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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => SR Forum Archives => LAB Subwoofer FUD Forum Archive => Topic started by: Berti Jacobs on December 28, 2009, 12:37:53 pm

Title: adavantage?
Post by: Berti Jacobs on December 28, 2009, 12:37:53 pm
Hello everybody,

I'm in the process of building 8 LAB's (4 per side)
I'm wondering whats the better way to power them:
1 Itech 8000 per 4 LAB's  or  2 Itech4000 per 4 LAB's ??

thanks for any advice

Berti
Title: Re: adavantage?
Post by: Art Welter on December 28, 2009, 04:52:26 pm
Berti,

The Lab Sub with the speakers in parallel is roughly a 4 ohm load.

I-T 8000 can deliver 3500 watts/4 =875, so it is adequate in that it has more than double the RMS rating of a Lab 12 speaker.

However, the I-T amps are current limited at 2 ohm operation, and put out less power at 2 Ohm than 4 ohm, so a pair of I-T 4000 at 4 ohm would deliver a bit more power without limiting than an I-T 8000 loaded to 2 ohm.
Current limited power generally does not sound so good as straight power, I would not want to use an amp that is current limiting under “full tilt” use.

If you are looking for a single amp for 4 Lab subs, you should look for something rated for more power at 2 than 4 ohms.index.php/fa/27089/0/

Art Welter
Title: Re: adavantage?
Post by: Elliot Thompson on December 29, 2009, 02:44:04 pm
Berti Jacobs wrote on Mon, 28 December 2009 17:37

Hello everybody,

I'm in the process of building 8 LAB's (4 per side)
I'm wondering whats the better way to power them:
1 Itech 8000 per 4 LAB's  or  2 Itech4000 per 4 LAB's ??

thanks for any advice

Berti




Two amplifiers are always better than one from a longevity standpoint. Many amplifiers claim they can deliver 2-ohms per channel however, how long is not guaranteed.

If you are looking for worry free results, buy two amplifiers, use them at 4-ohm per channel than, trying to extract every ounce of reserve energy from one amplifier loaded in a 2-ohm per channel load.

Best Regards,
Title: Re: adavantage?
Post by: jeffhtg (Jeff Kenney) on February 10, 2010, 12:01:13 am
Elliot Thompson wrote on Tue, 29 December 2009 19:44

Berti Jacobs wrote on Mon, 28 December 2009 17:37

Hello everybody,

I'm in the process of building 8 LAB's (4 per side)
I'm wondering whats the better way to power them:
1 Itech 8000 per 4 LAB's  or  2 Itech4000 per 4 LAB's ??

thanks for any advice

Berti




Two amplifiers are always better than one from a longevity standpoint. Many amplifiers claim they can deliver 2-ohms per channel however, how long is not guaranteed.

If you are looking for worry free results, buy two amplifiers, use them at 4-ohm per channel than, trying to extract every ounce of reserve energy from one amplifier loaded in a 2-ohm per channel load.

Best Regards,



unless your talking about powersoft..
Title: Re: adavantage?
Post by: Gary Perrett on February 11, 2010, 11:29:45 pm
Ya... ain't that a bitch Laughing  Laughing  Laughing We have run 6 labs, 3 per side off a Digam 5000 INSANE output and NEVER a failure in 3 years of outdoors shows. This was for playback of music files on a lazer light show... did a show with a big outdoor stage and bands with a JBL rig with 4 JBL 4719's per side with each cab running off a Mac 5002VZ... that's 16, 18" woofers with 40,000 watts of crown power... couldn't even hear his system when the block of 6 labs was running...HE WAS SO PISSED...that was BEFORE he came over to see what was driving the labs...THAT sent him over the edge!
G
Title: Re: adavantage?
Post by: Elliot Thompson on February 12, 2010, 01:38:03 pm
jeffhtg (Jeff Kenney) wrote on Wed, 10 February 2010 05:01

Elliot Thompson wrote on Tue, 29 December 2009 19:44

Berti Jacobs wrote on Mon, 28 December 2009 17:37

Hello everybody,

I'm in the process of building 8 LAB's (4 per side)
I'm wondering whats the better way to power them:
1 Itech 8000 per 4 LAB's  or  2 Itech4000 per 4 LAB's ??

thanks for any advice

Berti




Two amplifiers are always better than one from a longevity standpoint. Many amplifiers claim they can deliver 2-ohms per channel however, how long is not guaranteed.

If you are looking for worry free results, buy two amplifiers, use them at 4-ohm per channel than, trying to extract every ounce of reserve energy from one amplifier loaded in a 2-ohm per channel load.

Best Regards,



unless your talking about powersoft..



Powersoft is just like any other amplifier. The only magic is the marketing. Smile

Best Regards,
Title: Re: adavantage?
Post by: Nathan Short on February 15, 2010, 12:34:15 pm
Powersoft K10dsp is the only amp I have torture tested that will put out a continuous 50hz sine wave minute after minute  @ 76v per output, off a 20 amp circuit.

Damn hard to find another amp that can do that.  

Not magic, just damn fine engineering.  
Title: Re: adavantage?
Post by: Elliot Thompson on February 15, 2010, 04:20:28 pm
Nathan Short wrote on Mon, 15 February 2010 17:34

Powersoft K10dsp is the only amp I have torture tested that will put out a continuous 50hz sine wave minute after minute  @ 76v per output, off a 20 amp circuit.

Damn hard to find another amp that can do that.  

Not magic, just damn fine engineering.  



I don't see the logic trying to operate an amplifier designed to deliver 12,000 watts on a 20-amp circuit more so feeding it a 50Hz sine wave.

