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Author Topic: QSC PLD amps  (Read 15856 times)

Tim McCulloch

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Re: QSC PLD amps
« Reply #20 on: November 30, 2015, 04:42:26 pm »


FWIW, the output stage on the Labs is actually class AB. The "class D" section is actually just used to feed the high voltage rail for the output devices, not for audio. Said class D section tracks the incoming audio signal and supplies rail voltage as needed in tandem. I emailed Lab's engineers several years ago to understand what was really happening in their amps as they were calling them "class TD" instead of just "class D." Interesting stuff.

Very cool, Greg!  Thanks for the 'amplification'. ;)
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Mark Wilkinson

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Re: QSC PLD amps
« Reply #21 on: December 02, 2015, 01:29:29 pm »

I'll defer to Bob Lee's answer if he posts one and I'll of course defer to JR or someone else with real electronic circuit design knowledge but I'll venture to guess...

In instances where the "dynamic" or "peak" power is very close to the continuous power, the power is likely limited by voltage not current. It may not be possible for the amplifier circuit to produce more than X voltage per channel but it can produce that X voltage all day long. This corresponds to the area to the right of each peak in the chart below.

When more "dynamic" power is available the power is likely limited by current not voltage. Running over current rating for brief periods can be handled because it's mostly a matter of dissipating that extra heat over time. Continuous power is limited by the heat dissipation rate. This corresponds to the area to the left of each peak in the chart below.

This chart is helpful to look at some of the relationships. You can see Ohm's law at play.

Thx Corey,

Yes, I've read and studied the FAST paper a few times, and hold the same views about current vs voltage constraints you describe.

But I still have a hard time fully tying the PLD 4.5 specs with the charts in the FAST paper.....particularly the dynamic vs continuous numbers.

And if you'll note the continuous 2 channel outputs vs the FAST graph you posted, the 8 ohm output can only be obtained bridging a pair of channels, while the 2 and 4 ohm outputs need to be paralleled. 
Seems like this should be footnoted or something, because it appears that you really need to know which side of the 5.5 ohm single channel peak  the amps will be pushing.... when you go to combine channels...
Also seems like this means the amp might have a bit of difficulty with subs that have relatively large minimum and maximum impedances ???



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Dale Sandberg

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Re: QSC PLD amps
« Reply #22 on: December 02, 2015, 07:25:24 pm »

Thx Corey,

Yes, I've read and studied the FAST paper a few times, and hold the same views about current vs voltage constraints you describe.

But I still have a hard time fully tying the PLD 4.5 specs with the charts in the FAST paper.....particularly the dynamic vs continuous numbers.

And if you'll note the continuous 2 channel outputs vs the FAST graph you posted, the 8 ohm output can only be obtained bridging a pair of channels, while the 2 and 4 ohm outputs need to be paralleled. 
Seems like this should be footnoted or something, because it appears that you really need to know which side of the 5.5 ohm single channel peak  the amps will be pushing.... when you go to combine channels...
Also seems like this means the amp might have a bit of difficulty with subs that have relatively large minimum and maximum impedances ???

Hi All,
As the Product Manager who along with our engineering staff developed the PLD amps, I will try to answer some of the lingering questions.

First of all Amplifier Design: Corey appears to have a good understanding of the principles that govern our Flexible Amplifier Summing Technology.  To add to what he has said, all single amp channels (from any manufacturer) run up against two limiting factors, the voltage rails and the current limits.  In the PLD we chose to put the Voltage/Current Peak right around 5Ω to make it more flexible.  When the load is above this peak the power amp output typically runs out of voltage rail before it runs out of current limit, and when the impedance is below this peak it will run out of current capability before it runs out of voltage.  What this means in real world applications is that in the case of an 8Ω (or higher) loads the PLD voltage rails (PLD4.5 Vrails are +/- 155V) are more than stout enough to support the majority of applications.  If you do want or need larger voltage, you can bridge two channels like any other amplifier out there – resulting in massive 310V potential.  When the load drops to 4Ω (or below) we recommend putting two channels in parallel, this doubles the current capability.  Again, in real world applications, even if you don’t parallel the channel the amp will run just fine, but the current limiters will kick in and you may not get as much power as you might like.  This is a bit simplified because current limiting is not a dead stop the way voltage rails are, so there is some short term additional power capability that you can get with transient material.     

Secondly Software/Firmware: Yes, this is a DSP driven amp and like any DSP product, there are bugs found and firmware updates that will squash those bugs along with providing greater functionality.  We always recommend that you run the latest software version. 

