I recognize. I still advocate letting speaker engineers choose your amps for you.

JR
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Scott Holtzman

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Re: Power amp specs and ratings....
« Reply #31 on: August 08, 2015, 12:59:55 am »


Amplifiers are a mature technology these days, while I would stick to names I recognize. I still advocate letting speaker engineers choose your amps for you.

JR

Clarification (I agree with the points you made).  I drifter when I made this comment.  My original intent is if one amplifier can drive a specific speaker to better audible performance than another similarly spec'd amp then we are missing a spec.  It appears that spec is duty cycle.  For all I know it could be transient intermodulation distortion.  A spec I haven't heard quoted in 20 years.

However when I made this comment I had broadened beyond the original scope to include speakers (both powered and unpowered).  I completely agree.  If a powered speaker exists at the performance point sought then I lean towards it.  People far smarter than I tuned the processing.  Next preferred would be an integrated system like the Vue AL4.  Last would be a processor by the speaker vendor such as EAW and QSC.  The Driverack to me is a processor that happens to have tunings for some JBL speakers.  A different Harmon division.

Lastly, I am stunned when a sound tech comes up to me extolling the virtue of their "tweaks".  Clearly this person is not in charge of a team of engineers.  If the tweeks extend outside the realm of environmental tuning or interfacing then IMHO they are nuts. 

One of Scott's rules to manage technician.  When asked to look at a problem start with what the technician last touched or was in proximity of (as they may lie about touching it).
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John Roberts {JR}

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Re: Power amp specs and ratings....
« Reply #32 on: August 08, 2015, 09:15:24 am »

Clarification (I agree with the points you made).  I drifter when I made this comment.  My original intent is if one amplifier can drive a specific speaker to better audible performance than another similarly spec'd amp then we are missing a spec.
If you can reliably hear a difference, we can measure that difference and quantify it.
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It appears that spec is duty cycle.
Duty cycle is a possible difference but in my judgement would not track with different speakers, but more likely track with musical genre.
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For all I know it could be transient intermodulation distortion.  A spec I haven't heard quoted in 20 years.
Shhh there's a good reason TIM has dropped from the popular vocabulary, it was a spec searching for a problem, and not useful for differentiating between properly designed amps.

A subtle interaction that tracks with speakers is current limiting. More likely to be audible when speakers are paralleled or bridged (i.e. amps run near their current limit). Amp designers try to be generous with extra current output. Customers (and speaker designers) keep consuming the headroom that designers try to provide. Passive crossovers could be a critical variable creating unexpected impedance loads. Large companies that sell both speakers and amps, are less likely to screw up a crossover than a speaker only company, but again large well known companies should be safer. 
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However when I made this comment I had broadened beyond the original scope to include speakers (both powered and unpowered).  I completely agree.  If a powered speaker exists at the performance point sought then I lean towards it.  People far smarter than I tuned the processing.  Next preferred would be an integrated system like the Vue AL4.  Last would be a processor by the speaker vendor such as EAW and QSC.  The Driverack to me is a processor that happens to have tunings for some JBL speakers.  A different Harmon division.

Lastly, I am stunned when a sound tech comes up to me extolling the virtue of their "tweaks".  Clearly this person is not in charge of a team of engineers.  If the tweeks extend outside the realm of environmental tuning or interfacing then IMHO they are nuts. 

One of Scott's rules to manage technician.  When asked to look at a problem start with what the technician last touched or was in proximity of (as they may lie about touching it).
yup..

JR
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Gordon Waters

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Re: Power amp specs and ratings....
« Reply #33 on: August 08, 2015, 09:27:54 am »

.Shhh there's a good reason TIM has dropped from the popular vocabulary, it was a spec searching for a problem, and not useful for differentiating between properly designed amps.


More accurately, it could be said that it was a problem that was very specifically related to the limitations on the transistors used in early solid-state designs, back in the 1960s and 1970s. These transistors had very slow slew rates compared to modern designs (and by that, I mean pretty much any transistor designed after 1980), and to compound the problem, designers were producing amp topologies that pretty much ignored the slew rate- making no real effort to compensate for it.

After the seminal papers by Matti Otala and Dr. William Marshall Leach in the early 1970s exposing the problem and offering several avenues for correcting it, there was a rapid advancement in design- to where, after about the mid 1970s, there were no real further instances of significant problems related to transient intermodulation distortion in amplifiers. I can't think of a design from any significant manufacturer made since then, that had any sort of significant issue with this.

In essence, it's a problem that has, fortunately, been laid to rest in the sands of time, so to speak. As such, it's really not a spec that warrants any real concern, now. 

BTW: I am not an EE by trade, but I studied under the late Dr. William Marshall Leach, back at the Georgia Institute of Technology, so I am somewhat familiar with this topic on a first-hand basis...