I hope you know 76 volts only brings forth the following wattage:

76 volts @ 8 ohms = 722 watts

76 volts @ 4 ohms = 1444 watts

76 volts @ 2 ohms = 2888 watts

If you would like to believe you are going to attain more than that on a 20-amp circuit longterm you have a right to your own opinion. However, the ohms law states otherwise.   Cool

Best Regards,
Title: Re: adavantage?
Post by: Nathan Short on February 15, 2010, 05:58:57 pm
Oops,

I just went back over my notes The powersoft had to go back to an old business partner before I did round 2 of my last 2 weeks of torture testing.

The Powersoft K10dsp would max out at 67v drawing an average of 26amps @ 50Hz near input limit

PL380 60v drawing 29 amps consistantly.  

Those were the only 2 real sub amps I tested, the rest were a range of 4 OEM new amps of varying topology.  PL340, crown 3600, PL3602, and RMX 2402.

Detailed notes about voltage and current draw over time, were taken at Pink 6db crest, 30Hz, 50Hz, 1k, 15k, Dance Music, Dance Music into hard limit.

Power was supplied 100amps in from the wall, into a custom Military Server Farm Surge supressor with current meter, out to amplifiers through a heavy duty 20amp outlet wired to heavy 10 guage cable.

Amps were wired into a heavy resistor bank, and water heater bucket at alternating tests.

The most interesting thing was finding a flaw in the PLX3602 that at 30Hz and 15K  channel 2 would behave normally and channel 1 would try and put out 81v!!!  Danger Danger!  and the amp would go into overdrive and try and pull 33amps from the wall.  All in all, I got all the info I needed, and a good refresher course on amp layouts with a little help from Fred Merkle.  

My point being, that Powersoft to me was able to do the most, for longer, than any other brand.  

New Crown was not invited to the test, as I don't care for it.

Title: Re: adavantage?
Post by: Elliot Thompson on February 16, 2010, 09:32:05 pm
Nathan Short wrote on Mon, 15 February 2010 22:58

Oops,

I just went back over my notes The powersoft had to go back to an old business partner before I did round 2 of my last 2 weeks of torture testing.

The Powersoft K10dsp would max out at 67v drawing an average of 26amps @ 50Hz near input limit

PL380 60v drawing 29 amps consistantly.  







Upon reading your review, it seems the K 10 current limit itself from drawing more amperage even though the line source was not the limiting factor.

67 volts using a dummy load would yield:

571 watts @ 8 ohms

1122.25 watts @ 4 ohms

QSC introduced a white paper on the PL 380 (in addition to the lower wattage models) that states 30 amperes is to be expected long term so, 29 amperes is not surprising.

60 volts would bring forth the following results:

450 watts @ 8 ohms

900 watts @ 4 ohms

Using the wattage versus dB equation (60 volts versus 67 volts), you would achieve a 0.7 dB gain.

I would imagine the user would determine which option is best for his or her needs.

Personally, I rather have an amplifier that chokes based on not enough current from the receptacle than, one that limits itself when there is more than enough current available. I do not own any Class D amplifiers as of yet so, it is always good to hear how they compare amongst one another.

Best Regards,

Title: Re: adavantage?
Post by: Nathan Short on February 16, 2010, 10:29:14 pm
The powersoft k10dsp actually let's the end user set the mains current limit within the dsp.  Can be a lifesaver in a difficult situation.   Unfortunately the test was done off of a heavy duty 20 amp plug on a 30 amp circuit off the mains panel in the shop. I did not feel comfortable wiring the amp into a bigger circuit and the k10dsp went back before the new distro arrived or I would gave taken the powersoft tests much much further. I had an old hearty 3600vz drawing 44 amps for a few minutes before getting scared!  That was neat to measure  
Title: Re: adavantage?
Post by: Elliot Thompson on February 16, 2010, 11:25:49 pm
The old Crown’s were known for being current hogs. I really believe many never understood the amount of current those amplifiers will draw (based on the high current limit setting) and, assumed the size of the power cable was means of decoration. If I remember correctly, the MA 3600 current limit is 90 amperes.

Amplifiers that fall within the K10 range, really shine on voltage ranging from 200V on wards. If the United States offered 240 volts, as a standard Crown & QSC would have more than likely not be at a stand still on how much power they could offer to their customers.

Best Regards,
Title: Re: adavantage?
Post by: Nathan Short on February 17, 2010, 02:20:13 am
Now if only we could put tiny little ZPM's inside them.....

Zero point modules, come on science!
Title: Re: adavantage?
Post by: Matt F Castle on February 18, 2010, 10:42:04 am
Interested in any more info on what you found with the PLX3602, any likly hood of that happerning in a realy world usage situation?

Cheers

matt
Title: Re: adavantage?
Post by: Nathan Short on February 18, 2010, 02:19:27 pm
My guess would be absolutely ,  keyboard or synth music, feedback near 15K which is very likely would be devastating to tweeters, low feedback near 30hz might be bad in a DJ situation.

I have had a few instances over the years where I had a properly limited system and for some unknown reason during some hip hop nights I had 1 or two subs basically blown right out of the basket, and leads ripped off without any other damage,  a quick burst of near 70-80v at 30hz would absolutely be a culprit.  As no coils were burned and just leads ripped out.

Who knows maybe Bob Lee could shed some light?

Title: Re: adavantage?
Post by: Matt F Castle on February 18, 2010, 06:42:58 pm
What sort of input was causing this to happen?  I would like to think QSC test these things fairly well and you were doing somthing faily out of the ordinary for what you describe to happen?

