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Author Topic: adavantage?  (Read 16790 times)

John Roberts {JR}

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Re: adavantage?
« Reply #40 on: February 23, 2010, 03:03:18 pm »

Nathan Short wrote on Tue, 23 February 2010 13:07

Sorry Bob,

As I am not an EE, this is just a losing conversation for me.

I wish you would be more helpful, but at the same time, I guess that other people have actually helped me more understand what I am trying to explain, so all in all, I have my answers.  

To put it better.  The PL380 and 340 both do excellent jobs of only drawing a certain ammount of amperage under a heavy load.
They do their jobs well, and with repeatable results that satisfy my needs.  

And to be very hones, I have had zero failures of PLX and PLX2 in the last 7 years. Good power, good ventilation, and proper wiring on the installs, and bam the amps do their job.

The 3602 testing I did just confirmed my personal experience that every now and then, with the right DJ doing the right thing.  Ch1 will blow a VC right out of the basket, while Ch2 will not.  Clip limiter on Ch1 not effective. But who knows.  Evidently not me.


The PL340 is class H just like PLX so it will be very similar. The PL380 with class D output will be somewhat more efficient which could make a marginal difference in a wimpy mains power application.

A difference between two amp channels is easy enough to bench test.

JR  
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Bob Lee (QSC)

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Re: adavantage?
« Reply #41 on: February 23, 2010, 05:31:54 pm »

I'd like to be more helpful, Nathan, but I have very sketchy (and sometimes contradictory) information from you to work with. I'd like to know what fault you have found, but you won't say.

If you conclude that the PL340 is better at current draw than the PLX3602, then you haven't actually put them through equal tests.
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Bob Lee
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Nathan Short

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Re: adavantage?
« Reply #42 on: February 23, 2010, 07:45:15 pm »

Holy crap.  Wow.

Take oscillator, run into console, plug into amp, run amp on 4ohm high wattage resistors each channel.  Measure voltage at 15kHz and 30Hz at different times. For however you feel is long enough.

NOT THAT HARD.  Channel 1 on 3602 will put out near 81v when clip light starts to flash,  Channel 2 on 3602 will behave more normally at 60v and lower.  

There has to be a reason I can do this easily.  

 

For the love of god, this is not that crazy of a scenario.

Any help, or is this too sketchy of a signal flow for my results.
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John Roberts {JR}

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Re: adavantage?
« Reply #43 on: February 23, 2010, 08:34:02 pm »

Nathan Short wrote on Tue, 23 February 2010 18:45

Holy crap.  Wow.

Take oscillator, run into console, plug into amp, run amp on 4ohm high wattage resistors each channel.  Measure voltage at 15kHz and 30Hz at different times. For however you feel is long enough.

NOT THAT HARD.  Channel 1 on 3602 will put out near 81v when clip light starts to flash,  Channel 2 on 3602 will behave more normally at 60v and lower.  

There has to be a reason I can do this easily.  

 

For the love of god, this is not that crazy of a scenario.

Any help, or is this too sketchy of a signal flow for my results.


You are describing a near doubling of power between channel 1 and channel 2. If you swap the input cables around and still get that same doubling of power in the same output channel, with identical inputs, YOU HAVE A FAULTY AMPLIFIER that needs to be repaired.


JR


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Nathan Short

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Re: adavantage?
« Reply #44 on: February 23, 2010, 08:38:45 pm »

Well then as this is the case with both of the new 3602's I just got in over a week ago , now I know.

The amps sound great though.  But this problem makes me want to go back over a lot of my installs and make damn sure ch1 never even gets near clip.  I will sacrifice the sound a bit for safety.
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John Roberts {JR}

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Re: adavantage?
« Reply #45 on: February 24, 2010, 09:00:53 am »

Nathan Short wrote on Tue, 23 February 2010 19:38

Well then as this is the case with both of the new 3602's I just got in over a week ago , now I know.

The amps sound great though.  But this problem makes me want to go back over a lot of my installs and make damn sure ch1 never even gets near clip.  I will sacrifice the sound a bit for safety.

Have you swapped the inputs to confirm that both channels were getting the same exact signal?

If yes, and you still get a near 3 dB difference at the output, you need to be talking with QSC service not us...

JR

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Bob Lee (QSC)

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Re: adavantage?
« Reply #46 on: February 24, 2010, 02:35:13 pm »

Nathan, thank you for the 60V information. It adds something, though your test conditions are still quite sketchy.

Can you take a moment to define "behave [more] normally"? Does channel 2 behave abnormally at above 60V?

Does one channel have more gain than the other (when you set the gain controls to the same position)? Is either channel distorted? Do you know what signal levels you're putting into the amp channels? Is there a possibility that you could get a technician to work with you on this?
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Bob Lee
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Nathan Short

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Re: adavantage?
« Reply #47 on: February 24, 2010, 02:44:29 pm »

I am a firm believer that the pots on my amps should always be at full.  This makes sure that the only fiddling that can go on is in a reduction direction.

The amp when driven by a 15kHz or 30Hz sine wave at full gain on the amp, driven into slightly flashing limit,  regardless of switching the inputs back and forth,

Channel 1    81v  unwaivering

Channel 2    60v + or minus  3v   very stable

Going further in input level will cause a solid clip indication on the amp leading me to believe I am doing nothing positive.

Going on what I measured many many amps on in similar conditions

The 3602  should be putting out about 60v per channel for the test I was doing.  60v  would be in the expected range of the rest of my results.   81v  is the anomalous reading.
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Bob Lee (QSC)

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Re: adavantage?
« Reply #48 on: February 24, 2010, 05:26:22 pm »

Thanks.

Keep in mind that 81V on the output is not inherently abnormal, depending on the gain, the input signal, etc. I'm not certain what you feel is normal with that or other power amps. The basic idea is that a power amp in its linear range is a voltage multiplier. IOW, input voltage
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Bob Lee
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John Roberts {JR}

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Re: adavantage?
« Reply #49 on: February 24, 2010, 06:00:53 pm »

Nathan Short wrote on Wed, 24 February 2010 13:44

I am a firm believer that the pots on my amps should always be at full.  This makes sure that the only fiddling that can go on is in a reduction direction.


always good to eliminate variables.
Quote:


The amp when driven by a 15kHz or 30Hz sine wave at full gain on the amp, driven into slightly flashing limit,  regardless of switching the inputs back and forth,

Channel 1    81v  unwaivering

Channel 2    60v + or minus  3v   very stable



OK, this is still unclear (to me).
Are you saying that with the exact same input signal, one channel makes 60V and the other  80V?  Or are you saying one channel flashes the clip light at 60V (output) and the other flashes the clip light at 80V (output), but with different input voltages too?

One situation suggests a seriously flawed amplifier (3 dB gain mismatch), the other a less than precise clip indication. If both outputs actually make 80V clean for same input voltage, this is a premature clip light.  
Quote:


Going further in input level will cause a solid clip indication on the amp leading me to believe I am doing nothing positive.

Going on what I measured many many amps on in similar conditions

The 3602  should be putting out about 60v per channel for the test I was doing.  60v  would be in the expected range of the rest of my results.   81v  is the anomalous reading.


A simple trick to confirm how long the outputs track each other, is to connect a speaker across the two hots. With both amps set for same gain, with the same input voltage there should be only a tiny signal (null of gain error) in that speaker. If one output actually clips 20V before the other, you will get a lot of sound from that speaker between when the first channel stops and the second one keeps going. If they both clip about the same time, not so much noise.

Of course a scope would be easier yet.

JR

PS: My apologies to Bob who is on the case.. but it's always the last thing you check.




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