Third, and probably the biggest one of all, Power Ratings: Due to the flexibility of our FAST channel combining it was nearly impossible to post the power ratings for all channel configurations in the given space so we had to come up with a way to show the flexibility as well as giving the reader some idea of the capabilities of this amplifier family.  What I did, was I chose from the various configurations in 4CH, 2CH, and 1CH and posted the power for those.  In some cases this was a parallel channel combination, and in others this is a bridged channel combination.  Of course the other big issue is the fact that our industry has gone away from continuous power ratings to a more “Dynamic” power testing.  I put dynamic in quotes because the actual test method is called “Burst” (using tone bursts meant to emulate an actual dynamic audio signal to measure power instead of a continuous sine wave).  One of the problems that we have as an industry is that we all do these tests somewhat differently so you can see where that is headed.  We are actually working on a project with the AES, Crown, and Lab Gruppen to create a standardized dynamic power test, but it may be a while before this is completed.

I hope that answers the majority of questions.

Thanks,
Dale
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Mac Kerr

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« Reply #23 on: December 02, 2015, 08:30:40 pm »

Hi All,
As the Product Manager who along with our engineering staff developed the PLD amps, I will try to answer some of the lingering questions.

Please go to your profile and change the "Name" field to your real first and last name as required by the posting rules displayed in the header at the top of the section, and in the Site Rules and Suggestions in the Forum Announcements section, and on the registration page when you registered.

Also, as a representative of a manufacturer, please include that affiliation in a sig line.

Mac
« Last Edit: December 02, 2015, 08:36:51 pm by Mac Kerr »
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Bob Faulkner

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Re: QSC PLD amps
« Reply #24 on: December 02, 2015, 08:33:53 pm »

Great information Dale!  Thank you for posting!

You mention terms of "burst" and "continuous".  What amount of "time" was used when speaking of burst or continuous ratings.  For example, is "bust" time a 20ms unit of time?  How much time references "continuous"?

Thanks again,
Bob
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Mark Wilkinson

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Re: QSC PLD amps
« Reply #25 on: December 03, 2015, 11:57:45 am »

Thanks for your reply Dale,

And also thanks for the information you provide in your white papers....they really help trying to understand.
I wish all the manufactures published the voltages and currents available, and design vs impedance curve.

I've been very happy using the pld4.5 to triamp a MF/HF/VHF box, paralleling two channels for the MF.
I also use the amp to drive a sub with 2 channels, and a two-way top.

So I do use it for subwoofer duty....and with today's high power LF drivers it's made me question things a little

One horn loaded sub I use with the amp has a couple of impedance minimums of about 3.5 ohms, and a couple of maximums at about 15 ohms, within a 30-100hz passband.
Unless I'm missing something, if I parallel two channels, the 15 ohm frequencies will have about 800 continuous watts available.
And if I bridge two channels, the 3.5 ohm freqs will have about 1100 watts available.
(I used 17.7arms from the FAST paper ....and 109vrms from your post, assuming the 155v you mentioned is peak????)

Either way, at certain freqs the sub will be getting less than half the power i might have expected from just glancing at specs.
(Maybe this sub has unusually high impedance peaks??? I certainly dunno...)

But all this to me begs...don't all amps have this trade-off going on?
And is this the gist of what makes a "good" sub amp?  Or knowing what amps can give headroom to high-power high-impedance compression drivers?

Sorry if this has been too much of a tangent to the OP's reliability questions.. but might reliability be a function of suitably matching channel combining to the load..?


Thanks again, Mark



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Dale Sandberg

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Re: QSC PLD amps
« Reply #26 on: December 03, 2015, 05:24:16 pm »

Great information Dale!  Thank you for posting!

You mention terms of "burst" and "continuous".  What amount of "time" was used when speaking of burst or continuous ratings.  For example, is "burst" time a 20ms unit of time?  How much time references "continuous"?

Thanks again,
Bob

Hi Bob,
Continuous power testing is a great way to find the long-term thermal capacity of the amplifier but it isn't the best way to measure the real-world power capability that can be put into an loudspeaker.  When we do continuous power testing we typically do either an FTC or an EIA power test (sometimes both).  Continuous power testing is done long term (many hours), so the the amp is essentially at steady state and will run indefinitely at those ratings.

Our burst power testing (I prefer the term Dynamic Power Testing) is done using a 30ms sine wave tone burst.  These types of signals do a better job at emulating the duty cycle of dynamic music (LF in particular).