Cheers

Matt (interested as I have a few 3602's)
Title: Re: adavantage?
Post by: Nathan Short on February 19, 2010, 12:08:57 am
Hey grab an oscillator and find out. Turns out the new ones I just go do exibot this behavior.  Let me ask a few EEs and and designers then get back to you.
Title: Re: adavantage?
Post by: Bob Lee (QSC) on February 19, 2010, 05:24:57 pm
Nathan Short wrote on Mon, 15 February 2010 14:58

The most interesting thing was finding a flaw in the PLX3602 that at 30Hz and 15K  channel 2 would behave normally and channel 1 would try and put out 81v!!!  Danger Danger!  and the amp would go into overdrive and try and pull 33amps from the wall.


What were you testing, and how?

What does "go into overdrive" mean?

And what is an RMX2402?
Title: Re: adavantage?
Post by: Nathan Short on February 19, 2010, 11:20:33 pm
Sorry Bob, it seems you have not read the thread.

I have been in the process of torture testing about 4 OEM amps, and a couple of prototypes from new companies.  Included with these tests I had a pile of QSC  new and old, the new ones slated for an install in NYC coming up.  Some rebuilt RMX2450's, damn input pots are one of the only flaws with that amp,  always failing.  Thank god for my pile of replacements, and a good ol can of Caig labs deoxit and pro gold to protect them when they go into installs with any humidity in the air.

My apologies on saying 2402  when I meant Rmx2450.  

A slew of tests were run on all amps into two types of dummy load, various passive loudspeakers, and a couple of huge port assisted hybrid horn loaded 18's with 5in voice coils and a lot of abuse room.

Overdrive is what I would call when the amp in question, the PLX3602's fan kicked into high speed, and the meter showed that it was pulling a ridiculous amount of current I have never seen that amp draw before.  But granted this is the first time I have torture tested that amp other than the near 100 units I have put in my medium budget DJ rigs over the past 5 years or so since it came out and replaced the older plx model.

The current meter/distro unit I was using I cant actually say, Signed the NDA already.  But needless to say it is ridiculously accurate, and is the same unit that protects every server farm on nearly every US military server farm.

I don't know if it is just the batch I just received, but I also found one other amp that did this,  an A/B Bipolar Mosfet Hybrid,  channel one was more than double the voltage at 15Khz but not 30hz. Current draw went up a bit, but the 3602's took the prize for jumping to 33amps and 81v.
Title: Re: adavantage?
Post by: Matt F Castle on February 20, 2010, 01:36:05 pm
It would be helpful if you could actually tell us how you achieved this happening, what were your test conditions, what were you putting in to the amp etc?!

I would like to think QSC do fairly comprehensive testing on there products and the condition you describe was not as a result of the amp being connected to a normal speaker and just driven with a test tone or normal program music with an amount of the frequency's you mention? and this something that will happen consistently with a *normal* use input??

Cheers

matt
Title: Re: adavantage?
Post by: Nathan Short on February 20, 2010, 03:09:44 pm
Damnit, I had a very well worded and long winded response, then my damn computer glitched.

So here to sum up again.  

Noise generator->midas Venice console->amp->load->2 types of multimeter B&K and Extech= Sencore Audio consultant to measure THD%

I actually love these amps, but have noticed in the past, that my subs on a few installs in which the system was properly limited, had the symptoms of 2 subs on channel 1 of a 3602  would be blown out of the basket. Not literally.  Surround partially ripped, spider partially torn, leads ripped off the input, slightly rubbed coil, but not burnt. Basically very fast Xmech damage.  The other subs would be fine on the other channel, and in most cases still work to this day.  I am always delighted to go do a service call 4 years later to find everything working fine even when they have DJ's 5 nights a week.  I try to make my systems very Idiot resistant.  

So, the real question is this.  Is this an isolated batch?  An individual component tolerance issue?  Has anyone else noticed this behavior?

Go ahead and try it.  But I will make damn sure that from now on, that little red clip light on channel 1  never lights up EVER.


Title: Re: adavantage?
Post by: Nathan Short on February 20, 2010, 03:16:01 pm
My purpose in widening the scope of my testing with tone into near limit on the amps is the growing use by us in electronic music.

Dubstep is changing the way we need to measure amplifiers as many of the bass lines are long droning synth lines.  

This will reveal a weak sub amp very quickly, and many amp manufacturers are stepping up the robustness of the amplifiers.

I would hate to be doing a show where the DJ or Electronic musician played a high note, or used a "sweep" effect while performing, and 1- I nuke my drivers on Ch 1, 2 - I blow half my PD.  Who knows?  This is why I try to abuse the amps any way I can think of before I let the professional Idiots beat them up in my installs.
Title: Re: adavantage?
Post by: Art Welter on February 20, 2010, 03:26:36 pm
Nathan,

You wrote previously:

“The most interesting thing was finding a flaw in the PLX3602 that at 30 Hz and 15K channel 2 would behave normally and channel 1 would try and put out 81v!!! Danger Danger! and the amp would go into overdrive and try and pull 33amps from the wall.”

Did you mean 30 Hz and 15K simultaneously, or either tone separately caused the runaway?
Title: Re: adavantage?
Post by: Nathan Short on February 20, 2010, 03:29:02 pm
Detailed notes about voltage and current draw over time, were taken at Pink 6db crest, 30Hz, 50Hz, 1k, 15k, Dance Music, Dance Music into hard limit.  Separate.
Title: Re: adavantage?
Post by: John Roberts {JR} on February 20, 2010, 04:34:10 pm
Nathan Short wrote on Sat, 20 February 2010 14:16

My purpose in widening the scope of my testing with tone into near limit on the amps is the growing use by us in electronic music.

