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Luke's basic amplifier tests

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That's awesome!  But I'm still curious why it can't be pushed farther at 5 Hz.  Are you out of displacement there?  Or does it throttle down too quickly?  Am just curious what your thoughts are.

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That's awesome!  But I'm still curious why it can't be pushed farther at 5 Hz.  Are you out of displacement there?  Or does it throttle down too quickly?  Am just curious what your thoughts are.

 

It would throttle down after about 1 second at 5hz.  

 

I recall a post from Brian a few years ago saying the amp would reach the full 6kw down to 20hz, but by 12hz it was down to ~4,800w or something.

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Luke

It seems odd that the voltage recorded at 40Hz was lower than at 20Hz in both tests even with a higher impedance.

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Luke

It seems odd that the voltage recorded at 40Hz was lower than at 20Hz in both tests even with a higher impedance.

 

Yeah...you're right...  What the heck is up with that?

 

There's never any wiring changes or anything when going from 40hz to 20hz for any amp test, and I was running the CC4000 through it's paces last night and it also did the same thing. 

 

Could my system resonance around 35-36hz somehow hinder the amps performance near there?  The 40hz numbers across the board for the 3 amps I've tested do look a little on the low side. 

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Typically if the impedance gets higher the amplifier will be limited by the amount of voltage it can swing. If voltage is the limitation, rather than current demand, which will drop as the impedance increases, then the voltage limit should plateau with little fluctuation as long as the signals used are similar.

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Typically if the impedance gets higher the amplifier will be limited by the amount of voltage it can swing. If voltage is the limitation, rather than current demand, which will drop as the impedance increases, then the voltage limit should plateau with little fluctuation as long as the signals used are similar.

 

I re-ran the 40hz testing on the XLS and got the same result.  Pushing past clipping, 100-101v is like a brick wall and the amp will lower itself to ~95v after a second.

 

I checked the signal on the scope and pushing passed 95v does show distortion at the top of the sine wave, so it does appear the amp is reaching it's limit.

 

I'll do the same on the SP2 to see if I can squeeze anymore out of it. 

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Might be interesting to try a test at the impedance maximum and see what type of voltage reading you get there.

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Yeah...you're right...  What the heck is up with that?

 

There's never any wiring changes or anything when going from 40hz to 20hz for any amp test, and I was running the CC4000 through it's paces last night and it also did the same thing. 

 

Could my system resonance around 35-36hz somehow hinder the amps performance near there?  The 40hz numbers across the board for the 3 amps I've tested do look a little on the low side. 

 

Interesting.  I wonder if there is some measurement error somewhere?  Are you using two Fluke meters for measurement?  I don't know if yours has different capabilities than mine, but mine does supposedly offer True RMS.  However, I notice that the readings oscillate more the lower the frequency I measure.  IIRC, I went with the middle of the range for my number, but now I wonder if that might have been a high estimate.  My signal chain roll-off measurements did seem a bit too good to be true.

 

Maybe the 40 Hz measurement is correct and the 5 Hz and 10 Hz measurements are inflated.  Just a thought.

 

Edit: So to test this, maybe just run different tones at different frequencies but the same level to the amp.  If you get a higher voltage for 20 Hz than 40 Hz, then the issue probably is measurement.

Edited by SME

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Is the Fluke meter 'True RMS' only for a 60Hz AC signal?  

 

JSS

From casual observation, my meter seems to have a pretty wide range of validity.  It only started oscillating badly below 20 Hz or so, IIRC.  It also seems to exhibit some mild roll-off > 1 kHz, which may start at lower frequency.  I'm not sure.  I'll have to try playing with it some more.

 

I'm thinking of getting a calibrated SPL meter to get my absolute levels a bit closer as I suspect my rat-shack meter is reading 3 dB or so too low, in which case, I'm calibrating everything 3 dB too hot.

 

Edit: Don't know if this is the right meter, but here is an extended specs page that gives a range of 45-500 Hz for both volt and amp readings (page 3):

 

   http://media.fluke.com/documents/2155a.pdf

Edited by SME

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More thoughts.

 

If the meter itself has a flat response down to DC and the loss of accuracy below 40 Hz is due to the "true RMS" integration time not being long enough, I think this would explain the oscillation.  Furthermore, one would expect the spread of measurements to widen with decreasing frequency until the DC limit (vanishingly small frequencies) where it would oscillate between 0 and the the peak voltage.

 

So if you always write down is the highest voltage you see, you'll be off by between 0 and +3 dB.  Or you can subtract 1.5 dB, and now you're off by +/- 1.5 dB.  It'd be nice to be able to do a better correction, but I think we'd need to know what kind of averaging filter was used in the meter.  In time, I might think of a way to fudge things "a little better" though.

 

There may be an easier way.  Does the meter have a "peak hold" function?  Assuming it samples quickly enough, you might have an easier time just measuring voltage peaks and subtracting 3 dB.  For low frequency signals especially, it shouldn't be hard at all for the meter to get accurate voltage peak measurements.  You do have to test with sine signals for this method to work, of course.

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Might be interesting to try a test at the impedance maximum and see what type of voltage reading you get there.

 

 

More thoughts.

 

If the meter itself has a flat response down to DC and the loss of accuracy below 40 Hz is due to the "true RMS" integration time not being long enough, I think this would explain the oscillation.  Furthermore, one would expect the spread of measurements to widen with decreasing frequency until the DC limit (vanishingly small frequencies) where it would oscillate between 0 and the the peak voltage.

 

So if you always write down is the highest voltage you see, you'll be off by between 0 and +3 dB.  Or you can subtract 1.5 dB, and now you're off by +/- 1.5 dB.  It'd be nice to be able to do a better correction, but I think we'd need to know what kind of averaging filter was used in the meter.  In time, I might think of a way to fudge things "a little better" though.