Regards,
Gordon.
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John Roberts {JR}

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Re: Power amp specs and ratings....
« Reply #34 on: August 08, 2015, 10:33:17 am »

More accurately, it could be said that it was a problem that was very specifically related to the limitations on the transistors used in early solid-state designs, back in the 1960s and 1970s. These transistors had very slow slew rates compared to modern designs (and by that, I mean pretty much any transistor designed after 1980), and to compound the problem, designers were producing amp topologies that pretty much ignored the slew rate- making no real effort to compensate for it.
Not to quibble but you may be referring to gain-bandwidth product. Transistors do not have slew rates, while circuits using them could*** .  Many early transistor amp designs just dropped transistors into old tube circuits with little appreciation for the differences.
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After the seminal papers by Matti Otala and Dr. William Marshall Leach in the early 1970s exposing the problem and offering several avenues for correcting it, there was a rapid advancement in design- to where, after about the mid 1970s, there were no real further instances of significant problems related to transient intermodulation distortion in amplifiers. I can't think of a design from any significant manufacturer made since then, that had any sort of significant issue with this.
I recall seeing an interview of the older Peter Baxandall before he died, when they asked him about TIM (and I seem to recall a few similar proposed new classes of distortion.. SID?). He shrugged it all off as already well known to designers skilled in the art. (Baxandall was involved in HF radar circuit design during WWII so understood rate of change considerations).
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In essence, it's a problem that has, fortunately, been laid to rest in the sands of time, so to speak. As such, it's really not a spec that warrants any real concern, now. 
Kind of what I said... while at the time proponents of the sundry new distortions had high hopes for their significance. Coincidentally there have been many products designed over the years by engineers who were IMO NOT skilled in the art.  8)
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BTW: I am not an EE by trade, but I studied under the late Dr. William Marshall Leach, back at the Georgia Institute of Technology, so I am somewhat familiar with this topic on a first-hand basis...

Regards,
Gordon.

**** Slew rate. If you studied under Leach (RIP) you should be familiar with his classic paper describing how to rise-time limit an amplifier circuit such that it can never slew-rate limit. Unfortunately customers (who are always right) didn't get that memo, so major amp makers would literally defeat their rise-time limiting, so they could force slew rate limiting to have a number to publish. Try to explain rise-time to audio-phools and their eyes glaze over. ???

I considered Leach's contribution to the art positive, Otala and his fan boys, struck me as much ado about nothing (or very little). The same crowd made hyperbolic claims about known capacitor metrics (dielectric absorption), etc. For some reason audio attracts a population that still believes the available measurements do not completely describe the technology. I consider audio technology mature so I discourage mystical searches for some unknown missing specification. More often esoteric specs get hyperbolically inflated by marketers trying to differentiate their product where little real difference actually exists (like damping factor, etc).   

JR

PS If you studied at Ga tech did you know Dr Patronis? He was another solid citizen for the audio community (IMO).
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Gordon Waters

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Re: Power amp specs and ratings....
« Reply #35 on: August 08, 2015, 03:28:48 pm »


PS If you studied at Ga tech did you know Dr Patronis? He was another solid citizen for the audio community (IMO).

Yes indeed. Learned a lot from him. He, at that time, was THE MAN in terms of distributed delay speaker configuration (taking advantage of the Haas Effect).

He oversaw the system setup when the Fox Theater in Atlanta was refurbed back in the 80s- and a LOT of the basic principles he put in place in that system, are STILL being used. The equipment has been upgraded several times, but the basic setup has remained a lot the same, in terms of the delay parameters and such that he specified. And it still sounds good, IMHO...

Regards,
Gordon
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John Roberts {JR}

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Re: Power amp specs and ratings....
« Reply #36 on: August 08, 2015, 05:14:53 pm »

Yes indeed. Learned a lot from him. He, at that time, was THE MAN in terms of distributed delay speaker configuration (taking advantage of the Haas Effect).
Yup, Dr Patronis was the man for lots of stuff, and IIRC he also felt like he needed to oversee sound for the Ga Tech basketball games.  ;D
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He oversaw the system setup when the Fox Theater in Atlanta was refurbed back in the 80s- and a LOT of the basic principles he put in place in that system, are STILL being used. The equipment has been upgraded several times, but the basic setup has remained a lot the same, in terms of the delay parameters and such that he specified. And it still sounds good, IMHO...

Regards,
Gordon
If the room hasn't changed, the basic sound system design shouldn't change, but yes he was on top of the physics (and psycho-acoutics).

Dr Patronis patented a feedback killer decades before the electronic technology advanced enough to make them cost effective. I brought him into Peavey in the mid-80s to investigate licensing his invention, but it was legally complicated by the first company he licensed it to, so we never reached an agreement. Too bad, I would have loved working with him to bring that technology to a wider market (very hip technology for 30+ years ago).   

JR
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Ivan Beaver

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Re: Power amp specs and ratings....
« Reply #37 on: August 08, 2015, 06:02:13 pm »

Yes indeed. Learned a lot from him. He, at that time, was THE MAN in terms of distributed delay speaker configuration (taking advantage of the Haas Effect).

He oversaw the system setup when the Fox Theater in Atlanta was refurbed back in the 80s- and a LOT of the basic principles he put in place in that system, are STILL being used. The equipment has been upgraded several times, but the basic setup has remained a lot the same, in terms of the delay parameters and such that he specified. And it still sounds good, IMHO...

Regards,
Gordon
Dr.P is a great guy, and very entertaining and informative to listen to.

What I love about him is that he can talk on whatever level you want to and go as deep or shallow as you want.

And he has no problem speaking his mind-whether you like it or not.

I have the unique distinction of the only person who has been shoot by one of his guns.

It was a 45 with a hair trigger and long barrel.

And I shot myself-we won't go into the details.
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Re: Power amp specs and ratings....
« Reply #37 on: August 08, 2015, 06:02:13 pm »


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