Dubstep is changing the way we need to measure amplifiers as many of the bass lines are long droning synth lines.  

This will reveal a weak sub amp very quickly, and many amp manufacturers are stepping up the robustness of the amplifiers.

I would hate to be doing a show where the DJ or Electronic musician played a high note, or used a "sweep" effect while performing, and 1- I nuke my drivers on Ch 1, 2 - I blow half my PD.  Who knows?  This is why I try to abuse the amps any way I can think of before I let the professional Idiots beat them up in my installs.


There is nothing wrong with sine wave (tone) testing on amps, loudspeakers not so much.

I'm sure Bob Lee will sort this out but 81V sounds a little hi for a single channel of 3602 under load, unless square waved.

Excessive current at HF can be caused by a condition called mutual conduction, when both the pull up power devices and pull down power devices are pulling both directions at the same time. This was an issue with early mosfet power amps (due to poor driver stage design) but not common in modern power amps. If such an amp doesn't trip mains breakers it could easily release device smoke, since the power devices are working up a good sweat playing tug of war with each other.

I doubt this is happening inside a properly functioning PLX, just saying what the symptom sounds like.

JR
Title: Re: adavantage?
Post by: Bob Lee (QSC) on February 22, 2010, 02:18:30 pm
Sorry Nathan, but when I read the thread it seemed that your "test" was (and is) not described in any detail.

Do you mean the gain pots on the RMX2450? There are no pots on the inputs. The gain pots are a weak spot on the RMX amps; it's probably good that they don't also have input pots.

Your description of "overdrive" is still sketchy. Can you describe your test setup, what you were measuring, etc.? "Jumped to 81V" and "drew 33 A" doesn't tell us much. I can make a perfectly good PLX3602 put out 81 V and draw 33 A too. Can you describe what the amp did versus what it was supposed to do?
Title: Re: adavantage?
Post by: Nathan Short on February 22, 2010, 02:38:12 pm
Never mind Bob,

I don't have time to argue semantics over my quickly worded posts.

Everyone knows that the potentiometers on the RMX series blow, are cheap, degrade quickly, and otherwise ruin a perfectly good budget amp line.  I will never sell them for an install, but make plenty of money upgrading and fixing them then reselling to my friends.

It is wonderful To see that the PL340 and 380 series have wonderful current management and do what they are supposed to do.

It is very odd that I can easily, get one channel of a 3602 to put out 81 volts while the other measures perfectly,  and that the amp even tries to pull 33amps from the wall.  

This is my only question.  How and why is the 3602 able to do this.  Seems pretty dangerous, and a complete failure of the "clip limiter" circuit in the amp. Whatever that might be.
Title: Re: advantage?
Post by: Mac Kerr on February 22, 2010, 02:55:28 pm
Nathan Short wrote on Mon, 22 February 2010 14:38

Never mind Bob,

I don't have time to argue semantics over my quickly worded posts.

Everyone knows that the potentiometers on the RMX series blow, are cheap, degrade quickly, and otherwise ruin a perfectly good budget amp line.  I will never sell them for an install, but make plenty of money upgrading and fixing them then reselling to my friends.

It is wonderful To see that the PL340 and 380 series have wonderful current management and do what they are supposed to do.

It is very odd that I can easily, get one channel of a 3602 to put out 81 volts while the other measures perfectly,  and that the amp even tries to pull 33amps from the wall.  

This is my only question.  How and why is the 3602 able to do this.  Seems pretty dangerous, and a complete failure of the "clip limiter" circuit in the amp. Whatever that might be.


This is a pretty cheap shot that basically puts your credibility at zero. Bob asked a couple of questions about your testing methods that you have never explained.

Bob is not the only one who can't figure out what you are talking about.

Mac
Title: Re: advantage?
Post by: Nathan Short on February 22, 2010, 03:31:08 pm
I don't see how it is a cheap shot, just the truth.  

The quick and easy method to reproduce my findings.

Take an oscillator, Run it into a midas venice, set the level to 0.
Buss the channel.  Run Masters to 0.  Masters are plugged directly into the amplifier.  Amplifier was tested on 2 dummy loads. I will explain the first.  A 4 element 4x 7.8-8.2 ohm 2000w water heater elements suspended in a bucket of water.  This provides an easy impedance that doesn't seem to drift more than .2ohm when hot.

Test leads from a BnK multimeter were attached to the individual leads one one of the resistors , and at the amp different times.

The amp was plugged into a military grade distro, with a current meter.  The distro was plugged into two seperate 100amp breakers, 1 per circuit of the 2 circuit unit.  The unit has 40amp breakers.  both circuits went back to the same panel ground, and seperate neutral bus.

Channel 1 was found, when starting to engage the clip led by driving the output of the midas past +4, to start putting out a lot more voltage than Channel 2.  Channel 2 behaved normally like almost every other amp at the test.  Channel 1 could be made to put out 81v when the clip light was moderately active.
The current meter on the Distro showed 33amp draw.

What else can I tell you.  A few of my EE friends, as I am not an EE, gave the testing a thumbs up, and a great way to check for thermal limits also.  

The 3602 was found to exibit this behavior at 30Hz and 15kHz, I did not push the test further for fear of ruining the amp.
Title: Re: advantage?
Post by: John Roberts {JR} on February 22, 2010, 04:10:36 pm
Nathan Short wrote on Mon, 22 February 2010 14:31

I don't see how it is a cheap shot, just the truth.  

The quick and easy method to reproduce my findings.