Thanks,
Dale
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Dale Sandberg

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Re: QSC PLD amps
« Reply #27 on: December 03, 2015, 05:47:38 pm »


I've been very happy using the pld4.5 to triamp a MF/HF/VHF box, paralleling two channels for the MF.
I also use the amp to drive a sub with 2 channels, and a two-way top.

So I do use it for subwoofer duty....and with today's high power LF drivers it's made me question things a little

One horn loaded sub I use with the amp has a couple of impedance minimums of about 3.5 ohms, and a couple of maximums at about 15 ohms, within a 30-100hz passband.
Unless I'm missing something, if I parallel two channels, the 15 ohm frequencies will have about 800 continuous watts available.
And if I bridge two channels, the 3.5 ohm freqs will have about 1100 watts available.
(I used 17.7arms from the FAST paper ....and 109vrms from your post, assuming the 155v you mentioned is peak????)

Either way, at certain freqs the sub will be getting less than half the power i might have expected from just glancing at specs.
(Maybe this sub has unusually high impedance peaks??? I certainly dunno...)

But all this to me begs...don't all amps have this trade-off going on?
And is this the gist of what makes a "good" sub amp?  Or knowing what amps can give headroom to high-power high-impedance compression drivers?

Sorry if this has been too much of a tangent to the OP's reliability questions.. but might reliability be a function of suitably matching channel combining to the load..?


Great questions Mark,
Yes, every single amplifier out there has the same issues, balancing Low-Z current performance with high-Z voltage performance; the difference that we offer with the PLD amps is the ability to parallel channels for high current output.
 
Yes, reliability is absolutly a function of matching the drivers with an amp that can drive them well.  Not to toot our own horn, but one of the major successes of the QSC K Series has been how well matched the drivers were to the amplifiers and vice versa - it creates a very robust system that repeatably sounds great. 

If it were me, I would error on the Low-Z side of the impedance curve and parallel the channels.  With a 155V rail there is almost always enough voltage to push a high impedance load but current limiting will steal your thunder.  Yes, you can get quick transients through the current limiter, but with LF content there can be a lot of them and so at some point either the thermal or current limiter is going to kick in.  When you parallel channels you have twice the current capability.  Our own testing at QSC has shown me time and again that while bridging can be useful in some applications, paralleling outputs (especially with subwoofers) often produces more useful output.  If you are using the latest firmware I would recommend you do your own testing and see.  You will not hurt the amp to run into the current limiter, nor will you hurt it if you bridge into a 4 ohm load, in either case if you run into trouble the limiters will protect you.

Thanks,
Dale
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Brian Jojade

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Re: QSC PLD amps
« Reply #28 on: December 03, 2015, 08:24:29 pm »

Hi Bob,
Continuous power testing is a great way to find the long-term thermal capacity of the amplifier but it isn't the best way to measure the real-world power capability that can be put into an loudspeaker.  When we do continuous power testing we typically do either an FTC or an EIA power test (sometimes both).  Continuous power testing is done long term (many hours), so the the amp is essentially at steady state and will run indefinitely at those ratings.

Our burst power testing (I prefer the term Dynamic Power Testing) is done using a 30ms sine wave tone burst.  These types of signals do a better job at emulating the duty cycle of dynamic music (LF in particular).

Thanks,
Dale

This is like RMS, program, and peak ratings of the speaker.  If you design your system so that within the continuous rating of the amp you are at or below the RMS rating of the speaker and you can achieve the maximum volume needed for your event, you will almost never have to worry about blown equipment. Thrown in low pass filters to prevent mechanical over excursion and you can create a system nearly DJ proof. :)
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Brian Jojade

Bob Faulkner

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Re: QSC PLD amps
« Reply #29 on: December 03, 2015, 11:43:49 pm »

Hi Bob,
Continuous power testing is a great way to find the long-term thermal capacity of the amplifier but it isn't the best way to measure the real-world power capability that can be put into an loudspeaker.  When we do continuous power testing we typically do either an FTC or an EIA power test (sometimes both).  Continuous power testing is done long term (many hours), so the the amp is essentially at steady state and will run indefinitely at those ratings.

Our burst power testing (I prefer the term Dynamic Power Testing) is done using a 30ms sine wave tone burst.  These types of signals do a better job at emulating the duty cycle of dynamic music (LF in particular).

Thanks,
Dale
Looks good.  Thanks Dale.
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Re: QSC PLD amps
« Reply #29 on: December 03, 2015, 11:43:49 pm »


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