 

There may be an easier way.  Does the meter have a "peak hold" function?  Assuming it samples quickly enough, you might have an easier time just measuring voltage peaks and subtracting 3 dB.  For low frequency signals especially, it shouldn't be hard at all for the meter to get accurate voltage peak measurements.  You do have to test with sine signals for this method to work, of course.

 

 

Or use a calibrated O-Scope.

 

JSS

 

I did some comparisons between the scope and the fluke meters with no load connected to the amp.  I did this from the sub output, but I raised the XO to it's highest setting of 250hz.

 

scopevsfluke1_zpsewqhvyit.png

 

 

The time in between the lowest and highest readings on the Fluke was 2-3 seconds at 5hz/10hz, so it's not oscillating very quickly.  However, when doing actual power testing, I don't let the sine wave run for that long, so my 5hz/10hz readings so far haven't been accurate since I don't know if I was taking the number from the high or low end.  

 

However, things got more interesting when I raised the voltage:

 

scopevsfluke2_zps9t4r1weq.png

 

 

The readings were always identical between the Fluke 117 and the Fluke 115, however, the higher the voltage got, the more the scope read a higher voltage than the Flukes, and that gap slowly gets larger the higher the voltage is.  

 

My thoughts are that I should use the scope to measure Vrms directly off the amps, and use a fluke to measure voltage across the resistor (I actually have no choice since the scope can't do this as I mentioned a few days ago).  The voltage across the resistor, which will never exceed ~650mv doesn't fluctuate very much at all even at 5hz.  

 

Thoughts?

 

 

Also, during my first comparison while the amp was in stereo I took some screenshots from the scope at each frequency:

 

DS1Z_QuickPrint1_zps0p7pzwkx.png

 

DS1Z_QuickPrint2_zpsr9vkiehu.png

 

DS1Z_QuickPrint3_zpsu2a6vobi.png

 

DS1Z_QuickPrint4_zps1s5qugyx.png

 

DS1Z_QuickPrint5_zpscwokuf3v.png

 

DS1Z_QuickPrint6_zpsriuydepq.png

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I'm also going to do more testing with the differential probe.  The "noise floor" was too high to get impedance measurements at levels of only 1-3 volts (it reads ~12-14mV even when not connected to anything), but once the load exceeds ~2A across the resistor (20mV) it should be accurate but I need to put it through it's paces.

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Crown XLS 2500 (Bridged/Bypass mode with clip limiting turned off)

 

 

XLS%202500_zpsa9inbyji.jpg

 

 

Even though the clip limiters were off, once clipping started the voltage wouldn't increase even when turning up the main volume.

 

At 10hz and lower, rather than seeing clip lights, the subs would start "pulsing" rapidly.  I've seen similar behavior from my Lab Gruppen IPD 2400. 

 

I re-ran the testing for the XLS 2500, this time using the scope to measure Vrms and also letting the sine waves run a little longer at 5/10hz to capture the range of current across the resistor.  

 

xls2500_v2_zpsghxaskwj.jpg

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SpeakerPower SP2-12000 (1ch driven)

 

SP2-12K_zpsfntvbryl.jpg

 

Once the clip lights were on I didn't push any further, and at 5hz it wouldn't have made any difference.  However, I've read several things that indicate on this specific amp, once the clip lights are seen, there's still 1-2db left before actual clipping occurs.  It's possible there was some power left in the tank.  

 

This time I ignored the clip lights and watched for any distortion in the wave form.  At 5hz it can only maintain that power for about a second, and that's not long enough to get a low/high range of the fluctuation.

 

SP2-12K_v2_zpscaxmc5ki.jpg

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Luke, Have you measured the 7500 or crest compared to the Inuke 6000?  At 4ohms and then what did the 7500 do at 2 ohms? I might buy another amp if significant. Which speakerpower amp is equivalent? 

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Sorry to keep bugging you Luke but the results still don't look right to me. Do you have impedance curve measurements of the subs in the cabs covering say 2-200Hz?

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Luke, Have you measured the 7500 or crest compared to the Inuke 6000?  At 4ohms and then what did the 7500 do at 2 ohms? I might buy another amp if significant. Which speakerpower amp is equivalent? 

 

Unfortunately I sold the 7.5 so I can't do any further testing.  However, I do have a NU4-6000 I'll be testing. 

 

 

Sorry to keep bugging you Luke but the results still don't look right to me. Do you have impedance curve measurements of the subs in the cabs covering say 2-200Hz?

 

No problem man!  Is it still the voltage that seems off, or the current?

 

On the XLS, the voltage at 20hz can only be maintained for 1 second or so, which I think is misleading since all the other voltages can be maintained for much longer.

 

I'll take another impedance sweep.  Should I take it from a single cabinet, or all 4 wired to a single 1.5ohm load like I used for most of the testing?

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Can you do all 4 cabs as wired and a single? It's the voltage being less at 40Hz than 20Hz despite a 20% higher impedance. Just doesn't seem right to me. 

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Can you do all 4 cabs as wired and a single? It's the voltage being less at 40Hz than 20Hz despite a 20% higher impedance. Just doesn't seem right to me. 

 

Yeah I can do that.

 

On the XLS, even without a load connected the most voltage I can get out of it was ~100v.  On the SP2, both 20hz and 40hz are clipping and are beyond the voltage specs listed.  I did get more voltage at 40hz than 20hz on the CC amps though. 

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Sorry to keep bugging you Luke but the results still don't look right to me. Do you have impedance curve measurements of the subs in the cabs covering say 2-200Hz?

 

Is it possible that by design the SP's are capable of lots of current but not wide enough voltage swings?

 

Isn't your K20 a voltage monster?

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