Take an oscillator, Run it into a midas venice, set the level to 0.


doesn't matter
Quote:


Buss the channel.  Run Masters to 0.  Masters are plugged directly into the amplifier.


bus.. Console outputs, I presume. Same as level coming from the Oscillator?  IIRC Midas uses peak meters, but I'm not sure how they calibrate for sine waves.  Probably doesn't matter.
Quote:


Amplifier was tested on 2 dummy loads. I will explain the first.  A 4 element 4x 7.8-8.2 ohm 2000w water heater elements suspended in a bucket of water.  This provides an easy impedance that doesn't seem to drift more than .2ohm when hot.


nominal 2 ohm load.. one on each channel I assume.
Quote:


Test leads from a BnK multimeter were attached to the individual leads one one of the resistors , and at the amp different times.


Average AC volts (even if RMS it doesn't tell us the full story since it's clipping).
Quote:


The amp was plugged into a military grade distro, with a current meter.  The distro was plugged into two seperate 100amp breakers, 1 per circuit of the 2 circuit unit.  The unit has 40amp breakers.  both circuits went back to the same panel ground, and seperate neutral bus.


I assume nominal 115-120V mains?
Quote:


Channel 1 was found, when starting to engage the clip led by driving the output of the midas past +4, to start putting out a lot more voltage than Channel 2.  Channel 2 behaved normally like almost every other amp at the test.  Channel 1 could be made to put out 81v when the clip light was moderately active.
The current meter on the Distro showed 33amp draw.


81V/2 ohm is 40A. A perfect amp would require 24A line cord draw for just this one channel, if mains were stiff 120V. Since the amp isn't 100% efficient (even class D amps aren't) the mains draw needs to also supply current to account for losses.

A clip light being "moderately" active on a sine wave means IT'S CLIPPING.  
Quote:


What else can I tell you.  A few of my EE friends, as I am not an EE, gave the testing a thumbs up, and a great way to check for thermal limits also.  

The 3602 was found to exibit this behavior at 30Hz and 15kHz, I did not push the test further for fear of ruining the amp.


My EE friends would ask you for more data.. like current draw and output voltage at 1kHz for the same input voltage.

I wouldn't be surprised to see clipping earlier at 30Hz than 1kHz. More data might help reveal a problem (or not). In general the clip light being on, makes the actual (clean) output voltage rather difficult to know from a clipped waveform measurement.  

If your EE friends approve voltage measurements with clipped waveforms, why stop at only moderate clipping? You could double the power with a full square wave.

JR

PS: Inexpensive pots in a value amp... I'm shocked.  Shocked
Title: Re: advantage?
Post by: Nathan Short on February 22, 2010, 04:21:23 pm
The bucket was set up to run 2 elements per load in parallel.  4ohm.

I will go ahead and type up my full measured list for the 3602.

since the amp has no "0" level the near "0" measurement was set to at "no clip" "solid -6"  The second measurement was taken as an average of moderate clip light flashing, but not solid or heavy.

It will go as follows  Noise     Vat"0"/V at "clip",  amps/amps

Pink 6db crest    33v/45v    15a/21a
30Hz              na   /60v     na  /33a
50Hz              40v/60v     24a/33a
85Hz              50v/60v     27a/32a
1kHz              45v/60v     24a/31a
15kHz             na/na          33a
Dance Music    
Light ,hard, limit   20v/41v/50v    10a/21a/26/




Title: Re: advantage?
Post by: Mac Kerr on February 22, 2010, 04:34:08 pm
Nathan Short wrote on Mon, 22 February 2010 16:21

since the amp has no "0" level the near "0" measurement was set to at "no clip" "solid -6"  The second measurement was taken as an average of moderate clip light flashing, but not solid or heavy.


This would have been a lot more useful if instead of "no clip" and "solid -6" that have no real reference you measured the actual input voltage and the actual output voltage at each frequency, and looked at the waveform on a scope to determine the onset of clipping. The actual input and output voltages at clipping at each frequency would also be good to know, as would the line voltage at each point.

If you are going to all the trouble of making measurements you may as well make complete measurements.

Given complete information on the test procedures Bob could duplicate the test on another amp and perhaps tell you whether your amps are broken or if it is a problem in the deign (or not).

Mac
Title: Re: advantage?
Post by: Elliot Thompson on February 22, 2010, 04:41:14 pm
Nathan,

I don’t see anything wrong with the amplifier. If you look below QSC states the PLX 3602 will draw 33 amperes when you feed it sine waves.

If the PLX 3602 was drawing 33 amperes, that would equate to 3960 watts on a 120-volt line source. It seems to me it exceeds
it's advertised wattage of 3600.  Smile


http://i46.tinypic.com/2euijbt.jpg



Best Regards,
Title: Re: advantage?
Post by: Art Welter on February 22, 2010, 05:18:27 pm
Nathan Short wrote on Mon, 22 February 2010 14:21

The bucket was set up to run 2 elements per load in parallel.  4ohm.

I will go ahead and type up my full measured list for the 3602.

since the amp has no "0" level the near "0" measurement was set to at "no clip" "solid -6"  The second measurement was taken as an average of moderate clip light flashing, but not solid or heavy.

It will go as follows  Noise     Vat"0"/V at "clip",  amps/amps

Pink 6db crest    33v/45v    15a/21a
30Hz              na   /60v     na  /33a
50Hz              40v/60v     24a/33a
85Hz              50v/60v     27a/32a
1kHz              45v/60v     24a/31a
15kHz             na/na          33a
Dance Music    
Light ,hard, limit   20v/41v/50v    10a/21a/26/




Nathan,

I can’t tell from what you wrote if these test results are both channels driven at 4 ohm, or just one channel.

Also, the PLX 3602 has no -6 indicator, it has SIG, -10 and CLIP.

The 60v sine wave figures you posted above would be a bit below the 1100 rated 20-20kHz output, 60 v at 4 ohm would be 900 watts.

By the current draw and output voltage you state, sounds like you were not quite hitting clip.
Always a question whether an amp’s clip lights come on a bit before or a bit after onset of clipping, from my experience with the PLX 3602, they come on before, but that is just “by ear”.

As Elliot pointed out, the PLX 3602 specifications state that with both channels driven with sine wave, it draws 25 amps at 8 ohms, 40 at 4 ohms, 63 amps at 2 ohms.

Back to the previous anomaly causing your concern, 81 volts into 2 ohms would be about 3280 watts, not an “abnormal” amount if the amp was hard clipping, 60 volts would be 1800 watts at 2 ohms, less than a 3 dB difference.
An amp producing a square wave can produce almost double the power output as a sine wave.

33 amps at 120 volts would not be out of line for 3280 watts delivered, 82.7 % efficiency.

Granted, both sides should perform the same, but the difference sounds like it could be a funky Clip Limiter on /off switch, with the limiter engaging on channel 2, but not on channel one.

The limiter only responds to actual clipping. When the limiter is engaged, the CLIP light indicates both clip and limit.

Art Welter
Title: Re: advantage?
Post by: Bob Lee (QSC) on February 22, 2010, 08:05:43 pm
Nathan, you keep saying that one channel behaved normally but you don't tell us how you define that.

And you complain that one channel will put out 81 volts, which is entirely possible. With no load and a normal mains voltage, yeah, it'll probably clip at around 81 V rms. With a load it'll clip at a lower voltage but can put out a higher RMS voltage by running it into clipping. You don't tell us anything about that 81 volts, like whether it's clipped or not, whether below clipping it's proportional to the input signal
Title: Re: adavantage?
Post by: Nathan Short on February 22, 2010, 09:19:27 pm
Hey guys, I am ordering this http://www.syscompdesign.com/OrderStep1.html

I have heard great things about it.  

Once I have my scope set up, I should be able to very much further refine my process.  

Thanks for all the replies, but this is beating a dead horse now.  The people who obviously have the rest of the proper testing equipment I do not have, want answers I cannot give them at this time.  I am slowly building my equipment up.

Maybe we could approach this more constructively with a full set of amps from various brands, and set some sort of "real world" standard.

I approached this out of frustration from what I believe to be inflated specs from manufacturers.  And wanted to gather my own tables of real output voltage at common levels of use and abuse.

IE see the performance of low and high level gear in my own situation.  

Plain and simple 3602 has an issue.  So I will be happy and glad to continue putting PL340's and 380's on my rigs.  They are rock solid, and a joy to work with.  
Title: Re: adavantage?
Post by: John Roberts {JR} on February 22, 2010, 11:39:09 pm
Nathan Short wrote on Mon, 22 February 2010 20:19

Hey guys, I am ordering this http://www.syscompdesign.com/OrderStep1.html

I have heard great things about it.  


I'm not familiar with that brand.. USB scopes range all over from crap to OK. You can probably get a real scope used for that, but if you know folks who have checked that one out and like it, go for it.  
Quote:


Once I have my scope set up, I should be able to very much further refine my process.  

Thanks for all the replies, but this is beating a dead horse now.  The people who obviously have the rest of the proper testing equipment I do not have, want answers I cannot give them at this time.  I am slowly building my equipment up.

Maybe we could approach this more constructively with a full set of amps from various brands, and set some sort of "real world" standard.

I approached this out of frustration from what I believe to be inflated specs from manufacturers.  And wanted to gather my own tables of real output voltage at common levels of use and abuse.


Please understand and take this the right way. There is frustration also from amp makers, some who have had to deal with uninformed criticisms for decades about amp specs with suggestions that the amp makers are dishonest.

I applaud you willingness to educate yourself and folks here will help you, but I would suggest a starting from a more neutral premise than manufacturer's specs are "inflated".  
Quote:


IE see the performance of low and high level gear in my own situation.  

Plain and simple 3602 has an issue.  So I will be happy and glad to continue putting PL340's and 380's on my rigs.  They are rock solid, and a joy to work with.  



I would also suggest doing a search and checking out the power amp test series that Bink did several years ago. There was a great deal of useful information in the advance work, and I probably wrote too many words on the subject of possible new ways to characterize duty cycle for audio amps.

Good luck. I expect you'll find this isn't as simple as you suspect.

JR

PS: You are starting from a good place with a good distro (I think). That is a common mistake made by many who blame their amps for things caused by bad mains power. Of course the scope will be useful to help confirm the quality of the mains too.
Title: Re: adavantage?
Post by: Brian Seefeldt on February 23, 2010, 01:32:41 am
Nathan Short wrote on Mon, 22 February 2010 20:19



Plain and simple 3602 has an issue.  So I will be happy and glad to continue putting PL340's and 380's on my rigs.  They are rock solid, and a joy to work with.  


Well...I'll buy Bob Lee and the guys @ QSC a beer anytime.
I have (4) 3602's and they have been damn near flawless for 3.5 years. (1 had a power supply go down) It was fixed under warranty,
(Very quickly i might add)
And these are amps that do not live in a perfect studio environment. There out working sometimes 4-5 nights a week.

I will be buying 2 more here very shortly.
Title: Re: adavantage?
Post by: Bob Lee (QSC) on February 23, 2010, 01:26:06 pm
Nathan, what do you mean by "current management"? The PL340 doesn't have any "current management" that the PLX3602 doesn't have. They're actually very similar in their core power supply and audio circuitry, and differ mainly in the features. You can download and compare the schematics if you like.
Title: Re: adavantage?
Post by: Nathan Short on February 23, 2010, 02:07:52 pm
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.
Title: Re: adavantage?
Post by: John Roberts {JR} 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  
Title: Re: adavantage?
Post by: Bob Lee (QSC) 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.
Title: Re: adavantage?
Post by: Nathan Short 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.
Title: Re: adavantage?
Post by: John Roberts {JR} 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


Title: Re: adavantage?
Post by: Nathan Short 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.
Title: Re: adavantage?
Post by: John Roberts {JR} 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

Title: Re: adavantage?
Post by: Bob Lee (QSC) 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?
Title: Re: adavantage?
Post by: Nathan Short 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.
Title: Re: adavantage?
Post by: Bob Lee (QSC) 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
Title: Re: adavantage?
Post by: John Roberts {JR} 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.




Title: Re: adavantage?
Post by: Nathan Short on February 26, 2010, 11:13:43 pm
Thanks for the cool info Jr,  Now just gotta wait for my scope to arrive, and then I should have some much better info.

I really wish some other people would try this, or QSC itself, and Just say what they find.  But I don't really expect them to say if they find a flaw...  

Gotta wait for my new scope.
Title: Re: adavantage?
Post by: John Roberts {JR} on February 27, 2010, 09:28:52 am
Nathan Short wrote on Fri, 26 February 2010 22:13


I really wish some other people would try this, or QSC itself, and Just say what they find.  But I don't really expect them to say if they find a flaw...  

Gotta wait for my new scope.

Why would people not talk about a problem..? You sound a little paranoid. Don't assume everybody is trying to cheat or lie to you. Amplifier specifications are generally more accurate than customer measurements.

I have no horse in this race. I used to compete with QSC, but in my judgment and experience, when they say an amp will do XYZ, it does. That and I have known Bob Lee some 30 years, and there isn't a drop of BS in him.  

In my judgment if a QSC amp had a 3 dB gain mismatch between channels, or a 3dB output difference, they wouldn't make it through final QA at the factory.

The clip(?) lights are probably not measured for precise accuracy.

We are all curious to learn what really is going on with your amp(s).

JR

Title: Re: adavantage?
Post by: Nathan Short on March 01, 2010, 05:41:40 pm
I am not paranoid.  But If I was a manufacturer had known there was an issue, a nice PM would have shut me up right quick.

The second issue, is this, why cant QSC go out to the warehouse, open up a couple and give this a try?  Two things might happen,  "hey wow, we made this happen too!"  or "damn, we could not do this , no matter how hard we tried, and we blew up the power supply trying"  

Not one word from other users, or the manufacturer. So unless, my scope could come more quickly...   I could go around with my oscillator and bucket to any number of 10 bars in the city, and see if it was just something I never noticed, or it is just these two new ones.

And a public forum is probably not the best place for this problem to be hashed out,  but again,  no one contacted me as this happened/unfolded.  And I can return these amps any time.
Title: How deep to dig
Post by: Mac Kerr on March 01, 2010, 05:48:39 pm
Put down the shovel, the hole is deep enough.

Mac
Title: Re: How deep to dig
Post by: Nathan Short on March 01, 2010, 06:02:26 pm
But Mac This Rabbit Hole is still here.
Title: Re: adavantage?
Post by: Chris Van Duker on March 01, 2010, 06:32:44 pm
Nathan,
In my experience, QSC has always been exceptionally transparent and proactive when there have been problems with their products. Two examples:

1) The original version of the PLX series had a problem with a ribbon cable which could fail after a certain period of time. QSC continued to repair that problem with these amps for free, even after the warranty had expired.

2) They had problems with the PFC power supply in the Powerlight 6.0 and 9.0 which affected (as I understand it) a fairly small minority of the units in the field. When they couldn't come up with an adequate fix for the problem, they acknowledged it and stopped selling them. I never dealt with this issue personally, but my understanding is that those who experienced the problem were well taken care of.

If I could air one complaint about QSC, it would be this: I'd really like it if they sold a 6-8KW, lightweight, mid-market amp.
Title: Re: adavantage?
Post by: John Roberts {JR} on March 01, 2010, 07:23:19 pm
Nathan Short wrote on Mon, 01 March 2010 16:41

I am not paranoid.  But If I was a manufacturer had known there was an issue, a nice PM would have shut me up right quick.


Would it work if I PM'd you.  Laughing

Seriously nobody wants to shut you up.. People would like to help you clear up "your" problem.

Since a large number of customers have used those models without the problems you report, there is justification to assume this may be a personal problem.  
Quote:


The second issue, is this, why cant QSC go out to the warehouse, open up a couple and give this a try?  Two things might happen,  "hey wow, we made this happen too!"  or "damn, we could not do this , no matter how hard we tried, and we blew up the power supply trying"  


You haven't been adequately precise or lucid in describing the exact problem.

I have tried a number of times to coax a clear description of what is going on from you without success so far. At the moment we are waiting for you to get some test equipment while I have suggested some simple MacGuyver tests you could do for free. A speaker across the two hots will surely identify a 3 dB gain mismatch. Likewise a Y cable into both inputs could help identify gain matching or even OL LED threshold discrepancy between channels.  

Quote:


Not one word from other users, or the manufacturer. So unless, my scope could come more quickly...   I could go around with my oscillator and bucket to any number of 10 bars in the city, and see if it was just something I never noticed, or it is just these two new ones.


If you've got the time to go bar hopping with test loads, you should be able to find the time to answer Bob's questions.

I am still unclear what this error is? As I have said a number of times, if you actually have a 3 dB gain difference between the two channels, QSC will fix it..  But that is not proved to be the case at least to me.
Quote:


And a public forum is probably not the best place for this problem to be hashed out,  but again,  no one contacted me as this happened/unfolded.  And I can return these amps any time.


No this is a fine forum, but you need to meet QSC half way and answer Bob's questions... So he can determine what you are seeing. This is not semantic games play. In technical troubleshooting, and I suspect Bob has decades of experience doing so, there is a need for following a very concise checklist of questions.

This is not a "the world" against Nathan thread, but you are testing the patience of people who aren't being paid to be nice to you, like Bob is.  Laughing  

We all want you to resolve you questions with insight and awareness, not conspiracy theories and suspicion of deviousness.

JR
Title: Re: adavantage?
Post by: Nathan Short on March 01, 2010, 08:00:25 pm
I have pretty much figured that the problem is at the onset of clip light indication.  

Unfortunately I do not have the luxury of doing more tests this past week as bids and shop work have taken all my time and a lot of 15hr days.

At different gain pot settings, driven by different levels of input, swapping both sides of the amps input to make sure the problem is not upstream in the signal flow.  My basic conclusion is that at 30Hz and lower, and at 15kHz and higher,  the ability of channel one to grossly outperform the output of channel 2 is unmistakable and possibly dangerous to my loudspeakers.

I would suspect the clip limiter is in some way at fault.

Channel 1 is consistantly able to put out 81v as measured by a voltmeter at the load of 4ohm.  Channel 2 is able to consistantly put out in the 60v range.

This is the symptom, and I need a doctor to help diagnose the disease. Or design flaw,  I would normally not drive an amp quite this hard in an install, but the test is to show me these weaknesses and I need to be prepared for all sorts of DJ abuse scenarios.  

I will most certainly keep at it, when I have more time, and my new tools, I will gather many more notes, post a video, and grab some big old beater subs To also use as a load.

I just wish someone else would try it.

Until then I am going to enjoy the PL340's and 380's that are proving to do an excellent job.
Title: Re: adavantage?
Post by: John Roberts {JR} on March 01, 2010, 08:42:21 pm
Nathan Short wrote on Mon, 01 March 2010 19:00

I have pretty much figured that the problem is at the onset of clip light indication.  

Unfortunately I do not have the luxury of doing more tests this past week as bids and shop work have taken all my time and a lot of 15hr days.

At different gain pot settings, driven by different levels of input, swapping both sides of the amps input to make sure the problem is not upstream in the signal flow.  My basic conclusion is that at 30Hz and lower, and at 15kHz and higher,  the ability of channel one to grossly outperform the output of channel 2 is unmistakable and possibly dangerous to my loudspeakers.

I would suspect the clip limiter is in some way at fault.

Channel 1 is consistantly able to put out 81v as measured by a voltmeter at the load of 4ohm.  Channel 2 is able to consistantly put out in the 60v range.

This is the symptom, and I need a doctor to help diagnose the disease. Or design flaw,  I would normally not drive an amp quite this hard in an install, but the test is to show me these weaknesses and I need to be prepared for all sorts of DJ abuse scenarios.  

I will most certainly keep at it, when I have more time, and my new tools, I will gather many more notes, post a video, and grab some big old beater subs To also use as a load.

I just wish someone else would try it.

Until then I am going to enjoy the PL340's and 380's that are proving to do an excellent job.


I believe it has already been suggested that 81V is a possible valid output level, so if one channel only puts out 60, That is the suspicious one. If it is limiting at 60V due to a valid external reason (like too low load impedance) or invalid reason like internal circuit fault, the OL indicator would fire early for either case.

The obvious test is, do both channels put out rated power into the identical, valid  test load, with identical input voltage?

I don't think your worry is one channel putting out too much in some faulty mode, but if anything one channel not putting out rated voltage.

If the loads and inputs are the same (and valid), and the outputs are that different, the amp is bad. Keep in mind that exact symptom could also be caused by a bad load, so that is worth confirming.  

JR
Title: Re: adavantage?
Post by: Nathan Short on March 01, 2010, 10:06:18 pm
You know JR  that brings up a great point, there might be some odd impedance problem at both ends of the audio spectrum.  I am gonna hook up my woofer tester and run an impedance sweep on both of the bucket loads.  I am pretty sure I tried both channels on of most of the amps on both dummy loads, but as I was taking mostly averages, when the amplifiers put out within 6v +or-  I was pretty happy.  That is why the 81v>60v was the one that really got me worried.
Title: Re: advantage?
Post by: Mac Kerr on March 01, 2010, 10:42:22 pm
Nathan Short wrote on Mon, 01 March 2010 22:06

You know JR  that brings up a great point, there might be some odd impedance problem at both ends of the audio spectrum.  I am gonna hook up my woofer tester and run an impedance sweep on both of the bucket loads.  I am pretty sure I tried both channels on of most of the amps on both dummy loads, but as I was taking mostly averages, when the amplifiers put out within 6v +or-  I was pretty happy.  That is why the 81v>60v was the one that really got me worried.


It might help to stop thinking in terms of volts and start thinking in terms of dB. While the difference between 60V and 81V looks like a lot of volts, it is only 2.5dB.

You don't need to test both bucket loads, you need to use just one of them so you have the same test jig on each channel, and measure the voltage at the input to the amp, as well as the voltage at the output of the amp. Having 2 signal paths in the test just adds another variable. The test of each channel needs to be exactly the same as the other. Since your area of interest is what is happening just before and after clipping ensues you should probably record the results with several voltages before and after the clip light comes on so you can see where the voltage gain between input and output varies from the published spec of 36dB.

Mac